A FAST AND DIRECT METHOD FOR CHARACTERIZING HYDRAULIC ROUGHNESS

We describe a fast direct numerical simulation (DNS) method that promises to directly characterize the hydraulic resistance of any given rough surface, from the hydraulically smooth to the fully rough regime. The method circumvents the unfavorable computational cost associated with simulating high-Reynolds-number flows by employing minimal-span channels (Jiménez & Moin, 1991). Proofof-concept simulations, employing the parametric-forcing roughness model (Busse & Sandham, 2012), demonstrate that flows simulated in minimal-span channels are sufficient for capturing the downward velocity shift predicted by flows in full-span channels. Owing to the minimal cost, we are able to conduct parametric DNSs with increasing roughness Reynolds numbers while maintaining a fixed roughness height that is 40 times smaller than the half-channel height. When coupled to an unstructured-grid code, the present method promises a practical, fast and accurate tool for characterizing hydraulic resistance directly from profilometry data of rough surfaces.