Instability and transition in axisymmetric wakes

Axisymmetric wakes created by bodies of revolution of a varity of bluntness, at a fixed body-diameter Reynolds number of 3600, are studied. The first wake producing body is a streamlined model with a sharp trailing tip of NACA 0006 cross-sectional shape, on which no separation takes place, and the second is a prolate spheroid with an enclosed recirculation region, which nevertheless does not break away. The experiment is performed in a running water channel, the hot-film anemometry being the principal technique. In both cases the basic oscillation at the beginning of the wake is that of 1 Hz component. In the wake of the streamlined body, this component acquires a reasonable helical mode as the stability theory considers, and an ensuing 2 Hz component is amplified quickly and dominates the instability and final breakdown. Behind the blunt prolate spheroid, on the contrary, the 1 Hz component dominates. In both cases, the dominating mode of oscillation, i.e., the 2 Hz component in the former and the 1 Hz in the latter, is identified to be a heavily squashed helical mode, which may be regarded as merely a side-to-side oscillation, and appears to play a decisive role in the breakdown.