Large flow structures in a turbulent open channel flow

The outer region flow structure in an open channel flow is studied by means of flow visualization and velocity measurements. The structure can be thought of as formed by large streamwise vortices scaling with the flow depth, creating upwelling and downwelling motions at the free surface. Associated with the downwelling motion induced by these vortices are regions with higher longitudinal velocity on the free surface. Similarly, "boils" and eddies with a vertical axis are detected in the zones corresponding to the upwelling. Three temporal components are used to analyze velocity measurements: a temporal mean value, a component associated with slow fluctuations, and a third component associated with fast fluctuations. It is shown that the contribution of fast fluctuations to the turbulent intensities is important near the wall, at z⁺ < 1000. Slow fluctuations are the primary contributors in the remainder of the boundary layer's outer region. It is proposed that the large streamwise vortices, a manifestation of the largest turbulence scales, are a major contributor to slow fluctuations.