Author(s): Wangru Wei, Jun Deng
Keywords: Free surface; Air entrainment; High-speed observation; Open channel;
Abstract: Individual air bubbles are the typical air-water mixture structures in high speed open channel flows. The mechanism of air bubble formation, affected by the free surface deformation process over time and space in high speed flows, is not yet fully understood. This paper aims to describe the process of free-surface entrapped deformation and air entrainment in supercritical open channel flows. The detailed information on entrapped free surface curve and the process of surface deformation and air entrainment are observed using a high speed camera system. The data from the image analysis indicate the Gaussian-curve type is determined to fitting the entrapped free surface shape. Based on the shape stability and the deformation intensity variation, four stages, namely, generation, development, instability and bubble entrainment, are introduced to describe the entire free surface entrapment deformation and air bubble entrainment process. Once the deformation intensity exceeds a critical condition, the shape of deformed surface collapses and the two-side entrapped surface gets unstable. Air bubble is entrained into the water when two-side surface gets closure at a certain location in the depth dimension of entrapped deformation. The total duration from the entrapped generation to the air bubble entrainment is extremely short, less than 20 milliseconds for the present condition. In order to make further conclusions, theoretical studies and experiments in self-aerated open-channel flows are recommended to be done including non-intrusive measurements for the entrained air bubble size and the relationship between the free-surface deformation and bubble distributions.