A fast direct numerical simulation method for characterising hydraulic roughness

Abstract

We describe a fast direct numerical simulation (DNS) method that promises to di- rectly characterise the hydraulic roughness of any given rough surface, from the hy- draulically smooth to the fully rough regime. The method circumvents the unfavourable computational cost associated with simulating high-Reynolds-number flows by employ- ing minimal-span channels (Jiḿenez & Moin 1991). Proof-of-concept simulations demon- strate that flows in minimal-span channels are sufficient for capturing the downward velocity shift, that is, the Hama roughness function, predicted by flows in full-span chan- nels. We consider two sets of simulations, first with modelled roughness imposed by body forces, and second with explicit roughness described by roughness-conforming grids. Ow- ing to the minimal cost, we are able to conduct DNSs with increasing roughness Reynolds numbers while maintaining a fixed blockage ratio, as is typical in full-scale applications. The present method promises a practical, fast and accurate tool for characterising hy- draulic resistance directly from profilometry data of rough surfaces.