  The critical velocity vc for the onset of turbulent drag of small spheres
oscillating in superfluid 4He is frequency dependent (omega/2pi from 100 Hz to
700 Hz) and is described by vc = 2.6sqrt(kappa*omega), where kappa is the
circulation quantum. A qualitative analysis based on a recent theory of the
onset of superfluid turbulence gives vc = sqrt(8*kappa*omega/beta), where beta
is of order 1 and depends on the coefficients of mutual friction. This agrees
well with the data and implies that vc is a universal critical velocity that is
independent of geometry, size, and surface properties of the oscillating body.
This is confirmed by comparing our data on spheres with vc obtained with other
oscillating structures by other groups. Numerical simulations indicate somewhat
larger critical velocity, above which a rapid increase in vortex length is
observed.
