Generation of Coherent Structures in Turbulent Boundary Layers

Abstract

This paper presents the results of hydrogen-bubble flow visualization and hot-film velocity measurements of the turbulent flow structure along a flat wall. The objectives of the study were to examine the interaction of the vortices and the generation of new structures during this process. A new digital technique is introduced for analysis of visual records. An alternative view for the generation and evolution of the vortices is presented, and accounts for the generation of horseshoe and ring vortices through the interaction of lifted U-loop vortices with streamwise vortices. The interaction of near-wall horseshoe vortices with the sublayer plays an important role in forming U-loop vortices. The main mechanism appears to be spanwise vorticity concentration caused by the inflow associated with near-wall horseshoe vortices. Horseshoe and ring vortices are created by interacting streamwise and U-loop vortices. During this process the circulations of the two vortices are almost equal. Two modes are possible: one is vortex connection and detachment into a horseshoe vortex through a bridging-type mechanism; the other is deformation of the vortex contact zone into a ring vortex.