Alfredsson, “ Measurements with a flow direction boundary-layer probe in a two-dimensional laminar separation bubble,” Exp. Watmuff, “ Evolution of a wave packet into vortex loops in a laminar separation bubble,” J. Pauley, “ Low-Reynolds-number separation on an airfoil,” AIAA J. Michalke, “ Laminar boundary layer separation: instability and associated phenomena,” Prog. Tang, “ Aerodynamic performance and flow structure studies of a low Reynolds number airfoil,” AIAA J. Mueller, “ Boundary-layer measurements on an airfoil at low Reynolds numbers,” J. Tani, “ Low speed flows involving bubble separations,” Prog. Carmichael, “ Low Reynolds number airfoil survey,” NASA Contractor Report No. Lissaman, “ Low-Reynolds-number airfoils,” Annu. It is shown that the boundary layer behavior has a profound effect on the identified coherent structures, and each of the two flow regimes is associated with distinctly different vortex shedding characteristics. Vortex shedding also occurs in the airfoil wake and vortices form in the near-wake region. The development of the roll-up vortices leads to boundary layer transition, and the vortices break down during the transition process. Linear stability theory can be employed to adequately describe the salient characteristics of such vortices and the initial stage of the separated shear layer transition. The formation of the roll-up vortices in the separated shear layer is linked to inviscid spatial growth of disturbances and is attributed to the Kelvin-Helmholtz instability. The results suggest that coherent structures form in the separated flow region and the wake of the airfoil for both flow regimes. Two flow regimes are investigated (i) boundary layer separation without reattachment and (ii) separation bubble formation. Laminar boundary layer separation occurs on the upper surface of the airfoil for all Reynolds numbers and angles of attack examined. Wind tunnel experiments were carried out for a range of Reynolds numbers and three angles of attack. ![]() Boundary layer and turbulent wake development for a NACA 0025 airfoil at low Reynolds numbers was studied experimentally.
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