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• 1.
Department of Information Technology, Uppsala University.
Department of Information Technology, Uppsala University.
Discrete adjoint-based shape optimization for an edge-based finite-volume solver2003In: Computational Fluid and Solid Mechanics 2003 / [ed] K. J. Bathe, 2003Conference paper (Refereed)
• 2.
Department of Information Technology, Uppsala University.
Universita di Salerno, Italy. FOI, Swedish Defence Research Agency, Stockholm. FOI, Swedish Defence Research Agency, Stockholm. FOI, Swedish Defence Research Agency, Stockholm.
Shape Optimization for Delay of Laminar-Turbulent Transition2006In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 44, no 5, p. 1009-1024Article in journal (Refereed)
• 3.
Uppsala University, Sweden.
Salermo University, Italy. FOI, Sweden. FOI, Sweden. KTH, Sweden.
Adjoint-Based Shape Optimization for Natural Laminar Flow Design2004In: ERCOFTAC Design Optimization: Methods & Applications / [ed] K.C. Giannakoglou, W. Haase, 2004Conference paper (Refereed)
• 4.
Department of Mechanics, KTH, Stockholm.
FFA, the Aeronautical Research Institute of Sweden. Department of Mechanics, KTH, Stockholm.
Optimal Disturbances in Boundary Layers1998In: Computational Methods for Optimal Design and Control, / [ed] J. Borggaard, J. Burns, E. Cliff, and S. Schreck, 1998Conference paper (Refereed)
• 5.
FFA, the Aeronautical Research Institute of Sweden, Bromma.
FFA, the Aeronautical Research Institute of Sweden, Bromma. FFA, the Aeronautical Research Institute of Sweden, Bromma.
Optimal disturbances and bypass transition in boundary layers1999In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 11, no 1, p. 134-150Article in journal (Refereed)
Umeå University, Faculty of Science and Technology, Department of Computing Science. Department of Electronics and Electrical Communications, Menouﬁa University, Menouf, Egypt. 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.
Characterization of the Fat Channel for Intra-Body Communication at R-Band Frequencies2018In: Sensors, ISSN 1424-8220, E-ISSN 1424-8220, Vol. 18, no 9, article id 2752Article in journal (Refereed)

In this paper, we investigate the use of fat tissue as a communication channel between in-body, implanted devices at R-band frequencies (1.7–2.6 GHz). The proposed fat channel is based on an anatomical model of the human body. We propose a novel probe that is optimized to efficiently radiate the R-band frequencies into the fat tissue. We use our probe to evaluate the path loss of the fat channel by studying the channel transmission coefficient over the R-band frequencies. We conduct extensive simulation studies and validate our results by experimentation on phantom and ex-vivo porcine tissue, with good agreement between simulations and experiments. We demonstrate a performance comparison between the fat channel and similar waveguide structures. Our characterization of the fat channel reveals propagation path loss of ∼0.7 dB and ∼1.9 dB per cm for phantom and ex-vivo porcine tissue, respectively. These results demonstrate that fat tissue can be used as a communication channel for high data rate intra-body networks.

• 7.
Sandia National Laboratories.
Sandia National Laboratories. Department of Information Technology, Uppsala University.
Hybrid Differentiation Strategies for Simulation and Analysis of Applications in C++2008In: ACM Transactions on Mathematical Software, ISSN 0098-3500, E-ISSN 1557-7295, Vol. 35, no 1Article in journal (Refereed)
• 8.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
A unified discrete-continuous sensitivity analysis method for shape optimization2010In: Applied and Numerical Partial Differential Equations: Scientific Computing in Simulation, Optimization and Control in a Multidisciplinary Context / [ed] W. Fitzgibbon, Y.A. Kuznetsov, P. Neittaanmäki, J. Periaux, O. Pironneau, Springer, 2010, p. 25-39Conference paper (Refereed)

Boundary shape optimization problems for systems governed by partial differential equations involve a calculus of variation with respect to boundary modifications. As typically presented in the literature, the first-order necessary conditions of optimality are derived in a quite different manner for the problems before and after discretization, and the final directional-derivative expressions look very different. However, a systematic use of the material-derivative concept allows a unified treatment of the cases before and after discretization. The final expression when performing such a derivation includes the classical before-discretization (“continuous”) expression, which contains objects solely restricted to the design boundary, plus a number of “correction” terms that involve field variables inside the domain. Some or all of the correction terms vanish when the associated state and adjoint variables are smooth enough.

• 9.
Department of Information Technology, Uppsala University.
A vertex-centered dual discontinuous Galerkin method2006In: Journal of Computational and Applied Mathematics, ISSN 0377-0427, E-ISSN 1879-1778, Vol. 192, no 1, p. 175-181Article in journal (Refereed)
• 10.
Department of Information Technology, Uppsala University.
Approximations of very weak solutions to boundary-value problems2004In: SIAM Journal on Numerical Analysis, ISSN 0036-1429, E-ISSN 1095-7170, Vol. 42, no 2, p. 860-877Article in journal (Refereed)
• 11.
Department of Computational and Applied Methematics, Rice University, Houston.
Numerical solution of a flow-control problem: vorticity reduction by dynamic boundary action1998In: SIAM Journal on Scientific Computing, ISSN 1064-8275, E-ISSN 1095-7197, Vol. 19, no 3, p. 829-860Article in journal (Refereed)
• 12.
Department of Computational and Applied Mathematics, Rice University, Houston, TX.
Solving an advection-diffusion problem on the Connection Machine1994In: Concurrency: Practice and Experience, Vol. 6, no 1, p. 55-68Article in journal (Refereed)
• 13.
FFA, the Aeronautical Research Institute of Sweden.
The volume discharge approach to geometric conservation1999In: Computational Methods for Fluid-Structure Interaction / [ed] T. Kvamsdal, 1999Conference paper (Refereed)
• 14.
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.

• 15.
Department of Information Technology, Uppsala University.
Department of Information Technology, Uppsala University. Department of Information Technology, Uppsala University.
Multifrequency shape optimization of an acoustic horn2003In: Computational Fluid and Solid Mechanics 2003 / [ed] K. J. Bathe, 2003, p. 2204-2207Conference paper (Refereed)
• 16.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Department of Information Technology, Uppsala University. Department of Information Technology, Uppsala University.
A discontinuous Galerkin extension of the vertex-centered edge-based finite volume method2009In: Communications in Computational Physics, ISSN 1815-2406, Vol. 5, no 2-4, p. 456-468Article in journal (Refereed)

The finite volume (FV) method is the dominating discretization technique for computational fluid dynamics (CFD), particularly in the case of compressible fluids. The discontinuous Galerkin (DG) method has emerged as a promising high-accuracy alternative. The standard DG method reduces to a cell-centered FV method at lowest order. However, many of today's CFD codes use a vertex-centered FV method in which the data structures are edge based. We develop a new DG method that reduces to the vertex-centered FV method at lowest order, and examine here the new scheme for scalar hyperbolic problems. Numerically, the method shows optimal-order accuracy for a smooth linear problem. By applying a basic hp-adaption strategy, the method successfully handles shocks. We also discuss how to extend the FV edge-based data structure to support the new scheme. In this way, it will in principle be possible to extend an existing code employing the vertex-centered and edge-based FV discretization to encompass higher accuracy through the new DG method.

• 17.
Department of Computational and Applied Mathematics, Rice University, Houston TX.
Department of Mathematics, University of Houston, Houston, , TX .
A spectral preconditioner for control problems associated with linear evolution equations1995In: East-West Journal on Numerical Mathematics, Vol. 3, no 2, p. 81-109Article in journal (Refereed)
• 18.
FFA, the Aeronautical Research Institute of Sweden, Bromma.
Department of Mathematics, University of Houston. College de France.
A Computational Approach to Controllability Issues for Flow-Related Models. (I): Pointwise Control of the Viscous Burgers Equation1996In: International journal of computational fluid dynamics (Print), ISSN 1061-8562, E-ISSN 1029-0257, Vol. 7, no 3, p. 237-252Article in journal (Refereed)
• 19.
FFA, the Aeronautical Research Institute of Sweden, Bromma, Sweden.
Department of Mathematics, University of Houston, Houston. College de France, Rue d'Ulm, Paris, France .
A computational approach to controllability issues for flow-related models. (II): Control of two-dimensional, linear advection-diffusion and Stokes models.1996In: International journal of computational fluid dynamics (Print), ISSN 1061-8562, E-ISSN 1029-0257, Vol. 6, no 4, p. 253-274Article in journal (Refereed)
• 20.
FFA, the Aeronautical Research Institute of Sweden.
Department of Computational and Applied Mathematics, Rice University, Houston, TX.
Parallel Solution of Optimal-Control Problems by Time-Domain Decomposition1997In: Computational Science for the 21st Century / [ed] M.O. Bristeau, G. Etgen, W. Fitzgibbon, J.L. Lions, J. Périaux, and M. F. Wheeler, 1997Conference paper (Refereed)
• 21.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Weak material approximation of holes with traction-free boundaries2012In: SIAM Journal on Numerical Analysis, ISSN 0036-1429, E-ISSN 1095-7170, Vol. 50, no 4, p. 1827-1848Article in journal (Refereed)

Consider the solution of a boundary-value problem for steady linear elasticity in which the computational domain contains one or several holes with traction-free boundaries. The presence of holes in the material can be approximated using a weak material; that is, the relative density of material rho is set to 0 < epsilon = rho << 1 in the hole region. The weak material approach is a standard technique in the so-called material distribution approach to topology optimization, in which the inhomogeneous relative density of material is designated as the design variable in order to optimize the spatial distribution of material. The use of a weak material ensures that the elasticity problem is uniquely solvable for each admissible value rho is an element of [epsilon, 1] of the design variable. A finite-element approximation of the boundary-value problem in which the weak material approximation is used in the hole regions can be viewed as a nonconforming but convergent approximation of a version of the original problem in which the solution is continuously and elastically extended into the holes. The error in this approximation can be bounded by two terms that depend on epsilon. One term scales linearly with epsilon with a constant that is independent of the mesh size parameter h but that depends on the surface traction required to fit elastic material in the deformed holes. The other term scales like epsilon(1/2) times the finite-element approximation error inside the hole. The condition number of the weak material stiffness matrix scales like epsilon(-1), but the use of a suitable left preconditioner yields a matrix with a condition number that is bounded independently of epsilon. Moreover, the preconditioned matrix admits the limit value epsilon -> 0, and the solution of corresponding system of equations yields in the limit a finite-element approximation of the continuously and elastically extended problem.

• 22.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Sound vibration damping optimization with application to the design of speakerphone casings2013In: : Paper id 5569, 2013Conference paper (Refereed)

We optimize the thickness distribution in a 1D beam model of an elastic plate, subject to forced vibration at one of its ends, in order to minimize the structural vibration in a given area of the plate. The optimization is carried out both in broadband and band-pass cases. Geometric constraints, weight constraints, and constraints on the static compliance are imposed in the optimization. A broadband optimization over 50 frequencies, evenly distributed in the 300–3400 Hz range, reduces the vibration by around 5–10 dB on average throughout the frequency range. When targeting only the higher end of the above frequency range, it is possible to achieve more dramatic results. Vibration reductions of 20 dB and more can be achieved in the 2300–2800 Hz region. In the latter case, the results suggest that a band-gap phenomenon occurs, similarly as for phononic band gap materials. To validate the results, the best-performing optimal shape for the clamped case was imported into a 3D computational structural model, and the resulting forced vibration response agreed well with the the beam-model computations. These results were first announced in a technical report by Lacis et al. [5].

• 23.
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 shape optimization using cut finite elements2018In: International Journal for Numerical Methods in Engineering, ISSN 0029-5981, E-ISSN 1097-0207, Vol. 113, no 3, p. 432-449Article in journal (Refereed)

Fictitious domain methods are attractive for shape optimization applications, since they do not require deformed or regenerated meshes. A recently developed such method is the CutFEM approach, which allows crisp boundary representations and for which uniformly well-conditioned system matrices can be guaranteed. Here, we investigate the use of the CutFEM approach for acoustic shape optimization, using as test problem the design of an acoustic horn for favorable impedance-matching properties. The CutFEM approach is used to solve the Helmholtz equation, and the geometry of the horn is implicitly described by a level-set function. To promote smooth algorithmic updates of the geometry, we propose to use the nodal values of the Laplacian of the level-set function as design variables. This strategy also improves the algorithm's convergence rate, counteracts mesh dependence, and, in combination with Tikhonov regularization, controls small details in the optimized designs. An advantage with the proposed method is that the exact derivatives of the discrete objective function can be expressed as boundary integrals, as opposed to when using a traditional method that uses mesh deformations. The resulting horns possess excellent impedance-matching properties and exhibit surprising subwavelength structures, not previously seen, which are possible to capture due to the fixed mesh approach.

• 24.
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.
SHAPE OPTIMIZATION OF A COMPRESSION DRIVER PHASE PLUG2019In: SIAM Journal on Scientific Computing, ISSN 1064-8275, E-ISSN 1095-7197, Vol. 41, no 1, p. B181-B204Article in journal (Refereed)

A compression driver is an electro-acoustic transducer with considerably higher efficiency than direct radiating loudspeakers, thanks to the increased radiation resistance caused by a large vibrating diaphragm placed in a compression chamber with small openings. The transition section between compression chamber and output waveguide, the phase plug, must be carefully designed to avoid irregularities in the output sound pressure level (SPL) as a function of frequency. Here we present a shape optimization method based on an implicit level-set description and adjoint sensitivity analysis, which enables a large number of design parameters and vast design freedom. The CutFEM approach, a fictitious domain finite element method, removes the need for mesh updates and makes the method robust and computationally inexpensive. Numerical experiments for a generic annular diaphragm compression driver are presented, with optimized designs showing only minor frequency irregularities. Two different objective functions are considered: one for maximum SPL and one where the SPL is fitted to that of a hypothetical ideal design; the latter approach is found to be more effective in reducing irregularities. Visco-thermal boundary-layer losses are included in a post-processing step, and, though the influence of losses is clearly noticeable, the overall performance is similar and the optimized designs still outperform the original design.

• 25.
Department of Information Technology, Uppsala University.
Department of Information Technology, Uppsala University. Department of Information Technology, Uppsala University.
Shape optimization of an acoustic horn2003In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 192, p. 1533-1571Article in journal (Refereed)
• 26.
FFA, the Aeronautical Research Institute of Sweden.
FFA, the Aeronautical Research Institute of Sweden.
Accuray of Gradient Computations in Aerodynamic Shape Optimization2000In: Paper ICAS 2000-2.4.5, 2000Conference paper (Refereed)
• 27.
FOI, Stockholm.
FOI, Stockholm. Sandia National Laboratories. Department of Mechanics, KTH, Stockholm.
Linear and Nonlinear Optimal Control in Spatial Boundary Layers2002In: AIAA 3rd Theoretical Fluid Mechanics Meeting, St. Louis, MO.: AIAA Paper 2002-2755, 2002Conference paper (Refereed)
• 28.
Department of Information Technology, Uppsala University.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Agglomeration multigrid for the vertex-centered dual discontinuous galerkin method2010In: ADIGMA - a European initiative on the development of adaptive higher-order variational methods for aerospace applications: Results of a collaborative research project funded by the European Union, 2006-2009 / [ed] N. Krol, H. Bieler, H. Deconinck, V. Couaillier, H. van der Ven, and K. Sørensen, Springer Berlin/Heidelberg, 2010, p. 301-308Chapter in book (Refereed)

Agglomoration multigrid is used in many finite-volume codes for aerodynamic computations in order to reduce solution times. We show that an existing agglomeration multigrid solver developed for equations discretized with a vertex-centered, edge-based finite-volume scheme can be extended to accelerate convergence also for a vertex-centered discontinuous Galerkin method. Preliminary results for a subsonic as well as a transonic test case for the Euler equations in two space dimensions show a significant convergence acceleration for the discontinuous Galerkin equations using the agglomoration multigrid strategy.

• 29.
Department of Information Technology, Uppsala University.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Incorporating a Galerkin method into the existing vertex-centered edge-based finite volume solver edge2010In: ADIGMA - a European initiative on the development of adaptive higher-order variational methods for aerospace applications: Results of a collaborative research project funded by the European Union, 2006-2009 / [ed] N. Kroll and H. Bieler and H. Deconinck, V. Couaillier, H. van der Ven, and K. Sørensen, Springer Berlin/Heidelberg, 2010, p. 39-52Chapter in book (Refereed)

The discontinuous Galerkin (DG) method can be viewed as a generalization to higher orders of the finite volume method. At lowest order, the standard DG method reduces to the cell-centered finite volume method.We introduce for the Euler equations an alternative DG formulation that reduces to the vertex-centered version of the finite volume method at lowest order. The method has been successfully implemented for the Euler equations in two space dimensions, allowing a local polynomial order up to p=3 and supporting curved elements at the airfoil boundary. The implementation has been done as an extension within the existing edge-based vertex-centered finite-volume code Edge.

• 30.
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.

• 31.
Department of Mathematics, Technical University of Denmark, Lyngby, Denmark.
Department of Information Technology, Uppsala University. Informatics and Mathematical Modelling, Technical University of Denmark, Lyngby, Denmark.
Topology optimization of mass distribution problems in Stokes flow2006In: IUTAM Symposium on Topological Design Optimization of Structures, Machines, and Materials. Status and Perspectives / [ed] M. P. Bendsoe, N. Olhoff, O. Sigmund, 2006, p. 365-374Conference paper (Refereed)
• 32.
Umeå University, Faculty of Science and Technology, Department of Computing Science. Department of Electronics and Electrical Communications, Menoufia University, Menouf, Egypt.
Umeå University, Faculty of Science and Technology, Department of Computing Science. Institute for Electronics Engineering, Friedrich-Alexander University Erlangen-Nuremberg (FAU), 91058 Erlangen, Germany. Institute for Electronics Engineering, Friedrich-Alexander University Erlangen-Nuremberg (FAU), 91058 Erlangen, Germany. Institute for Electronics Engineering, Friedrich-Alexander University Erlangen-Nuremberg (FAU), 91058 Erlangen, Germany. Institute for Electronics Engineering, Friedrich-Alexander University Erlangen-Nuremberg (FAU), 91058 Erlangen, Germany.
Design of Planar Microstrip-to-Waveguide Transitions Using Topology Optimization2019In: 2019 IEEE Radio and Wireless Symposium (RWS), Orlando, USA, January 20-23, 2019, IEEE, 2019, p. -3Conference paper (Refereed)

This paper presents a topology optimization approach to design planar transitions between a microstrip line (MSL) and a rectangular waveguide (RWG) in the K-band. The transition comprises two sub-transitions: one from the MSL to a substrate integrated waveguide (SIW) and the second from the SIW to the RWG. Both are on the same substrate and can be manufactured with a standard printed circuit board process. This leads to a very costeffective solution compared with other approaches. A WR-42 waveguide can easily be surface mounted to the transitions using a standard flange. The transitions have been fabricated, and their measured performance shows good agreement with the simulations. The MSL-SIW transition has a broadband behavior and the SIW-RWG transition still reaches a relative bandwidth of 10%.

• 33.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Umeå University, Faculty of Science and Technology, Department of Computing Science. Uppsala University. 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 planar antennas for wideband near-field coupling2015In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 63, no 9, p. 4208-4213Article in journal (Refereed)

We present an approach to design from scratch planar microwave antennas for the purpose of ultra-wideband (UWB) near-field sensing. Up to about 120 000 design variables associated with square grids on planar substrates are subject to design, and a numerical optimization algorithm decides, after around 200 iterations, for each edge in the grid whether it should consist of metal or a dielectric. The antenna layouts produced with this approach show UWB impedance matching properties and near-field coupling coefficients that are flat over a much wider frequency range than a standard UWB antenna. The properties of the optimized antennas are successfully cross-verified with a commercial software and, for one of the designs, also validated experimentally. We demonstrate that an antenna optimized in this way shows a high sensitivity when used for near-field detection of a phantom with dielectric properties representative of muscle tissue.

• 34.
Umeå University, Faculty of Science and Technology, Department of Computing Science. Department of electronics and electrical communications, Menoufia University, Menouf, 32952, Egypt.
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.
Topology Optimisation of Wideband Coaxial-to-Waveguide Transitions2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 45110Article in journal (Refereed)

To maximize the matching between a coaxial cable and rectangular waveguides, we present a computational topology optimisation approach that decides for each point in a given domain whether to hold a good conductor or a good dielectric. The conductivity is determined by a gradient-based optimisation method that relies on finite-difference time-domain solutions to the 3D Maxwell’s equations. Unlike previously reported results in the literature for this kind of problems, our design algorithm can efficiently handle tens of thousands of design variables that can allow novel conceptual waveguide designs. We demonstrate the effectiveness of the approach by presenting optimised transitions with reflection coefficients lower than −15dB over more than a 60% bandwidth, both for right-angle and end-launcher configurations. The performance of the proposed transitions is crossverified with a commercial software, and one design case is validated experimentally.

• 35.
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.
Conductive material distribution optimization for ultrawideband antennas2013In: Proceedings 11th International Conference on Mathematical and Numerical Aspects of Waves: Waves 2013, Tunis: ENIT-LAMSIN , 2013, p. 171-172Conference paper (Refereed)

An Ultrawideband (UWB) planar monopole an-tenna is designed using the material distribution ap-proach to topology optimization. The design variablesare the local conductivity values in a 75 × 75 mm areawhere the radiating element can be located. Theantenna is optimized for maximum reception, in anattached coaxial cable, of incoming plane waves. Thewave propagation is modeled using the time domain3D Maxwell equations discretized using FDTD, andthe optimization is carried out using a gradient-basedoptimization method, in which the derivatives aresupplied through solving corresponding adjoint equa-tions. The outer dimensions of the optimized antennais 75 × 60 mm, and its reflection coefficient |S11 |,with respect to a feeding signal in the coaxial cable,stays below −10 dB throughout the frequency band1.2–9.7 GHz.

• 36.
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.
Patch and ground plane design of microstrip antennas by material distribution topologly optimization2014In: Progress in Electromagnetics Research B, ISSN 1937-6472, E-ISSN 1937-6472, Vol. 59, p. 89-102Article in journal (Refereed)

We use a gradient-based material distribution approach to design conductive parts of microstrip antennas in an efficient way. The approach is based on solutions of the 3D Maxwell's equation computed by the finite-difference time-domain (FDTD) method. Given a set of incoming waves, our objective is to maximize the received energy by determining the conductivity on each Yee-edge in the design domain. The objective function gradient is computed by the adjoint-field method. A microstrip antenna is designed to operate at 1.5 GHz with 0.3 GHz bandwidth. We present two design cases. In the first case, the radiating patch and the finite ground plane are designed in two separate phases, whereas in the second case, the radiating patch and the ground plane are simultaneously designed. We use more than 58,000 design variables and the algorithm converges in less than 150 iterations. The optimized designs have impedance bandwidths of 13% and 36% for the first and second design case, respectively.

• 37.
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.
Time-Domain Sensitivity Analysis for Conductivity Distribution in Maxwell's Equations2015Report (Other academic)

We present expressions for the derivatives of the outgoing signal in coaxial cables with respect to the conductivity distribution in a specific domain. The derived expressions can be used with gradient-based optimization methods to design metallic electromagnetic devices, such as antennas and waveguides. We use the adjoint-field method to derive the expressions and the derivation is based on the 3D time-domain Maxwell's equations. We present two derivative expressions; one expression is derived in the continuous case and the second is derived based on the FDTD discretization of Maxwell's equations, including the uniaxial perfectly match layer (UPML) to simulate the radiation boundary condition. The derivatives are validated through a numerical example, where derivatives computed by the adjoint-field method are compared against derivatives computed with finite differences. Up to 7 digits precision matching is obtained.

• 38.
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.
Topology optimization of metallic antennas2014In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 62, no 5, p. 2488-2500Article in journal (Refereed)

• 39.
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.
Topology optimization of UWB monopole antennas2013In: 7th European Conference on Antennas and Propagation (EuCAP2013), New York: IEEE conference proceedings, 2013, p. 1488-1492Conference paper (Refereed)

A Topology optimization technique is used for complete layout optimization of the radiating element of a planar monopole antenna. The design objective is to find a conductivity distribution that maximizes the energy received by the planar monopole over the frequency band 1-10 GHz. The finite difference time domain method (FDTD) is used for the numerical calculations, and an adjoint problem is derived to calculate the corresponding sensitivities. Numerical results show a promising use of topology optimization techniques for the systematic design of ultrawideband monopoles.

• 40.
Umeå University, Faculty of Science and Technology, Department of Computing Science. Department of Electronics and Electrical Communications, Menoufia University, Menouf, Egypt.
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.
Topology optimization of compact wideband coaxial-to-waveguide transitions with minimum-size control2018In: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 57, no 4, p. 1765-1777Article in journal (Refereed)

This paper presents a density-based topology optimization approach to design compact wideband coaxial-to-waveguide transitions. The underlying optimization problem shows a strong self penalization towards binary solutions, which entails mesh-dependent designs that generally exhibit poor performance. To address the self penalization issue, we develop a filtering approach that consists of two phases. The first phase aims to relax the self penalization by using a sequence of linear filters. The second phase relies on nonlinear filters and aims to obtain binary solutions and to impose minimum-size control on the final design. We present results for optimizing compact transitions between a 50-Ohm coaxial cable and a standard WR90 waveguide operating in the X-band (8-12 GHz).

• 41.
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. Umeå University, Faculty of Science and Technology, Department of Computing Science.
1D-model of the interaction between a stack of wood and an imposed electromagnetic wave2015Manuscript (preprint) (Other academic)

We have developed and investigated a 1D-model for the interaction between a stack of wood and an impinging electromagnetic field. Maxwell's equations are used to model the electromagnetic interaction and each layer in a stack of boards has been modeled as a homogenous lossy dielectric slab. The main reason for developing this model has been to investigate the possibility of measuring the moisture content of wood inside a drying kiln using electromagnetic waves. Our investigations show that it is in principle possible to measure the moisture content, since the electromagnetic field is sensitive to changes in the moisture content of the wood. We also show that it might be possible to measure the average moisture content, without detailed knowledge of the distribution of moisture content between different boards.

• 42.
Department of Mechanics, Royal Institute of Technology, Stockholm.
Department of Information Technology, Uppsala University.
Numerical Approaches to Optimal Control of a Model Equation for Shear Flow Instabilities2000In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 65, p. 299-320Article in journal (Refereed)
• 43.
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.
Fixed-mesh curvature-parameterized shape optimization of an acoustic horn2012In: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 46, no 5, p. 727-738Article in journal (Refereed)

We suggest a boundary shape optimization approach in which the optimization is carried out on the coefficients in a boundary parameterization based on a local, discrete curvature. A fixed mesh is used to numerically solve the governing equations, in which the geometry is represented through inhomogeneous coefficients, similarly as done in the material distribution approach to topology optimization. The method is applied to the optimization of an acoustic horn in two space dimensions. Numerical experiments show that this method can calculate the horn's transmission properties as accurately as a traditional, body-fitted approach. Moreover, the use of a fixed mesh allows the optimization to create shapes that would be difficult to handle with a traditional approach that uses deformations of a body-fitted mesh. The parameterization inherently promotes smooth designs without unduly restriction of the design flexibility. The optimized, smooth horns consistently show favorable transmission properties.

• 44.
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)
• 45.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Department of Information Technology, Uppsala University. Umeå University, Faculty of Science and Technology, Department of Computing Science.
A hybrid scheme for bore design optimization of a brass instrument2010In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 128, no 3, p. 1391-1400Article in journal (Refereed)

Thispaper presents how the shape of a brass instrument canbe optimized with respect to its intonation properties. The instrumentis modeled using a hybrid method between a lossy one-dimensionaltransmission line analogy for the slowly flaring part of theinstrument, and a two-dimensional finite element model for the rapidlyflaring part. The optimization employs gradient-based algorithms, and allows fora large number of design variables. Through the use ofan appropriate choice of design variables, the algorithm is capableof rapidly finding horn profiles that are optimal subject tovarious geometric constraints, such as increasing or convex bell flares.It is found that under a convexity constraint, brass windbells that are optimal with respect to an intonation conditioncan be constructed of piecewise conical sections.

• 46.
Umeå University, Faculty of Science and Technology, Department of Computing Science.
Uppsala University, Department of Information Technology, Division of Scientific Computing. D.A.S. Audio S.A., Valencia, Spain. Umeå University, Faculty of Science and Technology, Department of Computing Science. Umeå University, Faculty of Science and Technology, Department of Computing Science.
An efficient loudspeaker horn designed by numerical optimization: an experimental study2010Report (Other academic)

Using rapid prototyping, we manufacture an acoustic horn designed by gradient-based shape optimization to have virtually perfect impedance-matching properties. The horn is 161.5 mm long, has a mouth diameter of 300 mm, and a throat diameter intended for a 1.5 inch driver. We optimize the horn with the aim of having perfect radiation efficiency at 31 frequencies in the range 1.6–9.05 kHz, while satisfying a convexity constraint on the flare. The acoustical properties, as needed by the optimization algorithm, are calculated through numerical solutions with the finite-element method of the axisymmetric Helmholtz equation. The prototype has been analyzed in an anechoic chamber. In the design frequency band, the acoustic input impedance agrees reasonably well with the ideal characteristic impedance of a waveguide with the same cross sectional area as the horn throat.

• 47. Roos, M. W.
Umeå University, Faculty of Science and Technology, Department of Computing Science. Umeå University, Faculty of Science and Technology, Department of Computing Science.
Computational estimation of fluid mechanical benefits from a fluid deflector at the distal end of artificial vascular grafts2013In: Computers in Biology and Medicine, ISSN 0010-4825, E-ISSN 1879-0534, Vol. 43, no 2, p. 164-168Article in journal (Refereed)

Intimal hyperplasia at the distal anastomosis is considered to be an important determinant for arterial and arteriovenous graft failure. The connection between unhealthy hemodynamics and intimal hyperplasia motivates the use of computational fluid dynamics modeling to search for improved graft design. However, studies on the fluid mechanical impact on intimal hyperplasia at the suture line intrusion have previously been scanty. In the present work, we focus on intimal hyperplasia at the suture line and illustrate potential benefits from the introduction of a fluid deflector to shield the suture line from unhealthily high wall shear stress.

• 48. 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.

• 49.
Institut für Mathematik , Universität Würzburg.
Umeå University, Faculty of Science and Technology, Department of Computing Science. Umeå University, Faculty of Science and Technology, Department of Computing Science.
Large-Scale Three-Dimensional Acoustic Horn Optimization2016In: SIAM Journal on Scientific Computing, ISSN 1064-8275, E-ISSN 1095-7197, Vol. 38, no 6, p. B917-B940Article in journal (Refereed)

We consider techniques that enable large-scale gradient-based shape optimization of wave-guiding devices in the context of three-dimensional time-domain simulations. The approach relies on a memory efficient boundary representation of the shape gradient together with primal and adjoint solvers semiautomatically generated by the FEniCS framework. The hyperbolic character of the governing linear wave equation, written as a first-order system, is exploited through systematic use of the characteristic decomposition both to define the objective function and to obtain stable numerical fluxes in the discontinuous Galerkin spatial discretization. The methodology is successfully used to optimize the shape of a midrange acoustic horn, described by 1,762 design variables, for maximum transmission efficiency, where the parallel computations involve a total of $3.5\times10^9$ unknowns.

• 50.
Department of Information Technology, Uppsala University.
Department of Information Technology, Uppsala University.
Optimization of an acoustic horn with respect to efficiency and directivity2007In: International Journal for Numerical Methods in Engineering, ISSN 0029-5981, E-ISSN 1097-0207, Vol. 73, no 11, p. 1571-1606Article in journal (Refereed)
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