Decay of Classical and Superfluid Turbulence

Evgeny Kozik, Boris V. Svistunov

P.-E. Roche

Tyler W. Neely

Turbulence in classical fluids has been the subject of scientific study for centuries, yet there is still no complete general theory of classical turbulence connecting microscopic physics to macroscopic fluid flows, and this remains one of the open problems in physics. In contrast, the phenomenon of quantum turbulence in superfluids has well-defined theoretical descriptions, based on first principles and microscopic physics, and represents a realm of physics that can connect the classical and quantum worlds. Studies of quantum turbulence may thus be viewed as a path for progress on the long-standing problem of turbulence.A dilute-gas Bose-Einstein condensate (BEC) is, in most cases, a superfluid that supports quantized vortices, the primary structural elements of quantum turbulence. BECs are particularly convenient systems for the study of vortices, as standard techniques allow the microscopic structure and dynamics of the vortices to be investigated. Vortices in BECs can be created and manipulated using a variety of techniques, hence BECs are potentially powerful systems for the microscopic study of quantum turbulence.This dissertation focuses on quantized vortices in BECs, specifically experimental and numerical studies of their formation, dynamics, and decay, in an effort to understand the microscopic nature of vortices as elements of quantum turbulence. Four main experiments were performed, and are described in the main chapters of this dissertation, after introductions to vortices, experimental methods, and turbulence are presented. These experiments were aimed at understanding various aspects of how vortices are created and behave in a superfluid system. They involved vortex dipole nucleation in the breakdown of superfluidity, persistent current generation from a turbulent state in the presence of energy dissipation, decay of angular momentum of a BEC due to trapping potential impurities, and exploration of the spontaneous formation of vortices during the BEC phase transition. These experiments represent progress towards enhanced understanding of the formation, dynamics, and decay of vortices in BECs and thus may be foundational to more general studies of quantum turbulence in superfluids.

G. W. Stagg, Nicholas G. Parker, C. F. Barenghi

Using the classical field method, we study numerically the characteristics and decay of the turbulent tangle of superfluid vortices which is created in the evolution of a Bose gas from highly nonequilibrium initial conditions. By analyzing the vortex line density, the energy spectrum, and the velocity correlation function, we determine that the turbulence resulting from this effective thermal quench lacks the coherent structures and the Kolmogorov scaling; these properties are typical of both ordinary classical fluids and of superfluid helium when driven by grids or propellers. Instead, thermal quench turbulence has properties akin to a random flow, more similar to another turbulent regime called ultraquantum turbulence, which has been observed in superfluid helium.

Carlo F. Barenghi, Ladislav Skrbek, Katepalli R. Sreenivasan

Demetris Charalambous, P. C. Hendry, P. V. E. McClintock +1 more

The theory of how turbulent energy decays via a Richardson cascade is well‐established for classical fluids, and it also seems to apply to the case of so‐called co‐flowing He II turbulence in the range from the superfluid transition temperature Tλ down to ∼1 K, where its behaviour is similar to that of a classical fluid. For pure superfluids, e.g. He II in the mK range or 3He‐B in the μK range, where the normal fluid density ρn is near zero, the mode(s) through which quantum turbulence (QT) might decay have been much less clear because of the absence of viscosity to dissipate the turbulent energy on small length scales. Recent advances made in the theory of QT in this T → 0 limit are consistent with such experimental evidence as is available, but new experiments supported by new techniques for the production and detection of QT are urgently required. The experimental situation is reviewed and prospects for further advances are considered.