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

本研究旨在探讨微纹减少壁面射流中表面摩擦的影响，并明确这是由于改变了近壁面射流中的湍流结构。到目前为止，可以明确的是，从平均速度剖面估计的局部表面摩擦表明阻力的减少率。项目期内的主要成果如下：1。制作了更大尺寸的纹面，并认识到由平均速度剖面估计的局部表面摩擦比以前的无量纲纹宽在更大的位置显示出更大的减速比。在药物减少的地区，由于小颗粒的产量。壁面附近波动速度的概率密度函数近似为高斯分布。从VITA技术的分析来看，即使在壁面射流中也能观测到爆发波，但其时间尺度是湍流边界层的10倍。四象限法的结果表明，由于裂纹的存在，产生雷诺应力的机制变得较弱。在x方向和z方向上，脉动速度的相关长度由于波纹的存在而增加。4 .纹面上方近壁面沿展向的积分尺度比平面大，大涡沿展向的尺度也大。随着内层内波长的增加，壁面射流的展向光谱也随之增加。这说明了边界层湍流结构的不同，而发生最大减量比的纹线尺度的差异是由于这样的湍流结构的不同。直接法在平面壁面射流中得到的结果属于散射范围，其他研究人员用间接法得到的结果是可靠的。

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.