Form-induced stress in turbulent flow over riblets

Abstract

We carry out direct numerical simulation of minimal openchannel flow over riblets. Several riblet geometries are simulated, namely symmetric triangular, asymmetric triangular, blade and trapezoidal, and with this unprecedented high-fidelity dataset, we are able to obtain broad insights into the flow physics of riblets. We find that the roughness sublayer thickness, above which the flow is statistically homogeneous, is proportional to the square root of the riblet groove cross-sectional area ℓ+ g in both the drag-reducing and the drag-increasing regime, consistent with the ability of this parameter to collapse the roughness function corresponding to different groove geometries. Large grooves are associated with mean secondary velocities and they carry additional stress that contributes up to 40% of the total shear stress at the crest, comparable to the contribution from the turbulent fluctuations.