Ph.D. Dissertation

Unstructured Mesh Generation for Inviscid/Viscous Flow Simulations

Supervisor:Dr. Kazuhiro Nakahashi
Dept. of Aeronautics and Space Engineering, Tohoku University, Sendai, Japan, March 2003

Abstract

Efficient and robust algorithms are presented for surface triangulation and the generation of unstructured hybrid meshes comprised of tetrahedra, prisms and pyramids for 3D viscous flow simulations. The approaches of the surface triangulation are as follows: (a) use of repaired stereolithography (STL) files as surface definitions, that is, background grids for surface meshing, (b) automatic reconstruction of geometric features for initial front setup, (c) Graphical User Interface (GUI) for easily controlling surface mesh density by inserting source lines and points on the surface, (d) adoption of a direct advancing-front surface triangulation method, which minimizes the necessity to divide the surface into a number of patches, (e) outer boundary generation from templates in the system.

The hybrid mesh generation starts with isotropic tetrahedral meshes to enhance the robustness of the algorithm. Use of double normals improves grid quality around sharp corners. Prismatic layers are added fully automatically, while overall mesh validity was kept. All users have to do is to input boundary conditions and three parameters: the maximum number of layers, the minimum layer thickness and the stretching factor. An angle-based smoothing method is applied to a tetrahedral domain, which sufficiently improved mesh quality.

In this dissertation, the algorithms are discussed for the repair of STL files, the surface triangulation, and the hybrid mesh generation. The resulting unstructured grid generation system is applied to several airplanes, vehicles and so on. Viscous flow simulations are also carried out for experimental supersonic airplane models designed by the National Aerospace Laboratory of Japan. The numerical results are compared with experiment. The ability and efficiency of the presented unstructured mesh generation system are demonstrated.

Directly Related Papers

• Ito, Y. and Nakahashi, K., "Improvements in the Reliability and Quality of Unstructured Hybrid Mesh Generation," International Journal for Numerical Methods in Fluids, Vol. 45, Issue 1, May 2004, pp. 79-108.
• Ito, Y. and Nakahashi, K., "Surface Triangulation for Polygonal Models Based on CAD Data," International Journal for Numerical Methods in Fluids, Vol. 39, Issue 1, May 2002, pp. 75-96.
• Ito, Y. and Nakahashi, K., "Direct Surface Triangulation Using Stereolithography Data," AIAA Journal, Vol. 40, No. 3, March 2002, pp. 490-496.

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