Flow structures of a separating, reattaching, and recovering boundary layer for a large range of Reynolds number

Flow structures of a separating, reattaching, and recovering boundary layer over a smoothly contoured ramp are examined for an order of magnitude range of Reynolds number, which is achieved by increasing the wind tunnel pressure by up to 8 atm and varying the tunnel speed by a factor of three. The study was performed using instantaneous velocity vector plots and two-point correlations measured by PIV. The present paper discusses length scales of large-scale motions of the flow and roller vortices in the separated shear layer and their dependence on the Reynolds number or the flow geometry. Two-point correlation profiles show that the Reynolds number effects on the correlations vary strongly from region to region within the flow. Downstream of reattachment, correlation profiles show that the inner portion of the boundary layer recovers more rapidly than the outer portion, where excessively active eddies still persist. The Reynolds number effects seen at reattachment diminish in the recovery region, and the correlation profiles start to take on ordinary boundary layer characteristics.