Transition to a time-dependent state of fluid flow in the wake of a sphere.

In this paper, the results of laboratory investigation about the flow behind the sphere in the range of 150<Re<300 , where very interesting physical phenomenon occurs, are presented. After a first stationary transition where the flow breaks the axisymmetry, very controlled experiments allow one to define a threshold for the second transition from stationary to unstable flow, which includes three-dimensional peristaltic oscillations of the two trailing vortices prior to hairpins shedding. The scenario has been proposed to explain the hairpin formation, as pieces of a counter-rotating longitudinal vortex. Hairpin shedding is suggested to be the result of oscillations, which are powerful enough, and reconnection of the trailing vortices.