Applying tet-to-hex meshing method to complex nuclear reactor geometries for spectral element code

The spectral element code Nek5000 is an open-source, higher-order CFD code developed at Argonne National Laboratory. For spectral element code Nek5000, the computational domain is decomposed into second-order hexahedral elements. However, generating a quality pure-hexahedral mesh can be challenging for some problems. For geometries with relative complexity, blocking the geometry becomes hard or sometimes impossible. To address these issues, we adopted a tet-to-hex strategy to generate a pure hexahedral mesh. First, we generate a pure tetrahedral mesh for the geometry, and then one tetrahedral element is divided into four hexahedral elements. Many meshing codes could automatically generate a pure tetrahedral mesh. In this paper, we use the commercial codes ANSYS-meshing and ANSYS-ICEM to generate the pure tetrahedral mesh and then convert it to a pure hexahedral mesh. Several examples, including fuel assemblies with grid spacer, helical coil steam generator, and random pebble bed, are demonstrated to show the accuracy and robustness of this meshing method.