Abstract
Wall static-pressure and longitudinal-velocity fluctuations are measured in a pressure-induced turbulent separation bubble generated on a flat test surface by a combination of adverse and favorable pressure gradients. The Reynolds number, based on momentum thickness upstream of separation, is at a free-stream velocity of . The results indicate that the flow is characterized by two separate time-dependent phenomena: a low-frequency mode, with a Strouhal number , which is related to a global “breathing” motion (i.e., contraction/expansion) of the separation bubble, and a higher-frequency mode, with a Strouhal number , which is linked to the roll-up of vortical structures in the shear layer above the recirculating region and their shedding downstream of the bubble. These two phenomena are reminiscent of the “flapping” and “shedding” modes observed in fixed-separation experiments, though their normalized frequencies are different. The breathing mode is also shown to be strikingly similar to the low-frequency unsteadiness observed in shock-induced separated flows at supersonic speeds.
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