Experimental Study of a Vortex Generated at the Edge of a Channel with a Step

At the outlet of estuary-like systems, three vortices are formed: a dipole and a spanwise vortex. The spanwise vortex is formed due to the separation of the bottom boundary layer, just in front of the dipole. If a step occurs at the bottom in the channel output, a single vortex will be formed, the dipole becomes a part of a structure having a horseshoe shape. In a periodic driving flow, after a while this structure results in a dipole and a spanwise vortex. To study in laboratory this kind of vortices we made experiments in a system consisting of two domains connected by a channel in which the flow is induced by a periodic forcing. The channel layer depth is different with respect the depth in the two others domains. In order to investigate this system some measurements of velocity field using PIV were carried out in the vertical plane passing along the channel centerline. On the other hand, the detection of vortices composing the dipole was made with the synthetic Schlieren method. Vortices are low pressure regions, then they produce a deformation of the free surface which can be detected with this method. We observed that at the channel output a horseshoe vortex is formed by the flushing into the open domain. This structure remains for some time, after it decomposes in a dipole and a spanwise vortex. Finally all three vortices are destroyed.