247 research outputs found
Vortex density fluctuations in quantum turbulence
We compute the frequency spectrum of turbulent superfluid vortex density
fluctuations and obtain the same Kolmogorov scaling which has been observed in
a recent experiment in Helium-4. We show that the scaling can be interpreted in
terms of the spectrum of reconnecting material lines. The calculation is
performed using a vortex tree algorithm which considerably speeds up the
evaluation of Biot-Savart integrals.Comment: 7 Pages, 7 figure
Quantum vortex reconnections
We study reconnections of quantum vortices by numerically solving the
governing Gross-Pitaevskii equation. We find that the minimum distance between
vortices scales differently with time before and after the vortex reconnection.
We also compute vortex reconnections using the Biot-Savart law for vortex
filaments of infinitesimal thickness, and find that, in this model,
reconnection are time-symmetric. We argue that the likely cause of the
difference between the Gross-Pitaevskii model and the Biot-Savart model is the
intense rarefaction wave which is radiated away from a Gross-Pitaeveskii
reconnection. Finally we compare our results to experimental observations in
superfluid helium, and discuss the different length scales probed by the two
models and by experiments.Comment: 23 Pages, 12 Figure
Quasiclassical and ultraquantum decay of superfluid turbulence
This letter addresses the question which, after a decade-long discussion,
still remains open: what is the nature of the ultraquantum regime of decay of
quantum turbulence? The model developed in this work reproduces both the
ultraquantum and the quasiclassical decay regimes and explains their
hydrodynamical natures. In the case where turbulence is generated by forcing at
some intermediate lengthscale, e.g. by the beam of vortex rings in the
experiment of Walmsley and Golov [Phys. Rev. Lett. {\bf 100}, 245301 (2008)],
we explained the mechanisms of generation of both ultraquantum and
quasiclassical regimes. We also found that the anisotropy of the beam is
important for generating the large scale motion associated with the
quasiclassical regime
The Kelvin-wave cascade in the vortex filament model
The energy transfer mechanism in zero temperature superfluid turbulence of
helium-4 is still a widely debated topic. Currently, the main hypothesis is
that weakly nonlinear interacting Kelvin waves transfer energy to sufficiently
small scales such that energy is dissipated as heat via phonon excitations.
Theoretically, there are at least two proposed theories for Kelvin-wave
interactions. We perform the most comprehensive numerical simulation of weakly
nonlinear interacting Kelvin-waves to date and show, using a specially designed
numerical algorithm incorporating the full Biot-Savart equation, that our
results are consistent with nonlocal six-wave Kelvin wave interactions as
proposed by L'vov and Nazarenko.Comment: 6 pages, 6 figure
Vortex Solutions in a Binary Immiscible Bose-Einstein Condensate
We consider the mean-field vortex solutions and their stability within a
two-component Bose Einstein condensate in the immiscible limit. A variational
approach is employed to study a system consisting of a majority component which
contains a single quantised vortex and a minority component which fills the
vortex core. We show that a super-Gaussian function is a good approximation to
the two-component vortex solution for a range of atom numbers of the in-filling
component, by comparing the variational solutions to the full numerical
solutions of the coupled Gross-Pitaevskii equations. We subsequently examine
the stability of the vortex solutions by perturbing the in-filling component
away from the centre of the vortex core, thereby demonstrating their stability
to small perturbations.Comment: 10 pages, 5 figure
Coherent laminar and turbulent motion of toroidal vortex bundles
Motivated by experiments performed in superfluid helium, we study numerically
the motion of toroidal bundles of vortex filaments in an inviscid fluid. We
find that the evolution of these large-scale vortex structures involves the
generalised leapfrogging of the constituent vortex rings. Despite three
dimensional perturbations in the form of Kelvin waves and vortex reconnections,
toroidal vortex bundles retain their coherence over a relatively large distance
(compared to their size), in agreement with experimental observations.Comment: 22 pages, 12 figure
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