Turbulence structure and unsteadiness in a separation-reattachment flow

A wind tunnel experiment has been performed to obtain the unsteady nature of flow in a turbulent separation bubble formed at the leading edge of a blunt plate with right-angled corners. Reynolds number based on the free-stream velocity and the thickness of the plate was of the order of 10,000. The reverse-flow intermittency Ir was found to be the error function of the time-mean value of the local longitudinal velocity divided by its rms value in most of the separation bubble. This was because the longitudinal velocity fluctuation was approximately Gaussian except near the outer edge of the separated shear layer. The frequency at which the local flow changes its direction was a fairly unique function of Ir, attaining a maximum of about twice the central frequency of shedding of large-scale vortices from the reattaching zone. On the other hand, the bursting frequency in the relaxing boundary layer downstream of the separation bubble was equal to the vortex-shedding frequency. These features suggested a mechanism by which the heat-transfer rate attains a maximum near the time-mean reattachment line.