Abdelouahab Mohammed-Taifour

A new wind tunnel for the study of pressure-induced separating and reattaching flows

The design, construction, and validation of a new academic wind tunnel is described in detail. The wind tunnel is of a classical, blow-down type and generates a pressure-induced, turbulent separation bubble on a flat test surface by a combination of adverse and favorable pressure gradients. The Reynolds number, based on momentum thickness just upstream of separation, is Re θ ≃ 5,000 at a free-stream velocity of U ref = 25ms −1 . The length of the separation bubble is estimated at 0°42 ± 0°02m by three different methods. Results of a numerical simulation demonstrate the absence of flow separation in the wind-tunnel contraction. This results in a turbulence level of about 0·05% in the test section. Oil-film visualisation experiments show that the flow near the wall is strongly three-dimensional in the recirculating region and that the topology of the limiting streamlines is consistent with experiments performed on configurations with fixed separation. Finally, spatial variations of the forward-flow fraction have been documented using a thermal-tuft probe and are shown to compare well with the results of the oil-film visualisation.

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 Reθ≃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≃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≃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 str...