Superfluid turbulence in a nonuniform rectangular channel

We therefore find that even a small perturbation imposed on a uniform flow radically changes the nature of the [ital T]-II state, and the inhomogeneous [ital T]-II state we observe is markedly different from that of homogeneous turbulence. This rather surprising result points to the need for a proper theory, or direct numerical simulation, of inhomogeneous turbulence in nonuniform flow. By introducing a substantial modification to the Schwarz line density, we identity a modified line density that constitutes a three-parameter fit yielding excellent agreement with our results. Finally, we find that the transition from laminar to turbulent flow occurs at a stationary front. Observations of the location of this front as a function of [ital ];DQ show that the critical condition for maintaining turbulence is more consistent with a minimum line density than a critical velocity.