On the receptivity of separated and attached shear layers to free-stream turbulence

The transition to turbulence has a profound impact on the performance of large-scale thermofluidic systems. The ability to control its onset, particularly by passive means, is therefore highly desirable, and this in turn requires a fundamental understanding of its underlying physical mechanisms. Now although computational techniques have evolved to the point where many transition scenarios can be reproduced and studied numerically, experiments continue to provide invaluable support and insight—especially in the form of quantitative visualisations from methods such as particle image velocimetry (PIV). In this work, the PIV technique is used to provide detailed perspective on the receptivity and development of unsteady disturbances in both attached and separated shear layers. The experiments are carried out in a subsonic wind tunnel facility over a flat plate, typically under conditions of elevated freestream turbulence (FST). A flap at the trailing edge of the plate is used to control whether or not the flow separates. In the attached scenario under elevated FST, PIV flow fields reveal the presence of long streamwise streaks in the laminar boundary layer. High-amplitude, lifted negative streaks—known precursors to breakdown—are shown to dominate increasingly along the mean boundary-layer edge as transition is approached. Correlation maps of the wall-normal fluctuation velocity possess a distinct ‘comma’ shape in this region, indicating the diffusion of disturbances from the free-stream to the boundary layer under shear. Some new visual data on turbulent breakdown events are also provided. In a second experiment, an array of vortex generators (VGs) is used to modulate the boundary layer into alternating streamwise streaks in the spanwise direction. Whilst nominally stable, these particular devices are shown to produce a growing unsteadiness in the downstream direction. The observed fluctuations seem to result from a meandering motion of the generated streaks and suggest another, possibly new, form of spanwise shearing instability. In the separated transition scenario, PIV reveals that the free shear layer rolls up into vortices via Kelvin-Helmholtz instability. Under elevated FST, the laminar part of the shear layer is also pervaded by slender, highly coherent streamwise fluctuations that resemble the boundary-layer streaks usually observed in the attached case. Ultimately, the separated shear layer (under both low and elevated FST) breaks down to turbulence and reattaches to the wall to form a bubble of reverse flow. Instantaneous flow fields from the low-FST case show that the bubble often breaks up into two or more parts, thereby obscuring the reattachment process. It is therefore suggested that the downstream end of the ‘main’ bubble (the one anchored to the leading edge) be used to determine the true instantaneous reattachment point. A histogram of this point is found to be bimodal: the upstream peak relates to roll-up of the shear layer, whereas the downstream peak may suggest a ‘flapping’ motion.