Simulating electromagnetic waves is of increasing importance, for example, due to the rapidly growing demand of wireless communication in the fields of antenna design, photonics and electromagnetic compatibility (EMC). Many numerical and asymptotic techniques have been developed and one of the most common is the Finite-Difference Time-Domain (FDTD) method, also known as the Yee scheme. This centered difference scheme was introduced by Yee in 1966. The success of the Yee scheme is based on its relatively high accuracy, energy conservation and superior memory efficiency from the staggered form of defining unknowns. The scheme uses a structured Cartesian grid, which is excellent for implementations on modern computer architectures. However, the structured grid results in loss of accuracy due to general geometry of boundaries and material interfaces. A natural challenge is thus to keep the overall structure of Yee scheme while modifying the coefficients in the algorithm near boundaries and interfaces in order to improve the overall accuracy. Initial results in this direction have been presented by Engquist, Gustafsson, Tornberg and Wahlund in a series of papers. Our contributions are new formulations and extensions to higher dimensions. These new formulations give improved stability properties, suitable for longer simulation times. The development of the algorithmsis supported by rigorous stability analysis. We also tackle the problem of controlling the divergence free property of the solution—which is of extra importance in three dimensions—and present results of a number of numerical tests. / QC 20101101

Understanding the mechanics of turbulent fluid flow is of key importance for industry and society as for example in aerodynamics and aero-acoustics. The massive computational cost for resolving all turbulent scales in a realistic problem makes direct numerical simulation of the underlying Navier-Stokes equations impossible. Recent advances in adaptive finite element methods offer a new powerful tool in Computational Fluid Dynamics (CFD). The computational cost for simulating turbulent flow can be minimized where the mesh is adaptively resolved, based on a posteriori error control. These adaptive methods have been implemented for efficient serial computations, but the extension to an efficient parallel solver is a challenging task. This work concerns the development of an adaptive finite element method for modern parallel computer architectures. We present efficient data structures and data decomposition methods for distributed unstructured tetrahedral meshes. Our work also concerns an efficient parallellization of local mesh refinement methods such as recursive longest edge bisection. We also address the load balance problem with the development of an a priori predictive dynamic load balancing method. Current results are encouraging with almost linear strong scaling to thousands of cores on several modern architectures. / QC 20110223

This thesis proposes a way of simulating the continuous casting process of steel using Smoothed Particle Hydrodynamics (SPH). It deals with the SPH modeling of mass, momentum and the energy equations. The interpolation kernel functions required for the SPH modeling of these equations are calculated. Solidification is modeled by some particles are used to represent fluids and others solids. Elastic forces are calculated between the particle neighbors to create deformable bodies. The fluid solidifies into the elastic body when it cools down and the elastic body melts as it is heated. In continuous casting the molten metal solidifies forming a shell when it comes in contact with the cold wall. The mold of the continuous casting is modeled with a cold oscillating wall and a symmetric wall. Once the shell is formed water is sprayed on the solidified metal. If the shell is thin and cooling is not sufficient, the elastic body melts due to the effect of the hot fluid. / Den klassiska SPH-modellen för vätskor med fri yta kompletteras med värmeledning med fasomvandling och stelning: partiklar kan byta mellan vätske-tillstånd och solid-tillstånd beroende på temperaturen. Elastiska krafter beroende på avstånd mellan partiklarna aktiveras i solid-tillståndet och slås av i fluid-tillstånd så att vätskan kan stelna och senare smälta igen om så behövs. Vid stränggjutning stelnar smältan, som fylls på via ett rör, vid kontakt med en oscillerande, kall kokill-vägg, till ett elastiskt skal. Detta kyls fortlöpande genom påsprutning av vatten utanpå kokillen och direkt på skalet, som förångas. Skalet deformeras nedanför kokillen av det hydrostatiska trycket från smältan; om det ar för tunt brister det. Som demonstration gjordes en simulering där ett skal skapas, varpå man slår av vattenkylningen på ett parti: då smälter skalet och blir tunnare och till sist brister det och all smälta rinner ut genom hålet. Noggrannheten i simuleringen lämnar en del att önska men det vore mycket svårt att bygga en så komplex modell med vanlig CFD.

Molecular Dynamics has been a field using the most advanced computer systems for decades to perform simulations. Sophisticated methods with complicated potentials and parallelized implementations have been developed starting from simple models and a few hundred atoms within one simulation in the beginning. Nowadays, we see an increasing need for large-scale parallel simulations in molecular dynamics. Computer simulations are a cost-effective research tool. Furthermore, a shift to multi-core processors provides computer systems allowing a much higher degree of parallelism in applications. It is possible to use implementations on these systems, which are based on established programming models and techniques. Nevertheless, it is necessary to analyze the limitations of such implementations on significant larger systems then common until now. Such an analysis work is an important step to the development of new programming models that make efficient use of modern multi-core processors. This thesis project has been focused on the implementation and performance analysis of a tree code that is an important algorithm for molecular dynamic simulations. The tree code algorithm has been parallelized for distributed memory computer systems using MPI. The load-balancing applies space-filling curves for work decomposition. The performance of the implementation was tested finally with input data covering a large range of parameters like the number of processors in the jobs and the number of particles per processor.

A patient-specific poroelastic model of the brain was constructed in COMSOL Multiphysics and evaluated for its usability in a clinical setting. Image processing of magnetic resonance (MR) images of a standard (uninjured) brain and a computed tomography (CT) scan of a patient with a subdural hematoma was used to make an estimation of the shape patient’s pre-injury brain and obtain a deformation map describing the displacement due to the hematoma. A finite-element mesh of the normal brain was generated from a standard MRI-based brain atlas. The steady-state solution of the normal brain was similar to that found in a previous study. The steady-state solution of the deformed brain had the same pressure distribution as the normal brain, which was predicted by analysis of the equations. When the hematoma was simulated over 15000 seconds, the progression of the pressure over time seemed qualitatively plausible. The strain in the steady-state deformed brain and time-based studies generally behaved as expected, but a few regions near the surface of the brain adjacent to the hematoma had unexpectedly high values. When the magnitude of hematoma deformation was altered, it was seen that intracranial pressure, maximum pressure and strain had an exponential-recovery relationship over time for all factors. Strain rate decreased exponentially with time. The intracranial pressure, maximum pressure and strain had a linear relationship with a scale factor that was used to change to magnitude of deformation of the outer surface. Cortical cell death increased exponentially with scale factor and with time. Hematoma volume had a linear relationship with scale factor. Simulated pressure-volume curves did not have the same shape as experimental curves and the observed deformation had noticeable differences in mid-line shift when compared with the real brain. Conditions of hyper- and hypotension had big effects on the pressure, but not the strain. The results indicate that the model has the potential to be a good tool in brain injury evaluation, but more work must be done to increase accuracy and to validate the model.

An aerodynamic propeller model, which can contribute to the prediction of structural loads experienced by aircraft in different flight maneuvers is presented.The model is based on Blade Element Momentum theory and is able to predict the unsymmetrical and frequency-dependent forces and moments induced by the propeller on the airplane structure at steady and unsteady inflow-conditions.In order to validate the model, a comparison with experimental results was performed and it can be seen that the model is in agreement with the experimental data providing that the aerodynamic data used for the calculations has good accuracy. / En modell har utvecklats för att beräkna aerodynamiska krafter som orsakas av propellern vid manöverflygning. Modellen använder sig av klassiska bladelementteorin för predikering av osymmetriska stationära krafter som uppstår vid snedanblåsning av propellerskivan. Modellen kommer att användas inom ett forskningsprojekt om effektiv beräkning av aerodynamiska laster vid flygmanövrar och i vindbyar. En vidareutveckling av den klassiska metoden används för att ta fram instationära kraftbidrag i frekvensplanet i en form som är lämpligt för aeroelastiska stabilitetsanalys och beräkning av vindbylasterna.Jämförelser med omfattande experimentella resultat har genomförts för att validera modellen. Inom ramen för antaganden och noggrannheten i modellens indata kan modellens tillförlitighet bedömas som tillräckligt för ändamålet. Däremot visar sig att propellermodellen är -- som förväntat -- mindre lämpligt för att bedöma propellerlasterna utanför propellerns reguljära driftområdet.

An image matching system for object recognition in scenes of varying complexity was constructed in Matlab for evaluating the recognition quality of two types of image features: SURF (Speeded-Up Robust Features) and SIFT (Scale Invariant Feature Transform) using the affine Hough matching algorithm for finding matches between training and test images. In the experimental part of the thesis, the matching was algorithm tested for varying number of image features extracted from the train and test images, namely 1000, 2000 and 3000 interest points. These experiments were carried out against 9 objects of different complexity, including difficulties such as repeating patterns on the image, down and upscaling of the object, cluttered scenes, silhouette features, partly occluded object and multiple known objects in the scene. The work provides the directions for improvement of the given view-based recognition algorithm and suggests other possible ways to perform the object matching with higher quality.

We study the problem of packing spheres of different radii into a rigid container. The spheres are packed as densely as possible so they neither overlap nor cross the boundaries of the container. The packed spheres are then meshed in preparation for finite element analysis. The spheres are meshed individually (separately) without taking care of the contact surface between spheres or containers wall. The last step is the hydrostatic pressing of the spheres made of elastic-plastic materials and with simplified contact conditions. Such simulations are of interest to the metallurgist because they can show differences in structure between grain surfaces and centres.

Numerical simulations play an important role to uncover the dynamic behaviour at the cellular and network levels and accelerate the work in the field of Neuroscience. The modern computational technologies have made it possible to simulate a huge network of neurons which was possible only in theory two decades ago. The simulations of networks of thousand of neurons are carried out on the parallel machine Cray XE6 system, based on the AMD Opteron 12-core which shows good scaling properties. These models can be beneficial for the generation of global behaviour which could not be produced by fewer cells. For example, the effect of inhibition in a striatal network of MSNs is only seen if the number of cells and the synapses are increased sufficiently. The simulated responses of cells are greatly influenced by the numerical scheme. This has been demonstrated using gap junctions between striatal fast spiking inter-neurons. Implicit numerical schemes need to be used in order to get stable and accurate results. The simulations are carried out using serial and parallel implementations on the GENESIS and PGENESIS simulator respectively. The limitations of the simulators have been highlighted by performing several simulation experiments. After exploiting the shortcomings presented in this work, it would be possible to use the insight to investigate biologically relevant questions. / Numeriska simuleringar har en stor betydelse när man vill undersöka och förstå dynamiska fenomen på cell- och nätverksnivå. Detta är mycket viktigt för hela neuroområdet. Dagens beräkningsteknologier har gjort det möjligt att simulera stora nätverk av neuroner, vilket knappast var realistiskt för 10-20 år sedan. Simuleringar av nätverk som består av tusentals neuroner kan göras lokalt på KTH på en parallelldator som heter Cray XE6 och som är baserad på AMD Opteron 12-core,med goda skalningsegenskaper. Sådana nätverkssimuleringar är nödvändiga för att undersöka det globala beteendet i nätverket vilket inte kan produceras med färre antal celler. Ett exempel på en nätverkseffekt som endast kan ses i en storskalig modell är hur inhibitionen mellan s.k. medium spiny neurons (MSNs) i det striatal nätverket fungerar. Eftersom inhibitionen mellan varje cellpar är mycket svag behövs input från många celler för att nätverket skall påverkas. Simuleringsresultaten påverkas signifikant av vilken numerisk metod som används. Detta demonstreras med ett striatalt nätverk innehållande s.k. gap junctions (elektriska synapser) mellan striatala fast-spiking interneurons (FS). Implicita numeriska metoder blir nödvändiga för att få stabila och riktiga resultat. Simuleringar görs både seriellt och parallellt med hjälp m.h.a. Genesis simulatorn (PGenesis för parallella implementationer), vilket är en standardsimulator för biofysikaliskt detaljerade neuronmodeller. För att utvärdera Genesis simulatorn och dess svagheter har flera simuleringsexperiment utförts. Insikter från dessa simuleringar, vilka diskuteras i detta arbete, kan hjälpa till att lägga en grund för framtida användning av Genesis för storskaliga simuleringar.

Some cases of infant skull fracture fall under the category of forensic study where it is not obvious whether the head trauma happened due to an accident or abuse. To be able to determine the cause of the head trauma with sufficient accuracy, biomechanical analysis using finite element modeling of the infant cranium has been established. By simulating the trauma, one may be able to obtain the fracture propagation of the skull and from it determine if the scenario narrative is plausible. Geometry of skull, sutures, scalp and brain of a 2 month old infant head was obtained using CT images and meshed using voxel hexahedral meshing. Simulation of an impact to the head from a fall of 0.82 m height, to a rigid floor, was carried out in the non-linear finite element program LS-Dyna. Two scenarios were simulated: an impact to the occipitalparietal bones and an impact to the right parietal bone. The fracture propagation was obtained using the Chang-Chang Composite Failure Model as a constitutive model for the skull bones. The amount of material parameters gathered in the present study to predict fracture of the infant skull has not been obtained before, to the best knowledge of author. Validation of the models’ ability to show relatively correct fracture propagation was carried out by comparing the obtained fracture pattern from the parietal-occipital impact against published fracture patterns of infant PMHS skulls from a free fall onto a hard surface. The fracture pattern was found to be in good compliance with the published data. The fracture pattern in the parietal bone from the impact was compared against a fracture pattern from a previously constructed model at STH. The patterns of the models show some similarities but improvements to the model and further validations need to be carried out. / Några skallskador hos spädbarn ger grund till kriminaltekniska studier där det inte är självklart om skallskadan skett på grund av en olycka eller misshandel. För att kunna fastställa orsaken till skallskadan med tillräcklig noggrannhet har biomekaniska analyser med finita element modeler av barns huvud genomförts. Genom att simulera traumat kan man kunna få sprickpropagering I skallbenet och från den avgöra om scenariot är rimligt. Geometrin för skallen, suturer, hårbotten och hjärnan hos ett 2 månader gammalt spädbarns huvud erhölls genom CT-bilder och Voxel hexahedermeshning. Simulering av påverkan på huvudet från ett fall på 0,82 m höjd mot ett hart golv simulerades i det icke-linjära finita element programmet LS-Dyna. Två scenarier simulerades: ett islag mot nack-hjässbenet och ett mot det högra hjässbenet. Sprickpropagering simulerades med en Chang-Chang Composite konstitutiv frakturmodell för skallbenet. Den omfattande mängd materialparametrar som sammanfattades i denna studie för att prediktera skallbensfrakturer hos spädbarnets har, enligt författarens kännedom, inte erhållits tidigare. Validering av modellernas förmåga att visa relativt korrekt sprickpropagering genomfördes genom att jämföra det erhållna frakturmönstret från simuleringarna med publicerade frakturmönster från spädbarn för fritt fall mot en hård yta mot nack-hjässbenet. Frakturmönstret befanns vara i god överensstämmelse med publicerade data. Brottmönstret i hjässbenet jämfördes med frakturmönstret från en tidigare konstruerad modell på KTH. Brottmönstren från modellerna visar vissa likheter men förbättringar av modellen och ytterligare valideringar måste genomföras.