Flow Resistance in High Velocity Supercritical Flows

On large chutes and spillways air is entrained through the free surface. When the air reaches the shear layers close to the invert the air bubbles interact with the turbulence mechanisms and induce drag reduction. This reduction of the friction factor was first described by WOOD (1983) using model data. In this paper a large number of prototype and model data are re-analyzed and confirm the drag reduction. A correlation between the reduction of friction factor and the quantity of air entrained is presented. The author suggests that the mechanisms of drag reduction are associated with the presence of rigid air bubbles next to the invert and linked with the air concentration boundary layer