ترغب بنشر مسار تعليمي؟ اضغط هنا

Inverse Energy Cascade in Forced 2D Quantum Turbulence

147   0   0.0 ( 0 )
 نشر من قبل Thomas Billam
 تاريخ النشر 2012
  مجال البحث فيزياء
والبحث باللغة English




اسأل ChatGPT حول البحث

We demonstrate an inverse energy cascade in a minimal model of forced 2D quantum vortex turbulence. We simulate the Gross-Pitaevskii equation for a moving superfluid subject to forcing by a stationary grid of obstacle potentials, and damping by a stationary thermal cloud. The forcing injects large amounts of vortex energy into the system at the scale of a few healing lengths. A regime of forcing and damping is identified where vortex energy is efficiently transported to large length scales via an inverse energy cascade associated with the growth of clusters of same-circulation vortices, a Kolmogorov scaling law in the kinetic energy spectrum over a substantial inertial range, and spectral condensation of kinetic energy at the scale of the system size. Our results provide clear evidence that the inverse energy cascade phenomenon, previously observed in a diverse range of classical systems, can also occur in quantum fluids.



قيم البحث

اقرأ أيضاً

Despite the prominence of Onsagers point-vortex model as a statistical description of 2D classical turbulence, a first-principles development of the model for a realistic superfluid has remained an open problem. Here we develop a mapping of a system of quantum vortices described by the homogeneous 2D Gross-Pitaevskii equation (GPE) to the point-vortex model, enabling Monte-Carlo sampling of the vortex microcanonical ensemble. We use this approach to survey the full range of vortex states in a 2D superfluid, from the vortex-dipole gas at positive temperature to negative-temperature states exhibiting both macroscopic vortex clustering and kinetic energy condensation, which we term an Onsager-Kraichnan condensate (OKC). Damped GPE simulations reveal that such OKC states can emerge dynamically, via aggregation of small-scale clusters into giant OKC-clusters, as the end states of decaying 2D quantum turbulence in a compressible, finite-temperature superfluid. These statistical equilibrium states should be accessible in atomic Bose-Einstein condensate experiments.
83 - Petri J. Kapyla 2019
(abridged) Context: Turbulent diffusion of large-scale flows and magnetic fields play major roles in many astrophysical systems. Aims: Our goal is to compute turbulent viscosity and magnetic diffusivity, relevant for diffusing large-scale flows and m agnetic fields, respectively, and their ratio, the turbulent magnetic Prandtl number, ${rm Pm}_{rm t}$, for isotropically forced homogeneous turbulence. Methods: We use simulations of forced turbulence in fully periodic cubes composed of isothermal gas with an imposed large-scale sinusoidal shear flow. Turbulent viscosity is computed either from the resulting Reynolds stress or from the decay rate of the large-scale flow. Turbulent magnetic diffusivity is computed using the test-field method. The scale dependence of the coefficients is studied by varying the wavenumber of the imposed sinusoidal shear and test fields. Results: We find that turbulent viscosity and magnetic diffusivity are in general of the same order of magnitude. Furthermore, the turbulent viscosity depends on the fluid Reynolds number (${rm Re}$) and scale separation ratio of turbulence. The scale dependence of the turbulent viscosity is found to be well approximated by a Lorentzian. The results for the turbulent transport coefficients appear to converge at sufficiently high values of ${rm Re}$ and the scale separation ratio. However, a weak decreasing trend is found even at the largest values of ${rm Re}$. The turbulent magnetic Prandtl number converges to a value that is slightly below unity for large ${rm Re}$ whereas for small ${rm Re}$, we find values between 0.5 and 0.6. Conclusions: The turbulent magnetic diffusivity is in general consistently higher than the turbulent viscosity. The actual value of ${rm Pm}_{rm t}$ found from the simulations ($approx0.9ldots0.95$) at large ${rm Re}$ and scale separation ratio is higher than any of the analytic predictions.
Adding energy to a system through transient stirring usually leads to more disorder. In contrast, point-like vortices in a bounded two-dimensional fluid are predicted to reorder above a certain energy, forming persistent vortex clusters. Here we real ize experimentally these vortex clusters in a planar superfluid: a $^{87}$Rb Bose-Einstein condensate confined to an elliptical geometry. We demonstrate that the clusters persist for long times, maintaining the superfluid system in a high energy state far from global equilibrium. Our experiments explore a regime of vortex matter at negative absolute temperatures, and have relevance to the dynamics of topological defects, two-dimensional turbulence, and systems such as helium films, nonlinear optical materials, fermion superfluids, and quark-gluon plasmas.
We develop a coarse-grained description of the point-vortex model, finding that a large number of planar vortices and antivortices behave as an inviscid non-Eulerian fluid at large scales. The emergent binary vortex fluid is subject to anomalous stre sses absent from Eulers equation, caused by the singular nature of quantum vortices. The binary vortex fluid is compressible, and has an asymmetric Cauchy stress tensor allowing orbital angular momentum exchange with the vorticity and vortex density. An analytic solution for vortex shear flow driven by anomalous stresses is in excellent agreement with numerical simulations of the point-vortex model.
We experimentally study the emergence of high-energy equilibrium states in a chiral vortex gas of like-circulation vortices realized within a disk-shaped atomic Bose-Einstein condensate. In contrast to the familiar triangular Abrikosov lattice, the l owest-energy state of the superfluid in a rotating frame, we observe the formation of rotating vortex equilibria that are highly disordered and have significant energy per vortex. Experimental stirring protocols realize equilibrium states at both positive and negative absolute temperatures of the vortex gas. At sufficiently high energies the system exhibits a symmetry breaking transition, resulting in an off-axis equilibrium phase that no longer shares the symmetry of the container. By initializing vortices in a non-equilibrium distribution with sufficient energy, relaxation to equilibrium is observed within experimental timescales and an off-axis equilibrium state emerges at negative absolute temperature. The observed equilibria are in close agreement with mean field theory of the microcanonical ensemble of the vortex gas.
التعليقات
جاري جلب التعليقات جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا