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

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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.