Flow visualization tests have been performed to examine the structure of the near-wall flow over a low-aspect ratio straight wing installed at various angles of attack IX and chord Reynolds number Re,» Ucct» = 1.76 X 10 5 The experiments were carried out at two free-stream turbulence levels, E = 0.1% and E = 1%, the latter one having been achieved using a baffling grid. To visualize the flow, termochromic cholesteric liquid crystals and digital processing of video images were used. At the low turbulence level and IX = 27", a flow stall on the lee side of the wing was observed, with a pair oflargescale vortices rotating in the wing plane. Simultaneously, no vortex structures were observed on the windward wing surface. It was found the flow patterns on either side of the wing significantly changed with increasing free-stream turbulence level. A separation bubble appeared near the leading edge on the lee side of the airfoil at E = 1%, and large-scale stationary longitudinal vortices originated over the wing windward surface. The number and sizes ofthe longitudinal structures were found to be dependent on the angle of attack.

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An effect of sound on the flow with a transitional separation bubble on a wing model at low subsonic freestream velocity was experimentally investigated. Using hot-wire measurements and liquid-crystal visualization, stationary streamwise structures were found to appear in the reattachment region. Quantitative characteristics of the disturbances upcoming during the breakdown to turbulence were measured.At present we observe general increased interest in streamwise structures originating in shear flows because of their substantial role both in the transition to turbulence and in engineering applications. In this paper we consider the origin of stationary streamwise structures in the local region of laminar boundary layer separation on a straight wing model.Some known results concerning the origin of threedimensionality in subsonic separated flows are presented by Watmuff 1 and Häggmark et al. 2 Watmuff 1 conducted his experiments with a separation bubble on a flat plate caused by an induced adverse pressure gradient. The evolution of threedimensional impulsive controlled disturbances being introduced through a hole in the plate was investigated in the originally two-dimensional separation bubble. In this the evolution of the forced wave packet leads at first to the Kelvin-Helmholtz spanwise structure and then to the formation of a three-dimensional vortex structure such as -vortex in the reattachment region. Häggmark et al. 2 conducted their experiments in similar conditions. In contrast to the previous study, these results were obtained for the case of twodimensional controlled disturbances being generated by harmonic suction and blowing through a slot in the plate. Nevertheless, streamwise three-dimensional structures caused by the forced two-dimensional instability wave in the separated boundary layer appeared before the reattachment region.The phenomenon of the origin of streamwise structures in supersonic separated flows has been known for a long time. Data obtained for separation in a corner and on a flat plate are presented by, for instance, Brazhko. 3 The visualization of limiting streamlines shows well-ordered streamwise vortex cells to appear in the separation region and downstream the reattachment line. The author attributes the generation of the streamwise structures to the Görtler instability which may take place in the reattachment region where the streamlines are strongly curved. There is also information about the origin of similar structures at turbulent boundary layer separation in a rectangular semichannel ͑Kornilov 4 ͒.The purpose of this work was to study the streamwise structures induced by acoustic disturbances in a separation bubble and downstream. The differences between this work and the experiments of Watmuff 1 and Häggmark et al. 2 are that our experiments were performed not with a flat plate but with a wing model for a flow with a separation bubble near the leading edge, and acoustic waves were used as controlled disturbances.The experiments with the laminar separation bubble were carri...