Direct numerical simulation of the flow through a structured pebble bed near a wall boundary

Packed pebble beds occur in many industrial applications, including the very high temperature and molten salt nuclear reactor design concepts. These designs are currently being researched as possible fourth-generation nuclear power system designs. In order to ensure proper cooling of the reactor cores in these systems during normal operation, as well as under accident conditions, a detailed understanding of the coolant flow behavior is required. Direct numerical simulation (DNS) can be used to simulate specific pebble bed flow and geometry conditions in order to develop high-fidelity fluid flow data and hence improve scientists' understanding and enhance lowerfidelity modeling. We have used Nek5000, a spectral-element computational fluid dynamics code, to develop DNS fluid flow data for pebble bed flow, including budgets for the Reynolds stresses. The geometry is a structured pebble bed with a face-centered cubic packing arrangement. The flow domain features periodic boundaries in both streamwise and spanwise directions except for a single bounding wall parallel to the flow direction. In a fully periodic domain, the flow was found to be asymmetric. In this work we focus on turbulence properties in the near-wall region and their effect on the overall flow behavior. A set of preliminary Reynolds-averaged Navier-Stokes calculations was also performed to investigate the effect of geometric parameters such as the distance of the wall from the first row of pebbles.