Vortex formation in the wake of a flat plate for subsonic and supersonic freestream mach numbers

The interaction of a straight oblique shock with a supersonic turbulent wake of a flat plate was studied experimentally by means of Laser-Doppler-Anemometry and Mach-Zehnder interferometry. Investigated were the changes of the time-averaged velocity, the Reynolds stresses, the turbulence intensity across the shock and instantaneous density profiles. After completion of the experiments with a smooth plate, the investigation was extended to flows about flat plates with rough surfaces. The roughening of the surfaces was achieved by pasting sandpaper on to both sides of the plate. It was found that a detectable vortex street was formed in the wake in subsonic and supersonic flow, if the following three conditions are met: First, the relative roughness had to exceed a certain critical value, second, the trailing edge had to be sharp, i.e. no rounded corners, and third, the sandpaper should not extend to the trailing edge, but should end at a certain distance upstream from it. In a second series of experiments the shedding frequency was measured with Laser-Schlieren optics and the density drop in the vortex cores was evaluated with holographic interferometry in two wind tunnels. In addition, the time averaged velocity profiles were measured as before for undisturbed and shock-disturbed flow. The comparison of the results is discussed in this paper. Finally, an attempt was made to explain the formation of the vortex street in terms of absolute and convective instability. In order to do so, the Raleigh-equation was solved on the basis of time-averaged velocity profiles, measured in the near wake. First results indicate that the predicted Strouhal number is in rather close agreement with the one observed in the corresponding experiment