Chapter 18 - Summary: Air bubble diffusion in shear flows

Abstract

This chapter outlines the similarity among different turbulent shear flows. In high velocity water flows, large quantities of air are entrained at the free-surfaces. The process of self-aeration induces a modification of the flow properties as compared with clear-water flows. The entrained air bubbles interact with the flow turbulence and these interactions modify the flow behavior. The presence of large amounts of air bubble modify some turbulence characteristics while the turbulence controls the mechanism of bubble break-up. In turbulent shear flows, the bubble break-up is determined by the balance between the surface tension and the turbulent shear stress. And the presence of bubbles increase the level of turbulence in air-water flows. The distributions of air concentration are represented by a simple advective diffusion theory and the mean velocity distributions have the same shape as for non-aerated monophase flows. The level of turbulence affects the bubble size distributions, and simple calculations of maximum bubble size are developed. The presence of air bubbles within the shear layers modifies the turbulence characteristics and induces an increase of momentum losses, some drag reduction in open channel flows or an increase of turbulence level in pipe flows. In air water flows, the resulting effects of interactions between air bubbles and turbulence show interesting analogies with turbulence modifications, observed in dilute-polymer flows, suspended particle flows and flows with turbulence manipulators.