Spatio-temporal reconstruction of vortex dynamics in axisymmetric wakes

With time recording Digital-Particle-Image Velocimetry and spatio-temporal reconstruction technique, we obtained detailed quantitative results of the evolution of the velocity and vorticity field in the wake of axisymmetric bluff bodies-a sphere and an axially oriented cylinder with an elliptic nose and a blunt base. Experiments were carried out for Reynolds numbers of Re = 500, 700 and 1000 in the transition range from '"regular" to "'irregular" shedding. DPIV-recordings in radial cross-sections at several distances downstream of the bodies allowed us to reconstruct the dynamics of the streamwise vorticity over a large number of shedding cycles. Our results prove that the wake in this regime consists of a double-sided chain of oppositely oriented hairpin vortices. In addition, the results show a well-defined low-frequency modulation of the vortex shedding, with a distinct peak in the frequency spectrum at a Strouhal number of about Sr approximate to 0.05 (in case of the sphere). The wake pattern in this "irregular" shedding regime typically exhibits periodic packets of 3-4 "regular" shedding cycles which are interrupted by phases with less action. The results indicate the coexistence of a long-wave instability of axisymmetric wakes against helical waves in addition to the primary instability causing the vortex shedding process. (C) 2001 Academic Press.