Unsteady Behavior of a Pressure-Induced Turbulent Separation Bubble

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 Re similar or equal to 5000 at a free-stream velocity of Uref=25m/s. The results indicate that the flow is characterized by two separate time-dependent phenomena: a low-frequency mode, with a Strouhal number St1 similar or equal to 0.01, 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 St2 similar or equal to 0.35, 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.