Intermittent Switching Between Turbulent and Potential Flow Around a Sphere in He II at mK Temperatures

Abstract

Intermittent switching between potential flow and turbulence is observed with an oscillating sphere in HeII below 0.5 K, where there is no normal fluid component and no viscosity. The remaining dilute phonon gas is in the ballistic regime and therefore turbulence in the pure superfluid can be investigated. The amplitude of the driven oscillations is a measure of the damping which in case of potential flow is due to residual ballistic phonon scattering or, when the flow is turbulent, is due to a large nonlinear turbulent drag. In an intermediate range of driving forces the flow is observed to be unstable, intermittently switching between both patterns. We have investigated this phenomenon down to 25 mK and have made a statistical analysis of the time series measured at various constant driving forces and temperatures. We obtain a temperature independent probability density for switching, different for both directions. We find a regime of metastable laminar flow whose lifetime is limited by natural radioactivity and cosmic rays.