Turbulent boundary layer to single-stream shear layer: the transition region

Abstract

This communication presents the results and conclusions of an experimental study of the near-separation region of a single-stream shear layer. The momentum thickness at separation ($x\,{=}\,0$) was $\theta_0\,{=}\,9.6$ mm, with Reynolds number $\hbox{\itshape Re}_\theta\,{=}\,4650$. Boundary layer separation was caused by a sharp $90^{\circ}$ edge. Detailed single- and multi-point measurements of the velocity field were acquired at the streamwise locations $0\,{<}\,x/\theta_0\,{<}\,100$. This represents the transition region between two of the canonical turbulent shear flows: the zero-pressure-gradient turbulent boundary layer and the single-stream shear layer. From the viewpoint of a separating boundary layer, the results describe how the turbulent flow reacts to a sudden change in wall boundary conditions. From the viewpoint of the developed shear layer, the results describe the transition to the self-similar region. The data acquired suggest that the initial shear layer instability occurs in the region very near separation ($x\,{\approx}\,\theta_0$), and that it involves only the vorticity filaments which originate in the near-wall region of the upstream boundary layer. This ‘near-wall region’ roughly defines the origin of a narrow wedge-shaped domain that was identified from the velocity statistics. This domain is termed the ‘sub-shear layer’. The statistics of the velocity field in the region bounded by the sub-shear layer and the free-stream flow were found to represent the normative continuation of the upstream boundary layer. The sub-shear layer has been found to exhibit many of the standard features observed in fully developed shear layers. For example, velocity measurements on the entrainment side of the shear layer indicate that large-scale motions with spanwise coherence were observed. The streamwise dependence of the dominant frequency, convection velocity, and spanwise velocity correlation have been documented in order to characterize the sub-shear layer phenomenon.