Physics of shallow recirculation zones downstream lateral expansions

This paper deals with the behaviour of the recirculation zones occurring downstream sudden lateral expansions of shallow, open-channel flows. Such recirculations are essential in river engineering as they form silting zones or favour the development of specific fauna and flora. Although the quite similar "classical" flow over backward facing step brings some interesting knowledge, the present situation is sufficiently different to justify a specific study as the recirculation zone is confined vertically between the free-surface and the river bed. In the literature, the importance of this confinement is accounted for by a stability parameter, the "bed friction number". The present experimental results, at odd with the ones of the literature, clearly show that the bed friction number cannot by itself account for the length of the recirculation: the dimensionless flow depth and the geometrical expansion ratio have comparable influences, as demonstrated through numerical simulations. The momentum balance reveals why the use of a stability parameter, relevant at the mixing layer scale, cannot explain the physics of the whole recirculation.