In supercritical open channel flows, air is entrained at the free surface. Such air-water flows, called selfaerated flows, exhibit smaller friction losses than non-aerated flows. New data on drag reduction in selfaerated 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.
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