Winkelmann, Ralf

Strategies for Parallel and Numerical Scalability of CFD Codes

Winkelmann R., Hauser J., Williams R.D.

Published in 1998

In this article we discuss a strategy for speeding up the solution of the Navier-Stokes equations on highly complex solution domains such as complete aircraft, spacecraft, or turbomachinery equipment. We have used a finite-volume code for the (non-turbulent) Navier-Stokes equations as a testbed for implementation of linked numerical and parallel processing techniques. Speedup is achieved by the Tangled Web of advanced grid topology generation, adaptive coupling, and sophisticated parallel computing techniques.

An optimized grid topology is used to generate an optimized grid: on the block level such a grid is unstructured whereas within a block a structured mesh is constructed, thus retaining the geometrical exibility of the nite element method while maintaining the numerical efficiency of the nite dierence technique. To achieve a steady state solution, we use grid-sequencing: proceeding from coarse to finer grids, where the scheme is explicit in time. Adaptive coupling is derived from the observation that numerical schemes have differing efficiency during the solution process. Coupling strength between grid points is increased by using an implicit scheme at the sub-block level, then at the block level, ultimately fully implicit across the whole computational domain. Other techniques include switching numerical schemes and the physics model during the solution, and dynamic deactivation of blocks. Because the computational work per block is very variable with adaptive coupling, especially for very complex ows, we have implemented parallel dynamic load-balancing to dynamically transfer blocks between processors. Several 2D and 3D examples illustrate the functioning of the Tangled Web approach on different parallel architectures.

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A Tangled Web Strategy for Numerical and Parallel Scalability in Aerospace Simulation

Hauser J., Williams R.D., Winkelmann R.

Published in 1997

Parallel computing has become a key component of high performance computing in the 90’s. In order to exploit this technology in science and engineering, in particular for aerospace, automotive, and turbomachinery applications as well as in environmental simulation, highly complex geometries have to be dealt with, often generated by CAD systems. In order to bring CFD in the design loop, quick turnaround times are mandatory.

Grids are multiblock hexahedra allowing any kind of topology and comprise any number of blocks. Grid generation is completely non-interactive. Only a wireframe and the geometry description are provided. Surface and volume grids are generated together. Grids are slope continuous. Grids are constructed using a high-level grid generation language. Grids consist of objects that are reusable. A problem/user specific grid database may be constructed that allows to use more complex entities from which to generate new grids. Complex grids are built in an object oriented fashion.

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