External flows in the high-speed (compressible) regime with regions of flow separation often present rich fluid-dynamical features. An aspect of such flows that is particularly interesting is temporal unsteadiness, which is typically driven by interaction of shock waves with flow regions of high shear and/or separation. In certain cases, the unsteadiness is characterized by periodic shock wave motion with large spatial amplitudes, which gives the flows a visually spect acular character!

This talk will present experimental studies at Mach 6 of two canonical flows that are characterized by shock-wave/separation-region interactions – one is flow over a double cone and the other is flow in the wake of a 2D circular cylinder. The double-cone flow exhibits two distinct states of unsteadiness, with large- and small-amplitude shock-wave oscillations. Physical mechanisms will be deduced from experimental results to explain the oscillations and the transition between flow states. Coherent flow oscillations are also found in the high-speed cylinder wake, with some broad similarities to small-amplitude shock-wave oscillations in the double-cone flow. The Strouhal number (non-dimensional frequency) of the cylinder wake shows universal behavior, and further, oscillatory activity in the two statistically identical halves of the flow is anti-symmetric.

The relationship observed between the local flow properties, instability of the shear layer, and geometric constraints on the flow suggests that an aeroacoustic feedback mechanism sustains coherent oscillations in both the canonical flows. Based on this physical insight, a simple model with no empiricism is developed for the Strouhal number. The model predictions are found to match well with experimental measurements for both the flows. The model provides helpful physical insight into the nature of flow oscillations in a class of flows dominated by shock-wave/separation-region interactions.