V. Kalro

Parachute fluid-structure interactions: 3-D computation

Abstract We present a parallel computational strategy for carrying out 3-D simulations of parachute fluid–structure interaction (FSI), and apply this strategy to a round parachute. The strategy uses a stabilized space-time finite element formulation for the fluid dynamics (FD), and a finite element formulation derived from the principle of virtual work for the structural dynamics (SD). The fluid–structure coupling is implemented over compatible surface meshes in the SD and FD meshes. Large deformations of the structure are handled in the FD mesh by using an automatic mesh moving scheme with remeshing as needed.

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.

Parallel 3D computation of unsteady flows around circular cylinders

Abstract In this article we present parallel 3D finite element computation of unsteady incompressible flows around circular cylinders. We employ stabilized finite element formulations to solve the Navier-Stokes equations on a thinking machine CM-5 supercomputer. The time integration is based on an implicit method, and the coupled, nonlinear equations generated every time step are solved iteratively, with an element-vector based evaluation technique. This strategy enables us to carry out these computations with millions of coupled, nonlinear equations, and thus resolve the flow features in great detail. At Reynolds number 300 and 800, our results indicate strong 3D features arising from the instability of the columnar vortices forming the Karman street. At Re = 10 000 we employ a large eddy simulation (LES) turbulence model.

Parallel Computation of Parachute Fluid-Structure Interactions

The Army currently lacks the ability to predict accurately the behavior of parachute fluid-structure interaction phenomena. The interactions between the parachute system and the surrounding flow field are dominant in most parachute operations and thus the ability to predict parachute fluid-structure interaction phenomena is of interest to the Army. This paper presents the current status of an ongoing research effort to couple the finite element formulations for the fluid dynamics and structural dynamics encountered in parachute problems. An application of the opening of an axisymmetric round parachute is presented. Future considerations for the coupled model and potential applications of interest to the Army are presented.

Implementation of implicit finite element methods for incompressible flows on the CM-5

Abstract A parallel implementation of an implicit finite element formulation for incompressible fluids on a distributed-memory massively parallel computer is presented. The dominant issue that distinguishes the implementation of finite element problems on distributed-memory computers from that on traditional shared-memory scalar or vector computers is the distribution of data (and hence workload) to the processors and the non-uniform memory hierarchy associated with the processors, particularly the non-uniform costs associated with on-processor and off-processor memory references. Accessing data stored in a remote processor requires computing resources an order of magnitude greater than accessing data locally in a processor. This distribution of data motivates the development of alternatives to traditional algorithms and data structures designed for shared-memory computers, which must now account for distributed-memory architectures. Data structures as well as data decomposition and data communication algorithms designed for distributed-memory computers are presented in the context of high level language constructs from High Performance Fortran. The discussion relies primarily on abstract features of the hardware and software environment and should be applicable, in principle, to a variety of distributed-memory system. The actual implementation is carried out on a Connection Machine CM-5 system with high performance communication functions.

Parallel computational methods for 3D simulation of a parafoil with prescribed shape changes

Abstract In this paper we describe parallel computational methods for 3D simulation of the dynamics and fluid dynamics of a parafoil with prescribed, time-dependent shape changes. The mathematical model is based on the time-dependent, 3D Navier-Stokes equations governing the incompressible flow around the parafoil and Newtons law of motion governing the dynamics of the parafoil, with the aerodynamic forces acting on the parafoil calculated from the flow field. The computational methods developed for these 3D simulations include a stabilized space-time finite element formulation to accommodate for the shape changes, special mesh generation and mesh moving strategies developed for this purpose, iterative solution techniques for the large, coupled nonlinear equation systems involved, and parallel implementation of all these methods on scalable computing systems such as the Thinking Machines CM-5. As an example, we report 3D simulation of a flare maneuver in which the parafoil velocity is reduced by pulling down the flaps. This simulation requires solution of over 3.6 million coupled, nonlinear equations at every time step of the simulation.

3d computation of unsteady flow past a sphere with a parallel finite element method

Abstract We present parallel computation of 3D, unsteady, incompressible flow past a sphere. The Navier-Stokes equations of incompressible flows are solved using a stabilized finite element formulation. Equal-order interpolation functions are used for velocity and pressure. The second-order accurate time-marching within the solution process is carried out in an implicit fashion. The coupled, nonlinear equations generated at each time step are solved using an element-vector-based iteration technique. The computed value of the primary frequency associated with vortex shedding is in close agreement with experimental measurements. The computation was performed on the Thinking Machines CM-5.

Parallel finite element computation of the dynamics of large ram air parachutes

Future airdrop systems require the development of very large gliding parachutes capable of delivering 21-ton payloads. This airdrop requirement presents new technology barriers which cannot be addressed by previous methods such as extensively costly airdrop testing and wind tunnel testing; hence the need to look towards high-performance computing as a viable alternative. In this paper we present a methodology to simulate the dynamics of ram-air parachutes using stabilized finite element Navier-Stokes solvers. Highly optimized coding techniques and algorithms provide us with the potential to solve systems with millions of coupled nonlinear equations. These computations are carried out on the massively parallel supercomputer CM-5.

Multi-domain parallel computation of wake flows

Abstract We present a new, multi-domain parallel computational method for simulation of unsteady flows involving a primary object, a long wake region and, possibly, a secondary object affected by the wake flow. The method is based on the stabilized finite element formulation of the time-dependent Navier—Stokes equations of incompressible flows. In the multi-domain computational method the entire simulation domain is divided into an ordered sequence of overlapping subdomains. The flow data computed over the leading subdomain is used for specifying the inflow boundary conditions for the next subdomain. The subdomain corresponding to the wake would not involve any objects, hence the mesh constructed over this domain would be structured. A special-purpose finite element implementation for structured meshes is used for the wake domain to achieve much higher computational speeds compared to a general-purpose implementation. We present verification studies for the multi-domain method and special-purpose implementation, followed by two numerical examples. The first example is the wake behavior behind a circular cylinder. The second one is the aerodynamic effect of tip vortices released from a leading wing on a trailing wing placed in the far wake.

3D Simulation of Flow Problems with Parallel Finite Element Computations on the Cray T3D

In this article, we present our recent work in finite element computations using the Cray T3D massively parallel supercomputer. Several fluid flow problems encountered in real-life applications are numerically simulated with highly optimized coding techniques and efficient algorithms implemented on the Cray T3D. The superior technical features of the Cray T3D combined with our efficient implementations give us a unique capability to solve a large class of practical problems involving millions of degrees of freedom. These new computational capabilities are allowing us to solve problems which we were not able to attempt few years ago. All computations reported in this article are carried out on unstructured meshes with high resolution and with emphasis on the precise representation of the complex 3D model.