Hydrodynamics of Superfluid Helium below 0.6 °K.I. Viscosity of the Normal Fluid

The normal-fluid viscosity of superfluid ${\mathrm{He}}^4$ has been calculated for temperatures below about 0.6 °K. The calculation assumes that the dominant scattering mechanism is the three-phonon process in the first order of perturbation theory. This contrasts with the previous calculation of Landau and Khalatnikov, who assumed a second-order process involving four phonons. The theory is compared with the experiments of Whitworth on heat flow through tubes. Good agreement is obtained, as regards both the magnitude of the viscosity and its temperature dependence.