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（3-D PTV）研究了波纹表面湍流流场的结构。测量了所有三种速度分量的统计数据，并与光滑壁面上的统计数据进行了比较。在减阻条件下，波纹表面附近的湍流速度波动和雷诺数剪切应力均减小，但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。湍流动能从顺流分量向展向分量的再分配机制在纹谷上方区域被明显抑制。另一方面，在中性阻力条件下，肋部附近存在明显的横流二次流；可达最大速度的0.8%。此外，对目前获得的速度数据库进行了象限分析，并采用条件采样技术对引起雷诺剪切应力的近壁准相干结构进行了检验。纹状表面还存在准流向涡和条纹结构。在减阻条件下，顺流涡与光滑壁面上的涡具有基本相同的特征。涡核从肋面移出，这些涡不受肋面强相互作用的影响。在这种情况下，近壁条纹结构的空间长度尺度不变。另一方面，在阻力增加的条件下，掠流运动渗透到肋谷中，涡结构受到肋的显著影响。还发现，在这种情况下，条纹结构的顺流持久性大大降低。

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.