3,813 research outputs found
Phase diagram of the 2D He in the density-temperature plane
Thin He films adsorbed to weakly attractive substrates form nearly 2D
layers. We describe the vortices in 2D superfluid He like quasiparticles.
With the aid of a variational many-body calculation we estimate their inertial
mass and describe their interactions with the He particles and other
vortices. Third sound measurements revealed anomalous behavior below the
BKT-phase transition temperature. We ascribe this to the sound mode traveling
in the fluid of vortex-antivortex pairs. These pairs forms a crystal (or liquid
crystal) when the film thickness increases, the third sound mode splits into
two separate modes as seen in experiments. Our many-body calculation predicts
the critical density, at which the phase transition into the vortex-antivortex
state at zero temperature occurs. We also describe the phase diagram of thin
He films.Comment: Contribution paper to LT21 (to be published in Physica B
Critical angular velocity for vortex lines formation
For helium II inside a rotating cylinder, it is proposed that the formation
of vortex lines of the frictionless superfluid component of the liquid is
caused by the presence of the rotating quasi-particles gas. By minimising the
free energy of the system, the critical value Omega_0 of the angular velocity
for the formation of the first vortex line is determined. This value
nontrivially depends on the temperature, and numerical estimations of its
temperature behaviour are produced. It is shown that the latent heat for a
vortex formation and the associated discontinuous change in the angular
momentum of the quasi-particles gas determine the slope of Omega_0 (T) via some
kind of Clapeyron equation.Comment: 16 page
Kolmogorov Spectrum of Quantum Turbulence
There is a growing interest in the relation between classical turbulence and
quantum turbulence. Classical turbulence arises from complicated dynamics of
eddies in a classical fluid. In contrast, quantum turbulence consists of a
tangle of stable topological defects called quantized vortices, and thus
quantum turbulence provides a simpler prototype of turbulence than classical
turbulence. In this paper, we investigate the dynamics and statistics of
quantized vortices in quantum turbulence by numerically solving a modified
Gross-Pitaevskii equation. First, to make decaying turbulence, we introduce a
dissipation term that works only at scales below the healing length. Second, to
obtain steady turbulence through the balance between injection and decay, we
add energy injection at large scales. The energy spectrum is quantitatively
consistent with the Kolmogorov law in both decaying and steady turbulence.
Consequently, this is the first study that confirms the inertial range of
quantum turbulence.Comment: 14pages, 24 figures and 1 table. Appeared in Journal of the Physical
Society of Japan, Vol.74, No.12, p.3248-325
Preferential accumulation of bubbles in Couette-Taylor flow patterns
We investigate the migration of bubbles in several flow patterns occurring within the gap between a rotating inner cylinder and a concentric fixed outer cylinder. The time-dependent evolution of the two-phase flow is predicted through three-dimensional Euler-Lagrange simulations. Lagrangian tracking of spherical bubbles is coupled with direct numerical simulation of the Navier-Stokes equations. We assume that bubbles do not influence the background flow (one-way coupling simulations). The force balance on each bubble takes into account buoyancy, added-mass, viscous drag and shear-induced lift forces. For increasing velocities of the rotating inner cylinder, the flow in the fluid gap evolves from the purely azimuthal steady Couette flow to Taylor toroidal vortices and eventually a wavy vortex flow. The migration of bubbles is highly dependent on the balance between buoyancy and centripetal forces (mostly due to the centripetal pressure gradient) directed toward the inner cylinder and the vortex cores. Depending on the rotation rate of the inner cylinder, bubbles tend to accumulate alternatively along the inner wall, inside the core of Taylor vortices or at particular locations within the wavy vortices. A stability analysis of the fixed points associated with bubble trajectories provides a clear understanding of their migration and preferential accumulation. The location of the accumulation points is parameterized by two dimensionless parameters expressing the balance of buoyancy, centripetal attraction toward the inner rotating cylinder, and entrapment in Taylor vortices. A complete phase diagram summarizing the various regimes of bubble migration is built. Several experimental conditions considered by Djéridi et al.1 are reproduced; the numerical results reveal a very good agreement with the experiments. When the rotation rate is further increased, the numerical results indicate the formation of oscillating bubble strings, as observed experimentally by Djéridi et al.2. After a transient state, bubbles collect at the crests or troughs of the wavy vortices. An analysis of the flow characteristics clearly indicates that bubbles accumulate in the low-pressure regions of the flow field
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