Vortex shedding from a microsphere oscillating in superfluid ^4He at mK temperatures and from a laser beam moving in a Bose-Einstein condensate

Abstract

Turbulent drag of an oscillating microsphere, that is levitating in superfluid $^4$He at mK temperatures, is unstable slightly above a critical velocity amplitude $v_c$. The lifetime $\tau$ of the turbulent state is determined by the number $n$ of vortices shed per half-period. It is found that this number is identical to the superfluid Reynolds number. The possibility of moving a levitating sphere through superfluid $^3$He at microkelvin temperatures is considered. A laser beam moving through a Bose-Einstein condensate (BEC) (as observed by other authors) also produces vortices in the BEC. In particular, in either case a linear dependence of the shedding frequency $f_v$ on $\Delta v = v - v_c$ is observed, where $v$ is the velocity amplitude of the sphere or the constant velocity of the laser beam above $v_c$ for the onset of turbulent flow: $f_v = a \Delta v$, where the coefficient $a$ is proportional to the oscillation frequency $ \omega $ above some characteristic frequency $\omega_k$ and assumes a finite value for steady motion $\omega \rightarrow 0$. A relation between the superfluid Reynolds number and the superfluid Strouhal number is presented that is different from classical turbulence.