Transformation From Octree Grid to Unstructured Hybrid Grid

Unstructured grids can be used in numerical simulation to deal with arbitrary complex computational regions in a unified scheme, but it is difficult to generate the grid and control the quality. Tree-structured grid can be considered as a kind of grid between structured grid and unstructured one. At present, there are relatively mature methods to quickly generate two-dimensional quadtree grid and three-dimensional octree grid in complex regions. In practical applications, discrete numerical methods often need to be applied on unstructured grids with coordinated connection, which is limited by tree-structured ones. In this paper, the transformation from tree-structured grid to unstructured hybrid grid was realized. In two-dimensional cases, the quadtree grid was transformed into unstructured triangle and quadrilateral hybrid grid, and in three-dimensional cases, the octree grid was transformed to unstructured hybrid grid. The difficulty of this transformation process is that thousands of different octree elements need to be considered, and an unstructured hybrid grid partition that can realize coordinated connection is given. The grid elements include four different cases: hexahedron, triangular prism, pyramid and tetrahedron. Through special classification, the automatic generation of programs was realized, which on the one hand avoids the error of writing a large number of programs manually, and on the other hand makes it possible to deal with thousands of different cases. Through testing several grids, the grid transformation method was preliminarily verified.