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

An important difficulty in understanding turbulence in natural flows is tha t one does not see what is being measured by devices recording velocity flu ctuations. This study shows how visualization of turbulent structures can b e effectively implemented in natural river flows. In order to match turbule nt flow structures with their velocity signatures, flow visualization is co mbined with high-frequency (20 Hz) velocity measurements from electromagnet ic current meters. Two fluvial environments are examined. In a river conflu ence where the difference in turbidity provides a natural tracer, very stro ng negative velocity fluctuations for the streamwise component are concomit ant with an upwelling motion. A video recording made from a bridge of the p assage of turbulent structures delineated by turbid and clear water allowed us to relate the marked fluctuations in velocity to eddies of a duration o f 1-2 s in the mixing layer. In a gravel-bed river we have examined pattern s of eddy shedding in the lee of a pebble cluster. These patterns were high lighted by dye (milk) injected in the separation zone downstream. Black and white images were obtained from an underwater lens linked to a video camer a recorder. We observed a complex dynamic characterized by two dominant pat terns 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 structu res. This pattern is related to the passage of intermittent wedges of low-s peed fluid separated by wedges of fast moving fluid.