Low Reynolds number two-dimensional separated and reattaching turbulent shear flow

Abstract

Pulsed-wire measurements of the streamwise mean velocity and Reynolds stress, mean and fluctuating surface shear stress, and other statistics have been made in a sharp- edged turbulent separation bubble formed behind a normal flat plate mounted on the front of a long splitter plate, covering nearly a decade range of low Reynolds number. The streamwise Reynolds stress increases appreciably with Reynolds number, from a level comparable with that in an isolated plane mixing layer to more than twice that level, while the change in the mean flow is at most slight. It is inferred using previous measurements that the other stresses do not change as much, thereby leaving the mean flow relatively unaffected. The mean wall shear stress and the r.m.s. of the (streamwise) fluctuations decrease in fixed proportion with increasing Reynolds number. Normalized p.d.f. distributions of velocity and wall shear stress do not change, except in the vicinity of the secondary separation bubble. At low Reynolds number the development length of the overlying mixing layer is an appreciable proportion of the bubble length, the development length being a function of the momentum thickness at separation. It is argued that the observed changes with Reynolds number in the bulk of the flow arise primarily as a result of a change in response of this layer to the fluctuating rates of strain imposed by the recirculating flow; at a low Reynolds number the response of the shear layer structures to the fluctuating rates of strain is less than it is at a high Reynolds number. As a consequence of the sensitivity to fluctuating strain rates the structure of the layer differs fundamentally from that of an isolated mixing layer.The measurements help to reconcile previous measurements in this flow geometry which otherwise appear to be inconsistent. Moreover, in most of the previous measurements the flow width was not sufficient for end effects to have been negligible, most noticeably in the near-wall flow, where such effects are largest.