Turbulent drag reduction mechanism above a riblet surface

The structure of a turbulent flowfield along a riblet surface was investigated with the aid of a three-dimensional particle tracking velocimetry. The statistics of all three velocity components were measured and compared with those above a smooth wall. Under a drag-reducing condition, all of the turbulent velocity fluctuations and the Reynolds shear stress were decreased near the riblet surface, although the flow characteristics in most of the flowfield were quite similar to those above the smooth wall. It was also found that the redistribution mechanism of the turbulent kinetic energy from the streamwise component to the spanwise one was considerably suppressed in the region above the riblet valley. On the other hand, under a neutral drag condition, a cross-stream secondary flow was apparent near the ribs. This fluid motion should enhance the turbulent momentum transport and deteriorate the drag-reducing effect of the riblet.