Observation Investigation on Free Surface Deformation and Air Entrainment Process in Supercritical Open Channel Flows

Individual air bubbles are the typical air-water mixture structures formatting 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. Direct observation on free surface and the behavior of air entrainment in high speed flowing water has the potential to understanding the air-water multiphase flow and mass transfer. This paper aims to describe the relationship between free-surface entrapped deformation and air entrainment in open channel flows. The performed experiments have been carried out in a physical scaled model for the evaluation of free-surface curve and the process of surface deformation and air entrainment. After analysing images taken by a camera during the experiments, the result has been divided in two parts, namely, to describe the air-entrainment phenomenon using the relevant curve type and fitting the obtained curves into the known curves, and to obtain the critical radius of curvature. Based on the experimental data from the image analysis and one sample t-test, the Gaussian-curve type has been decided to describe the phenomenon. After the image analysis of the free-surface deformation curve type has been done, the process of surface deformation and air entrainment has been evaluated. As a result, it is found that the surface achieves closure at a relatively low position at a range of 7 – 19 milliseconds. The results of the radius of curvature reveal that the radius of curvature varies for a short period of time, it increases and decreases until the process becomes unstable, ranges between 10 % – 60 % of the process time and between 0,5 – 2 mm. In order to make final conclusions, further studies and experiments in self-aerated open-channel flows should be done including non-intrusive measurements for the entrained air bubble size and the relationship between the free-surface deformation and bubble distributions.