Combined visual and quantitative techniques in the study of natural turbulent flows

An important difficulty in understanding turbulence in natural flows is that one does not see what is being measured by devices recording velocity fluctuations. This study shows how visualization of turbulent structures can be effectively implemented in natural river flows. In order to match turbulent flow structures with their velocity signatures, flow visualization is combined with high-frequency (20 Hz) velocity measurements from electromagnetic current meters. Two fluvial environments are examined. In a river confluence where the difference in turbidity provides a natural tracer, very strong negative velocity fluctuations for the streamwise component are concomitant with an upwelling motion. A video recording made from a bridge of the passage of turbulent structures delineated by turbid and clear water allowed us to relate the marked fluctuations in velocity to eddies of a duration of 1-2 s in the mixing layer. In a gravel-bed river we have examined patterns of eddy shedding in the lee of a pebble cluster. These patterns were highlighted by dye (milk) injected in the separation zone downstream. Black and white images were obtained from an underwater lens linked to a video camera recorder. We observed a complex dynamic characterized by two dominant patterns of eddy shedding: one with a clear motion of eddies toward the water surface and another with a downward expansion toward the bed of the structures. This pattern is related to the passage of intermittent wedges of low-speed fluid separated by wedges of fast moving fluid.