Réduction des pertes de frottement pour des écoulements à surface libre avec entraînement d'air. Augmentation de la débitance d'un coursier d'évacuateur de crues

Drag reduction in open channel flows with air entrainment. Augmentation of the maximum discharge capacity of a spillway

Maître de Conférences en Hydraulique et Mécanique des Fluides Department of Civil Engineering, The University of Queensland, Brisbane QLD 4072, Australie

Abstract

In supercritical open channel flows air is entrained at the free surface. Such air-water flows, called self-aerated flows, exhibit smaller friction losses than non-aerated flows. New data on drag reduction in self-aerated flows are presented. It is shown that the drag reduction process is linked with the presence of an air concentration boundary layer next to the channel bottom. An analogy with dilute polymer solutions and micro bubble modified boundary layers is developed and it is suggested that the presence of air next to the bottom increases the effective viscosity of the mixture and the sublayer thickness*. The process of drag reduction could be used to increase the maximum discharge capacity of a spillway. Combining self-aeration and the use of bottom aeration devices, the friction factor along the spillway can be reduced, inducing an increase of maximum discharge.