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[Theta][approximate]5000 at a free-stream velocity of Uref=25 m/s. The results indicate that the flow is characterized by two separate time-dependent phenomena: a low-frequency mode, with a Strouhal number St1[approximate]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[approximate]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. Presented as Paper 2015-1289 at the 53rd AIAA Aerospace Sciences Meeting, Kissemmee, FL, 5-9 January 2015
Unsteady Behavior of a Pressure-Induced Turbulent Separation Bubble
AIAA journal ; 53 , 9
2015
Article (Journal)
English