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

利用三维粒子跟踪测速仪(3DPTV)研究了沟槽表面的湍流流场结构。测量三个速度分量的统计量，并与光滑壁上的速度分量进行比较。在减阻条件下，尽管大部分流场的流动特性与光滑壁面上的流动特性非常相似，但在波纹面附近，所有的湍流速度脉动和雷诺剪应力都减小了。结果还发现，在Riblet谷以上区域，湍流动能从流向分量到跨向分量的再分配机制受到了很大的抑制。另一方面，在中性阻力条件下，肋骨附近存在明显的横流二次流，其最大速度可达最大速度的0.8%。此外，对已有的速度数据库进行了象限分析，并采用条件采样技术检验了导致雷诺剪应力的近壁准相干结构。在肋片表面上方也发现了准流向涡和条纹状结构。在减阻条件下，流向涡的特性与光滑壁面上的基本相同。涡核从肋片表面移开，这些涡核脱离了与肋片的强烈相互作用。在这种情况下，近壁条纹结构的空间长度尺度保持不变。另一方面，在阻力增加的情况下，掠过运动穿透到肋谷，肋骨对涡结构有明显的影响。研究还发现，在这种情况下，条状结构的流向持续性显著降低。

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.