Vortex Shedding from a Microsphere Oscillating in Superfluid ⁴He at mK Temperatures and from a Laser Beam Moving in a Bose–Einstein Condensate

Turbulent drag of an oscillating microsphere that is levitating in superfluid ⁴He at mK temperatures, is unstable slightly above a critical velocity amplitude vc. The lifetime τ 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 ³He 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 fv on Δv=v−vc is observed, where v is the velocity amplitude of the sphere or the constant velocity of the laser beam above vc for the onset of turbulent flow: fv=aΔv, where the coefficient a is proportional to the oscillation frequency ω above some characteristic frequency ωk and assumes a finite value for steady motion ω→0. A relation between the superfluid Reynolds number and the superfluid Strouhal number is presented that is different from classical turbulence