Axisymmetric bodies with fixed and free separation: Base-pressure and near-wake fluctuations

The fluctuations of the base pressure and of the near-wake velocity are experimentally characterized for a constant-diameter axisymmetric blunt-based body with different boundary-layer thicknesses and for boat-tailed bodies with various extents of flow separation. The tests are carried out at Re=u∞D/ν=9.6×104. The analysis is performed through time-frequency techniques based on the wavelet and Hilbert transforms. Two frequencies are detected: a higher frequency, which corresponds to the shedding of hairpin vortices, and a lower one, associated with the pulsation of the recirculation region. The shedding occurs with a preferential vertical symmetry plane due to the presence of the support and the corresponding frequency becomes dominant downstream of the recirculation region and over the base contour. The lower frequency is present in the pressure signals from the central region of the base and in the adjacent velocity field. The two dominant frequencies decrease with increasing boundary-layer thickness and significantly increase with decreasing boat-tail diameter ratio. Despite the differences in geometry and flow conditions, the lower frequency is always found equal to 1/3 of the higher one. Furthermore, a Strouhal number based on the wake width and the velocity defect at a suitable cross section downstream of the recirculation region remains practically constant.