PROPAGATING AND STATIONARY SUPERFLUID TURBULENT FRONTS

We have observed that the critical heat current for the transition to superfluid turbulence in weakly nonuniform circular channels depends s trongly on the flow direction, This observation is particularly surpri sing since no other property of the turbulence appears to have such a dependence. In a nonuniform channel the critical heat current is assoc iated with a stationary front between the laminar and turbulent flow W e propose a new model for superfluid turbulent fronts which explains t he asymmetry of the critical heat currents in a simple way. The model is based on the subcritical nature of the transition, and the generic description of such a bifurcation by the Ginzburg-Landau equation. As a bonus, the model also explains a long-standing problem in superfluid physics - the nature of propagating fronts in uniform channels. The r esults of our analysis of both the uniform and nonuniform channel data also provide new information about the vortex line drift velocity.