Breakdown of Potential Flow to Turbulence around a Sphere Oscillating in Superfluid He-4 above the Critical Velocity
The onset of turbulent flow around an oscillating sphere in superfluid $^4$He is known to occur at a critical velocity $v_c \sim \sqrt{κω}$ where $κ$ is the circulation quantum and $ω$ is the oscillation frequency. But it is also well known that initially in a first up-sweep of the oscillation amplitude, $v_c$ can be considerably exceeded before the transition occurs, thus leading to a strong hysteresis in the velocity sweeps. The velocity amplitude $v_c^* > v_c$ where the transition finally occurs is related to the density $L_0$ of the remanent vortices in the superfluid. Moreover, at temperatures below ca. 0.5 K and in a small interval of velocity amplitudes between $v_c$ and a velocity that is about 2% larger, the flow pattern is found to be unstable, switching intermittently between potential flow and turbulence. From time series recorded at constant temperature and driving force the distribution of the excess velocities $Δv = v_c^* - v_c$ is obtained and from that the failure rate. Below 0.1 K we also can determine the distribution of the lifetimes of the phases of potential flow. Finally, the frequency dependence of these results is discussed.