Second sound in counterflowing normal and superfluid Helium II

From ordinary hydrodynamics it is known that an "entrainment" of sound occurs in a moving fluid. A similar effect is expected to take place in the hydrodynamics of Helium II. A measurement has been made of the change in the time of flight of a second sound pulse as a function of the relative velocity (v[lowered n] - v[lowered s]), where v[lowered n] is the normal fluid velocity and v[lowered s] is the superfluid velocity. A temperature range of 2.10°K through 1.00°K was covered. A typical time of flight at 1.75°K for a second sound pulse for stationary conditions was 5.8622 milliseconds. The change in the time of flight for a normal fluid velocity of 0.4 cm/sec at 1.75°K was 0.7 microseconds. Each time of flight for the second sound pulses was measured within [plus-minus] 0.1 microseconds.