A Study on Drag Reduction Mechanism on the Basis of Quasi-Coherent Structures in Wall Turbulence

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The structure of turbulent flow field along a riblet surface was investigated with the aid of a three-dimensional particle tracking velocimetry (3-D PTV). The statistics of all three velocity components were measured and compared with those above a smooth wall. Under a drag reducing condition, all the turbulent velocity fluctuations and the Reynolds shear stress were decreased near the riblet surface, although the flow characteristics in most of the flow field 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 ; as large as 0.8% of the maximum velocity.Moreover, a quadrant analysis was applied to the velocity database presently obtained and the near-wall quasi-coherent structures, which are responsible for the Reynolds shear stress are examined by using a conditional sampling technique. The quasi-streamwise vortices and streaky structures were found also above the riblet surface. Under the drag reducing condition, streamwise vortices have almost same characteristics as those above the smooth wall. The cores of vortex are displaced from the riblet surface and these vortices are free from the strong interaction with the ribs. The spatial length scales of near-wall streaky structures are unchanged in this case. On the other hand, sweep motions penetrate into the riblet valley under the drag increasing condition and the vortex structures are markedly affected by the ribs. It is also found that the streamwise persistence of the streaky structures are considerably decreased in this case.