Free surface-vorticity interactions in an open channel flow

The interactions between vortical structures, free surface, and background shear in an open channel flow is experimentally investigated. Deterministic structures are generated by a vertical injection from the channel bottom and a number of disturbances characterized by different jet amplitudes and Reynolds number are considered. Three cases are selected as representatives of the whole phenomenology and their evolution is discussed in detail. The streamwise perturbation is visualized by fluorescein while the streamwise component of velocity is measured by a laser Doppler anemometer. The perturbation in the cross stream plane is analyzed quantitatively and the velocity field is reconstructed on the symmetry plane, to extract the traces of the individual vortical structures. Depending on the intensity of the external perturbation, the structures either evolve as in the absence of the interface (small intensity) or they approach the free surface to undergo a topological change due to their reconnection with the free surface. The process originating in the so-called blockage and viscous layers is analyzed in detail. As a major conclusion, the interaction with the free surface is found to deplete the velocity gradients generated by the liftup of the quasi-streamwise vortices. This effect is suggested to be a major cause of stabilization of the open channel flow, with respect to a closed channel at the same Reynolds number