Lmit and shakedown analysis based on solid shell models

The paper treats the formulation of the shakedown problem and, as special case, of the limit analysis problem, using solid shell models and ES-FEM discratization technology. In this proposal the Discrete shear gap method is applied to alleviate the shear locking phenomenon

Validation of Transcranial Electrical Stimulation (TES) Finite Element Modeling Against MREIT Current Density Imaging in Human Subjects

abstract: Transcranial electrical stimulation (tES) is a non-invasive brain stimulation therapy that has shown potential in improving motor, physiological and cognitive functions in healthy and diseased population. Typical tES procedures involve application of weak current (< 2 mA) to the brain via a pair of large electrodes placed on the scalp. While the therapeutic benefits of tES are promising, the efficacy of tES treatments is limited by the knowledge of how current travels in the brain. It has been assumed that the current density and electric fields are the largest, and thus have the most effect, in brain structures nearby the electrodes. Recent studies using finite element modeling (FEM) have suggested that current patterns in the brain are diffuse and not concentrated in any particular brain structure. Although current flow modeling is useful means of informing tES target optimization, few studies have validated tES FEM models against experimental measurements. MREIT-CDI can be used to recover magnetic flux density caused by current flow in a conducting object. This dissertation reports the first comparisons between experimental data from in-vivo human MREIT-CDI during tES and results from tES FEM using head models derived from the same subjects. First, tES FEM pipelines were verified by confirming FEM predictions agreed with analytic results at the mesh sizes used and that a sufficiently large head extent was modeled to approximate results on human subjects. Second, models were used to predict magnetic flux density, and predicted and MREIT-CDI results were compared to validate and refine modeling outcomes. Finally, models were used to investigate inter-subject variability and biological side effects reported by tES subjects. The study demonstrated good agreements in patterns between magnetic flux distributions from experimental and simulation data. However, the discrepancy in scales between simulation and experimental data suggested that tissue conductivities typically used in tES FEM might be incorrect, and thus performing in-vivo conductivity measurements in humans is desirable. Overall, in-vivo MREIT-CDI in human heads has been established as a validation tool for tES predictions and to study the underlying mechanisms of tES therapies.Dissertation/ThesisDoctoral Dissertation Biomedical Engineering 201

CFD Techniques for simulation of flow in a scour hole around a bridge pier

The flow field around a bridge pier is complex in detail and the complexity is further increased with the development of a scour hole. As flow-induced scouring around piers can cause bridge failures, a good understanding of the flow field is important to the safe design of the hydraulic structures. The objective of this study is to simulate free-surface flow around a pier in a fixed scour hole, and to further determine shear stress distributions at the channel-bed. Simulations were mainly performed using mesh-based numerical models. The mesh-based numerical model was established using Reynolds averaged momentum and continuity equations in three dimensions, with the k-ɷ model for turbulence closure. The regions around the pier and near the channel-bed were resolved with sufficiently fine mesh so as to capture detailed velocity structures. To explore the appropriate procedures for applying smoothed particle hydrodynamics, a mesh-free model was formulated with kernel approximation of the field variables and particle approximation. The governing equations for dynamic fluid flows for the mesh-free model were written as a set of partial differential equations in Lagrangian description, known as the Navier-Stokes equations. The simulations were carried out under the same geometric and hydraulic conditions as in available laboratory experiments. One of the major findings of this research is that, both models predict a downflow near the upstream nose of the pier which would affect the stability of pier foundations. The mesh-based model results exhibit realistic vortex features around the pier and in the wake region. Another new finding is the occurrence of flow separation and complex vortex stretching confined to the upper water column behind the pier. The predicted bed shear stress and turbulent kinetic energy are shown to compare well with the experimental data. Application of the mesh-free model to the flow in a scour hole around a bridge pier has been successful in generating desired approach flow. Velocity profiles extracted from the results of both models at selected locations in the approach channel, inside the scour hole, are compared. The results presented in this thesis are of practical values for prediction of sediment scour around bridge piers

Modelling Polycrystalline Materials: An Overview of Three-Dimensional Grain-Scale Mechanical Models

International audienc

Analysis and Generation of Quality Polytopal Meshes with Applications to the Virtual Element Method

This thesis explores the concept of the quality of a mesh, the latter being intended as the discretization of a two- or three- dimensional domain. The topic is interdisciplinary in nature, as meshes are massively used in several fields from both the geometry processing and the numerical analysis communities. The goal is to produce a mesh with good geometrical properties and the lowest possible number of elements, able to produce results in a target range of accuracy. In other words, a good quality mesh that is also cheap to handle, overcoming the typical trade-off between quality and computational cost. To reach this goal, we first need to answer the question: ''How, and how much, does the accuracy of a numerical simulation or a scientific computation (e.g., rendering, printing, modeling operations) depend on the particular mesh adopted to model the problem? And which geometrical features of the mesh most influence the result?'' We present a comparative study of the different mesh types, mesh generation techniques, and mesh quality measures currently available in the literature related to both engineering and computer graphics applications. This analysis leads to the precise definition of the notion of quality for a mesh, in the particular context of numerical simulations of partial differential equations with the virtual element method, and the consequent construction of criteria to determine and optimize the quality of a given mesh. Our main contribution consists in a new mesh quality indicator for polytopal meshes, able to predict the performance of the virtual element method over a particular mesh before running the simulation. Strictly related to this, we also define a quality agglomeration algorithm that optimizes the quality of a mesh by wisely agglomerating groups of neighboring elements. The accuracy and the reliability of both tools are thoroughly verified in a series of tests in different scenarios

Influence Of Topographic Elevation Error On Modeled Storm Surge

The following presents a method for determining topographic elevation error for overland unstructured finite element meshes derived from bare earth LiDAR for use in a shallow water equations model. This thesis investigates the development of an optimal interpolation method to produce minimal error for a given element size. In hydrodynamic studies, it is vital to represent the floodplain as accurately as possible since terrain is a critical factor that influences water flow. An essential step in the development of a coastal inundation model is processing and resampling dense bare earth LiDAR to a DEM and ultimately to the mesh nodes; however, it is crucial that the correct DEM grid size and interpolation method be employed for an accurate representation of the terrain. The following research serves two purposes: 1) to assess the resolution and interpolation scheme of bare earth LiDAR data points in terms of its ability to describe the bare earth topography and its subsequent performance during relevant tide and storm surge simulation

Automated Patient-Specific Modeling of Blood Flow and Vessel Wall Mechanics in Aortic

This work presents a numerical approach to non-invasively help the diagnosis of patients with vascular pathologies on an individual basis. Patient-specific computational modeling of cardiovascular biomechanics was conducted to simulate the hemodynamics, the elastomechanics and their interaction within the vessels. For the blood flow and vessel wall computations, individual mesh generation techniques and numerical models for CSM, CFD and FSI were generated and implemented based on CT/MRI images

ニュートン流体における粉体の二相動力学

Many scientific and technical problems which concern the dynamics of complex fluids such as multi-phase-flow and realistic flow in porous and granular media deal with the interaction between fluids and particles, rather than with the dynamics of the fluid alone. The research of how the surrounding fluid affects the dynamics of particles, or how to deal with the problem computationally for the microscopic level is still at the beginning. The aim of this study is to develop a microscopic simulation method (fluid goes around the particles) where granular particles can be simulated inside fluids to study those problems. This is done by combining the simulation method for granular particles with the simulation method for the incompressible Newtonian fluid. The granular particles are implemented via the discrete element method (DEM) where the elastic contact force between two undeformed contacting polygonal particles is proportional to the overlap area ("hard particle, soft contact"). The Gear Predictor-Corrector of 2nd-order (BDF2) is used as the time integrator to solve the equations of motion of the particles. For the fluid phase, the implementation of the incompressible Navier-Stokes equations via the Galerkin finite element method (FEM) is formulated as differential algebraic equations (DAE) with the pressures as the Lagrange parameters. The time integration is again via the BDF2 while the resulting non-linear equations are solved via the Newton-Raphson methods. The spatial discretization is via the Taylor-Hood elements from Delaunay triangulations with additional post-processing with the relaxation algorithm. The coupling of the DEM for the granular particles and the FEM for the fluid is via appropriate boundary conditions and the drag force (computed by the integration of the fluid stress tensor over the particle\u27s surface). This is being verified via the computation of wall correction factors of a sinking particle. The fluid simulation is extended to a simulation of free surfaces where the motion of the surface is integrated out according to the velocity on the surface which is obtained from the FEM-scheme. The second-order Adams-Bashforth method turns out to be the most suitable integrator for the surface motion. Compared to conventional efforts, which try to solve partial differential equations for the motion of the surface, the additional effort in our method with respect to new data structures etc. is minimal. The free surfaces code is verified by simulating the collapse of a water column. For the speed of the wavefronts, excellent agreement is obtained for large viscosity with the lubrication approximation. The agreement of the results with the experimental data for water is a further gratifying result. Two numerical experiments are conducted using the DEM-FEM code: one with a rather slow dynamics, another one relatively more "violent". The compaction simulation has shown that the addition of fluid to a granular assembly can increase the sound velocity in the system, compared to the dry case. The high viscosity slowed down the compaction, irrespective whether the system was tapped only on the ground or on the whole boundary. The granular column simulations show that for systems immersed under fluids, rolling of particles becomes less important than for the corresponding dry systems.電気通信大学201