Turbulent Model of Coherent Structures in a Transitional Boundary Layer

点击翻译按钮获取中文摘要

In order to experimentally study interaction phenomena between turbulent spots in a laminar boundary layer or horseshoe vortices in a turbulent boundary layer, a pair of turbulent spots or horseshoe vortices was artificially developed in each boundary layer. Then, a synthetic transitional boundary layer was generated by inducing many turbulent spots to clarify the effects of their interaction process on the structure of the boundary layer. To observe the vortex structure, coherent velocity and vorticity components were obtained trough conditional hot-wire measurement.(1)The longitudinal vortex at each inside wingtip comes into contact, as the two spots begin to merge. Mutual interaction between them generates a strong upwash and give birth to a strong velocity-defect region in the upper merged region.(2)The spanwise vortices constitute the heads of large-scale horseshoe vortices connecting the longitudinal vortices beneath and may grow into turbulent bulges in the downstream turbulent boundary layer. As the array of turbulent spots grows up into a turbulent boundary layer, its periodical structure survives still around the boundary layer's outer edge.(3)The merged horseshoe vortices grew to be a single larger horseshoe vortex in the downstream region, when two horseshoe vortices were induced simultaneously.(4)When the two horseshoe vortices were induced with time delay of 20msec, they are not connected with each other and apparently, their inside legs were not separated from each horseshoe vortex.