Visualisation of superfluid turbulence

Григорий Ефимович Воловик

Sergey K. Nemirovskii, Vladimir V. Lebedev

J. J. Hosio, V. B. Eltsov, R. de Graaf +2 more

Makoto Tsubota

This article reviews recent developments in quantum fluid dynamics and quantum turbulence (QT) for superfluid helium and atomic Bose–Einstein condensates. Quantum turbulence was discovered in superfluid 4He in the 1950s, but the field moved in a new direction starting around the mid-1990s. Quantum turbulence is comprised of quantised vortices that are definite topological defects arising from the order parameter appearing in Bose–Einstein condensation. Hence, QT is expected to yield a simpler model of turbulence than does conventional turbulence. A general introduction to this issue and a brief review of the basic concepts are followed by a description of vortex lattice formation in a rotating atomic Bose–Einstein condensate, typical of quantum fluid dynamics. Then we discuss recent developments in QT of superfluid helium such as the energy spectra and dissipative mechanisms at low temperatures, QT created by vibrating structures, and the visualisation of QT. As an application of these ideas, we end with a discussion of QT in atomic Bose–Einstein condensates.

Andrew Woods

Quantum turbulence is a conceptually simple form of turbulence, consisting of a tangle of quantised vortex lines. It provides a model system, through which it may be possible to understand features of the complex and not yet fully understood classical turbulence. A novel detector made from arrays of custom-designed tuning forks was developed and used to investigate properties of excitation beams and quantum turbulence in superfluid 3He-B at temperatures below 200 µK. The detector was constructed from 5 arrays of 5 tuning forks mounted in a copper block to create a 25 pixel square detector of excitation flux. The detector was situated in a cell such that it could be illuminated with a beam of thermal excitations, and that turbulence could be generated in the path of the beam, which will cast a shadow on the face. Characterisation of the detector response to beams generated by the black-body radiator and source wire were performed. We observe that the beam generated by a black-body radiator appears approximately symmetric, consistent with being emitted from a point-source of excitations. In addition to this we find that the profile of the beam generated by the black-body radiator was independent of the power applied to it, an important assumption of the properties of the black-body radiator that had not been previously observed. The beam emitted from a vibrating wire was found to be much narrower and has a angular profile that changes as the velocity of the wire is increased. Probing a turbulent tangle generated by a vibrating wire with this beam showed that the turbulence appears to fill all of the volume in between the radiator and detector. The vortex line density of the tangle appeared the be greatest in the vicinity of the wire, consistent with previous measurements of the vortex tangle generated by vibrating wires. In addition, we find that there are reproducible features in the development of the shadow as a function of the wire velocity, indicating that there is some structure in the development. The shadow is independent of the power of the beam used to probe the tangle. Measurements of fluctuations in the shadow cast by the vortex tangle show that the turbulence has a spectrum reminiscent of the Kolmogorov spectrum. In addition to this the resolution of the detector is such that it is possible in principle to measure the shadow cast by a single vortex line, and we examine candidate events for such a measurement. This represents the first such measurement of the motion of vortex lines in 3He-B.

Masahide Murakami