Direct Numerical Simulation of Incompressible Flows in Domains of Close Packed Spheres

This study aimed to investigate and quantify turbulent flow effects for incompressible, isothermal fluid flows in computational domains consisting of regularly packed spheres using high-fidelity computational fluid dynamics. The flow domains treated in this study are analogous in an idealized sense to those encountered in pebble bed based high temperature nuclear reactors. The quantification of turbulent flow effects serves two purposes. Firstly, it assists in the development of lower-fidelity engineering tools such as Reynolds averaged Navier-Stokes based methodologies. Secondly, the quantification of turbulent flow effects adds to our fundamental understanding of the physics of incompressible flows in complex geometries. The study was conducted using an open-source spectral element computational fluid dynamics code, Nek5000, which was used to perform a series of direct numerical simulations in several flow domains representing both theoretical geometries and idealized sections of a practical reactor core at low to moderate Reynolds numbers. Selected results include the development of a high-fidelity database of numerical data for an expanded unit-cell geometry, the identification of possible very low frequency temporal dynamics in domains featuring several close packed spheres, and the calculation of turbulence statistics in a domain approximating the near-wall region of a reactor core