Large-structure topology in a three-dimensional supersonic base flow

The near wake of a blunt-base cylinder at 10-deg angle of attack to a Mach 2.46 freestream flow is studied using Mie scattering flow visualization. Large-scale structures are clearly evident in side-view images of the windward and lateral parts of the shear layer but not in the leeward shear layer. Autocorrelation analysis shows that the average structures grow substantially in side views as the shear layer develops on the lateral plane and the windward shear layer, but not the leeward shear layer. 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 structures in end-view images are largest in the windward part of the shear layer and increase in size as the shear layer develops. The structures are inclined toward the leeward direction in the lateral plane, indicating circumferential flow from windward to leeward in the main shear layer. The appearance of the images suggests that this circumferential flow causes mass buildup on the leeward side of the wake, prompting the formation of the septum.