Measurement of superfluid critical velocities in wide channels utilizing phase coupling



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The superfluid component of He II flows without viscosity or dissipation below a critical velocity. Previously measured critical velocity values for flow in wide channels (>10[raised -3] cm) fall into two conflicting ranges. Values in the higher range vary with the characteristic channel size (d) as d[raised -(1/4)] and show no temperature dependence; while the lower reported values vary approximately as d[raised -1] with reported temperature dependence. It has been suggested that these lower values are the result of detecting normal fluid turbulence.. The higher reported values have no firm theoretical explanation. However, the lower-valued critical velocities are justifiable by the Feynman vortex model. Quantum mechanical phase coherence in He II has been previously observed, which demonstrates that a coupling between elements of the superfluid places restrictions on the flow profile. Here, this phase coupling is utilized to control the superfluid flow in parallel channels to indicate the first signs of flow impedance. A sensitive pressure measuring device is located in a flow channel to monitor the velocity and detect the superfluid critical velocity. Critical velocity data for isothermal as well as heat current flow are obtained. The results substantiate the concept of phase coupling and indicate that a superfluid critical velocity exists as a result of vortex generation and is not dependent upon normal fluid flow in the temperature range 1.71 to 2.15 °K. In this temperature range little or no temperature dependence is found. At velocities well above critical, flow transitions are observed which indicate the onset of superfluid turbulence. These turbulent critical velocities have values in the range corresponding to previous data with the d[raised (1/4)] dependence.