354 research outputs found
Ballistic propagation of thermal excitations near a vortex in superfluid He3-B
Andreev scattering of thermal excitations is a powerful tool for studying
quantized vortices and turbulence in superfluid He3-B at very low temperatures.
We write Hamilton's equations for a quasiparticle in the presence of a vortex
line, determine its trajectory, and find under wich conditions it is Andreev
reflected. To make contact with experiments, we generalize our results to the
Onsager vortex gas, and find values of the intervortex spacing in agreement
with less rigorous estimates
Cross-sections of Andreev scattering by quantized vortex rings in 3He-B
We studied numerically the Andreev scattering cross-sections of
three-dimensional isolated quantized vortex rings in superfluid 3He-B at
ultra-low temperatures. We calculated the dependence of the cross-section on
the ring's size and on the angle between the beam of incident thermal
quasiparticle excitations and the direction of the ring's motion. We also
introduced, and investigated numerically, the cross-section averaged over all
possible orientations of the vortex ring; such a cross-section may be
particularly relevant for the analysis of experimental data. We also analyzed
the role of screening effects for Andreev reflection of quasiparticles by
systems of vortex rings. Using the results obtained for isolated rings we found
that the screening factor for a system of unlinked rings depends strongly on
the average radius of the vortex ring, and that the screening effects increase
with decreasing the rings' size.Comment: 11 pages, 8 figures ; submitted to Physical Review
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 saturation of decaying counterflow turbulence in helium II
We are concerned with the problem of the decay of a tangle of quantized
vortices in He II generated by a heat current. Direct application of Vinen's
equation yields the temporal scaling of vortex line density .
Schwarz and Rozen [Phys. Rev. Lett. {\bf 66}, 1898 (1991); Phys. Rev. B {\bf
44}, 7563 (1991)] observed a faster decay followed by a slower decay. More
recently, Skrbek and collaborators [Phys. Rev. E {\bf 67}, 047302 (2003)] found
an initial transient followed by the same classical scaling observed
in the decay of grid-generated turbulence. We present a simple theoretical
model which, we argue, contains the essential physical ingredients, and
accounts for these apparently contradictory results.Comment: 19 pages, 5 figure
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
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