Spatially Averaged Turbulent Flow over Square Ribs

Abstract

A series of experiments was undertaken to assess fully rough turbulent subcritical flow over two-dimensional transverse repeated-rib roughness of varying spacing λ∕h=1–16 (roughness spacing/height ratio), with h∕H=0.09 (roughness height/flow depth). Each of the 11 experiments involved centerline measurements of three-dimensional velocity vectors, water surface profiles, and bed pressures and forces. The structure of flow over rib roughness consists of a pair of principal vortices of opposite sign set up by the ribs. These vortices are superimposed on an overall double- (time and space)-averaged velocity profile that is (quasi-) logarithmic above roughness tops, and that below roughness tops changes with increasing rib spacing from exponential (λ∕h<10) to linear (λ∕h≥10) to logarithmic (λ∕h≫10). The measured double-averaged velocity profiles are parameterized herein, and double-averaged Reynolds and form-induced stress profiles and trends are also identified. Maximum drag due to wall roughness is found to occur for λ∕h≈8. The experimentally determined momentum balance, including effects of secondary currents and flow acceleration, is found to agree well with theoretical expectations. It is shown that the knowledge of the effects of form-induced stresses, secondary currents, and flow nonuniformity may be particularly important for describing and modeling flows over roughness elements.