• base drag
• base pressure
• bluff body
• hairpin vortices
• near-wake flow
• wavelet spectra

A wind-tunnel investigation is carried out in order to study the influence of the thickness of the boundary layer developing over the lateral surface of an axisymmetric blunt-body upon its base pressure and near-wake flow features.
Three different boundary layer thicknesses have been obtained by wrapping strips of emery cloth on the smooth model at a Reynolds number Re=550000, at which the boundary layer over the lateral surface becomes turbulent before reaching the base contour. Pressure measurements show that the suctions on the base - and thus the pressure drag of the model - are reduced by increasing the boundary layer thickness. This phenomenon can be connected with a reduction of the curvature of the streamlines at the separation point and with a corresponding increase of the length of the mean recirculation region behind the body.

The results of the frequency analysis on the pressures acting on the model (obtained from pressure taps) and on the velocity fluctuations (measured through hot-wire anemometry) show a dominating peak, whose frequency decreases by increasing the boundary layer thickness. In non-dimensional terms, a Strouhal number based on the wake width and the velocity defect at a suitable reference cross-section is found to remain almost constant for the different cases. Moreover, a second dominating frequency, connected with the fluctuations of the mean recirculation region, is found in the spectra of the pressure signals around the base centre and in the ones of the velocity signals in the near-wake centreline. Also the Strouhal number associated with this frequency, approximately equal to one third of the previous one, decreases by increasing the boundary layer thickness.
In order to better understand the dynamics of the fluid structures present in the wake of the body, statistical and time-frequency analyses (using the wavelet and Hilbert transforms) are carried out: the detected peak is related to the presence of coherent vortical structures (hairpin vortices) downstream of the main recirculation region behind the body. The hairpin vortices keep a planar symmetry, due to the presence of the faired strut that supports the model.