Large flow structures in a turbulent open channel flow

The outer region flow structure in an open channel flow is studied by means of now visualization and velocity measurements. The structure can be thoug ht of as formed by large streamwise vortices scaling with the flow depth, c reating upwelling and downwelling motions at the free surface. Associated w ith the downwelling motion induced by these vortices are regions with highe r longitudinal velocity on the free surface. Similarly, "boils" and eddies with a vertical axis are detected in the zones corresponding to the upwelli ng. Three temporal components an used to analyze velocity measurements: a t emporal mean value, a component associated with slow fluctuations, and a th ird component associated with fast fluctuations. It is shown that the contr ibution of fast fluctuations to the turbulent intensities is important near the wall, at z(+) < 1000. Slow fluctuations are the primary contributors i n the remainder of the boundary layer's outer region. It is proposed that t he large streamwise vortices, a manifestation of the largest turbulence sca les, are a major contributor to slow fluctuations.