Real-time Physics Based Simulation for 3D Computer Graphics

Restoration of realistic animation is a critical part in the area of computer graphics. The goal of this sort of simulation is to imitate the behavior of the transformation in real life to the greatest extent. Physics-based simulation provides a solid background and proficient theories that can be applied in the simulation. In this dissertation, I will present real-time simulations which are physics-based in the area of terrain deformation and ship oscillations. When ground vehicles navigate on soft terrains such as sand, snow and mud, they often leave distinctive tracks. The realistic simulation of such vehicle-terrain interaction is important for ground based visual simulations and many video games. However, the existing research in terrain deformation has not addressed this issue effectively. In this dissertation, I present a new terrain deformation algorithm for simulating vehicle-terrain interaction in real time. The algorithm is based on the classic terramechanics theories, and calculates terrain deformation according to the vehicle load, velocity, tire size, and soil concentration. As a result, this algorithm can simulate different vehicle tracks on different types of terrains with different vehicle properties. I demonstrate my algorithm by vehicle tracks on soft terrain. In the field of ship oscillation simulation, I propose a new method for simulating ship motions in waves. Although there have been plenty of previous work on physics based fluid-solid simulation, most of these methods are not suitable for real-time applications. In particular, few methods are designed specifically for simulating ship motion in waves. My method is based on physics theories of ship motion, but with necessary simplifications to ensure real-time performance. My results show that this method is well suited to simulate sophisticated ship motions in real time applications

An immersed boundary method for particles and bubbles in magnetohydrodynamic flows

This thesis presents a numerical method for the phase-resolving simulation of rigid particles and deformable bubbles in viscous, magnetohydrodynamic flows. The presented approach features solid robustness and high numerical efficiency. The implementation is three-dimensional and fully parallel suiting the needs of modern high-performance computing. In addition to the steps towards magnetohydrodynamics, the thesis covers method development with respect to the immersed boundary method which can be summarized in simple words by From rigid spherical particles to deformable bubbles. The development comprises the extension of an existing immersed boundary method to non-spherical particles and very low particle-to-fluid density ratios. A detailed study is dedicated to the complex interaction of particle shape, wake and particle dynamics. Furthermore, the representation of deformable bubble shapes, i.e. the coupling of the bubble shape to the fluid loads, is accounted for. The topic of bubble interaction is surveyed including bubble collision and coalescence and a new coalescence model is introduced. The thesis contains applications of the method to simulations of the rise of a single bubble and a bubble chain in liquid metal with and without magnetic field highlighting the major effects of the field on the bubble dynamics and the flow field. The effect of bubble coalescence is quantified for two closely adjacent bubble chains. A framework for large-scale simulations with many bubbles is provided to study complex multiphase phenomena like bubble-turbulence interaction in an efficient manner

Bedforms and associated sediment dynamics on the inner shelves at different spatio-temporal scales

This Thesis conducts the study of bedform development and dynamics in the inner shelf integrating observations at different spatial and temporal scales. An initial assumption is that different scales of sedimentary processes strongly interact between them and small-scale sedimentary processes intended to foster those of larger scale. The potential contribution of small-scale bedforms to the sediment transport is particularly addressed. The study includes the monitoring and analysis of sand ridges, ripples and near-bottom suspended sediment variations at the Ebro Delta (NW Mediterranean Sea) and Perranporth (Atlantic Ocean) inner shelves. A sand ridge field with maximum ridges heights of 2.5 m and 400 m spaced is located over a retreating lobe in the Ebro Delta. Ridges are mostly symmetric arranging obliquely to the shoreline. The change of the main Ebro River channel led to the progressive abandonment of the former river mouth and to the severe coastal retreatment, providing large amount of sediment available in the coastal zone. NW winds induce strong near-bottom currents flowing towards the SE, which are able to transport sediment and produce ridge formation and migration towards the SE at ~10 m/y. The characteristics of the Ebro sand ridges match well with those of shoreface-connected sand ridges and, particularly, with the initial stages of sand ridge development on storm-dominated continental shelves. Time-scales related to their genesis can be within a few decades. The presence of ripples on the different inner shelves is the most usual situation under low- to moderate-energetic conditions. In the wave-dominated and tideless coast (Ebro Delta) ripples were observed superimposed on sand ridges. Four types were identified: small undulations that were the precursor ripples, 2D wave-ripples, current-dominated 2D-3D-ripples, and combined wave-current 3D-ripples. The wave-ripples were static while the current-dominated ripples migrated. In a macro-tidal sandy beach exposed to high-energetic Atlantic storms (Perranporth), only wave-ripples were developed arranging orthogonal to wave approach and they were static. The size of ripples changed from larger (developed in equilibrium conditions) to smaller ripples, the latter interpreted as degraded ripples in wash-out conditions. The ripple prediction did not well-adjust to neither ripple appearance nor dimension. However, if the observed thresholds of seabed states are applied the model improves ripple appearance forecast. The near-bottom suspended sediment variability from seconds to months in the Ebro Delta is described distinguishing between waves, currents, and combined wave-currents conditions. In general, waves dominated the sediment resuspensions although strong currents also have an important contribution in the suspended sediment concentration (SSC) increases. The time-averaged SSC usually showed redundant structure by forming three layers with different patterns. In the lower and intermediate layers the SSC oscillated at gravity and infragravity wave frequencies. Time-varying and instantaneous profiles exhibit sediment patterns potentially related to ripples. The potential role of ripple migration as an additional long-term mechanism of sediment transport was analysed. In Perranporth, wave ripples are stationary and did not contribute to beach recovery because of wave orbital symmetry. The sediment transport is alongshore during low-energetic regimes and cross-shore during more energetic regimes when ripples are washed-out. In the Ebro Delta, 3D-ripples migration agrees with sand ridges migration direction towards the SE. The respective migration rates and their differences in size, support that a subordinate part of sand ridge migration can be the result of ripple migration contribution under low-, moderate-regimes, suggesting that in specific environments, dynamics of small-scale bedforms can play a subordinate but not negligible role in the evolution of larger bedformsLa Tesi estudia el desenvolupament i dinàmica de les formes de fons a la plataforma continental somera mitjançant la integració d’observacions a diferents escales espacials i temporals. Es parteix de la hipòtesi en què els processos sedimentaris a diferents escales estan relacionats i que els de petita escala fomenten als majors, fent especial èmfasi a la contribució de les formes de fons petites al transport de sediments. L’estudi inclou la monitorització i anàlisi de sand ridges, ripples i de les variacions de concentració de sediment en suspensió (CSS) prop del fons a la zona somera de les plataformes continentals del Delta de l’Ebre (NO Mar Mediterrània) i a Perranporth (oceà Atlàntic). Un camp de sand ridges amb crestes de fins 2.5 m d’alçada i 400 m espaiades es troba en un antic lòbul al Delta de l’Ebre. Les crestes són simètriques i obliqües respecte a la línia de costa. El canvi del canal principal del riu va suscitar l’abandonament progressiu de l’antiga desembocadura amb el retrocés sever de la línia de costa, proporcionant gran quantitat de sediments per formar els sand ridges que actualment migren ~10 m/any. Les característiques dels sand ridges de l’Ebre son anàlogues a les de sand ridges connectats a la costa i especialment a les seves etapes inicials a plataformes continentals dominades per tempestes. Les escales temporals relacionades amb la seva formació poden comprendre dècades. Les petites formes de fons observades inclouen: ripples superposats als sand ridges a la costa amb marees ínfimes dominada per tempestes de l’Ebre i ripples a la costa macro-mareal esposada a tempestes atlàntiques de Perranporth. Ambdues àrees, la presència de ripples és la situació habitual en condicions de baixa i mitja energia. A l’Ebre, es van diferenciar quatre morfologies: petites ondulacions precursores de 2D-ripples d’onatge, 2D-3D-ripples de corrents i 3D-ripples de la combinació d’onatge i corrents. Els ripples d’onatge eren estàtics mentre que els dominats per corrents migraven ~10 cm/h. A Perranporth, es van observar dues mides de ripples d’onatge amb crestes perpendiculars a la direcció de l’onatge i també estàtics. Els ripples més petits s’interpreten com la degradació dels grans. El model de predicció de ripples no s’ajusta ni per la seva formació ni dimensions. Tanmateix, quan els llindars d’estat de fons observats s’apliquen, el model millora el seu pronòstic. Les CSS prop del fons al Delta de l’Ebre es distingeix entre condicions d’onatge, corrents i la combinació d’onatge i corrents. En general, l’onatge domina la resuspensió de sediments tot i que els corrents també hi contribueixen de forma important. Les estimacions d’estrès total de cisallament és un bon indicador de pics de CSS i assenyala llindars d’inici de resuspensió de sediments. Les CSS van oscil·lar entre freqüències d’onatge gravitacional i infragravitacional. Les mitjanes dels perfils de CSS mostren una estructura vertical de tres capes mentre que els perfils instantanis mostren patrons en els CSS potencialment relacionats amb la presència de ripples. La migració de ripples en direcció a la costa com a mecanisme addicional a la recuperació de la platja a llarg termini a Perranporth va ser descartat degut a la estacionalitat del ripples i a la simetria de les velocitats orbitals de l’onatge. El transport de sediment és paral·lel a la costa durant condicions de baixa energia i perpendicular en condicions d’alta energia quan els ripples són erosionats. A l’Ebre, els sand ridges i de ripples migren cap a la mateixa direcció (SE). La relació entre velocitat de migració i dimensions entre les dues morfologies refermen que una part subordinada de la migració dels sand ridges podria ser conseqüència de la migració de ripples durant períodes de baixa i mitja energia. Llavors, en entorns específics, la dinàmica de les formes de fons petites podrien tenir un paper subordinat, però no menyspreable, en l’evolució de les formes més gransPostprint (published version

Efficient Light and Sound Propagation in Refractive Media with Analytic Ray Curve Tracer

Refractive media is ubiquitous in the natural world, and light and sound propagation in refractive media leads to characteristic visual and acoustic phenomena. Those phenomena are critical for engineering applications to simulate with high accuracy requirements, and they can add to the perceived realism and sense of immersion for training and entertainment applications. Existing methods can be roughly divided into two categories with regard to their handling of propagation in refractive media; first category of methods makes simplifying assumption about the media or entirely excludes the consideration of refraction in order to achieve efficient propagation, while the second category of methods accommodates refraction but remains computationally expensive. In this dissertation, we present algorithms that achieve efficient and scalable propagation simulation of light and sound in refractive media, handling fully general media and scene configurations. Our approaches are based on ray tracing, which traditionally assumes homogeneous media and rectilinear rays. We replace the rectilinear rays with analytic ray curves as tracing primitives, which represent closed-form trajectory solutions based on assumptions of a locally constant media gradient. For general media profiles, the media can be spatially decomposed into explicit or implicit cells, within which the media gradient can be assumed constant, leading to an analytic ray path within that cell. Ray traversal of the media can therefore proceed in segments of ray curves. The first source of speedup comes from the fact that for smooth media, a locally constant media gradient assumption tends to stay valid for a larger area than the assumption of a locally constant media property. The second source of speedup is the constant-cost intersection computation of the analytic ray curves with planar surfaces. The third source of speedup comes from making the size of each cell and therefore each ray curve segment adaptive to the magnitude of media gradient. Interactions with boundary surfaces in the scene can be efficiently handled within this framework in two alternative approaches. For static scenes, boundary surfaces can be embedded into the explicit mesh of tetrahedral cells, and the mesh can be traversed and the embedded surfaces intersected with by the analytic ray curve in a unified manner. For dynamic scenes, implicit cells are used for media traversal, and boundary surface intersections can be handled separately by constructing hierarchical acceleration structures adapted from rectilinear ray tracer. The efficient handling of boundary surfaces is the fourth source of speedup of our propagation path computation. We demonstrate over two orders-of-magnitude performance improvement of our analytic ray tracing algorithms over prior methods for refractive light and sound propagation. We additionally present a complete sound-propagation simulation solution that matches the path computation efficiency achieved by the ray curve tracer. We develop efficient pressure computation algorithm based on analytic evaluations and combine our algorithm with the Gaussian beam for fast acoustic field computation. We validate the accuracy of the simulation results on published benchmarks, and we show the application of our algorithms on complex and general three-dimensional outdoor scenes. Our algorithms enable simulation scenarios that are simply not feasible with existing methods, and they have the potential of being extended and complementing other propagation methods for capability beyond handling refractive media.Doctor of Philosoph

Large Model Visualization : Techniques and Applications

The size of datasets in scientific computing is rapidly increasing. This increase is caused by a boost of processing power in the past years, which in turn was invested in an increase of the accuracy and the size of the models. A similar trend enabled a significant improvement of medical scanners; more than 1000 slices of a resolution of 512x512 can be generated by modern scanners in daily practice. Even in computer-aided engineering typical models eas-ily contain several million polygons. Unfortunately, the data complexity is growing faster than the rendering performance of modern computer systems. This is not only due to the slower growing graphics performance of the graphics subsystems, but in particular because of the significantly slower growing memory bandwidth for the transfer of the geometry and image data from the main memory to the graphics accelerator. Large model visualization addresses this growing divide between data complexity and rendering performance. Most methods focus on the reduction of the geometric or pixel complexity, and hence also the memory bandwidth requirements are reduced. In this dissertation, we discuss new approaches from three different research areas. All approaches target at the reduction of the processing complexity to achieve an interactive visualization of large datasets. In the second part, we introduce applications of the presented ap-proaches. Specifically, we introduce the new VIVENDI system for the interactive virtual endoscopy and other applications from mechanical engineering, scientific computing, and architecture.The size of datasets in scientific computing is rapidly increasing. This increase is caused by a boost of processing power in the past years, which in turn was invested in an increase of the accuracy and the size of the models. A similar trend enabled a significant improvement of medical scanners; more than 1000 slices of a resolution of 512x512 can be generated by modern scanners in daily practice. Even in computer-aided engineering typical models eas-ily contain several million polygons. Unfortunately, the data complexity is growing faster than the rendering performance of modern computer systems. This is not only due to the slower growing graphics performance of the graphics subsystems, but in particular because of the significantly slower growing memory bandwidth for the transfer of the geometry and image data from the main memory to the graphics accelerator. Large model visualization addresses this growing divide between data complexity and rendering performance. Most methods focus on the reduction of the geometric or pixel complexity, and hence also the memory bandwidth requirements are reduced. In this dissertation, we discuss new approaches from three different research areas. All approaches target at the reduction of the processing complexity to achieve an interactive visualization of large datasets. In the second part, we introduce applications of the presented ap-proaches. Specifically, we introduce the new VIVENDI system for the interactive virtual endoscopy and other applications from mechanical engineering, scientific computing, and architecture

Hardware accelerated volume texturing.

The emergence of volume graphics, a sub field in computer graphics, has been evident for the last 15 years. Growing from scientific visualization problems, volume graphics has established itself as an important field in general computer graphics. However, the general graphics fraternity still favour the established surface graphics techniques. This is due to well founded and established techniques and a complete pipeline through software onto display hardware. This enables real-time applications to be constructed with ease and used by a wide range of end users due to the readily available graphics hardware adopted by many computer manufacturers. Volume graphics has traditionally been restricted to high-end systems due to the complexity involved with rendering volume datasets. Either specialised graphics hardware or powerful computers were required to generate images, many of these not in real-time. Although there have been specialised hardware solutions to the volume rendering problem, the adoption of the volume dataset as a primitive relies on end-users with commodity hardware being able to display images at interactive rates. The recent emergence of programmable consumer level graphics hardware is now allowing these platforms to compute volume rendering at interactive rates. Most of the work in this field is directed towards scientific visualisation. The work in this thesis addresses the issues in providing real-time volume graphics techniques to the general graphics community using commodity graphics hardware. Real-time texturing of volumetric data is explored as an important set of techniques in delivering volume datasets as a general graphics primitive. The main contributions of this work are; The introduction of efficient acceleration techniques; Interactive display of amorphous phenomena modelled outside an object defined in a volume dataset; Interactive procedural texture synthesis for volume data; 2D texturing techniques and extensions for volume data in real-time; A flexible surface detail mapping algorithm that removes many previous restrictions Parts of this work have been presented at the 4th International Workshop on Volume Graphics and also published in Volume Graphics 2005

Fibre-reinforced additive manufacturing: from design guidelines to advanced lattice structures

In pursuit of achieving ultimate lightweight designs with additive manufacturing (AM), engineers across industries are increasingly gravitating towards composites and architected cellular solids; more precisely, fibre-reinforced polymers and functionally graded lattices (FGLs). Control over material anisotropy and the cell topology in design for AM (DfAM) offer immense scope for customising a part’s properties and for the efficient use of material. This research expands the knowledge on the design with fibre-reinforced AM (FRAM) and the elastic-plastic performance of FGLs. Novel toolpath strategies, design guidelines and assessment criteria for FRAM were developed. For this purpose, an open-source solution was proposed, successfully overcoming the limitations of commercial printers. The effect of infill patterns on structural performance, economy, and manufacturability was examined. It was demonstrated how print paths informed by stress trajectories and key geometric features can outperform conventional patterns, laying the groundwork for more sophisticated process planning. A compilation of the first comprehensive database on fibre-reinforced FGLs provided insights into the effect of grading on the elastic performance and energy absorption capability, subject to strut-and surface-based lattices, build direction and fibre volume fraction. It was elucidated how grading the unit cell density within a lattice offers the possibility of tailoring the stiffness and achieving higher energy absorption than ungraded lattices. Vice versa, grading the unit cell size of lattices yielded no effect on the performance and is thus exclusively governed by the density. These findings help exploit the lightweight potential of FGLs through better informed DfAM. A new and efficient methodology for predicting the elastic-plastic characteristics of FGLs under large strain deformation, assuming homogenised material properties, was presented. A phenomenological constitutive model that was calibrated based upon interpolated material data of uniform density lattices facilitated a computationally inexpensive simulation approach and thus helps streamline the design workflow with architected lattices.Open Acces

4D ultra-short TE (UTE) phase-contrast MRI for assessing stenotic flow and hemodynamics.

Phase-contrast (PC) MRI is a non-invasive technique to assess cardiovascular blood flow. However, this technique is not accurate in the case of atherosclerotic disease and vascular and valvular stenosis due to intravoxel dephasing secondary to disturbed blood flow, flow recirculation, and turbulence distal to the narrowing, resulting in flow-related artifacts. Previous studies have shown that reducing the echo time (TE) decreases the errors associated with phase incoherence due to random motions as observed in unsteady and turbulent flows. As part of this dissertation, a novel 3-D cine Ultra-Short (UTE)-PC imaging method has been developed, and implemented to measure the blood velocity using a UTE center-out radial k-space trajectory with short TE time compared to standard PC MRI sequences. 3D UTE characterizes flow in one direction in a 3D volume, resulting in a single component of the flow velocities. In order to obtain a comprehensive flow assessment in three directions, the 3D UTE sequence needs to be repeated three times, which can be inefficient and time consuming. 4-D flow MRI has been recently used for quantitative flow assessment and visualization of complex flow patterns resulting in more anatomical information and comprehensive assessment of blood flow. With 4D flow MRI method, all the flow information in three direction in a 3D volume though the time can be achieved as part of a single scan. In this dissertation, a novel 4D UTE flow MRI technique has also been designed and implemented which is capable of deriving the three orthogonal components of the velocity field in the flow in a single scan, while achieving very short echo times. In flow phantom studies, comprehensive investigation of several different flow rates revealed significant improvement in flow quantification and reduction of flow artifacts when compared to conventional 4D flow. Furthermore, a reduced TE 4D Spiral flow MRI method has also been implemented which reduces scan times when compared to conventional 4D flow MRI (as well as 4D UTE flow). Despite reduction of scan time as well as TE relative to conventional 4D flow, the achieved TE with the 4D spiral technique is indeed longer than 4D UTE flow. In order to assess clinical feasibility and in order to perform further validation of 4D UTE flow, in an IRB-approved study, twelve aortic stenosis (AS) patients underwent Doppler Ultrasound, conventional 4D flow, and 4D UTE flow scans for a 3 way comparison. 4D UTE flow displayed good correlation with Doppler Ultrasound in patients with moderately severe aortic stenosis, though with the added benefit of not having confounding factors encountered in Doppler Ultrasound (e.g., angle dependence, 2D measurement, and difficulty in locating a proper acoustic window). The proposed 4D UTE flow permits 4D visualization of flow and true 3D measurement of all flow quantities, not possible with Doppler. Further investigations will be required to test the technique in patients with severe or critical aortic stenosis wherein conventional 4D flow will be less accurate due to intravoxel dephasing and spin incoherence