Quasi-DNS capabilities of OpenFOAM for different mesh types

• Quasi-DNS of pipe and channel flow configurations are performed using OpenFOAM.
• Structured and unstructured meshes are assessed for quasi-DNS calculations.
• Practically, DNS quality is achieved on hexahedral and extruded polyhedral grids.

Experimental limitations for certain nuclear reactor safety applications have pushed forward the demand for high fidelity DNS reference solutions for complex geometric configurations such as a T-junction or a spherical pebble bed. The application of traditional high-order DNS codes is limited to simple flow domains such as a periodic box or channel. As a possibility to create reference DNS solutions for more complex geometries, we have assessed the (quasi-)DNS capabilities of the OpenFOAM finite volume CFD solver for both structured hexahedral meshes and arbitrary polyhedral meshes. The feasibility of (quasi-)DNS analyses on polyhedral grids is of main interest, since this may offer the possibility to significantly expand the availability of (quasi-)DNS-quality data on arbitrarily complex geometries. In order to have a basis for the considered assessment, the mutual differences between generally recognized reference DNS data bases for turbulent channel and pipe flows are determined first. Subsequently, the differences between these reference DNS solutions and the present OpenFOAM (quasi-)DNS solutions are quantified for the considered mesh types. We use an existing finite volume CFD method and well known turbulent channel and pipe flow DNS reference cases for the assessments in this paper. New in this paper are the application of this CFD method to (quasi-)DNS analyses using arbitrary polyhedral meshes, and the quantification of respectively the mutual differences between generally recognized reference DNS data bases and the differences between the obtained OpenFOAM (quasi-)DNS data and these reference DNS data bases.Based on the presented assessment, it is observed that the differences between the OpenFOAM solutions and the considered reference DNS solutions are practically the same as the mutual differences between these reference DNS solutions when structured hexahedral meshes are used. Furthermore, it is observed that the differences as obtained by OpenFOAM on extruded polyhedral meshes are practically the same as those obtained for the structured hexahedral meshes. In contrast, the full polyhedral mesh shows somewhat larger differences near the peaks in the rms velocity profiles, whereas the differences in the bulk flow are again practically the same as those for the hexahedral grids.