Numerical Study of Resistance with Rib Roughness of Various Scales

A dynamically consistent method of locating the rough wall origin y0 is presented that is applicable to large- and small-scale rib roughness elements, where scale is defined relative to the depth of flow. Using the results of Reynolds-averaged computational fluid dynamics (CFD) calculations, a near-standard resistance law is derived in which ks, the equivalent sand roughness parameter, for large-scale roughness is flow-depth dependent. It is postulated that the relationship with depth of both y0 and ks, when scaled appropriately, is independent of rib roughness height and spacing. For ks this is supported by the CFD results.