Shedding of Vortex Rings from an Oscillating Sphere in Superfluid 4He Below 0.5 K

The onset of turbulent flow around an oscillating sphere is known to occur at a critical velocity where κ is the circulation quantum and ω is the oscillation frequency. However, in a small interval of driving force amplitudes F (or corresponding velocity amplitudes of few percent above v c ) the turbulent flow is found to be unstable. The flow pattern switches intermittently between potential flow and turbulence. The lifetimes of the turbulent phases have an exponential distribution and the mean lifetimes τ grow very rapidly with increasing driving force, namely as τ ( F )∼exp[( F / F 1 ) 2 ]. In this work this experimental result is analyzed in more detail than before, in particular the force F 1 is identified. As a result, the turbulent drag force can be ascribed quantitatively to the shedding of vortex rings having the size of the sphere.