Double-averaging turbulence characteristics in flows over a gravel bed

Abstract

Double-averaged turbulence characteristics in a uniform flow over a gravel bed are studied. The log-law for the velocity is preserved above the interfacial sublayer, while the velocity within that sublayer follows a polynomial law. The Reynolds shear stress distribution within the roughness sublayer has a considerable damping. A quadrant analysis suggests that the sweeps become the prevalent mechanism within the interfacial sublayer. Within the interfacial sublayer, the quadrant plots of the form-induced velocities form a tilted pseudo-elliptical shape, suggesting that the fluid undergoes a cyclic process of upward-streamwise deceleration and downward-streamwise acceleration. An upward-downstream flux of the turbulent kinetic energy is prevalent over the entire flow depth, while a downward-upstream flux corresponding to sweep events is the mechanism within the interfacial sublayer. The energy budget indicates that the turbulent production, dissipation and diffusion, having attained their peaks, reduce sharply within the interfacial sublayer, and the pressure energy diffusion changes drastically to a negative magnitude below the virtual bed level.