Relation between the large scale eddy in the plane wall jet and the turbulent generating mechanism

通过对平壁射流平均流场和湍流流场的测量，研究了大尺度涡对湍流产生机理的影响。研究了最大速度与外部速度之比的三种情况。结果表明：在各种速度比下，在流向距离x/S1=57 ~ 162范围内，湍流结构和平均流结构均达到自保持状态。随着速度比的增大，半宽的扩展率增大，内层厚度的扩展率略有减小。在外层，平均流几乎不依赖于速度比，但雷诺切应力和湍流强度的绝对值都随着速度比的增加而减小。在内层，雷诺正应力和剪应力随速度比的变化呈现不同的趋势。另外，利用VITA技术和UV象限法研究了涡结构的作用。在最大速度处，第三象限对雷诺切应力的贡献率随速度比的增大而增大。从u脉动速度的系综平均模式来看，一对反向旋转的展向涡，即大尺度涡的存在，通过目前的壁射流的大部分支持。结果表明，雷诺切应力为零的位置随着速度比的增大而减小，并逐渐接近内层厚度。

Measurement was made on both the mean flow and the turbulent flow field in the plane wall jets to investigate the effect of large scale eddy on the turbulent generating mechanism. Three cases for the ratios of the maximum velocity to the external velocity were studied. The results obtained are as follows.In each velocity ratio,the turbulent structure reaches self-preserving state in the range of streamwise distance x/S1=57 to 162,as well as the mean flow structure. With increasing the velocity ratio,the spreading rate of half width increases,conversely that of the inner layer thickness slightly decreases. In the outer layer,the mean flow hardly depends on the velocity ratio,however the absolute value of both the Reynolds shear stress and the turbulent intensity decreases with increasing of the velocity ratio. In the inner layer,all the Reynolds normal and shear stresses show the different tendency depending on the velocity ratio.In addition,both VITA technique and the uv-quadrant method have been utilized to clarify the role of vortical structure. At the location of maximum velocity,the ratio of the fractional contributions to the Reynolds shear stress from the third quadrant increases with increasing the value of velocity ratio. From the ensemble averaged pattern of u fluctuating velocity,the existence of a pair of counter-rotating spanwise vortices,namely the large scale eddies,was supported through most of the present wall jet. It is inferred from these results that the location where the Reynolds shear stress is zero decreases and approaches the inner layer thickness with increasing of the velocity ratio.