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 strongly on the flow direction. This observation is particularly surprising since no other property of the turbulence appears to have such a dependence. In a nonuniform channel the critical heat current is associated with a stationary front between the laminar and turbulent flow. We propose a new model for superfluid turbulent fronts which explains the 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 super fluid physics-the nature of propagating fronts in uniform channels. The results of our analysis of both the uniform and nonuniform channel data also provide new information about the vortex line drift velocity.