  The critical velocity v_c for the onset of turbulence in oscillatory flows of
superfluid helium is known to depend on the oscillation frequency omega, namely
v_c ~ sqrt(kappa*omega) where kappa is the circulation quantum. Only the
numerical prefactor may have some geometry dependence. This universal behaviour
was described earlier qualitatively either by employing the superfluid Reynolds
number or by extending known dc vortex dynamics to ac flow. In our present work
we emphasize that v_c(omega) ~ sqrt(omega) can also be derived rigorously by
means of dynamical scaling of equations of vortex dynamics as pointed out by
Kotsubo and Swift already two decades ago.
