Critical Velocity in Two-Fluid Flow of He II

We report the first measurements of the transition to superfluid turbulence in flows where the normal and superfluid velocities can be varied independently. The data reveal the existence of a critical line in the ($V_n$, $V_s$) plane. A simple model of a temperature-independent critical line gives reasonable agreement with the critical velocities measured in pure superflow, in two-fluid flow, and in thermal counterflow. Our results support the theoretical view that the critical velocity is dominated by boundary dynamics.