Clarification of Drag-Reduction-Mechanism over a Riblet surface and Exploration of Geometry of Riblet

本研究旨在探讨沟槽减少壁面射流中的表面摩擦的效果，并弄清楚这是由于修改近壁射流中的湍流结构。到目前为止，通过对工程实例的分析，得出了以下主要结论：（1）采用较大尺寸的沟槽表面，由平均速度剖面估算的局部表面摩擦力在较大位置处比非沟槽表面具有更大的减阻率;（2）采用非沟槽表面，由平均速度剖面估算的局部表面摩擦力在较大位置处具有更大的减阻率;维肋宽度。2.在由于肋产生药物减少的区域，壁面附近脉动速度的概率密度函数呈近似高斯分布。3.从VITA技术分析来看，即使在壁面射流中也观察到爆发波，但时间尺度是湍流边界层的10倍。象限法分析表明，条纹的存在减弱了雷诺应力产生的机制由于肋，x方向和z方向上的速度增加。与平板相比，沟槽表面上方展向近壁处的积分尺度较大，大涡展向尺度较大。5.壁面射流的展向谱在内层内随波长的增大而增大。这说明了与边界层湍流结构的差异以及沟槽尺度的差异，最大缩减比的出现是由于湍流结构的差异。6.在平面上的附壁射流中，直接法的结果属于其他研究者间接法的散射范围，这些数据是可靠的。

This research is intended to investigate the effect that riblets reduce skin-friction in a wall jet and to make it clear that this is due to modify the turbulent structure in the near wall jet. So far, it is cleared that the local skin-friction estimated from the mean velocity profiles indicates the reduction rate of the drag.The principal results got for the term of project concerned are as follows.1.The larger size riblet surface was made and it was recognized that the local skin-friction estimated from the mean velocity profiles indicates larger reduction ratio at the larger position than the former non-dimensional riblet width.2.In the region where the drug reduction due to the riblets yields.the probability density functions for fluctuating velocity near the wall show approximately Gaussian distribution.3.From the analysis of the VITA technique, the bursting wave is observed even in the wall jet, however the time scale is ten times as long as that in turbulent boundary layr.The results of the four-quadrant method exhibit that the mechanism of generating to the Reynolds stress is become weak due to the riblets.4.The correlation length of the fluctuating velocity increase in the x-direction and z-direction due to the riblets. The integral scales near the wall in spanwise direction above the riblet surface are larger in comparison with the flat surface and the spanwise scales of large eddy are large.5.The spanwise spectrum in the wall jets increase as the wave length increasing within the inner layr. This shows the difference from the turbulent structure in the boundary layr and a discrepancy of the riblet scale that the maximum reduction ratio occurred is attributed to the difference in the turbulent structure like this.6.The results in the direct method in a wall jet on the flat surface belong to the range of the scatter the indirect meathod by the other research worker and these data are reliable.