Free Surface Air Entrainment and Single-Bubble Movement in Supercritical Open-Channel Flow

Abstract

There has been little study on the microscopic bubble entrainment and diffusion process on high-speed self-aerated flows, although the problem under investigation is theoretically important and has important engineering applications. This study presents an experimental investigation on visual processes of free surface air entrainment and single-bubble diffusion in supercritical open-channel flows. The typical surface deformation, single air bubble rising, and penetration are recorded using a high-speed camera system. Results show that for a single-bubble formation process, surface entrapment development and bubble entrainment through a deformation evolution underneath the free surface are the two main features. The shape variation of local surface deformation with time follows an identical power law for different bubble size generations. The entrained bubble size depends on both the size scale and the shape of the entrapped free surface. As the single bubble moves downstream, its longitudinal velocity is approximately the same as that of the water flow surrounding it, while its vertical velocity for rising and penetration increases with the increase in water flow velocity. An empirical-linear relationship for the bubble rising and penetration velocity with water flow velocity is obtained. This study demonstrates that microscopic bubble movement can improve the self-aeration prediction in open-channel flow and advance our understanding of the macroscopic and microscopic air–water properties in hydraulic engineering.