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Journal Articles Journal of Hydraulic Engineering Year : 2017

Shallow mixing layer downstream from a sudden expansion

Abstract

This study aims at investigating the mixing layer located at the interface between the free stream and recirculation zone downstream from a sudden lateral open-channel expansion. Specific attention is paid to the interaction of the shallowness of the flow, characterized by the bed-friction number, with the lateral confinement due to the side wall. The velocity field for four flows with the same geometry but very different bed-friction numbers is measured in detail in order to characterize the mean velocity fields and Reynolds stresses across the mixing layers and evaluate the width of the mixing layers and their growth rates along with the typical oscillation frequencies. In the upstream region of the recirculation zone, the mixing-layer characteristics for the examined configurations are analogous to the ones of classical—laterally unbounded—mixing layers. In this region, the shallowness modifies the shape of the streamwise velocity profiles, extends the mean velocity gradient magnitudes and lowers the Reynolds stress terms, but hardly affects the mixing layer’s expansion rate. On the other hand, in the region near the flow reattachment, the mixing layer adopts very different behavior, with an abrupt drop of the its expansion. This change in behavior is linked to the dynamics of the two-dimensional (2D) vortices within the mixing layer, not a damping effect of the bed friction on these vortices, as the local bed-friction numbers remain much lower than the critical values reported in the literature. It is rather due to the interaction of the coherent structures with the side wall; the characteristics of this interaction itself are influenced by the flow shallowness. It is also shown that the damping effect due to bed friction is not responsible for the huge variations reported on the recirculation zones length. This imposes a distinction between local mixing-layer shallowness—derived using the mixing-layer width as length scale and governing the mixing layer’s characteristics—and a global flow shallowness—derived using the expansion width and governing the recirculation length.
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Dates and versions

hal-01724987 , version 1 (21-11-2018)

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Lei Han, Emmanuel Mignot, Nicolas Rivière. Shallow mixing layer downstream from a sudden expansion. Journal of Hydraulic Engineering, 2017, 143 (5), pp.04016105. ⟨10.1061/(ASCE)HY.1943-7900.0001274⟩. ⟨hal-01724987⟩
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