Marek Behr

A new strategy for finite element computations involving moving boundaries and interfaces—the deforming-spatial-domain/space-time procedure. I: The concept and the preliminary numerical tests

Abstract A new strategy based on the stabilized space-time finite element formulation is proposed for computations involving moving boundaries and interfaces. In the deforming-spatial-domain/space-time (DSD/ST) procedure the variational formulation of a problem is written over its space-time domain, and therefore the deformation of the spatial domain with respect to time is taken into account automatically. Because the space-time mesh is generated over the space-time domain of the problem, within each time step, the boundary (or interface) nodes move with the boundary (or interface). Whether the motion of the boundary is specified or not, the strategy is nearly the same. If the motion of the boundary is unknown, then the boundary nodes move as defined by the other unknowns at the boundary (such as the velocity or the displacement). At the end of each time step a new spatial mesh covers the new spatial domain. For computational feasibility, the finite element interpolation functions are chosen to be discontinuous in time, and the fully discretized equations are solved one space-time slab at a time.

A new strategy for finite element computations involving moving boundaries and interfaces—the deforming-spatial-domain/space-time procedure. II: Computation of free-surface flows, two-liquid flows, and flows with drifting cylinders

New finite element computational strategies for free-surface flows, two-liquid flows, and flows with drifting cylinders are presented. These strategies are based on the deforming spatial-domain/spacetime (DSD/ST) procedure. In the DSD/ST approach, the stabilized variational formulations for these types of flow problem are written over their space-time domains. One of the important features of the approach is that it enables one to circumvent the difficulty involved in remeshing every time step and thus reduces the projection errors introduced by such frequent remeshings. Computations are performed for various test problems mainly for the purpose of demonstrating the computational capability developed for this class of problems. In some of the test cases, such as the liquid drop problem, surface tension is taken into account. For flows involving drifting cylinders, the mesh moving and remeshing schemes proposed are convenient and reduce the frequency of remeshing.

Parallel finite-element computation of 3D flows

The authors describe their work on the massively parallel finite-element computation of compressible and incompressible flows with the CM-200 and CM-5 Connection Machines. Their computations are based on implicit methods, and their parallel implementations are based on the assumption that the mesh is unstructured. Computations for flow problems involving moving boundaries and interfaces are achieved by using the deformable-spatial-domain/stabilized-space-time method. Using special mesh update schemes, the frequency of remeshing is minimized to reduce the projection errors involved and also to make parallelizing the computations easier. This method and its implementation on massively parallel supercomputers provide a capability for solving a large class of practical problems involving free surfaces, two-liquid interfaces, and fluid-structure interactions.<<ETX>>

Flow simulation and high performance computing

Flow simulation is a computational tool for exploring science and technology involving flow applications. It can provide cost-effective alternatives or complements to laboratory experiments, field tests and prototyping. Flow simulation relies heavily on high performance computing (HPC). We view HPC as having two major components. One is advanced algorithms capable of accurately simulating complex, real-world problems. The other is advanced computer hardware and networking with sufficient power, memory and bandwidth to execute those simulations. While HPC enables flow simulation, flow simulation motivates development of novel HPC techniques. This paper focuses on demonstrating that flow simulation has come a long way and is being applied to many complex, real-world problems in different fields of engineering and applied sciences, particularly in aerospace engineering and applied fluid mechanics. Flow simulation has come a long way because HPC has come a long way. This paper also provides a brief review of some of the recently-developed HPC methods and tools that has played a major role in bringing flow simulation where it is today. A number of 3D flow simulations are presented in this paper as examples of the level of computational capability reached with recent HPC methods and hardware. These examples are, flow around a fighter aircraft, flow around two trains passing in a tunnel, large ram-air parachutes, flow over hydraulic structures, contaminant dispersion in a model subway station, airflow past an automobile, multiple spheres falling in a liquid-filled tube, and dynamics of a paratrooper jumping from a cargo aircraft.

Finite element solution strategies for large-scale flow simulations☆

Abstract Large-scale flow simulation strategies involving implicit finite element formulations are described in the context of incompressible flows. The stabilized space-time formulation for problems involving moving boundaries and interfaces is presented, followed by a discussion of mesh moving schemes. The methods of solution of large linear systems of equations are reviewed, and an implementation of the entire finite element code, permitting the use of totally unstructured meshes, on a massively parallel supercomputer is considered. As an example, this methodology is applied to a flow problem involving three-dimensional simulation of liquid sloshing in a tank subjected to vertical vibrations.

Massively parallel finite element simulation Of compressible and incompressible flows

We present a review of where our research group stands in parallel finite element simulation of flow problems on the Connection Machines, an effort that started for our group in the fourth quarter of 1991. This review includes an overview of our work on computation of flow problems involving moving boundaries and interfaces, such as free surfaces, two-liquid interfaces, and fluid-structure and fluid-particle interactions. With numerous examples, we demonstrate that, with these new computational capabilities, today we are at a point where we routinely solve practical flow problems, including those in 3D and those involving moving boundaries and interfaces. We solve these problems with unstructured grids and implicit methods, with some of the problem sizes exceeding 5 000 000 equations, and with computational speeds up to two orders of magnitude higher than what was previously available to us on the traditional vector supercomputers.

Incompressible flow past a circular cylinder: dependence of the computed flow field on the location of the lateral boundaries

The influence of the location of the lateral boundaries on 2D computation of unsteady incompressible flow past a circular cylinder is investigated. The case of Reynolds number 100 is used as a benchmark, and several quantities characterizing the unsteady flow are obtained for a range of lateral boundary locations. The computations are performed with two distinct finite element formulations - space-time velocity-pressure formulation and velocity-pressure-stress formulation. We conclude that the distance between the cylinder and the lateral boundaries can have a significant effect on the Strouhal number and other flow quantities. The minimum distance at which this influence vanishes has been found to be larger than what is commonly assumed.

Stabilized Finite Element Methods for the Velocity-Pressure-Stress Formulation of Incompressible Flows

Formulated in terms of velocity, pressure and the extra stress tensor, the incompressible Navier-Stokes equations are discretized by stabilized finite element methods. The stabilized methods proposed are analyzed for a linear model and extended to the Navier-Stokes equations. The numerical tests performed confirm the good stability characteristics of the methods. These methods are applicable to various combinations of interpolation functions, including the simplest equal-order linear and bilinear elements.

The Shear-Slip Mesh Update Method

Abstract The Shear-Slip Mesh Update Method, designed to handle certain classes of flow problems with moving boundaries and interfaces, is presented. Specifically, we focus on problems with large but regular boundary displacements, such as straight-line translation or rotation. These motions are accommodated by using a thin layer of deforming space—time elements, together with limited remeshing without any projection at space—time slab interfaces. As examples, 2D flow around two counter-rotating squares and 3D flow past a propeller are presented.

Shape optimization in steady blood flow: A numerical study of non-Newtonian effects

We investigate the influence of the fluid constitutive model on the outcome of shape optimization tasks, motivated by optimal design problems in biomedical engineering. Our computations are based on the Navier-Stokes equations generalized to non-Newtonian fluid, with the modified Cross model employed to account for the shear-thinning behavior of blood. The generalized Newtonian treatment exhibits striking differences in the velocity field for smaller shear rates. We apply sensitivity-based optimization procedure to a flow through an idealized arterial graft. For this problem we study the influence of the inflow velocity, and thus the shear rate. Furthermore, we introduce an additional factor in the form of a geometric parameter, and study its effect on the optimal shape obtained.