Time -resolved visualization and analysis of compressible blunt -base cylinder wakes

The near wake of blunt-base cylinders at 0° and 10° angle-of-attack to a Mach 2.46 freestream flow is studied using Mie scattering flow visualization. These experiments are performed to describe the overall behavior of these wakes, and to determine the mechanisms by which the structure of the wake changes with angle-of-attack. Single-frame imaging of the angle-of-attack wake shows that the coherent structures are larger and more obvious in side-view images of the windward and lateral parts of the shear layer than in the leeward part. End-view images show that a septum originates from the leeward part of the shear layer and extends to the windward part of the shear layer during reattachment, dividing the wake into two lobes after reattachment and causing a short reattachment length.;The recompression shock system in both blunt-base cylinder wakes is also visualized, allowing the instantaneous position of the shock to be determined over a wide region of the flow. The recompression shock at alpha=0° is highly branched and appears to be quite weak. The shock appears to be stronger at alpha=10°, with far less branching. The unsteadiness in the recompression shock position is much higher for the axisymmetric wake than the angle-of-attack wake.;High-speed flow visualization of these wakes is also performed. Both conventional cross-correlation analysis and cross-correlation analysis conditioned on the coherent structures are performed to measure the convection velocity of the coherent structures. The coherent structures in the developing shear layers in side-view images of both wakes convect significantly faster than the isentropic convection velocity, and the structure convection velocity is highly sensitive to changes in the freestream flow velocity. The convection velocity of the structures is also highly dependent on the transverse position of the structures in the shear layer. Convection velocity measurements in the end-view images confirm that there is a significant circumferential component of structure convection velocity in the angle-of-attack wake. The pulsation and flapping of the wake core in end-view images contain a short time scale component due to the passage of individual structures and a long time scale component due to more global mechanisms.