Quantum turbulence in superfluids

5 problems

• Steady inverse energy cascade in forced two-dimensional quantum fluids

  Partially resolved

  Flat superfluids made of ultracold atoms have been seen to gather same-direction vortices into giant clusters, as predicted by Lars Onsager, a theoretical chemist and physicist. Simulations show energy flowing steadily to large scales under stirring, but no experiment has yet shown such a lasting flow of energy.

• Energy transfer across the classical-quantum crossover scale

  Contested

  In superfluid turbulence, energy moves first through swirls made of many vortex lines and then along single lines. At the handover scale energy may pile up, and theories disagree on how.

• Is inertial-range intermittency of superfluid turbulence the same as classical?

  Contested

  Above about one kelvin, superfluid helium is a mixture of a superfluid and a normal fluid, and its turbulence shows the same average spectrum as water. Whether its rare violent fluctuations are also the same at every temperature is disputed.

• Energy spectrum of the Kelvin-wave cascade on quantized vortices

  Contested

  When superfluid vortex lines wiggle, the wiggles (Kelvin waves) are expected to pass energy to ever shorter wavelengths until it is radiated as sound. Two competing theories predict different spectra, and no measurement in a superfluid has decided between them.

• Effective viscosity of superfluid turbulence at zero temperature

  Open

  Even at absolute zero, where superfluid helium has no viscosity, a vortex tangle decays as if the fluid had an effective viscosity. Its size, and why it differs between kinds of tangles, has not been calculated.