Entraînement d'air dans les écoulements à surface libre : application aux évacuateurs de crues de barrage

Air entrainment in open channel flows Application to spillways

Maître de Conférences, Department of Civil Engineering, The University of Queensland, St Lucia QLD 4072, Australia

Abstract

In open channel flows an important design parameter is the amount of entrained air. The presence of air in high velocity flows or steep chutes increases the bulk of the flow and this is a design pardmeter that determines the height of chute sidewalls. Furthermore the presence of air within the boundary layer reduces the shear stress betweenflow layers and hence the shear forces. The resulting increase of fluid motion must be taken in account when designing ski jump and stilling basin downstream of a spillway. Besides the presence of air within high-velocity flows may prevent or reduce cavitation damage. Estimation of the quantity of air entrained along the spillway is required by the design engineer when sitting aerators to artificially introduce more air, or determining whether aerators are required. Recently air entrainment on chutes also been recognized for its contribution to air-water transfer of atmospheric gases such as oxygen and nitrogen.

This article describes a general method to compute the flow characteristics of self-aerated flows. Firstly the uniform flow conditions are presented and comparisons will be made between model and prototype results. Then the basic equations for gradually varied aerated flows will be developed, and these results are discussed and compared with experimental data.