On the experimental study of 3D turbulent shear layers*

Turbulent mixing layers are formed when parallel streams flowing at different speed merge. In the usual laboratory model, the mean flow varies mainly in the directions of the velocity difference and flow convection, with no significant changes in the other direction. However, real flows are three dimensional (3D)−­the incoming flows are not parallel and have not only different speeds but also different directions, thereby giving rise to 3D turbulent shear layers, i.e., skewed mixing layers, the detailed quantitative study of which is missing. Flow skewing in turbulent boundary layers is reported to cause a reduction in Reynolds stresses and drag. We experimentally investigated the mean flow and statistics of skewed mixing layers in a wind tunnel to find out if similar effects prevail. We generated 3D shear layers by skewing the mean flow with turning vanes at the trailing edge of a splitter plate, and used pitot-static tubes and x-wires to probe the flow at different cross-, span- and downstream distances. Preliminary results show that skewed mixing layers, like their 2D counterpart, also spread linearly with downstream distance, and that an approx. homogenous shear develops further downstream. At the resolution of our current measurements, the shear layer thickness and shear rate seem not to depend strongly on skewing of the flow. We seek an explanation for this in the turbulence statistics. Understanding the effect of three dimensionality on mixing layers will help us improve designs of engineering structures.