Air Entrainment on Chute and Tunnel Spillways

In supercritical open channel flows air is entrained at the free surface. This process is called self-aeration. A method to estimate the flow properties is presented. New results on drag reduction in self-aerated flows are developed. It is shown that the mechanisms of drag reduction are associated with the presence of an air concentration boundary layer next to the spillway invert. An analogy with dilute polymer solutions and microbubble modified boundary layers suggests that the presence of air next to the bottom increases the effective viscosity of the mixture and the sublayer thickness. In a second part self-aerated flow characteristics are discussed in the particular case of tunnel spillways