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Recirculation zone downstream lateral expansions of open channel flow

ABSTRACTThe present work combines experiments, computational fluid dynamics, and hydraulic models to explain the recirculation length variations downstream of sudden lateral expansions of an open-channel flow. This situation can be compared to the classical turbulent backward facing step situation, but the flow shallowness brings substantial differences. Experiments show that the recirculating zone length relative to the expansion width, L/d, varies in a quite wide range from 2 to 14. The expansion ratio Rb and mostly the bed friction number S influence these variations. Three dimensional numerical simulations confirm these results but also detail the role of a third parameter, the relative water depth h/d. A one dimensional hydraulic model reveals the mechanisms leading to so huge L/d variations. Finally, two dimensional depth-averaged flow equations reveal the prevailing role of bed friction for high S values. Thanks to all these results, the present paper explains the evolution of the recirculation zone length with the different parameters. The shortening of this length is caused by two different mechanisms, corresponding to two asymptotical regimes. For low bed friction numbers S and high relative water depth h/d, large-scale vortices shed from the expansion corner drive the momentum exchanges from the freestream to the recirculation. For high bed friction numbers, their role is negligible: the recirculation zone is accelerated by a velocity leveling due to bed friction, leading to L/d ∝ S−0.7.

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