PIV measurements of induced separation at the leading edge of a plate

Transitional separated flow at the leading edge of a flat plate has been studied with PIV for the cases of background and elevated free-stream turbulence (FST). The separation was generated by means of a hinged flap at the trailing edge of the plate. Detailed measurements around the resultant separation bubble are reported, including spatial distributions of mean velocity, turbulence intensity, quadrant-decomposed Reynolds stress and some instantaneous characteristics. We demonstrate that in the transitional regime, FST serves to shorten the time-mean bubble via accelerated breakdown of the separated shear layer that is analogous, in some respects, to attached bypass transition. Indeed, both positive and negative streaks have been observed in the pre-transitional shear layer under FST. Subsequent breakdown of the shear layer involves the roll-up of vortices. Isolated pockets of reverse flow are often found to pervade the separation zone on the instantaneous flow fields. These tend to obscure the reattachment process. We therefore suggest that the isoline for zero absolute velocity that extends unbroken from the leading edge be used to define the instantaneous reattachment point and therefore the bub- ble proper. In the case of background FST, the streamwise distribution of this point is bimodal: the upstream mode relates directly to transition of the shear layer, whereas the downstream mode suggests a 'flapping' behaviour.