Recent research highlights

      

Quantum turbulence without energy cascade

  • Experiments and numerical simulations of turbulent superfluid helium have established that, at hydrodynamic length scales larger than the average distance between quantum vortices, the energy spectrum obeys the same 5/3 Kolmogorov law which is observed in the homogeneous isotropic turbulence of ordinary fluids. The importance of the 5/3 law is that it points to the existence of a Richardson cascade which transfers kinetic energy from large eddies to small eddies. However, there is also experimental evidence of quantum turbulent regimes without Kolmogorov scaling. Why does the Kolmogorov spectrum fail to form in such regimes? What is the physical nature of turbulence without energy cascade ?
  • In this work we describe simple physical mechanisms which prevent the formation of Kolmogorov scaling in superfluid helium at both high and low temperatures. Thee first regime, at high temperatures, is turbulence generated by a heat flux (thermal counterflow). The second regime, at low temperatures, is turbulence generated by injecting vortex rings.

    • CF Barenghi, YA Sergeev, and AW Baggaley, Regimes of turbulence without an energy cascade' Nature Scientific Reports 6 35701 (2016) arXiv article

    		      

    Computed superfluid energy spectrum E(k) vs wavenumber k for thermal conterflow. Note that energy is concentrated at intermediate k (not at small k as in classical turbulence) and, at large k, differs from the classical k^(-5/3) Kolmogorov scaling (red dashed line).

          

    Quantum turbulence in trapped atomic Bose Einstein condensates

  • Turbulence, the complicated fluid behavior of nonlinear and statistical nature, arises in many physical systems across various disciplines, from tiny laboratory scales to geophysical and astrophysical ones. The notion of turbulence in the quantum world was conceived long ago by Onsager and Feynman, but the occurrence of turbulence in ultracold gases has been studied in the laboratory only very recently. Albeit new as a field, it already offers new paths and perspectives on the problem of turbulence.
  • Here we review the general properties of quantum gases at ultralow temperatures paying particular attention to vortices, their dynamics and turbulent behavior. We review the recent advances both from theory and experiment. We highlight the difficulties of identifying and characterizing turbulence in gaseous Bose-Einstein condensates compared to ordinary turbulence and turbulence in superfluid liquid helium and spotlight future directions of research.

    • MC Tsatsos, PES Tavares, A Cidrim, AR Fritsch, M Caracanhas, FE dos Santos, CF Barenghi, and VS Bagnato, Quantum turbulence in trapped atomic Bose-Einstein condensates' Physics Reports 622 1-52 (2016) arXiv article

    		      

    Top: experimental absorption images of condensates containing 1, 2, 3 and more quantum regular vortices. Bottom: images of condensates containing turbulent vortices.

          

    Visualizing pure quantum turbulence in superfluid He3

  • Superfluid He3-B at very low temperatures gives insight into quantum turbulence in its most pure and fundamental form - a tangle of vortex filaments in a fluid without viscosity which move under the influence of each vortex on all the others (Euler dynamics) and also reconenct with each other. At these extreme low tempertures (175 microKelvin) flow visualization is not trivial: it is based on Andreev scattering of ballistic thermal excitations, a peculiar scattering in which a quasiparticle is bounced off the velocity field of a quantum vortex, becoming a quasiholes and (unlike classical reflections) going back along the same direction it arrived (the same scattering affects incoming quasiholes).
  • In this work we show the relation between the vortex line density (which characterizes the intensity of the turbulence) and the Andreev reflectance of the vortex tangle, in the first simulations of Andreev reflections by a realistic turbulent tangle of vortices. We compare the results with experiments, probing for the first time the turbulence on length scales smaller than the vortex separation.

    • AW Baggaley, V Tsepelin, CF Barenghi, SN Fisher, GR Pickett, YA Sergeev, and N Suramlishvili, Visualizing Pure Quantum Turbulence in Superfluid 3He: Andreev Reflection and its Spectral Properties Phys. Rev. Lett. 115, 015302 (2015). arXiv article

    		      

    2D representation of the reflection coefficient of thermal excitations incident on a turbulent tangle from one side of the cell. The vortex lines are shown in yellow, the regions of high/low Andreev reflectivity are in dark/light/blue.