60 research outputs found
Acceleration statistics in thermally driven superfluid turbulence
New methods of flow visualization near absolute zero have opened the way to
directly compare quantum turbulence (in superfluid helium) to classical
turbulence (in ordinary fluids such as air or water) and explore analogies and
differences. We present results of numerical simulations in which we examine
the statistics of the superfluid acceleration in thermal counterflow. We find
that, unlike the velocity, the acceleration obeys scaling laws similar to
classical turbulence, in agreement with a recent quantum turbulence experiment
of La Mantia et al.Comment: 5 pages, 5 figures, to appear in PR
A primer on quantum fluids
This book introduces the theoretical description and properties of quantum
fluids. The focus is on gaseous atomic Bose-Einstein condensates and, to a
minor extent, superfluid helium, but the underlying concepts are relevant to
other forms of quantum fluids such as polariton and photonic condensates. The
book is pitched at the level of advanced undergraduates and early postgraduate
students, aiming to provide the reader with the knowledge and skills to develop
their own research project on quantum fluids. Indeed, the content for this book
grew from introductory notes provided to our own research students. It is
assumed that the reader has prior knowledge of undergraduate mathematics and/or
physics; otherwise, the concepts are introduced from scratch, often with
references for directed further reading.Comment: 132 pages. Published as SpringerBriefs in Physics book. Typos
corrected in this versio
Hydromagnetic Taylor--Couette flow: wavy modes
We investigate magnetic Taylor--Couette flow in the presence of an imposed
axial magnetic field. First we calculate nonlinear steady axisymmetric
solutions and determine how their strength depends on the applied magnetic
field. Then we perturb these solutions to find the critical Reynolds numbers
for the appearance of wavy modes, and the related wavespeeds, at increasing
magnetic field strength. We find that values of imposed magnetic field which
alter only slightly the transition from circular--Couette flow to
Taylor--vortex flow, can shift the transition from Taylor--vortex flow to wavy
modes by a substantial amount. The results are compared against onset in the
absence of a magnetic field.Comment: 12 pages, 8 figures. To appear in J. Fluid Mech. To appear in J.
Fluid Mec
Coupled normal fluid and superfluid profiles of turbulent helium II in channels
We perform fully coupled two--dimensional numerical simulations of plane
channel helium II counterflows with vortex--line density typical of
experiments. The main features of our approach are the inclusion of the back
reaction of the superfluid vortices on the normal fluid and the presence of
solid boundaries. Despite the reduced dimensionality, our model is realistic
enough to reproduce vortex density distributions across the channel recently
calculated in three--dimensions. We focus on the coarse--grained superfluid and
normal fluid velocity profiles, recovering the normal fluid profile recently
observed employing a technique based on laser--induced fluorescence of
metastable helium molecules.Comment: 26 pages, 8 Figures, accepted for publication in Phys. Rev.
Vortex nucleation by collapsing bubbles in Bose-Einstein condensates
The nucleation of vortex rings accompanies the collapse of ultrasound bubbles
in superfluids. Using the Gross-Pitaevskii equation for a uniform condensate we
elucidate the various stages of the collapse of a stationary spherically
symmetric bubble and establish conditions necessary for vortex nucleation. The
minimum radius of the stationary bubble, whose collapse leads to vortex
nucleation, was found to be about 28 healing lengths. The time after which the
nucleation becomes possible is determined as a function of bubble's radius. We
show that vortex nucleation takes place in moving bubbles of even smaller
radius if the motion made them sufficiently oblate.Comment: 4 pages, 5 figure
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
Polarization of superfluid turbulence
We show that normal fluid eddies in turbulent helium II polarize the tangle
of quantized vortex lines present in the flow, thus inducing superfluid
vorticity patterns similar to the driving normal fluid eddies. We also show
that the polarization is effective over the entire inertial range. The results
help explain the surprising analogies between classical and superfluid
turbulence which have been observed recently.Comment: 3 figure
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