323,231 research outputs found
Resonant Interactions in Rotating Homogeneous Three-dimensional Turbulence
Direct numerical simulations of three-dimensional (3D) homogeneous turbulence
under rapid rigid rotation are conducted to examine the predictions of resonant
wave theory for both small Rossby number and large Reynolds number. The
simulation results reveal that there is a clear inverse energy cascade to the
large scales, as predicted by 2D Navier-Stokes equations for resonant
interactions of slow modes. As the rotation rate increases, the
vertically-averaged horizontal velocity field from 3D Navier-Stokes converges
to the velocity field from 2D Navier-Stokes, as measured by the energy in their
difference field. Likewise, the vertically-averaged vertical velocity from 3D
Navier-Stokes converges to a solution of the 2D passive scalar equation. The
energy flux directly into small wave numbers in the plane from
non-resonant interactions decreases, while fast-mode energy concentrates closer
to that plane. The simulations are consistent with an increasingly dominant
role of resonant triads for more rapid rotation
Decay rate of a Wannier exciton in low dimensional systems
The superradiant decay rate of Wannier exciton in one dimensional system is
studied. The crossover behavior from 1D chain to 2D film is also examined. It
is found that the decay rate shows oscillatory dependence on channel width L.
When the quasi 1-D channel is embeded with planar microcavities, it is shown
that the dark mode exciton can be examined experimentally.Comment: 12 pages, 1 figur
Nonmonotonic External Field Dependence of the Magnetization in a Finite Ising Model: Theory and MC Simulation
Using field theory and Monte Carlo (MC) simulation we investigate
the finite-size effects of the magnetization for the three-dimensional
Ising model in a finite cubic geometry with periodic boundary conditions. The
field theory with infinite cutoff gives a scaling form of the equation of state
where
is the reduced temperature, is the external field and
is the size of system. Below and at the theory predicts a
nonmonotonic dependence of with respect to at fixed and a crossover
from nonmonotonic to monotonic behaviour when is further increased. These
results are confirmed by MC simulation. The scaling function obtained
from the field theory is in good quantitative agreement with the finite-size MC
data. Good agreement is also found for the bulk value at .Comment: LaTex, 12 page
Sparse support vector regression based on orthogonal forward selection for the generalised kernel model
Formation of ultracold LiRb molecules by photoassociation near the Li (2s 2S1/2) + Rb (5p 2P1/2) asymptote
We report the production of ultracold 7Li85Rb molecules by photoassociation
(PA) below the Li (2s 2S1/2) + Rb (5p 2P1/2) asymptote. We perform PA
spectroscopy in a dual-species 7Li-85Rb magneto-optical trap (MOT) and detect
the PA resonances using trap loss spectroscopy. We observe several strong PA
resonances corresponding to the last few bound states, assign the lines and
derive the long range C6 dispersion coefficients for the Li (2s 2S1/2) + Rb (5p
2P1/2) asymptote. We also report an excited-state molecule formation rate
(P_LiRb) of ~10^7 s^-1 and a PA rate coefficient (K_PA) of ~4x10^-11 cm^3/s,
which are both among the highest observed for heteronuclear bi-alkali
molecules. These suggest that PA is a promising route for the creation of
ultracold ground state LiRb molecules.Comment: 6 page
A connection between the Camassa-Holm equations and turbulent flows in channels and pipes
In this paper we discuss recent progress in using the Camassa-Holm equations
to model turbulent flows. The Camassa-Holm equations, given their special
geometric and physical properties, appear particularly well suited for studying
turbulent flows. We identify the steady solution of the Camassa-Holm equation
with the mean flow of the Reynolds equation and compare the results with
empirical data for turbulent flows in channels and pipes. The data suggests
that the constant version of the Camassa-Holm equations, derived under
the assumptions that the fluctuation statistics are isotropic and homogeneous,
holds to order distance from the boundaries. Near a boundary, these
assumptions are no longer valid and the length scale is seen to depend
on the distance to the nearest wall. Thus, a turbulent flow is divided into two
regions: the constant region away from boundaries, and the near wall
region. In the near wall region, Reynolds number scaling conditions imply that
decreases as Reynolds number increases. Away from boundaries, these
scaling conditions imply is independent of Reynolds number. Given the
agreement with empirical and numerical data, our current work indicates that
the Camassa-Holm equations provide a promising theoretical framework from which
to understand some turbulent flows.Comment: tex file, 29 pages, 4 figures, Physics of Fluids (in press
Non-universal size dependence of the free energy of confined systems near criticality
The singular part of the finite-size free energy density of the O(n)
symmetric field theory in the large-n limit is calculated at finite
cutoff for confined geometries of linear size L with periodic boundary
conditions in 2 < d < 4 dimensions. We find that a sharp cutoff
causes a non-universal leading size dependence
near which dominates the universal scaling term . This
implies a non-universal critical Casimir effect at and a leading
non-scaling term of the finite-size specific heat above .Comment: RevTex, 4 page
Symmetries and Lie algebra of the differential-difference Kadomstev-Petviashvili hierarchy
By introducing suitable non-isospectral flows we construct two sets of
symmetries for the isospectral differential-difference Kadomstev-Petviashvili
hierarchy. The symmetries form an infinite dimensional Lie algebra.Comment: 9 page
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