Self-sustained oscillations in the wake of a sphere

The periodic wake of a sphere maintained in a uniform flow is experimentally characterized. Visualizations of the vortical structures periodically shed in this regime are presented and measurements of amplitude and frequency of the streamwise velocity fluctuations are performed. The experiments show that the resulting self-sustained oscillations of the wake flow appear via a supercritical Hopf bifurcation. Then, the relevance of a Landau-type equation to describe the temporal dynamics of the shedding modes near the instability threshold is discussed. For this purpose transient experiments of growth of the shedding modes are performed under supercritical conditions. From the evolutions during these transients of the instantaneous growth rate and frequency, the parameters of the Landau model are estimated. Dependence on the Reynolds number of these parameters is also deduced from the measurements.