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  • 1.
    Abrahamsson, Eric
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Samuel, Palm
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Optimering av täthetsprovning av ventiler2022Independent thesis Basic level (professional degree), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Nordhydraulic is a company in Kramfors that manufactures hydraulic valves. Hydraulic valves are used in a variety of areas to control hydraulic machines. For hydraulic valves to function properly, they must be tight. At Nordhydraulic, a leak tester called Nolek S9 N is used to determine if valves are tight. The leak tester fills the valves with a predetermined pressure and then measures the flow of air into the valve required to maintain the pressure.

    Today, this method has major problems with false negative results in the assembly line, which means that tight valves may give a result that indicates leakage and thus can lead to a bottleneck in the assembly. This is because there are many spaces in the valve that are difficult to reach by the air. The purpose of this work was to develop a better program for the Nolek S9 N to be used in the leak test to reduce assembly stops. The development of this program is done by making measurements with different time settings as well as connection methods to find the most important parameters of the program. Then, measurements are made on valves assembly errors and casting errors to see what flow the leakage has at these errors.

    The work resulted in three different programs for the RS210 valve, which in tests in the assembly achieved approved results faster than the previous program and with fewer false negative results. The conclusion is that the developed program could provide an improvement of the leak test and that the current limit for approved leakage can be increased to 50 mm^3/s  without missing the assembly errors that may occur.

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    Optimering av täthetsprovning av ventiler
  • 2.
    Adolfsson, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Expanding operation ranges using active flow control in Francis turbines2014Independent thesis Basic level (university diploma), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    This report contains an investigation of fluid injection techniques used in the purpose of reducing deleterious flow effects occurring in the draft tube of Francis turbines when operating outside nominal load. There is a focus on implement ability at Jämtkrafts hydroelectric power plants and two power plants were investigated, located in series with each other named Lövhöjden and Ålviken. The only profitable scenario found with some degree of certainty was an increase in the operating range upwards to allow overload operation.

    Findings show that both air and water can be introduced in various locations to improve hydraulic efficiency around the turbine parts as well as reduce pressure pulsations in harmful operating regions. Investments in such systems have proven useful and profitable at several facilities with poorly adapted operating conditions. But due to losses in efficiency when operating injection systems, it turns out unprofitable in situations where it does not improve the operating range in a way that is resulting in increased annual or peak production.

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    Lastområdesutvidgning med aktiv flödeskontroll i Francisturbiner
  • 3.
    Akkerman, V'yacheslav
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bastiaans, R. J. M.
    de Goey, L. P. H.
    van Oijen, J. A.
    Eriksson, L. E.
    Flow-flame interaction in a closed chamber2008In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 5, p. 055107-055121Article in journal (Refereed)
    Abstract [en]

    Numerous studies of flame interaction with a single vortex andrecent simulations of burning in vortex arrays in open tubesdemonstrated the same tendency for the turbulent burning rate$\propto U_{rms}\lambda^{2/3}$, where  $U_{rms}$ is theroot-mean-square velocity and $\lambda$ is the vortex size. Here itis demonstrated that this tendency is not universal for turbulentburning. Flame interaction with vortex arrays is investigated forthe geometry of a closed burning chamber using direct numericalsimulations of the complete set of gas-dynamic combustion equations.Various initial conditions in the chamber are considered, includinggas at rest and several systems of vortices of different intensitiesand sizes. It is found that the burning rate in a closed chamber(inverse burning time) depends strongly on the vortex intensity; atsufficiently high intensities it increases with $U_{rms}$approximately linearly in agreement with the above tendency. On thecontrary, dependence of the burning rate on the vortex size isnon-monotonic and qualitatively different from the law$\lambda^{2/3}$. It is shown that there is an optimal vortex size ina closed chamber, which provides the fastest total burning rate. Inthe present work the optimal size is 6 times smaller than thechamber height.

  • 4.
    Akkerman, V'yacheslav
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, WV, Morgantown, United States.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kuznetsov, Mikhail
    Institute for Nuclear and Energy Technologies, Karlsruhe Institute of Technology, Karlsruhe, Germany.
    Law, Chung K.
    Department of Mechanical and Aerospace Engineering, Princeton University, NJ, Princeton, United States.
    Valiev, Damir
    Department of Mechanical and Aerospace Engineering, Princeton University, NJ, Princeton, United States.
    Wu, Ming-Hsun
    Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan.
    Fast flame acceleration and deflagration-to-detonation transition in smooth and obstructed tubes, channels and slits2013In: 8th US National Combustion Meeting 2013, Western States Section/Combustion Institute , 2013, p. 970-978Conference paper (Refereed)
    Abstract [en]

    This work is devoted to the comprehensive analytical, computational and experimental investigation of various stages of flame acceleration in narrow chambers. We consider mesoscale two-dimensional channels and cylindrical tubes, smooth and obstructed, and sub-millimeter gaps between two parallel plates. The evolution of the flame shape, propagation speed, acceleration rate, and velocity profiles nearby the flamefront are determined for each configuration, with the theories substantiated by the numerical simulations of the hydrodynamics and combustion equations with an Arrhenius reaction, and by the experiments on premixed hydrogen-oxygen and ethylene-oxygen flames. The detailed analyses demonstrate three different mechanisms of flame acceleration: 1) At the early stages of burning at the closed tube end, the flamefront acquires a finger-shape and demonstrates strong acceleration during a short time interval. While this precursor acceleration mechanism is terminated as soon as the flamefornt touches the side wall of the tube, having a little relation to the deflagration-to-detonation transition (DDT) for relatively slow, hydrocarbon flames; for fast (e.g. hydrogen-oxygen) flames, even a short finger-flame acceleration may amplify the flame propagation speed up to sonic values, with an important effect on the subsequent DDT process. 2) On the other hand, the classical mechanism of flame acceleration due to wall friction in smooth tubes is basically unlimited in time, but it depends noticeably on the tube width such that the acceleration rate decreases strongly with the Reynolds number. The entire DDT scenario includes four distinctive stages: (i) initial exponential acceleration at the quasi-incompressible state; (ii) moderation of the process because of gas compression; (iii) eventual saturation to a quasisteady, high-speed flames correlated with the Chapman-Jouguet deflagration; (iv) finally, the heating of the fuel mixture leads to the explosion ahead of the flame front, which develops into a self-supporting detonation. 3) In addition, we have revealed a physical mechanism of extremely fast flame acceleration in channels/tubes with obstacles. Combining the "benefits" of 1) and 2), this new mechanism is based on delayed burning between the obstacles, creating a powerful jet-flow and thereby driving the acceleration, which is extremely strong and independent of the Reynolds number, so the effect can be fruitfully utilized at industrial scales. Understanding of this mechanism provides the guide for optimization of the obstacle shape, while this task required tantalizing cut-and-try methods previously. On the other hand, our formulation opens new technological possibilities of DDT in micro-combustion.

  • 5.
    Akkerman, V’yacheslav
    et al.
    Nuclear Safety Institute of Russian Academy of Sciences B. Tulskaya 52, 115191 Moscow, Russia.
    Ivanov, Mikhail
    Department of Physics and Power Engineering, Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Moscow Region, Russia.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Turbulent flow produced by Piston Motion in a Spark-ignition engine2009In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 82, no 3, p. 317-337Article in journal (Refereed)
    Abstract [en]

    Turbulence produced by the piston motion in spark-ignition engines is studied by 2D axisymmetric numerical simulations in the cylindrical geometry as in the theoretical and experimental work by Breuer et al (Flow Turb. Combust. 74 (2005) 145, Ref. [1]). The simulations are based on the Navier-Stokes gas-dynamic equations including viscosity, thermal conduction and non-slip at the walls. Piston motion is taken into account as a boundary condition. The turbulent flow is investigated for a wide range of the engine speed, 1000-4000 rpm, assuming both zero and non-zero initial turbulence. The turbulent rms-velocity and the integral length scale are investigated in axial and radial directions. The rms-turbulent velocity is typically an order-of-magnitude smaller than the piston speed. In the case of zero initial turbulence, the flow at the top-dead-center may be described as a combination of two large-scale vortex rings of a size determined by the engine geometry. When initial turbulence is strong, then the integral turbulent length demonstrates self-similar properties in a large range of crank angles. The results obtained agree with the experimental observations of [1].

  • 6. Arnott, Russell N.
    et al.
    Cherif, Mehdi
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Bryant, Lee D.
    Wain, Danielle J.
    Artificially generated turbulence: a review of phycological nanocosm, microcosm, and mesocosm experiments2021In: Hydrobiologia, ISSN 0018-8158, E-ISSN 1573-5117, Vol. 848, p. 961-991Article, review/survey (Refereed)
    Abstract [en]

    Building on a summary of how turbulence influences biological systems, we reviewed key phytoplankton-turbulence laboratory experiments (after Peters and Redondo in Scientia Marina: Lectures on plankton and turbulence, International Centre for Coastal Resources, Barcelona, 1997) and Peters and Marrase (Marine Ecology Progress Series 205:291-306, 2000) to provide a current overview of artificial turbulence generation methods and quantification techniques. This review found that most phytoplankton studies using artificial turbulence feature some form of quantification of turbulence; it is recommended to use turbulent dissipation rates (epsilon) for consistency with physical oceanographic and limnological observations. Grid-generated turbulence is the dominant method used to generate artificial turbulence with most experiments providing quantified epsilon values. Couette cylinders are also commonly used due to the ease of quantification, albeit as shear rates not epsilon. Dinoflagellates were the primary phytoplanktonic group studied due to their propensity for forming harmful algal blooms (HAB) as well as their apparent sensitivity to turbulence. This study found that a majority of experimental setups are made from acrylate plastics that could emit toxins as these materials degrade under UV light. Furthermore, most cosm systems studied were not sufficiently large to accommodate the full range of turbulent length scales, omitting larger vertical overturns. Recognising that phytoplankton-turbulence interactions are extremely complex, the continued promotion of more interdisciplinary studies is recommended.

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  • 7.
    Baigmohammadi, Mohammadreza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Roussel, Olivier
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dion, Claude
    Umeå University, Faculty of Science and Technology, Department of Physics.
    A Numerical Study of Lean Propane-Air Flame Acceleration at the Early Stages of Burning in Cold and Hot Isothermal Walled Small-Size Tubes2020In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 104, no 1, p. 179-207Article in journal (Refereed)
    Abstract [en]

    In this study, the problem of lean propane-air premixed flame acceleration from closed to open end during the early stages of burning in small-size tubes with isothermal walls was considered. In particular, the effects of tube radius, slip/non-slip wall conditions, and the wall temperature on the flame propagation and shape were investigated numerically. Five stages of flame propagation are identified: 1) spherical expansion of the flame front; 2) finger shape expansion of the flame front before touching the wall; 3) flame propagation in the tube subjected to flame-wall interactions; 4) transformation of the flame shape into tulip form; 5) conversion of the tulip shape flame to finger. Our results show that the tube radius, wall condition and its temperature significantly affect flame propagation regimes even in the first instance of the flame propagation in the tubes. We find that increasing tube radius has, overall, the effect of increasing the flame propagation speed in isothermal tubes. Also, depending on tube radius and wall condition, the wall temperature can increase or decrease the flame propagation speed in the isothermal tubes. Furthermore, the results demonstrate that imposing either slip or non-slip condition on the tube’s walls impressively affects flame acceleration and its configuration in the early stages. We observe that, unlike flame propagation forms in the tubes with slip walls, the early exponential flame propagation phase in the tubes was generally followed by a linear flame propagation phase in the tubes with a non-slip wall condition. We obtain that flame propagation in tubes with slip wall conditions are more sensitive to variations in tube radius and wall temperature compared to non-slip conditions. We also see that, contrary to the exponential flame propagation phase, increasing the non-slip wall temperature reduces the flame propagation speed in the linear part of the flame propagation, while such an increase in temperature leads to oscillations in the flame propagation speed in the tubes with slip walls.

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  • 8.
    Baigmohammadi, Mohammadreza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Combustion and Turbulence Research Laboratory (CTL), Department of Aerospace Engineering, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave, Tehran, Iran.
    Tabejamaat, Sadegh
    Experimental study on the effect of external thermal pattern on the dynamics of methane-oxygen and methane-oxygen-carbon dioxide premixed flames in non-adiabatic meso-scale reactors2019In: International journal of thermal sciences, ISSN 1290-0729, E-ISSN 1778-4166, Vol. 137, p. 242-252Article in journal (Refereed)
    Abstract [en]

    In the current study, the effect of external thermal pattern on the dynamics and characteristics of methane oxygen and methane-oxygen-carbon dioxide premixed flames in non-adiabatic meso-scale cylindrical reactors is investigated experimentally. In this regard, two different external thermal patterns were imposed on the outer surface of the reactors. The results showed that imposing method/direction and also temperature level of the external thermal pattern have impressive effect on flame dynamics and chemiluminescence in the non-adiabatic meso-scale reactors. Also, it was shown that increasing the temperature level of the external thermal pattern could significantly extend the flame stability and its presence range in the meso-scale reactors, especially for the vitiated mixtures (methane-oxygen-carbon dioxide). Moreover, the results demonstrated that decreasing the inner diameter of a meso-scale reactor, which was subjected to an external thermal pattern, could increase the flame controllability and its presence range in the non-adiabatic meso-scale reactors.

  • 9.
    Baigmohammadi, Mohammadreza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Combustion and Turbulence Research Laboratory (CTL), Department of Aerospace Engineering, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., Tehran, 15875-4413, Iran.
    Tabejamaat, Sadegh
    Faghani-Lamraski, Morteza
    Experimental study on the effects of mixture flow rate, equivalence ratio, oxygen enhancement, and geometrical parameters on propane air premixed flame dynamics in non-adiabatic meso-scale reactors2017In: Energy, ISSN 0360-5442, E-ISSN 1873-6785, Vol. 121, p. 657-675Article in journal (Refereed)
    Abstract [en]

    In the present study, the effects of reactive mixture flow rate, adding oxygen to propane-air mixture, geometrical parameters, and equivalence ratio on propane-air/oxygen premixed flame dynamics in non adiabatic meso-scale reactors were experimentally investigated. During the experiments, seven flame regimes of blow-off, blow-out, asymmetric stationary, stationary-repetitive extinction and re-ignition (RERI), forced/self-RERI, RERI-flash-back, and flash-back were observed. The results showed that increasing the reactive mixture flow rate could generally promote variety of the flame regimes and also improve flame stability in the non-adiabatic meso-scale reactors, especially in 40% and 80% oxygen enhanced cases. Also, the results demonstrated that increasing the reactor inner diameter and equivalence ratio generally extended propane-air- oxygen flame stability and its presence range in the non adiabatic meso-scale reactors. Moreover, it was shown that increasing the reactor length and also increasing the added oxygen to propane-air mixture more than 40% promoted flame instability and consequently restricted propane-air-oxygen flame presence range in the non-adiabatic meso-scale reactors. Also, it was shown that variations in the mixture flow rate, the reactor length and inner diameter, equivalence ratio, and oxygen concentration in propane-air mixture could significantly influence the flame average propagation speed, acoustic, and chemiluminescence in the non-adiabatic meso-scale reactors.

  • 10.
    Baigmohammadi, Mohammadreza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tabejamaat, Sadegh
    Javanbakht, Zeinab
    Numerical Study of Methane-Oxygen Premixed Flame Characteristics in Non-adiabatic Cylindrical Meso-Scale Reactors with the Backward-Facing Step2019In: Iranian Journal of Science and Technology, Transactions of Mechanical Engineering, ISSN 2228-6187, Vol. 43, p. 117-140Article in journal (Refereed)
    Abstract [en]

    In the present study, the effects of reactor diameter, inlet velocity, velocity profile, equivalence ratio (Phi, Ф), and outer wall convective and radiative heat transfer coefficients on flame characteristics in cylindrical non-adiabatic meso-scale reactors with the backward-facing step were investigated numerically. The results showed that these parameters could strongly affect the mole fraction of radical species within the flame zone. Also, it was shown that as compared to the reactor with 3 mm inner diameter, increasing the inlet velocity in the reactor with 5 mm inner diameter may lead to the opposite effect on the flame location. In addition, it was observed that the velocity profile could sensibly affect the flame location, temperature, and the species mole fractions in the meso-scale reactors. Moreover, it was demonstrated that the effect of equivalence ratio on the flame characteristics was more crucial for the reactors with smaller diameters. Furthermore, it was maintained that the outer wall convective and radiative heat transfer coefficients could cause the flame instability in the meso-scale reactors because of decreasing the mole fraction of important species such as O, H, and OH in the vicinity of the reactor inner wall.

  • 11.
    Bejarano, Maria Dolores
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Sordo-Ward, Alvaro
    Alonso, Carlos
    Nilsson, Christer
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Characterizing effects of hydropower plants on sub-daily flow regimes2017In: Journal of Hydrology, ISSN 0022-1694, E-ISSN 1879-2707, Vol. 550, p. 186-200Article in journal (Refereed)
    Abstract [en]

    A characterization of short-term changes in river flow is essential for understanding the ecological effects of hydropower plants, which operate by turning the turbines on or off to generate electricity following variations in the market demand (i.e., hydropeaking). The goal of our study was to develop an approach for characterizing the effects of hydropower plant operations on within-day flow regimes across multiple dams and rivers. For this aim we first defined ecologically meaningful metrics that provide a full representation of the flow regime at short time scales from free-flowing rivers and rivers exposed to hydropeaking. We then defined metrics that enable quantification of the deviation of the altered short-term flow regime variables from those of the unaltered state. The approach was successfully tested in two rivers in northern Sweden, one free-flowing and another regulated by cascades of hydropower plants, which were additionally classified based on their impact on short-term flows in sites of similar management. The largest differences between study sites corresponded to metrics describing sub-daily flow magnitudes such as amplitude (i.e., difference between the highest and the lowest hourly flows) and rates (i.e., rise and fall rates of hourly flows). They were closely followed by frequency-related metrics accounting for the numbers of within-day hourly flow patterns (i.e., rises, falls and periods of stability of hourly flows). In comparison, between-site differences for the duration-related metrics were smallest. In general, hydropeaking resulted in higher within-day flow amplitudes and rates and more but shorter periods of a similar hourly flow patterns per day. The impacted flow feature and the characteristics of the impact (i.e., intensity and whether the impact increases or decreases whatever is being described by the metric) varied with season. Our approach is useful for catchment management planning, defining environmental flow targets, prioritizing river restoration or dam reoperation efforts and contributing information for relicensing hydropower dams. 

  • 12.
    Berggren, Martin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Bernland, Anders
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Noreland, Daniel
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Acoustic boundary layers as boundary conditions2018In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 371, p. 633-650Article in journal (Refereed)
    Abstract [en]

    The linearized, compressible Navier-Stokes equations can be used to model acoustic wave propagation in the presence of viscous and thermal boundary layers. However, acoustic boundary layers are notorious for invoking prohibitively high resolution requirements on numerical solutions of the equations. We derive and present a strategy for how viscous and thermal boundary-layer effects can be represented as a boundary condition on the standard Helmholtz equation for the acoustic pressure. This boundary condition constitutes an O (delta) perturbation, where delta is the boundary-layer thickness, of the vanishing Neumann condition for the acoustic pressure associated with a lossless sound-hard wall. The approximate model is valid when the wavelength and the minimum radius of curvature of the wall is much larger than the boundary layer thickness. In the special case of sound propagation in a cylindrical duct, the model collapses to the classical Kirchhoff solution. We assess the model in the case of sound propagation through a compression driver, a kind of transducer that is commonly used to feed horn loudspeakers. Due to the presence of shallow chambers and thin slits in the device, it is crucial to include modeling of visco-thermal losses in the acoustic analysis. The transmitted power spectrum through the device calculated numerically using our model agrees well with computations using a hybrid model, where the full linearized, compressible Navier-Stokes equations are solved in the narrow regions of the device and the inviscid Helmholtz equations elsewhere. However, our model needs about two orders of magnitude less memory and computational time than the more complete model. 

  • 13.
    Bilgili, Serdar
    et al.
    Computational Fluid Dynamics and Applied Multi-Physics Center, Center for Innovation in Gas Research and Utilization (CIGRU), Center for Alternative Fuels, Engines and Emissions (CAFEE), Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, United States.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Akkerman, V'Yacheslav
    Computational Fluid Dynamics and Applied Multi-Physics Center, Center for Innovation in Gas Research and Utilization (CIGRU), Center for Alternative Fuels, Engines and Emissions (CAFEE), Department of Mechanical and Aerospace Engineering, West Virginia University, 1306 Evansdale Drive, WV, Morgantown, United States; Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, United States.
    Impacts of the Lewis and Markstein numbers on premixed flame acceleration in channels due to wall friction2022In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 34, no 1, article id 013604Article in journal (Refereed)
    Abstract [en]

    The effects of flame stretch as well as that of thermal and molecular diffusion on the scenario of flame acceleration in channels are quantified by means of computational and analytical endeavors. The analytical formulation incorporates the internal transport flame properties into the theory of flame acceleration due to wall friction by means of the Markstein number, which characterizes the flame response to curvature and stretch. Being a positive or negative quantity and a function of the thermal-chemical combustion parameters, such as the thermal expansion ratio as well as the Lewis and Zeldovich numbers, the Markstein number either moderates or promotes flame acceleration. While the Markstein number may provide a substantial impact on the flame acceleration rate in narrow channels, this effect diminishes with increase in the channel width. The analytical formulation is accompanied by extensive computational simulations of the reacting flow equations, which clarify the impact of the Lewis number on flame acceleration. It is noted that for Lewis numbers below a certain critical value, at the initial stage of flame acceleration, a globally convex flame front splits into two or more finger-like segments, accompanied by a drastic increase in the flame front surface area and associated enhancement of flame acceleration. Later, however, these segments of the flame front meet, promptly consuming cavities and pockets, which substantially decreases the flame surface area and moderates acceleration. Eventually, this dynamics results in a single, globally convex flame, which keeps accelerating. Overall, the thermal-diffusive effects substantially facilitate flame acceleration, thereby advancing a potential deflagration-to-detonation transition.

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  • 14.
    Bodin, Kenneth
    et al.
    Umeå University, Faculty of Science and Technology, High Performance Computing Center North (HPC2N).
    Lacoursière, Claude
    Umeå University, Faculty of Science and Technology, High Performance Computing Center North (HPC2N).
    Nilsson, Martin
    Servin, Martin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Constraint based particle fluids on GPGPU2011Conference paper (Other academic)
    Abstract [en]

    We present a fluid simulation method adapted for stream parallelism on general purpose graphics processingunits (GPGPU). In this method the equations of Navier and Stokes are discretized using particles and kernelfunctions as in Smoothed Particle Hydrodynamics (SPH), but rather than using penalty methods or solving for a divergence free velocity field, incompressibility is enforced using holonomic kinematic constraints [1]. We useone constraint for each smoothed particle stating that the local density should be kept constant. Other constraintsare used for boundary conditions and multiphysics coupling. We also present a viscosity model in which theshear rate at each pseudo particle is constrained to satisfy a given constitutive law. The computation of theconstraint forces, namely, the pressure and the stresses, requires the solution system of linear equations whichhave a sparse, saddle point structure. These are solved using the Uzawa method of preconditioned conjugate gradients (CG) applied directly to the symmetric indefinite matrix. The overall simulation method has its rootsin a discrete variational principle and the SPOOK time stepping scheme for constrained mechanical systems [2].The SPOOK method is second order accurate on the positions and constraints violations, and is stable at largetime-steps, thus often allowing several orders of magnitude larger timesteps in our method compared to intraditional SPH methods. The numerical implementation on GPGPU that is the main result of this paper consistsof the following components: particle neighbour searches based on spatial decomposition; summation of kernel densities; construction of Jacobians representing the constraints on the density, boundary conditions, viscosityand multiphysics couplings; a Uzawa CG solver for the system of linear equations; and finally, discrete timestepping of velocities and positions. The CG solver is particularly suitable for stream computing since it is basedon matrix-vector multiplications. The sparse system data is stored in a compressed matrix format and the algorithms operating on this data on GPGPU are implemented in CUDA and OpenCL. Our simulation resultsinclude performance measurements, and validation of the method for benchmark problems. We achieve up totwo orders of magnitude speed-up from the GPGPU over traditional processors and together with the increased timestep efficiency of our method we arrive at interactive performance for systems with up to two million fluidparticles representing an incompressible fluid.

  • 15.
    Bokhari, Ahmad Hasnain
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Noreland, Daniel
    Umeå University, Faculty of Science and Technology, Department of Computing Science. Forest operations Uppsala Science Park SE-751 83 Uppsala.
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science. Department of Mathematics and Computer Science, Karlstad University.
    Topology optimization of a subwooferManuscript (preprint) (Other academic)
    Abstract [en]

    We use material distribution-based topology optimization to optimize the design of a bandpass subwoofer enclosure. The objective is to maximize the subwoofer's output power for a single frequency as well as for a range of frequencies. A linear electromechanical transducer model is combined with a hybrid 2D-3D model for sound propagation to model the subwoofer's performance. The adjoint variable approach is used to compute the gradients of the objective function with respect to the design variables, and the Method of Moving Asymptotes (MMA) is used to solve the topology optimization problem. To manage intermediate values of the material indicator function, a quadratic penalty is added to the objective function, and a non-linear filter is used to obtain a mesh independent design. By carefully selecting the target frequency range, we can guide the optimization algorithm to successfully generate a subwoofer design with the required bandpass character. This study constitutes, to the best of our knowledge, the first successful attempt to design the interior structure of a loudspeaker using topology optimization. The success is much due to the hybrid 2D-3D approach, which reduces the computational effort significantly with only small effects on the modeling accuracy. 

  • 16.
    Burman, Erik
    et al.
    Department of Mathematics, University College London, WC1E 6BT, London, United Kingdom.
    Hansbo, Peter
    Department of Mechanical Engineering, Jönköping University, Jönköping, Sweden.
    Larson, Mats G.
    Umeå University, Faculty of Science and Technology, Department of Mathematics and Mathematical Statistics.
    Error Estimates for the Smagorinsky Turbulence Model: Enhanced Stability Through Scale Separation and Numerical Stabilization2022In: Journal of Mathematical Fluid Mechanics, ISSN 1422-6928, E-ISSN 1422-6952, Vol. 24, no 1, article id 5Article in journal (Refereed)
    Abstract [en]

    In the present work we show some results on the effect of the Smagorinsky model on the stability of the associated perturbation equation. We show that in the presence of a spectral gap, such that the flow can be decomposed in a large scale with moderate gradient and a small amplitude fine scale with arbitratry gradient, the Smagorinsky model admits stability estimates for perturbations, with exponential growth depending only on the large scale gradient. We then show in the context of stabilized finite element methods that the same result carries over to the approximation and that in this context, for suitably chosen finite element spaces the Smagorinsky model acts as a stabilizer yielding close to optimal error estimates in the L2-norm for smooth flows in the pre-asymptotic high Reynolds number regime.

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  • 17.
    Bychkov, Vitaly
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Modestov, Mikhail
    Law, Chung K.
    Combustion Phenomena in Modern Physics: I. Inertial Confinement Fusion2015In: Progress in Energy and Combustion Science, ISSN 0360-1285, E-ISSN 1873-216X, Vol. 47, p. 32-59Article, review/survey (Refereed)
    Abstract [en]

    The overarching objective of the present endeavor is to demonstrate the universal character of combustion phenomena for various areas of modern physics, focusing on inertial confinement fusion (ICF) in this review. We present the key features of laser deflagration, and consider the similarities and differences between the laser plasma flow and the slow combustion front. We discuss the linear stage of the Rayleigh-Taylor instability in laser ablation, short-wavelength stabilization of the instability due to the mass flow, and demonstrate the importance of the concepts and methods of combustion science for an understanding of the corresponding ICF processes. We show the possibility of the Darrieus-Landau instability in the laser ablation flow and discuss the specific features of the instability at the linear and nonlinear stages as compared to the combustion counterpart of this phenomenon. We consider the nonlinear stage of the Rayleigh-Taylor instability in the ICF and generation of ultra-high magnetic field by the instability, and show that proper understanding of vorticity production in the laser plasma and, hence, of the magnetic field generation requires concepts from combustion science.

  • 18.
    Bychkov, Vitaly
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sadek, Jad
    Akkerman, V'yacheslav
    Analysis of flame acceleration in open or vented obstructed pipes2017In: PHYSICAL REVIEW E, ISSN 2470-0045, Vol. 95, no 1, article id 013111Article in journal (Refereed)
    Abstract [en]

    While flame propagation through obstacles is often associated with turbulence and/or shocks, Bychkov et al. [V. Bychkov et al., Phys. Rev. Lett. 101, 164501 (2008)] have revealed a shockless, conceptually laminar mechanism of extremely fast flame acceleration in semiopen obstructed pipes (one end of a pipe is closed; a flame is ignited at the closed end and propagates towards the open one). The acceleration is devoted to a powerful jet flow produced by delayed combustion in the spaces between the obstacles, with turbulence playing only a supplementary role in this process. In the present work, this formulation is extended to pipes with both ends open in order to describe the recent experiments and modeling by Yanez et al. [J. Yanez et al., arXiv: 1208.6453] as well as the simulations by Middha and Hansen [P. Middha and O. R. Hansen, Process Safety Prog. 27, 192 (2008)]. It is demonstrated that flames accelerate strongly in open or vented obstructed pipes and the acceleration mechanism is similar to that in semiopen ones (shockless and laminar), although acceleration is weaker in open pipes. Starting with an inviscid approximation, we subsequently incorporate hydraulic resistance (viscous forces) into the analysis for the sake of comparing its role to that of a jet flow driving acceleration. It is shown that hydraulic resistance is actually not required to drive flame acceleration. In contrast, this is a supplementary effect, which moderates acceleration. On the other hand, viscous forces are nevertheless an important effect because they are responsible for the initial delay occurring before the flame acceleration onset, which is observed in the experiments and simulations. Accounting for this effect provides good agreement between the experiments, modeling, and the present theory.

  • 19. Cops, Mark J.
    et al.
    McDaniel, J. Gregory
    Magliula, Elizabeth A.
    Bamford, David J.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Estimation of acoustic absorption in porous materials based on visco-thermal boundary layers modeled as boundary conditions2020In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 148, no 3, p. 1624-1635Article in journal (Refereed)
    Abstract [en]

    A method for estimating acoustic absorption in porous materials is presented in which the thermal and viscous boundary layers are modeled through boundary conditions to the Helmholtz equation for the acoustic pressure. The method is proposed for rigid-framed porous materials in which vibration of the frame is negligible compared to pressure fluctuations in air. The method reduces computation times by 2 orders of magnitude compared to a full thermoviscous acoustic solver. Furthermore, the method is shown to be highly accurate over geometrical features and frequencies of interest as long as thermal and viscous boundary layers do not overlap and the effects of the sharp changes in curvature are negligible. The method is demonstrated for a periodic sound absorber from the literature as well as a sound absorber with a randomly graded microstructure.

  • 20.
    De Spiegeleer, Alexandre
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pitkänen, Timo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Volwerk, M.
    Mann, Ingrid
    Umeå University, Faculty of Science and Technology, Department of Physics. Department of Physics and Technology, The Arctic University of Norway, Tromsø, Norway.
    Nilsson, H.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andersson, L.
    Vaverka, Jakub
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Low-frequency oscillatory flow signatures and high-speed flows in the Earth's magnetotail2017In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 7, p. 7042-7056Article in journal (Refereed)
    Abstract [en]

    Using plasma sheet data from Cluster 1 spacecraft from 2001 till 2011, we statistically investigate oscillatory signatures in the plasma bulk flow. These periodic oscillations are compared to high-speed and quiet flows. Periodic oscillations are observed approximately 8% of the time, while high-speed flows and quiet flows are observed around 0.5% and 12% of the time, respectively. We remark that periodic oscillations can roughly occur everywhere for x(gsm) < -10 R-E and |y(gsm)| < 10 RE, while quiet flows mainly occur toward the flanks of this region and toward x = -10 R-E. The relation between the geomagnetic and solar activity and the occurrence of periodic oscillations is investigated and reveal that periodic oscillations occur for most Kp values and solar activity, while quiet flows are more common during low magnetospheric and solar activity. We find that the median oscillation frequency of periodic oscillations is 1.7 mHz and the median duration of the oscillation events is 41 min. We also observe that their associated Poynting vectors show a tendency to be earthward (S-x >= 0). Finally, the distribution of high-speed flows and periodic oscillations as a function of the velocity is investigated and reveals that thresholds lower than 200 km/s should not be used to identify high-speed flows as it could result in misinterpreting a periodic oscillations for a high-speed flow.

  • 21.
    Demirgok, Berk
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, WV, Morgantown, United States.
    Almeyda, Orlando Jesus Ugarte
    Department of Mechanical and Aerospace Engineering, West Virginia University, WV, Morgantown, United States.
    Akkerman, V'yacheslav
    Department of Mechanical and Aerospace Engineering, West Virginia University, WV, Morgantown, United States.
    Valiev, Damir
    Department of Mechanical and Aerospace Engineering, Princeton University, NJ, Princeton, United States.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wu, Ming-Hsun
    Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan.
    Analysis of ethylene-oxygen combustion in micro-pipes2013In: Fall Technical Meeting of the Eastern States Section of the Combustion Institute 2013, Combustion Institute , 2013, p. 155-160Conference paper (Refereed)
    Abstract [en]

    Propagation of premixed stoichiometric ethylene-oxygen flames in cylindrical pipes of sub/near-millimeter radii is investigated-computationally, analytically and experimentally. Namely, various stages of flame evolution such as quasi-isobaric, exponential acceleration; its moderation due to gas compression; and eventual saturation to the Chapmen-Jouget deflagration are consdiered. Specifically, we have determined the dynamics and morphology of the flame front, its propagation velocity and acceleration rate. Due to viscous heating, the entire process can be followed by the detonation initiation ahead of the flame front. The computational component of this research includes numerical solution of the hydrodynamics and combustion equations with chemical kinetics represented by one-step Arrhenius reaction. The theoretical model accounts for small, but finite Mach number; and it assumes a plane-parallel flame-generated flow, zero flame thickness as well as large thermal expansion and flame-related Reynolds number. The overall study bridges the gap between the experiments of Wu et al. [Proc. Combust. Inst. 31 (2007) 2429] and the analytical formulation of Akkerman et al. [Combust. Flame 145 (2006) 206].

  • 22.
    Dion, Claude
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Demirgök, Berk
    Dept. Mechanical and aerospace Engineering, West Virginia University.
    Akkerman, Vyacheslav
    Dept. Mechanical and aerospace Engineering, West Virginia University.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Flames in channels with cold walls: acceleration versus extinction2015In: MCS 9, 2015Conference paper (Refereed)
    Abstract [en]

    The present work considers the problem of premixed flame front acceleration in microchannelswith smooth cold non-slip walls in the context of the deflagration-to-detonationtransition; the flame accelerates from the closed channel end to the open one. Recently, anumber of theoretical and computational papers have demonstrated the possibility of powerfulflame acceleration for micro-channels with adiabatic walls. In contrast to the previous studies,here we investigate the case of flame propagation in channels with isothermal cold walls. Theproblem is solved by using direct numerical simulations of the complete set of the Navier-Stokes combustion equations. We obtain flame extinction for narrow channels due to heat lossto the walls. However, for sufficiently wide channels, flame acceleration is found even for theconditions of cold walls in spite of the heat loss. Specifically, the flame accelerates in thelinear regime in that case. While this acceleration regime is quite different from theexponential acceleration predicted theoretically and obtained computationally for theadiabatic channels, it is consistent with the previous experimental observations, whichinevitably involve thermal losses to the walls. In this particular work, we focus on the effectof the Reynolds number of the flow on the manner of the flame acceleration.

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  • 23.
    Eliasson, Bengt
    Umeå University, Faculty of Science and Technology, Department of Physics. Theoretische Physik IV, Ruhr-Universität Bochum, Bochum, Germany.
    A nonuniform nested grid method for simulations of RF induced ionospheric turbulence2007In: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 178, p. 8-14Article in journal (Refereed)
    Abstract [en]

    We present a numerical scheme to simulate radio-frequency (RF) induced ionospheric turbulence, in which an electromagnetic wave is injected into the overhead ionospheric plasma. At the turning point of the ordinary mode, the electromagnetic wave undergoes linear mode-conversion to electrostatic Langmuir and upper hybrid waves that can have a much shorter wavelength than the electromagnetic wave. In order to resolve both the electromagnetic and electrostatic waves, avoiding severe restrictions on the time step due to the Courant–Friedrich–Lewy (CFL) condition, the equation of motion for the plasma particles is solved on a denser grid than that for the Maxwell equations near the mode-conversion region. An interpolation scheme is employed to calculate the electromagnetic field in the equation of motion of the plasma particles, and an averaging scheme is used to calculate the current density acting as a source in the Maxwell equation. Special care has to be taken to reduce numerical recurrence effects when the wavelength of the electrostatic wave is of the same order or shorter than the coarse grid spacing of the electromagnetic wave.

  • 24.
    Eriksson, Joakim
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Simulering av oljeflödet i Scanias växellådor2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    At Scania they want to shorten the development time on their products in order to continue being one of the top manufacturers of trucks in the world. To do this they have find new ways and new tools to increase the knowledge and speed up the development of new products. The oil flow in the gearbox is such an area in which they have examined the possibilities to increase the knowledge with the aid of computer simulations.

    In this thesis work a model for computer simulations of the oil flow through the shafts of the gearbox has been developed in GT-Suite, a module based simulation program in 1D. The model can simulate the flow of oil from the oil pump in the front of the gearbox, all the way back to the sun wheel in the planetary gear and in between the flow to bearings and synchronizers. The aim is to be able to use the model during further development of todays gearboxes and in the future development of new gearboxes.

    The simulations have been validated against measurements which have been performed in which the oil flow to the sun wheel have been measured for various speeds an configurations of the gearbox. The simulations have been able to replicate the measurements in many ways adequately. GT-Suite is well suited for simulations and to use as a reference when making adjustments to the gearbox and in the future when developing new gearboxes. The result from simulations in GT-Suite have also been validate against CFD-simulations in 3D for some specific components. The advantage of CFD-simulations is that more knowledge of the flow can be obtained even for complicated geometries. The disadvantage is that the complexity when simulating the entire system makes it almost impossible, so mostly subsystems must be studied.

    In the future more work in form of new measurements and simulations in CFD-programs will be necessary to further improve the model. But already now the results of simulations have been able to increase the knowledge of the oil flow in the gearbox. And this in turn have helped designers to solve some challenges they have had, but also discovered new ones that could be solved by simulations and examine what changes are required. 

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  • 25.
    Feng, Ruixue
    et al.
    Tsinghua University.
    Zhong, Hongtao
    Tsinghua University.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Center for Combustion Energy, Tsinghua University, Beijing, 100084, China; Department of Thermal Engineering, Tsinghua University, Beijing, 100084, China.
    Influence of gas expansion on the interaction between spatially periodic shear flow and premixed flame2017In: 11th Asia-Pacific Conference on Combustion 2017: Volume 1 of 2 / [ed] Dunn M., Hawkes E.R., Masri A.R., Cleary M., Kourmatzis A., Chan Q.N., Kook S., Curran Associates , 2017, p. 807-810Conference paper (Other academic)
    Abstract [en]

    It has been previously shown that thermal expansion may have a role in a boundary layer flashback of premixed turbulent flames [Gruber et al., J Fluid Mech 2012], enhancing the flame velocity through flame corrugation. The current study focuses on the two-dimensional numerical investigation of the interaction of a periodic shear flow and a laminar premixed flame. The periodic shear is a simplified model for the oncoming prolonged velocity streaks with alternating regions of high and low velocities found in a turbulent channel flow close to the wall. The parametric study focuses on the amplitude and wavelength of the periodic shear flow, and gas expansion ratio. It has been found that with the increase of the amplitude of the periodic shear flow, as well as the gas expansion, the curved flame velocity increases monotonically. The flame velocity dependence on the periodic shear wavelength is non-monotonic, which is consistent with previous theoretical studies of the curved premixed flame velocity. 

  • 26.
    Forsgren, Fritz
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Simulations to determine the drag coefficient of a floating photovoltaic system2021Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    A floating photovoltaic (FPV) system is a structure of solar cells placed on water, where the solar cells are mounted on floating modules that have to be anchored. To know the size of the anchoring equipment, the forces on the FPV need to be determined. The main force affecting the FPV is the wind force. The force from the wind is directly correlated with the drag coefficient, hence we need to determine the drag coefficient to understand the system.

    The goal of this thesis is to first find the difference in the drag coefficient between two configurations of FPVs and for a second case with a floater added in front of both setups. To determine the difference in drag coefficient, between the two cases, the wind flow over the FPVs were studied by simulations using computational fluid dynamics (CFD) and calculating the drag coefficient for each case.

    The simulations showed that the difference in drag coefficient in the cases without a floater had the biggest difference between the first FPVs where the difference was a factor of two. For the cases with the floater, the simulations gave a similar result for the two configurations, leading to a smaller difference between the two configurations.

    We conclude that if a system without a floater is built, the configurations of the FPVs are important, while if the floater is added in front of the FPV there is less importance in the configurations of the FPVs. 

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  • 27.
    Fotios, Kasolis
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Analysis of fictitious domain approximations of hard scatterers2015In: SIAM Journal on Numerical Analysis, ISSN 0036-1429, E-ISSN 1095-7170, Vol. 53, no 5, p. 2347-2362Article in journal (Refereed)
    Abstract [en]

    Consider the Helmholtz equation del center dot alpha del p+k(2 alpha)p = 0 in a domain that contains a so-called hard scatterer. The scatterer is represented by the value alpha = epsilon, for 0 < epsilon << 1, whereas alpha = 1 whenever the scatterer is absent. This scatterer model is often used for the purpose of design optimization and constitutes a fictitious domain approximation of a body characterized by homogeneous Neumann conditions on its boundary. However, such an approximation results in spurious resonances inside the scatterer at certain frequencies and causes, after discretization, ill-conditioned system matrices. Here, we present a stabilization strategy that removes these resonances. Furthermore, we prove that, in the limit epsilon -> 0, the stabilized problem provides linearly convergent approximations of the solution to the problem with an exactly modeled scatterer. Numerical experiments indicate that a finite element approximation of the stabilized problem is free from internal resonances, and they also suggest that the convergence rate is indeed linear with respect to epsilon.

  • 28. Gross, Steven
    Applying turbulence theory to respond effectively to critical incidents2019In: Applying turbulence theory to educational leadership in challenging times: a case-based approach, London: Routledge, 2019, p. 47-62Chapter in book (Refereed)
    Abstract [en]

    Educational leaders have a background in Turbulence Theory, the stage is set to learn how to use it in practice. This means it's time to make a transition to the steps and mechanics in putting Turbulence Theory to work in real educational contexts. In order to do this, this chapter suggests following this five-step process: reflect deeply on the turbulent incident; determine the role of the three drivers of turbulence: positionality, cascading, and stability; and decide on a general level of turbulence. The steps are including: consider whether to escalate or tone down the turbulence level; and organize constructive advice that responds to the turbulent incident that includes an effective way to share with key individuals and groups. Each of the three drivers has an impact on the turbulent situation at hand: positionality, cascading and stability.

  • 29.
    Hägg, Linus
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mätning och modellering av lufthastigheten i virkestorkar2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Wood drying is the most energy-intensive process at a sawmill and air circulation fans are a samills largest consumer of electrical power. Increasing prices for electrical power and increased environmental awareness makes it interresting to investigate if the consumption of electrical power during drying can be reduced without riskning the quality of the dried timber. To accomplish this, information about the velocity distribution within the kiln is vital.

    In this thesis 20 hot film anemometers were used to measure the air velocity in an intustrial batch kiln. The flow was also simulated in a reduced 2D-model. Due to limitations of the anemometers the measurements were conducted on already dried boards.

    The results show that the flow is relatively more homogenous at higher velocities. This indicates that drying conditions vary more between different locations within the kiln when the air speed is reduced. The simulations and measurements show that as much as 30 % of the flow is not activley involved in the drying processs, because it passes through the larger bolster spaces between packages. When the seal between timber stack 2 and the dryers right wall was removed no significant difference in air flow distribution was measured.

  • 30.
    Hägglund, Jesper
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Simulated cerebrospinal fluid motion due to pulsatile arterial flow: Master Thesis Project2021Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    All organs, including the brain, need a pathway to remove neurotoxic extracellular proteins. In the brain this is called the glymphatic system. The glymphatic system works by exchanging proteins from interstitial fluids to cerebrospinal fluids. The extracellular proteins are then removed through the cerebrospinal fluid drains. The glymphatic system is believed to be driven by arterial pulsatility, cerebrospinal fluid production and respiration. Cerebrospinal fluids enters the brain alongside arteries. In this project, we investigate if a simulated pulsatile flow in a common carotid artery can drive cerebrospinal fluid flow running along the artery, using computational simulations of a linearly elastic and fluid-structure multiphysical model in COMSOL. Our simulations show that a heartbeat pulse increases the arterial radius of the common carotid artery by 6 %. Experimental data, assessed using 4D magnetic resonance imaging of a living human, show an increase of 13 %. Moreover, our results indicate that arterial displacement itself is not able to drive cerebrospinal fluid flow. Instead, it seems to create a back and forth flow that possibly could help with the protein exchange between the cerebrospinal and interstitial fluids. Overall, the results indicate that the COMSOL Multiphysics linearly elastic model is not ideal for approximations of soft non-linearly elastic solids, such as soft polydimethylsiloxane and artery walls work for stiffer materials. The long term aim is to simulate a part of the glymphatic system and the present work is a starting point to reach this goal. As the simulations in this work are simplified there are more things to test in the future. For example, using the same geometries a non-linear elastic model could be tested. The pulsatile waveform or the geometry could be made more complex. Furthermore the model could be scaled down to represent a penetrating artery in the brain instead of the common carotid artery. 

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  • 31.
    Jonsson, Ulf G
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Umeå University, Faculty of Science and Technology, Centre for Biomedical Engineering and Physics (CMTF).
    Lindahl, Olof A
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics. Umeå University, Faculty of Science and Technology, Centre for Biomedical Engineering and Physics (CMTF). Dept. of Engineering Sciences and Mathematics, Luleå University of Technology, Sweden.
    Andersson, Britt M
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Umeå University, Faculty of Science and Technology, Centre for Biomedical Engineering and Physics (CMTF).
    Modeling the high-frequency complex modulus of a silicone rubber using standing lamb waves and an inverse finite element method2014In: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, ISSN 0885-3010, E-ISSN 1525-8955, Vol. 61, no 12, p. 2106-2120Article in journal (Refereed)
    Abstract [en]

    To gain an understanding of the high-frequency elastic properties of silicone rubber, a finite element model of a cylindrical piezoelectric element, in contact with a silicone rubber disk, was constructed. The frequency dependent elastic modulus of the silicone rubber was modeled by a four parameter fractional derivative viscoelastic model in the 100 kHz to 250 kHz frequency range. The calculations were carried out in the range of the first radial resonance frequency of the sensor. At the resonance, the hyperelastic effect of the silicone rubber was modeled by a hyperelastic compensating function. The calculated response was matched to the measured response by using the transitional peaks in the impedance spectrum that originates from the switching of standing Lamb wave modes in the silicone rubber. To validate the results, the impedance responses of three 5 mm thick silicone rubber disks, with different radial lengths, were measured. The calculated and measured transitional frequencies have been compared in detail. The comparison showed very good agreement, with average relative differences of 0.7 %, 0.6 %, and 0.7 % for the silicone rubber samples with radial lengths of 38.0 mm, 21.4 mm, and 11.0 mm, respectively. The average, complex, elastic modulus of the samples were: (0.97 + 0.009i) GPa at 100 kHz and (0.97 + 0.005i) GPa at 250 kHz.

  • 32.
    Kasolis, Fotios
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    The material distribution method: analysis and acoustics applications2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    For the purpose of numerically simulating continuum mechanical structures, different types of material may be represented by the extreme values {,1}, where 0<1, of a varying coefficient  in the governing equations. The paramter  is not allowed to vanish in order for the equations to be solvable, which means that the exact conditions are approximated. For example, for linear elasticity problems, presence of material is represented by the value  = 1, while  =  provides an approximation of void, meaning that material-free regions are approximated with a weak material. For acoustics applications, the value  = 1 corresponds to air and  to an approximation of sound-hard material using a dense fluid. Here we analyze the convergence properties of such material approximations as !0, and we employ this type of approximations to perform design optimization.

    In Paper I, we carry out boundary shape optimization of an acoustic horn. We suggest a shape parameterization based on a local, discrete curvature combined with a fixed mesh that does not conform to the generated shapes. The values of the coefficient , which enters in the governing equation, are obtained by projecting the generated shapes onto the underlying computational mesh. The optimized horns are smooth and exhibit good transmission properties. Due to the choice of parameterization, the smoothness of the designs is achieved without imposing severe restrictions on the design variables.

    In Paper II, we analyze the convergence properties of a linear elasticity problem in which void is approximated by a weak material. We show that the error introduced by the weak material approximation, after a finite element discretization, is bounded by terms that scale as  and 1/2hs, where h is the mesh size and s depends on the order of the finite element basis functions. In addition, we show that the condition number of the system matrix scales inversely proportional to , and we also construct a left preconditioner that yields a system matrix with a condition number independent of .

    In Paper III, we observe that the standard sound-hard material approximation with   gives rise to ill-conditioned system matrices at certain wavenumbers due to resonances within the approximated sound-hard material. To cure this defect, we propose a stabilization scheme that makes the condition number of the system matrix independent of the wavenumber. In addition, we demonstrate that the stabilized formulation performs well in the context of design optimization of an acoustic waveguide transmission device.

    In Paper IV, we analyze the convergence properties of a wave propagation problem in which sound-hard material is approximated by a dense fluid. To avoid the occurrence of internal resonances, we generalize the stabilization scheme presented in Paper III. We show that the error between the solution obtained using the stabilized soundhard material approximation and the solution to the problem with exactly modeled sound-hard material is bounded proportionally to .

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    The Material Distribution Method
  • 33.
    Kasolis, Fotios
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Preventing resonances within approximated sound-hard material in acoustic design optimization2014In: 1st International Conference on Engineering and Applied Sciences Optimization, 2014Conference paper (Other academic)
  • 34.
    Kiflemariam, Medet
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Development of a CFD Boundary Condition to Simulate a Perforated Surface2021Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In aircraft with jet propulsion engine intakes at supersonic speed, strong pressure waves referred to as shockwaves occur, which may interact with any present boundary layers along the intake surface. The adverse pressure gradients associated with Shock Wave-Boundary Layer Interaction (SWBLI) may cause boundary layer flow separation, which can result in disturbances of the flow that can be harmful to the device or decrease engine performance. A common way in dealing with the adverse effects of SWBLI is through removal of low-momentum flow in the boundary layer, a process referred to as boundary layer bleed. In the process of bleed, the boundary layer is subjected to a pressure difference promoting flow out of the system, through a porous surface, and into a plenum. The porous surfaces used in the mass flow removal process contain orifices in small scales. Thus, in Computational Fluid Dynamics (CFD), creating a mesh resolving both the orifice scales and the bulk flow is a cumbersome task, and the computational cost becomes substantially increased. To this end, several boundary conditions which effectively model the large-scale effects of bleed have been developed.

    The aim of this study is to implement the Boundary Condition (BC) developed by John W. Slater into M-EDGE, the in-house compressible CFD-solver of SAAB Aeronautics. The bleed boundary condition model is based on a dimensionless surface sonic flow coefficient, which is derived from empirical wind-tunnel measurements of the bleed mass flow. In previous work, the Slater bleed BC has been shown to correlate well with wind-tunnel data. Furthermore, a simple transpiration law formulated by Reynald Bur was implemented in order get familiarized with the M-EDGE Fortran source code. However, this model is expected to yield unsatisfactory results, as reported in previous work in the field. The implemented Slater BC is tested on two different two-dimensional flow cases; flow over a flat plate without SWBLI, and flow including a shock wave generator creating SWBLI. In the flat plate case, simulations were run at Mach numbers 1.27, 1.58, 1.98 and 2.46 over a 6.85cm plate of 19% porosity. In the SWBLI-case, only flow at Mach 2.46 was considered, with a 9.53cm plate of 21% porosity. The Reynolds number range used throughout was 1.39−1.76·10^7/m. Simulations were run at different bleed rates over a structured grid using steady state RANS with the Spalart-Allmaras one-equation turbulence model. The boundary condition performance was assessed by its ability to recreate the sonic flow coefficients on which it is based. Further, the shape of downstream pitot pressure profiles are compared with experimental data.

    Results from the studies indicate that the implementation manages to recreate the data for the sonic flow coefficient with small error margins. The implementation can be used to simulate porous plates of different dimensions and porosities, even though the bleed model is based on empirical mass flow measurements of a 6.85cmplate of 19% porosity. The implementation is able to predict global bleed effects in the flow field, as indicated by comparisons of pitot pressure profiles at various downstream reference planes, despite differences in reference boundary layer intake profiles. Further, the overall flow field was compared visually with other simulation-studies, indicating that the global Mach distributions of the geometries were in accordance with the reference data. However, pitot profiles should be further studied with better matched intake boundary layer profiles. The main limitation of the boundary condition is that it relies on the wind-tunnel data of the surface sonic flow coefficients for specific bleed plate configurations. Furthermore, the implementation has only been verified to work within specific Mach number range of the underlying empirical measurements. In future work, the generality of the model could be increased by extending the data to other configurations and Mach numbers by conducting new experiments or using other published empirical data.

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  • 35.
    Kodakoglu, Furkan
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506, USA.
    Demir, Sinan
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, USA.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Center for Combustion Energy, Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education of China, Department of Energy and Power Engineering, Tsinghua University, Beijing, China.
    Akkerman, V’yacheslav
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, USA.
    Analysis of Gaseous and Gaseous-Dusty, Premixed Flame Propagation in Obstructed Passages with Tightly Placed Obstacles2020In: Fluids, E-ISSN 2311-5521, Vol. 5, no 3, article id 115Article in journal (Refereed)
    Abstract [en]

    A recent predictive scenario of premixed flame propagation in unobstructed passages is extended to account for obstructions that can be encountered in facilities dealing with explosive materials such as in coalmines. Specifically, the theory of globally-spherical, self-accelerating premixed expanding flames and that of flame acceleration in obstructed conduits are combined to form a new analytical formulation. The coalmining configuration is imitated by two-dimensional and cylindrical passages of high aspect ratio, with a comb-shaped array of tightly placed obstacles attached to the walls. It is assumed that the spacing between the obstacles is much less or, at least, does not exceed the obstacle height. The passage has one extreme open end such that a flame is ignited at a closed end and propagates to an exit. The key stages of the flame evolution such as the velocity of the flame front and the run-up distance are scrutinized for variety of the flame and mining parameters. Starting with gaseous methane-air and propane-air flames, the analysis is subsequently extended to gaseous-dusty environments. Specifically, the coal (combustible, i.e., facilitating the fire) and inert (such as sand, moderating the process) dust and their combinations are considered, and the impact of the size and concentration of the dust particles on flame acceleration is quantified. Overall, the influence of both the obstacles and the combustion instability on the fire scenario is substantial, and it gets stronger with the blockage ratio.

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  • 36.
    Kuoppala, Oskar
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Cavitation analysis on test rig.: An experimental and CFD study executed in collaboration with Epiroc AB2021Independent thesis Advanced level (degree of Master (Two Years)), 300 HE creditsStudent thesis
    Abstract [en]

    This master thesis project was done in collaboration with Epiroc Group Ab. Epiroc supplies high-quality drills of various types that can be used both above and below ground. A major problem in their percussive rock drills is that that cavitation is formed. Cavitation is a phenomenon that occurs when a fluid is subject to a sudden pressure drop. This pressure drop causes the liquid to vaporize and create gas bubbles. These gas bubbles will cause erosion to the walls when imploded. These cavitation damages lead to drills breaking and parts having to be replaced preserved.

    An experimental rig was used to create cavitation. From the experimental rig, it was possible to measure the hydraulic transients that are created when the valve was closed. In this study, we examined whether one can visually see these damages occurring inside the pipe on valve parts that are subjected to these cavitation damages.

    CFD simulations were used to re-create the closing of the valve in the experimental rig. By exporting pressure data from the experiments one could compare the numerical result to the experimental data. It was also investigated if it is possible to see some connection between the gas formation and the damages seen visually from the experimental part. For the simulation the realizable k − ε methods were implemented with enhanced wall treatment. The mixture model was used since we have a multi-phase flow. Some visual damages were recognized during the experiments. However, no distinguished pattern or specific areas was established. From the simulations, it could be determined that they generated gas when the valve was closed. However, the pressure transients could not be replicated in the numerical result.

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  • 37.
    Kälvelid, Fredrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Numerical Modeling of Plain Journal Bearings within a Heavy-Duty Engine Oil System using GT-SUITE2016Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Main and connecting rod bearings along the crankshaft in a heavy-duty engine oil system requires efficient feed of oil flow to ensure the performance of lubrication, cooling and cleaning. The bearings and builded bearing models considered in this work are based on a Volvo 13L engine. Two main models to predict the oil flow in main and connecting rod bearings have been investigated using the system analysis software GT-SUITE. These two models are based on two numerical methods called  the Mobility method and the Reynold's equation respectively. The main bearing and the connecting rod large end bearing loads calculated in the present work is similar to the reference bearing loads, especially for connecting rod large end bearing and at lower engine speeds. For higher engine speeds, the present load results are far off,  compared to the reference load results. This difference at the higher engine speeds showed also an increased difference in the oil volume flow rates. But more information regarding the calculations of the reference loads is needed to analyze the differences further. Considering the "split lines", where the upper and the lower bearing shell meet, in the bearing models increased the oil volume flow rate with 13.3\% on average for the main bearing. Main bearing stand for 86\% of the total flow whilst connecting rod large end and small end bearing stand for the rest 13\% and 1\% of the total flow. The feed of oil to main bearing and connecting rod large end bearing appears continuously whilst the feed of oil to connecting rod small end bearing appears in the form of a few pulsations. Comparing to the numerical reference flow results, the results in this work are similar at high oil temperatures and at low engine speeds, otherwise the results are far off. Considering the Reynold's equation when predicting the oil volume flow rates in plain journal bearings is a more accurate way than using the Mobility method since it takes more specific geometrical irregularities into account. The oil volume flow rate for main bearing differ with 2.7\% on average when solving the Reynold's equation compared to the Mobility method.

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  • 38.
    Lacis, Ugis
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Modelling air flow in the larynx.2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
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  • 39.
    Lundberg, Petter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Investigation of the transient nature of rolling resistance on an operating Heavy Duty Vehicle2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    An operating vehicle requires energy to oppose the subjected driving resistances. This energy is supplied via the fuel combustion in the engine. Decreasing the opposing driving resistances for an operating vehicle increases its fuel efficiency: an effect which is highly valued in today’s industry, both from an environmental and economical point of view. Therefore a lot of progress has been made during recent years in the area of fuel efficient vehicles, even though some driving resistances still rises perplexity. These resistances are the air drag Fd generated by the viscous air opposing the vehicles propulsion and the rolling resistance Frr generated mainly by the hysteresis caused by the deformation cycle of the viscoelastic pneumatic tires.

    The energy losses associated with the air drag and rolling resistance account for the majority of the driving resistances facing an operating vehicle, and depends on numerous stochastic and ambient parameters, some of which are highly correlated both within and between the two resistances. To increase the understanding of the driving mechanics behind the energy losses associated with the complexity that is rolling resistance, a set of complete vehicle tests has been carried out. These tests were carried out on the test track Malmby Fairground, using a Scania CV AB developed R440 truck equipped with various sensors connected in one measurement system. Under certain conditions, these parameters can allow for an investigation of the rolling resistance, and a separation of the rolling resistance and air drag via explicit subtraction of the air drag from the measured traction force. This method is possible since the aerodynamic property AHDVCd(β) to some extent can be generated from wind tunnel tests and CFD simulations.

    Two measurement series that enable the above formulated method of separation were designed and carried out, using two separate measurement methods. One which enables the investigation of the transient nature of rolling resistance as it strives for stationarity, where the vehicle is operated under constant velocities i.e. no acceleration, and one using the well established method of coastdown, where no driving torque is applied.

    The drive cycles spanned a range of velocities, which allowed for dynamic and stationary analyses of both the tire temperature- and the velocity dependence of rolling resistance. When analysing the results of the transient analysis, a strong dependence upon tire temperature for given constant low velocity i.e. v ≤ 60 kmh−1 was clearly visible. The indicated dependency showed that the rolling resistance decreased as the tire temperature increased over time at a given velocity, and vice versa, towards a stationary temperature and thereby rolling resistance. The tire temperature evolution from one constant velocity to another, took place well within 50 min to a somewhat stationary value. However, even though the tire temperature had reached stationarity, rolling resistance did not; there seemed to be a delay between stationary tire temperature, and rolling resistance. The results did not indicate any clear trends for v ≥ 60 kmh−1, where the results at v = 80 kmh−1 were chaotic. This suggests that some additional forces were uncompensated for, or that the compensation for air drag was somehow wrongly treated at higher velocities.

    Several factors ruled out any attempts at proposing a new rolling resistance model. These included: the chaotic results for v = 80 kmh−1, the delayed rolling resistance response upon tire temperature stabilization, and the lack of literature support for the observed tendency. The results from the coastdown series on the other hand, showed good agreement with a dynamical model suggested in literature. The stationary temperature behaviour for the considered velocity range at assumed constant condition is also supported in literature.

    Finally, an investigation of the aerodynamic property AHDVCd inspired by ongoing work in ACEA (European Automobile Manufacturers’ Association), was carried out assuming both zero and non-zero air drag at low velocities. The results indicated surprisingly good agreement with wind tunnel measurements, especially when neglecting air drag at low velocities: as suggested by ACEA. 

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  • 40.
    Löfgren, Erik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Accuracy of transient versus steady state forces on a rudder operating in a propeller slipstream2015Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In computational fluid dynamics (CFD), a transient simulation is in general more costly than computing the steady state of the system, if such a state exists. The velocity field produced by the propeller blades upstream of a rudder is transient in nature, and rudder design using CFD may therefore become very time-consuming. If a steady solution could accurately predict the performance of the rudder, such an approach would be favourable. The aim of the present study was to assess the possibility to accurately predict the performance of a rudder operating in a propeller slipstream using steady state simulations, e.g. an actuator disk model (ADM). For this reason, the performance of the two-dimensional NACA 0021 rudder section submitted to a sinusoidal transverse gust, representing a transient propeller slipstream, was simulated using ANSYS Fluent. The predicted force coefficients are presented for a number of gust amplitudes, mean angles of attack and reduced frequencies of the transverse gust. The simulations have shown that the modelling error introduced when predicting the performance in a steady state is highly dependent on all these parameters of the actual transient flow, and that the steady result may be a severe over- or under-prediction of the real performance of the rudder. Heavily loaded propellers are suspected to be less suitable for ADM modelling in rudder performance prediction. The predicted unsteady lift coefficient was compared to the linear theories of Horlock and Sears, and the agreement was fair at zero mean angle of attack but poor at a mean angle of attack of 10°. It was also found that the predicted performance of the rudder was significantly altered when the chord based Reynolds number was increased by a factor of 10, which has implications on the validity of model-scale simulations. The effect of including turbulent transition modelling for some of the simulations was also investigated, and the discrepancy in predicted performance was found to be considerable. Due to the formation of a laminar separation bubble the predicted trailing edge separation and viscous stress on the rudder were significantly decreased, leading to better overall performance.

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  • 41.
    Madison, Guy
    et al.
    Umeå University, Faculty of Social Sciences, Department of Psychology.
    Paulin, Johan
    Umeå University, Faculty of Social Sciences, Department of Psychology.
    Ratings of speed in real music as a function of both original and manipulated tempo2010In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 128, no 5, p. 3032-3040Article in journal (Refereed)
    Abstract [en]

    There is an apparent contradiction between the narrow range of tempi optimal for perceptualjudgment and motor synchronization and the wide range of beat tempi found in real music. Therelation between listeners’ perception of speed and beat tempo was therefore investigated, both forreal music excerpts (ME) and metronome sequences. Tempi ranged from 42 to 200 beats per minute (BPM), and some excerpts were further tempo manipulated in four levels from from ±5 to ±20%. Regression analyses showed that speed was a shallower function of original tempo for fast (> 150 BPM) and slow (< 95 BPM) MEs than for MEs with intermediate tempi, describing anon-linear, sigmoid function. Manipulated tempo had twice as large an effect on speed as hadoriginal tempo. In contrast, speed was an almost linear function of tempo for metronome sequences.Taken together, these results show that the non-linearity stems from properties of the musical signal,rather than being a subjective perceptual effect. They indicate an inverse relation between tempo andrelative event density in real music, and demonstrate that the perception of periodic signals isaffected not only by the beat level, but also by faster and slower levels.© 2010 Acoustical Society of America.

  • 42.
    Mousavi, Abbas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Computational analysis and design optimization for acoustic devices2023Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis focuses on material distribution topology optimization for acoustic waveguides. The limitations of the material distribution approach are discussed in the context of acoustic waveguides with extensive viscous and thermal boundary losses. An extension of the material distribution method is introduced which is capable of incorporating these boundary losses in the optimization process. Furthermore, a computational analysis of waveguide acoustic black holes (WABs) is also provided followed by a topology optimization approach for the conceptual design of a WAB with enhanced wave-focusing capabilities, utilizing the novel method introduced in the first part of the thesis.  The thesis commences with a comprehensive literature review to set the context for the subsequent research. The material distribution topology optimization is then discussed in detail, focusing on the design of a transition section for impedance matching between two cylindrical waveguides with different radii to maximize planar wave transmission. The linear wave propagation in the device is modeled using the Helmholtz equation and solved utilizing the finite element method to obtain acoustic pressure distribution. Nonlinear density filters are used to impose a size control on the design, and the design optimization problem is formulated and solved utilizing the method of moving asymptotes (MMA) with the sensitivity information provided through an ad-joint method. Selected results are provided for the considered design optimization problem. We expanded the analysis to encompass viscothermal acoustics and introduced a novel material distribution method capable of incorporating complex interface conditions. The new method is then applied to design acoustic absorbers with the aim of maximizing boundary losses in a targeted frequency range. The selected results represent the effectiveness of the proposed method.  The thesis further explores the limitations of the classical ribbed design of WABs in achieving true wave-focusing capabilities. To address this, a design optimization problem is formulated to obtain a conceptual design of a WAB. Utilizing the novel material distribution method for viscothermal acoustics introduced in this thesis, the optimization problem is solved, and the optimized design is compared with the results of a classical lossless approach and the ribbed design WAB. The numerical simulations demonstrate the superior wave-focusing capabilities of the optimized design, especially when incorporating boundary losses in the optimization process.   

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  • 43.
    Mousavi, Abbas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Wadbro, Eddie
    Department of Mathematics and Computer Science, Karlstad University, Karlstad, Sweden.
    How the waveguide acoustic black hole works: A study of possible damping mechanisms2022In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 151, no 6, p. 4279-4290Article in journal (Refereed)
    Abstract [en]

    The acoustic black hole (ABH) effect in waveguides is studied using frequency-domain finite element simulations of a cylindrical waveguide with an embedded ABH termination composed of retarding rings. This design is adopted from an experimental study in the literature, which surprisingly showed, contrary to the structural counterpart, that the addition of damping material to the end of the waveguide does not significantly reduce the reflection coefficient any further. To investigate this unexpected behavior, we model different damping mechanisms involved in the attenuation of sound waves in this setup. A sequence of computed pressure distributions indicates occurrences of frequency-dependent resonances in the device. The axial position of the cavity where the resonance occurs can be predicted by a more elaborate wall admittance model than the one that was initially used to study and design ABHs. The results of our simulations show that at higher frequencies, the visco-thermal losses and the damping material added to the end of the setup do not contribute significantly to the performance of the device. Our results suggest that the primary source of damping, responsible for the low reflection coefficients at higher frequencies, is local absorption effects at the outer surface of the cylinder.

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  • 44.
    Mousavi, Abbas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Wadbro, Eddie
    Department of Mathematics and Computer Science, Karlstad University, Karlstad, Sweden.
    On the acoustic black-hole effect in waveguides2021In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 149, no 4, article id A108Article in journal (Refereed)
    Abstract [en]

    The acoustic black-hole (ABH) effect is a well-known way of controlling structural vibrations in solid beams and plates. The theory behind this effect is to reduce the velocity of waves by altering the physical properties of the domain according to a power-law profile. For an ideal ABH, this leads to vanishing reflections from the end of the termination. In practice, there will be a truncation in the profile, which leads to some reflections. A well-known way of minimizing this truncation error is to add damping material to the end of the ABH termination.

    For a waveguide embedding a set of rings with retarding inner radius according to a power-law profile, the velocity of sound waves tends to zero. However, unlike the structural counterpart, experimental results in the literature show that adding damping material to reduce the truncation error is not effective for waveguides. Here, we present a finite element simulation of the considered cylindrical setup. Our results confirm that the addition of damping material to the end of the waveguide is ineffective while suggesting that the local absorption effects at the lateral surface of the cylinder are a primary source of damping to achieve the ABH effect.

  • 45.
    Mousavi, Abbas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Uihlein, Andrian
    Department of Mathematics, Chair of Applied Mathematics (Continuous Optimization), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany.
    Pflug, Lukas
    Department of Mathematics, Chair of Applied Mathematics (Continuous Optimization), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany; FAU Competence Center Scientific Computing, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany.
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science. Department of Mathematics and Computer Science, Karlstad University, Karlstad, Sweden.
    Topology optimization of broadband acoustic transition section: a comparison between deterministic and stochastic approaches2024In: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 67, no 5, article id 67Article in journal (Refereed)
    Abstract [en]

    This paper focuses on the topology optimization of a broadband acoustic transition section that connects two cylindricalwaveguides with different radii. The primary objective is to design a transition section that maximizes the transmission of aplanar acoustic wave while ensuring that the transmitted wave exhibits a planar shape. Helmholtz equation is used to modellinear wave propagation in the device. We utilize the finite element method to solve the state equation on a structured meshof square elements. Subsequently, a material distribution topology optimization problem is formulated to optimize the dis-tribution of sound-hard material in the transition section. We employ two different gradient-based approaches to solve theoptimization problem: namely, a deterministic approach using the method of moving asymptotes (MMA), and a stochasticapproach utilizing both stochastic gradient (SG) and continuous stochastic gradient (CSG) methods. A comparative analysisis provided among these methodologies concerning the design feasibility and the transmission performance of the optimizeddesigns, and the computational efficiency. The outcomes highlight the effectiveness of stochastic techniques in achievingenhanced broadband acoustic performance with reduced computational demands and improved design practicality. Theinsights from this investigation demonstrate the potential of stochastic approaches in acoustic applications, especially whenbroadband acoustic performance is desired.

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  • 46.
    Nobis, Harrison
    et al.
    FLOW and SeRC, KTH Engineering Mechanics, Royal Institute of Technology, Stockholm, Sweden.
    Schlatter, Philipp
    FLOW and SeRC, KTH Engineering Mechanics, Royal Institute of Technology, Stockholm, Sweden; Friedrich-Alexander-Universität (FAU) Erlangen–Nürnberg, Germany.
    Wadbro, Eddie
    Department of Mathematics and Computer Science, Karlstad University, Karlstad, Sweden.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Henningson, Dan S.
    FLOW and SeRC, KTH Engineering Mechanics, Royal Institute of Technology, Stockholm, Sweden.
    Topology optimization of Superhydrophobic Surfaces to delay spatially developing modal laminar–turbulent transition2023In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 104, article id 109231Article in journal (Refereed)
    Abstract [en]

    Super-Hydrophobic Surfaces (SHSs) have been shown to reduce skin friction of an overlying fluid as a consequence of gas pockets trapped within the surface's microstructure. More recently, they have also been shown capable of delaying laminar–turbulent transition. This article investigates the applicability of topology optimization in designing the macroscopic layout of SHSs in a channel that are able to further delay K-type transition in a spatial setting. Unsteady direct numerical simulations are performed to simulate the transition scenario. This is coupled with adjoint–based sensitivity analysis and gradient based optimization. The optimized designs found through this procedure are capable of moving the transition location further downstream compared to a homogeneous counterpart by inhibiting the growth of secondary instability modes. This article provides the first application of topology optimization to a spatially developing transition scenario.

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  • 47.
    Odevik, Josef
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Geometry dependency of cerebral arterial pressure, and estimation of wall shear stress in patients with carotid stenosis: a CFD approach2020Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This thesis have investigated how the cerebral arterial pressure can be estimated using computational fluid dynamics (CFD). Specifically, a sensitivity study of how the blood vessel diameter affects the arterial pressure in patients with carotid stenosis, has been conducted. A method for estimating the distribution of low shear stress at the vessel wall of the cerebral arterial system has also been developed. There are studies which point towards that low wall shear stress (WSS) could stimulate the build up of plaque inside the vessel lumen, called atherosclerosis. As it progresses, the lumen narrows and the blood supply to the brain become obstructed. It was therefore of interest to identify where such areas are located in patients with carotid stenosis. The first objective was to conduct the sensitivity study, by investigating how the arterial pressure changed when the diameter of the cerebral arteries was increased and decreased with approximately ±0.15and ±0.30 mm. The second objective was to develop a method estimating the distribution of low wall shear stress areas of the cerebral system. The pressure difference between left and right middle carotid artery was evaluated in a plane perpendicular to the artery, for five patients, each with five different arterial diameters. A Student’s T-test was performed to see whether the resulting mean pressure difference was significantly different for the different diameters. The anterior circulation of the cerebrovascular system was divided into regions of interest before and after each bifurcation, and the total area of low WSS was measured in each region, for each lateral sides. Wilcoxon sign rank test was used in order to see whether the area of low WSS was greater on the ipsilateral side of the cerebral arterial system. The main finding was that increasing the arterial diameter affected the resulting pressure less than decreasing the diameter. The mean pressure difference of the reference geometry was 4.8 ± 1.4 mmHg, and dilating the diameter with +0.15 mm yielded it to be 2.9±1.5 mmHg. Decreasing the diameter with-0.15 mm presented the mean pressure difference as 8.6 ± 3.7 mmHg. The Student’s T-test showed that the mean pressure difference was significantly different between the reference geometry and the dilated geometries. It could not be confirmed for the eroded diameters. The total area of low WSS was presented as significantly larger on ipsilateral side of the anterior circulation. It was also shown for the internal carotid artery and the first segment of the anterior cerebral artery. The result indicates that ipsilateral arteries of the anterior circulation has an increased risk for atherosclerosis initialization. The method developed for estimating areas of low WSS was promising, and need to be verified in similar studies for the progression of atherosclerosis in the cerebral arteries for carotid stenosic patients. The findings of the sensitivity study point towards that the uncertainty related to the segmentation were within reasonable limits, supporting the use of CFD to assess the cerebrovascular system in patients with carotid stenosis.

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  • 48.
    Pelland, Charlie
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Inverse Atmospheric Dispersion Modeling in Complex Geometries2022Independent thesis Advanced level (professional degree), 300 HE creditsStudent thesis
    Abstract [en]

    In the event of a radioactive release in an urban environment the consequent response mustbe swift and precise. As soon as first responders have correct information, they can make anaccurate risk assessment. However, if the position, release rate and time of the radioactiverelease is unknown it is hard to know how the pollutant will spread. This thesis aims to testa model which approximates these three unknowns using weather data (wind and rain) as wellas measurement data collected at sensors placed around an urban environment.

    An atmospheric dispersion model based on an existing Reynolds Averaged Navier-Stokes modelis set up in two geometries of different complexity to create forward mode synthetic depositiondata and adjoint mode concentration fields resulting from a fixed dry deposition velocity andscavenging effect for wet deposition. Variations of time- and space-dependent rainfall is simu-lated. The resulting data is used in an existing optimization model, where a parameter studyis conducted regarding regularization coefficients.

    This thesis shows that the optimization model accurately estimates position and its approximaterelease rate of a 2D geometry of radioactive releases using a logarithmic optimization approach,and fail to do so using a linear optimization approach. The logarithmic optimization model alsoapproximately estimates position and release rate in a 3D geometry. Regularization parametersshould be within the range of 0.1 and 1.2 depending on rain. More rain requires smallerparameters and will estimate a lower release rate. Time-dependent rainfall is shown to have amajor negative effect on simulation time.iii   

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  • 49. Piquet, A.
    et al.
    Roussel, Olivier
    Umeå University, Faculty of Science and Technology, Department of Physics. a INSA and University of Normandy, Avenue de l’Université, 76801 Saint-Étienne-du-Rouvray, France.
    Hadjadj, A.
    A comparative study of Brinkman penalization and direct-forcing immersed boundary methods for compressible viscous flows2016In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 136, p. 272-284Article in journal (Refereed)
    Abstract [en]

    This paper deals with the comparison between two methods to treat immersed boundary conditions: on the one hand, the Brinkman penalization method (BPM); on the other hand, the direct-forcing method (DFM). The penalty method treats the solid as a porous medium with a very low permeability. It provides a simple and efficient approach for solving the Navier-Stokes equations in complex geometries with fixed boundaries or in the presence of moving objects. A new approach for the penalty-operator integration is proposed, based on a Strang splitting between the penalization terms and the convection-diffusion terms. Doing so, the penalization term can be computed exactly. The momentum term can then be computed first and then introduced into the continuity equation in an implicit manner. The direct-forcing method however uses ghost-cells to reconstruct the values inside the solid boundaries by projection of the image points from the interface. This method is comparatively hard to implement in 3D cases and for moving boundaries. In the present paper, the performance of both methods is assessed through a variety of test problems. The application concerns the unsteady transonic and supersonic fluid flows. Examples include a normal shock reflection off a solid wall, transonic shock/boundary layer in a viscous shock tube, supersonic shock/cylinder interaction, and supersonic turbulent channel flow. The obtained results are validated against either analytical or reference solutions. The numerical comparison shows that, with sufficient mesh resolution, the BPM and the DFM methods yield qualitatively similar results. In all considered cases, the BPM is found to be a suitable and a possibly competitive method for viscous-IBM in terms of predictive performance, accuracy and computational cost. However, despite its simplicity, the method suffers from a lack of regularity in the very near-wall pressure fluctuations, especially for the turbulent case. This is attributed to the fact that the method requires no specific pressure condition at the fluid/solid interface.

  • 50. Saglietti, Clio
    et al.
    Schlatter, Philipp
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Henningson, Dan
    Topology optimization of heat sinks in a square differentially heated cavity2018In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 74, p. 36-52Article in journal (Refereed)
    Abstract [en]

    Innovative designs of heat sinks are generated in the present paper through numerical optimization, by applying a material distribution topology optimization approach. The potential of the method is demonstrated in a two-dimensional differentially heated cavity, in which the heat transfer is increased by means of introducing a solid structure that acts as a heat sink. We simulate the heat transfer in the whole system by performing direct numerical simulations of the conjugated problem, i.e. temperature diffusion and convection in the entire domain and momentum conservation in the fluid surrounding the solid. The flow is driven by the buoyancy force, under the Boussinesq approximation, and we describe the presence of solid material as the action of a Brinkman friction force in the Navier–Stokes equations. To obtain a design with a given length scale, we apply regularization techniques by filtering the material distribution. Two different types of filters are applied and compared for obtaining the most realistic solution. Given the large scale of the problem, the optimization is solved with a gradient based method that relies on adjoint sensitivity analysis. The results show the applicability of the method by presenting innovative geometries that are increasing the heat flux. Moreover, the effect of various factors is studied: We investigate the impact of boundary conditions, initial designs, and Rayleigh number. Complex tree-like structures are favored when a horizontal temperature gradient is imposed on the boundary and when we limit the amount of solid volume in the cavity. The choice of the initial design affects the final topology of the generated solid structures, but not their performance for the studied cases. Additionally, when the Rayleigh number increases, the topology of the heat exchanger is able to substantially enhance the convection contribution to the heat transfer.

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