Coherent structure eduction and convection velocity in compressible blunt-body wakes

An iterative eduction procedure has been developed to study coherent structures in blunt-base cylinder-wakes at 0 and 10 deg angles of attack to an M = 2.46 freestream flow. The eduction procedure uses an initial wavelet pattern to find candidate cohered structures of a particular scale and then uses the average intensity pattern of the educed structures to create an updated pattern. This iterative procedure continues until the average structure converges, indicating that a self-consistent intensity pattern has been described. Using this eduction procedure, the average appearance of the structures is described. Moreover, a convection velocity measurement is made by directly tracking individual coherent structures, unlike conventional imaging-based convection velocity measurements, which are not conditioned on coherent structures. The evolutionary trends of the structures are also quantified. The coherent structure averages show many of the same trends found in more conventional autocorrelation analysis. For example, the structures are inclined toward the freestream flow direction, and the angle of this inclination is affected by the recompression and reattachment processes. Convection velocity measurements show that the convection velocity is dependent on the transverse position of the structure in the shear layer, particularly for small structures. Finally, the coherent structures evolve in such a way that they become poorly representative of the average coherent structure pattern within 5-15 mu s after being first identified.