Boundary layer development after a region of three-dimensional separated flow

Abstract

The development of the boundary layer flow down stream of a region of three-dimensional turbulent separated flow has been investigated using hot-wire anemometry and pulsed-wire anemometry for wall shear stress. The flow, generated by means of a ‘v-shaped’ separation line, was set up so as to generate a central region of fully three-dimensional flow, bounded on each side by the degenerate case of spanwise-invariant flow, which is closely related to two-dimensional co-planar flow. The flow developing in the central region has fundamentally different features from that after a region of two-dimensional separated flow. A bulge in the boundary layer, generated in the separated flow, has a strong wake-like characteristic that sits very persistently in the outer part of the layer, its shape and relative size changing little in ∼25 reattachment length scales. The Reynolds stresses in this bulge region, driven by the mean velocity gradient of the wake-like flow, are very much larger than the normal levels in the boundary layer or in the developing region downstream of a two-dimensional separation. The inner layer that lies beneath the bulge is subjected to a high level of non-shear-stress-containing motion, the normal stresses being much higher than the shear stress which exhibits a constant level in the inner layer despite much higher levels further out.