Abstract:

The two fluid nature of superfluid [sup 4]He can lead to interesting behavior when it is used in a Helmholtz oscillator. If the oscillator's constriction is narrow enough, the normal fluid can be locked by its own viscosity leaving only the superfluid component to flow in and out of the oscillator volume. This leads to an accompanying temperature oscillation that can cause dissipation and/or frequency shifts depending on the thermal properties of the walls of the oscillator. If the constriction is somewhat larger, the normal component is able to move and the thermal effects are partially canceled. These same effects lead to dissipation when steady flow is forced through the constriction. Equations have been derived which include superfluid flow, normal fluid flow, compressibility, thermal expansion, and the properties of the oscillator walls. These have been solved for the resistance to steady flow and the resonant frequency and Q of a Helmholtz oscillation. The calculations will be compared with a wide array of recent measurements. [Work partially supported by the Office of Naval Research and NASA.]