Abstract
The concept of "effective viscosity" v(eff) of superfluid helium, widely used to interpret decaying turbulence, is tested in the steady-state case. We deduce.eff from measurements of the vortex line density, L, in a grid flow.
The scaling of L with velocity confirms the validity of the heuristic relation defining v(eff), epsilon =v(eff) (kappa L)(2), where epsilon is the energy dissipation rate and. the circulation quantum. Within 1.17-2.16K, v(eff) is consistent with that from decays, allowing for uncertainties in flow parameters.
Numerical simulations of the two-fluid equations yield a second estimation of v(eff) within an order of magnitude with all experiments. Its temperature dependence, more pronounced in numerics than experiments, shows a crossover from a viscous-dominated to a mutual-friction-based dissipation as temperature decreases, supporting the idea that the effective viscosity of a quantum turbulent flow is an indicator of the dissipative mechanisms at play.