525 research outputs found

    Conformation constraints for efficient viscoelastic fluid simulation

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    The simulation of high viscoelasticity poses important computational challenges. One is the difficulty to robustly measure strain and its derivatives in a medium without permanent structure. Another is the high stiffness of the governing differential equations. Solutions that tackle these challenges exist, but they are computationally slow. We propose a constraint-based model of viscoelasticity that enables efficient simulation of highly viscous and viscoelastic phenomena. Our model reformulates, in a constraint-based fashion, a constitutive model of viscoelasticity for polymeric fluids, which defines simple governing equations for a conformation tensor. The model can represent a diverse palette of materials, spanning elastoplastic, highly viscous, and inviscid liquid behaviors. In addition, we have designed a constrained dynamics solver that extends the position-based dynamics method to handle efficiently both position-based and velocity-based constraints. We show results that range from interactive simulation of viscoelastic effects to large-scale simulation of high viscosity with competitive performance

    Viscoelastic hydrodynamics and holography

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    Doctor of Philosophy

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    dissertationPhysical simulation has become an essential tool in computer animation. As the use of visual effects increases, the need for simulating real-world materials increases. In this dissertation, we consider three problems in physics-based animation: large-scale splashing liquids, elastoplastic material simulation, and dimensionality reduction techniques for fluid simulation. Fluid simulation has been one of the greatest successes of physics-based animation, generating hundreds of research papers and a great many special effects over the last fifteen years. However, the animation of large-scale, splashing liquids remains challenging. We show that a novel combination of unilateral incompressibility, mass-full FLIP, and blurred boundaries is extremely well-suited to the animation of large-scale, violent, splashing liquids. Materials that incorporate both plastic and elastic deformations, also referred to as elastioplastic materials, are frequently encountered in everyday life. Methods for animating such common real-world materials are useful for effects practitioners and have been successfully employed in films. We describe a point-based method for animating elastoplastic materials. Our primary contribution is a simple method for computing the deformation gradient for each particle in the simulation. Given the deformation gradient, we can apply arbitrary constitutive models and compute the resulting elastic forces. Our method has two primary advantages: we do not store or compare to an initial rest configuration and we work directly with the deformation gradient. The first advantage avoids poor numerical conditioning and the second naturally leads to a multiplicative model of deformation appropriate for finite deformations. One of the most significant drawbacks of physics-based animation is that ever-higher fidelity leads to an explosion in the number of degrees of freedom

    An effective interface tracking method for simulating the extrudate swell phenomenon

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    The extrudate swell, i.e., the geometrical modifications that take place when the flowing material leaves the confined flow inside a channel and moves freely without the restrictions that are promoted by the walls, is a relevant phenomenon in several polymer processing techniques. For instance, in profile extrusion, the extrudate cross-section is subjected to a number of distortions that are motivated by the swell, which are very difficult to anticipate, especially for complex geometries. As happens in many industrial processes, numerical modelling might provide useful information to support design tasks, i.e., to allow for identifying the best strategy to compensate the changes promoted by the extrudate swell. This study reports the development of an improved interface tracking algorithm that employs the least-squares volume-to-point interpolation method for the grid movement. The formulation is enriched further with the consistent second-order time-accurate non-iterative Pressure-Implicit with Splitting of Operators (PISO) algorithm, which allows for efficiently simulating free-surface flows. The accuracy and robustness of the proposed solver is illustrated through the simulation of the steady planar and asymmetric extrudate swell flows of Newtonian fluids. The role of inertia on the extrudate swell is studied, and the results that are obtained with the newly improved solver show good agreement with reference data that are found in the scientific literatureSearch-ON2 (NORTE-07-0162-FEDER-000086) the HPC infrastructure of UMinho under NSRF through ERDF; and FCT I.P. through the Advanced Computing Project CPCA/A00/6057/2020 using the Minho Advanced Computing Center (MACC

    Animating physical phenomena with embedded surface meshes

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    Accurate computational representations of highly deformable surfaces are indispensable in the fields of computer animation, medical simulation, computer vision, digital modeling, and computational physics. The focus of this dissertation is on the animation of physics-based phenomena with highly detailed deformable surfaces represented by triangle meshes. We first present results from an algorithm that generates continuum mechanics animations with intricate surface features. This method combines a finite element method with a tetrahedral mesh generator and a high resolution surface mesh, and it is orders of magnitude more efficient than previous approaches. Next, we present an efficient solution for the challenging problem of computing topological changes in detailed dynamic surface meshes. We then introduce a new physics-inspired surface tracking algorithm that is capable of preserving arbitrarily thin features and reproducing realistic fine-scale topological changes like Rayleigh-Plateau instabilities. This physics-inspired surface tracking technique also opens the door for a unique coupling between surficial finite element methods and volumetric finite difference methods, in order to simulate liquid surface tension phenomena more efficiently than any previous method. Due to its dramatic increase in computational resolution and efficiency, this method yielded the first computer simulations of a fully developed crown splash with droplet pinch off.Ph.D.Committee Chair: Turk, Greg; Committee Member: Essa, Irfan; Committee Member: Liu, Karen; Committee Member: Mucha, Peter J.; Committee Member: Rossignac, Jare

    Boundary-Conforming Free-Surface Flow Computations: Interface Tracking for Linear, Higher-Order and Isogeometric Finite Elements

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    The simulation of certain flow problems requires a means for modeling a free fluid surface; examples being viscoelastic die swell or fluid sloshing in tanks. In a finite-element context, this type of problem can, among many other options, be dealt with using an interface-tracking approach with the Deforming-Spatial-Domain/Stabilized-Space-Time (DSD/SST) formulation. A difficult issue that is connected with this type of approach is the determination of a suitable coupling mechanism between the fluid velocity at the boundary and the displacement of the boundary mesh nodes. In order to avoid large mesh distortions, one goal is to keep the nodal movements as small as possible; but of course still compliant with the no-penetration boundary condition. Standard displacement techniques are full velocity, velocity in a specific coordinate direction, and velocity in normal direction. In this work, we investigate how the interface-tracking approach can be combined with isogeometric analysis for the spatial discretization. If NURBS basis functions of sufficient order are used for both the geometry and the solution, both a continuous normal vector as well as the velocity are available on the entire boundary. This circumstance allows the weak imposition of the no-penetration boundary condition. We compare this option with an alternative that relies on strong imposition at discrete points. Furthermore, we examine several coupling methods between the fluid equations, boundary conditions, and equations for the adjustment of interior control point positions.Comment: 20 pages, 16 figure

    Planning Framework for Robotic Pizza Dough Stretching with a Rolling Pin

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    Stretching a pizza dough with a rolling pin is a nonprehensile manipulation. Since the object is deformable, force closure cannot be established, and the manipulation is carried out in a nonprehensile way. The framework of this pizza dough stretching application that is explained in this chapter consists of four sub-procedures: (i) recognition of the pizza dough on a plate, (ii) planning the necessary steps to shape the pizza dough to the desired form, (iii) path generation for a rolling pin to execute the output of the pizza dough planner, and (iv) inverse kinematics for the bi-manual robot to grasp and control the rolling pin properly. Using the deformable object model described in Chap. 3, each sub-procedure of the proposed framework is explained sequentially

    Thermocapillary motion of droplets in complex fluid flows

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    Understanding how the presence of thermal gradients affects the motion of bubbles and drops is a subject of great relevance both from a theoretical and a practical standpoint, particularly when gravitational effects are minimal or completely uninfluential. In the past half century, considerable progress has been made onthe investigation of the so-called thermocapillary phenomenon in an attempt to clarify the mechanisms at work in multiphase systems with liquid-liquid or liquid-gas interfaces.;Given the complexity of the problem, most of these investigations have been carried out under simplified conditions, assuming unbounded flows or considering relatively simple geometries in which the presence of solid boundaries was not explicitly taken into account. Additionally, even though non-Newtonian fluids are ubiquitous in engineering and science, the majority of these works have been carried out assuming Newtonian phases.;The aim of the present thesis is to study the thermocapillary migration of a droplet in systems exhibiting an added level of complexity, specifically in terms of wall effects, domain shape and rheological properties of the fluids. To accomplish these objectives, we rely on a concerted approach based on well-established numerical strategies and, where possible, we derive analytical solutions.;A thermocapillary solver based on a hybrid Level Set-Volume of Fluid method availablein OpenFOAM has been implemented and validated against previous analytical results, numerical solutions and experimental observations obtained in reduced gravity conditions (Sect. 3.5). In the first part of the study, we investigate the problem of a droplet interacting with the boundaries of a parallelepipedic domain.;The case study has been assessed by releasing the droplet in proximity to the lateral walls of the domain considering both adiabatic and purely conductive boundary conditions. The results showed that the droplet can experience a secondary motion perpendicular to the main direction of motion. In particular, it was observed that the droplet can either move away or towards the walls depending on the thermal boundary conditions at the wall (i.e., whether the wall is adiabatic or purely conductive) and on the extent of convective phenomena.;The investigation was then extended by adopting more complex geometries (converging and diverging channels), which were found to produce distortion of the thermal field distribution with direct consequences on the migration process (Sect. 4.2.1 and 4.2.2). In the second part of the thesis, non-Newtonian effects have been expressly considered. Specifically, the role played by the fluid's elasticity (while neglecting convective transport of energy and momentum) has been accounted for by modelling the continuous phase on the basis of constant-viscosity viscoelastic models, namely the Oldroyd-B model and FENE-CR model.;The numerical simulations were carried out for a specific value of the Capillary number and assuming thesame material properties for both phases. We investigated the effects of the various model parameters (i.e., polymer concentration and extensibility parameter) and Deborah number on the droplet motion. The results showed that the droplet speed, evaluated as a function of the Deborah number, initially decreases following a quadratic trend.;For larger Deborah number, the trend reverts its concavity and eventually reaches a plateau. In terms of shape, the results have shown that under the prescribed conditions the droplet deforms in a prolate manner and, for sufficiently large values of the Deborah number (having fixed the Capillary number), the viscoelastic stresses localised at the rear stagnation point are responsible for the formation of a pointed tail.;The viscoelastic problem was also tackled by means of perturbation techniques under the assumption of absence of confinement and weak viscoelastic effects, which allowed the derivation of corrective formulae for the droplet migration velocity and expressions describing the shape of the deformed drop. The results of the analytical solutions were found to be in fairly good agreement with the outcomes of the computations, both interms of drop shape and migration speed.Understanding how the presence of thermal gradients affects the motion of bubbles and drops is a subject of great relevance both from a theoretical and a practical standpoint, particularly when gravitational effects are minimal or completely uninfluential. In the past half century, considerable progress has been made onthe investigation of the so-called thermocapillary phenomenon in an attempt to clarify the mechanisms at work in multiphase systems with liquid-liquid or liquid-gas interfaces.;Given the complexity of the problem, most of these investigations have been carried out under simplified conditions, assuming unbounded flows or considering relatively simple geometries in which the presence of solid boundaries was not explicitly taken into account. Additionally, even though non-Newtonian fluids are ubiquitous in engineering and science, the majority of these works have been carried out assuming Newtonian phases.;The aim of the present thesis is to study the thermocapillary migration of a droplet in systems exhibiting an added level of complexity, specifically in terms of wall effects, domain shape and rheological properties of the fluids. To accomplish these objectives, we rely on a concerted approach based on well-established numerical strategies and, where possible, we derive analytical solutions.;A thermocapillary solver based on a hybrid Level Set-Volume of Fluid method availablein OpenFOAM has been implemented and validated against previous analytical results, numerical solutions and experimental observations obtained in reduced gravity conditions (Sect. 3.5). In the first part of the study, we investigate the problem of a droplet interacting with the boundaries of a parallelepipedic domain.;The case study has been assessed by releasing the droplet in proximity to the lateral walls of the domain considering both adiabatic and purely conductive boundary conditions. The results showed that the droplet can experience a secondary motion perpendicular to the main direction of motion. In particular, it was observed that the droplet can either move away or towards the walls depending on the thermal boundary conditions at the wall (i.e., whether the wall is adiabatic or purely conductive) and on the extent of convective phenomena.;The investigation was then extended by adopting more complex geometries (converging and diverging channels), which were found to produce distortion of the thermal field distribution with direct consequences on the migration process (Sect. 4.2.1 and 4.2.2). In the second part of the thesis, non-Newtonian effects have been expressly considered. Specifically, the role played by the fluid's elasticity (while neglecting convective transport of energy and momentum) has been accounted for by modelling the continuous phase on the basis of constant-viscosity viscoelastic models, namely the Oldroyd-B model and FENE-CR model.;The numerical simulations were carried out for a specific value of the Capillary number and assuming thesame material properties for both phases. We investigated the effects of the various model parameters (i.e., polymer concentration and extensibility parameter) and Deborah number on the droplet motion. The results showed that the droplet speed, evaluated as a function of the Deborah number, initially decreases following a quadratic trend.;For larger Deborah number, the trend reverts its concavity and eventually reaches a plateau. In terms of shape, the results have shown that under the prescribed conditions the droplet deforms in a prolate manner and, for sufficiently large values of the Deborah number (having fixed the Capillary number), the viscoelastic stresses localised at the rear stagnation point are responsible for the formation of a pointed tail.;The viscoelastic problem was also tackled by means of perturbation techniques under the assumption of absence of confinement and weak viscoelastic effects, which allowed the derivation of corrective formulae for the droplet migration velocity and expressions describing the shape of the deformed drop. The results of the analytical solutions were found to be in fairly good agreement with the outcomes of the computations, both interms of drop shape and migration speed
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