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

本研究旨在研究肋条对壁面射流的减摩效果，并明确这是由于改变了近壁射流中的湍流结构。到目前为止，根据平均速度分布估计的局部表面摩擦表明了阻力的减小率。对于有关项目而言，得到的主要结果如下：1.制作了较大尺寸的沟槽表面，并且认识到，根据平均速度剖面估计的局部壁面摩阻在较大位置处的减阻率比先前的无量纲沟槽宽度大。2.在由于沟槽引起的药物减少的区域中，壁面附近的脉动速度的概率密度函数呈近似的高斯分布。3.从Vita技术的分析，四象限方法的计算结果表明，沟槽对雷诺应力的产生机制变得较弱。4.沟槽使x向和z向脉动速度的关联长度增加。壁面附近波纹面以上沿展向方向的积分尺度比平坦表面大，大涡的展向尺度也较大。5.壁面射流的展向谱随内层波长的增加而增大。这说明了与边界层湍流结构的差异和Riblet尺度的差异，最大折减率的出现是由于湍流结构的不同。6.直接法在平板壁面射流中的计算结果属于其他研究人员间接法的散布范围，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.