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• 1.
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

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.

• 2.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
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)

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.

• 3.
Nuclear Safety Institute of Russian Academy of Sciences B. Tulskaya 52, 115191 Moscow, Russia.
Department of Physics and Power Engineering, Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Moscow Region, Russia. 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)

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].

• 4.
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.
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)

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.

• 5.
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.
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)

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.

• 6.
Umeå University, Faculty of Science and Technology, Department of Physics.
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)

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.

• 7.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
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)

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.

• 8.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Umeå University, Faculty of Science and Technology, Department of Computing Science. 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)

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.

• 9.
Umeå University, Faculty of Science and Technology, High Performance Computing Center North (HPC2N).
Umeå University, Faculty of Science and Technology, High Performance Computing Center North (HPC2N). Umeå University, Faculty of Science and Technology, Department of Physics.
Constraint based particle fluids on GPGPU2011Conference paper (Other academic)

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.

• 10.
Umeå University, Faculty of Science and Technology, Department of Physics.
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)

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.

• 11.
Umeå University, Faculty of Science and Technology, Department of Physics.
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)

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.

• 12.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Department of Physics and Technology, The Arctic University of Norway, Tromsø, Norway. Umeå University, Faculty of Science and Technology, Department of Physics. 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)

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.

• 13.
Umeå University, Faculty of Science and Technology, Department of Physics.
Dept. Mechanical and aerospace Engineering, West Virginia University. Dept. Mechanical and aerospace Engineering, West Virginia University. Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Umeå University, Faculty of Science and Technology, Department of Physics.
Flames in channels with cold walls: acceleration versus extinction2015In: MCS 9, 2015Conference paper (Refereed)

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.

• 14.
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

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.

• 15.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Umeå University, Faculty of Science and Technology, Department of Computing Science. 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)

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.

• 16.
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

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.

• 17.
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).
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. 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)

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.

• 18.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
The material distribution method: analysis and acoustics applications2014Doctoral thesis, comprehensive summary (Other academic)

For the purpose of numerically simulating continuum mechanical structures, different types of material may be represented by the extreme values {$\epsilon$,1}, where 0<$\epsilon$$\ll$1, of a varying coefficient $\alpha$ in the governing equations. The paramter $\epsilon$ 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 $\alpha$ = 1, while $\alpha$ = $\epsilon$ provides an approximation of void, meaning that material-free regions are approximated with a weak material. For acoustics applications, the value $\alpha$ = 1 corresponds to air and $\alpha$ $\epsilon$ to an approximation of sound-hard material using a dense fluid. Here we analyze the convergence properties of such material approximations as $\epsilon$!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 $\alpha$, 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 $\epsilon$ and $\epsilon$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 $\epsilon$, and we also construct a left preconditioner that yields a system matrix with a condition number independent of $\epsilon$.

In Paper III, we observe that the standard sound-hard material approximation with $\alpha$ $\epsilon$ 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 $\epsilon$.

• 19.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Umeå University, Faculty of Science and Technology, Department of Computing Science. 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)
• 20.
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

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.

• 21.
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
• 22.
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

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.

• 23.
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

• 24.
Umeå University, Faculty of Social Sciences, Department of Psychology.
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)

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.

• 25. Piquet, A.
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.
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)

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.

• 26. Saglietti, Clio
Umeå University, Faculty of Science and Technology, Department of Computing Science. Umeå University, Faculty of Science and Technology, Department of Computing Science.
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)

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.

• 27.
Umeå University, Faculty of Science and Technology, Department of Physics.
Vibro-acoustic analysis of a satellite reflector antenna using FEM2011Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis

The acoustic environment generated during launch is the most demanding structural load case for large, lightweight satellite reflector antennas. The reflector is exposed to extremely high sound pressure levels originating from the structural excitation of the rocket engines and exterior air flow turbulence. This thesis aims to predict the structural responses in the reflector due to the acoustic pressure load with a model based on Finite Element Modelling (FEM). The FE-model is validated against a previously performed Boundary Element Method (BEM) analysis. An approach called Split Loading together with a combination of BEM and FEM will be utilized to handle the surrounding air mass and the applied sound pressures. The idea of Split Loading is to divide the structure into several patches and apply a unit pressure load to each patch separately. In the last step the unit pressure is scaled and correlated by a power spectral density calculated from the acoustic pressures. Split Loading will be implemented in software packages MSC.NASTRAN/PATRAN. The model developed in this thesis handles both the added mass of the surrounding air and the sound pressure applied to the reflector. The model can qualitatively well reproduce the results of the BEM-analysis and the test data. However, the model tends to overestimate responses at low frequencies and underestimate them at high frequencies. The end results is that the model becomes too conservative at low frequencies to be used without further development.

• 28.
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
Prediction of ignition limits with respect to fuel fraction of inert gases.: Evaluation of cost effective CFD-method using cold flow simulations2015Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis

Improving fuel flexibility for gas turbines is one advantageous property on the market. It may lead to increased feasibility by potential customers and thereby give increased competiveness for production and retail companies of gas turbines such as Siemens Industrial Turbomachinery in Finspång. For this reason among others SIT assigned Anton Berg to perform several ignition tests at SIT’s atmospheric combustion rig (ACR) as his master thesis project. In the ACR he tested the limits for how high amounts of inert gases (N2 and CO2) that the rig, prepared with the 3rd generation DLE-burner operative in both the SGT-700 and SGT-800 engine, could ignite on (Berg, 2012).

Research made by Abdel-Gay and Bradley already in 1985 summarized methane and propane combustion articles showing that a Karlovitz number (Chemical time scale/Turbulent time scale) of 1.5 could be used as a quenching limit for turbulent combustion (Abdel-Gayed & Bradley, 1985). Furthermore in 2010 Shy et al. showed that the Karlovitz number showed good correlation to ignition transition from a flamelet to distributed regime (Shy, et al., 2010). They also showed that this ignition transition affected the ignition probability significantly.

Based on the results of these studies among others a CFD concept predicting ignition probability from cold flow simulations were created and tested in several applications at Cambridge University (Soworka, et al., 2014) (Neophytou, et al., 2012). With Berg’s ignition tests as reference results and a draft for a cost effective ignition prediction model this thesis where started.

With the objectives of evaluating the ignition prediction against Berg’s results and at the same time analyze if there would be any better suited igniter spot 15 cold flow simulations on the ACR burner and combustor geometry were conducted. Boundary conditions according to selected tests were chosen with fuels composition ranging from pure methane/propane to fractions of 40/60 mole% CO2 and 50/75 mole% N2.

By evaluating the average Karlovitz number in spherical ignition volumes around the igniter position successful ignition could be predicted if the Karlovitz number were below 1.5. The results showed promising tendencies but no straightforward prediction could be concluded from the evaluated approach. A conclusion regarding that the turbulence model probably didn’t predict mixing good enough was made which implied that no improved igniter position could be recommended. However by development of the approach by using a more accurate turbulence model as LES for example may improve the mixing and confirm the good prediction tendencies found. Possibilities for significantly improved ignition limits were also showed for 3-19% increase in equivalence ratio around the vicinity of the igniter.

• 29.
Umeå University, Faculty of Science and Technology, Department of Physics.
Probabilistic Multidisciplinary Design Optimization on a high-pressure sandwich wall in a rocket engine application2017Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis

A need to find better achievement has always been required in the space industrythrough time. Advanced technologies are provided to accomplish goals for humanityfor space explorer and space missions, to apprehend answers and widen knowledges. These are the goals of improvement, and in this thesis, is to strive and demandto understand and improve the mass of a space nozzle, utilized in an upperstage of space mission, with an expander cycle engine. The study is carried out by creating design of experiment using Latin HypercubeSampling (LHS) with a consideration to number of design and simulation expense.A surrogate model based optimization with Multidisciplinary Design Optimization(MDO) method for two different approaches, Analytical Target Cascading (ATC) and Multidisciplinary Feasible (MDF) are used for comparison and emend the conclusion.

In the optimization, three different limitations are being investigated, designspace limit, industrial limit and industrial limit with tolerance. Optimized results have shown an incompatibility between two optimization approaches, ATC and MDF which are expected to be similar, but for the two limitations, design space limit and industrial limit appear to be less agreeable. The ATC formalist in this case dictates by the main objective, where the children/subproblems only focus to find a solution that satisfies the main objective and its constraint. For the MDF, the main objective function is described as a single function and solved subject to all the constraints. Furthermore, the problem is not divided into subproblems as in the ATC.

Surrogate model based optimization, its solution influences by the accuracy ofthe model, and this is being investigated with another DoE. A DoE of the full factorial analysis is created and selected to study in a region near the optimal solution.In such region, the result has evidently shown to be quite accurate for almost allthe surrogate models, except for max temperature, damage and strain at the hottestregion, with the largest common impact on inner wall thickness of the space nozzle. Results of the new structure of the space nozzle have shown an improvement of mass by ≈ 50%, ≈ 15% and ≈ -4%, for the three different limitations, design spacelimit, industrial limit and industrial limit with tolerance, relative to a reference value,and ≈ 10%, ≈ 35% and ≈ 25% cheaper to manufacture accordingly to the defined producibility model.

• 30.
Umeå University, Faculty of Science and Technology, Department of Physics.
Evaluation of a CFD method for estimating aerodynamic loads on external stores on JAS 39 Gripen2011Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis

Loads determination for external stores on fighter aircraft is an important task for manufacturers in ensuring the safe operation of their aircraft. Due to the large number of possible store combinations, wind tunnel tests – the primary approach to obtaining loads data – cannot be performed for all configurations. Instead, supplementary techniques to estimating loads are necessary. One approach is to use information from another store and adapt it, using so-called scaling methods, to the non-tested store.

In this thesis, a scaling method combining the results of computational fluid dynamics (CFD) simulations, for both a non-tested and a reference store, with existing wind tunnel data for the reference store, is thoroughly examined for a number of different stores, angles of attack, sideslip angles and Mach numbers. The performance of the proposed scaling method is assessed in relation to currently used scaling methods, using non-parametric and multivariate statistics.

The results show no definitive improvement in performance for the proposed scaling method over the current methods. Although the proposed method is slightly more conservative, considerable variability in the estimates and an increased time consumption for scaling leads the author to advise against using the proposed method for scaling aerodynamic loads on external stores.

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