1,276 research outputs found
Isotropisation at small scales of rotating helically-driven turbulence
We present numerical evidence of how three-dimensionalization occurs at small
scale in rotating turbulence with Beltrami (ABC) forcing, creating helical
flow. The Zeman scale at which the inertial and eddy turn-over
times are equal is more than one order of magnitude larger than the dissipation
scale, with the relevant domains (large-scale inverse cascade of energy, dual
regime in the direct cascade of energy and helicity , and dissipation)
each moderately resolved. These results stem from the analysis of a large
direct numerical simulation on a grid of points, with Rossby and
Reynolds numbers respectively equal to 0.07 and . At scales
smaller than the forcing, a helical wave-modulated inertial law for the energy
and helicity spectra is followed beyond by Kolmogorov spectra
for and . Looking at the two-dimensional slow manifold, we also show
that the helicity spectrum breaks down at , a clear sign of
recovery of three-dimensionality in the small scales.Comment: 13 pages, 6 figure
Effect of helicity and rotation on the free decay of turbulent flows
The self-similar decay of energy in a turbulent flow is studied in direct
numerical simulations with and without rotation. Two initial conditions are
considered: one non-helical (mirror-symmetric), and one with maximal helicity.
The results show that, while in the absence of rotation the energy in the
helical and non-helical cases decays with the same rate, in rotating flows the
helicity content has a major impact on the decay rate. These differences are
associated with differences in the energy and helicity cascades when rotation
is present. Properties of the structures that arise in the flow at late times
in each time are also discussed.Comment: 4 pages, 4 figure
The decay of Batchelor and Saffman rotating turbulence
The decay rate of isotropic and homogeneous turbulence is known to be
affected by the large-scale spectrum of the initial perturbations, associated
with at least two cannonical self-preserving solutions of the von
K\'arm\'an-Howarth equation: the so-called Batchelor and Saffman spectra. The
effect of long-range correlations in the decay of anisotropic flows is less
clear, and recently it has been proposed that the decay rate of rotating
turbulence may be independent of the large-scale spectrum of the initial
perturbations. We analyze numerical simulations of freely decaying rotating
turbulence with initial energy spectra (Batchelor turbulence) and
(Saffman turbulence) and show that, while a self-similar decay
cannot be identified for the total energy, the decay is indeed affected by
long-range correlations. The decay of two-dimensional and three-dimensional
modes follows distinct power laws in each case, which are consistent with
predictions derived from the anisotropic von K\'arm\'an-Howarth equation, and
with conservation of anisotropic integral quantities by the flow evolution
The decay of turbulence in rotating flows
We present a parametric space study of the decay of turbulence in rotating
flows combining direct numerical simulations, large eddy simulations, and
phenomenological theory. Several cases are considered: (1) the effect of
varying the characteristic scale of the initial conditions when compared with
the size of the box, to mimic "bounded" and "unbounded" flows; (2) the effect
of helicity (correlation between the velocity and vorticity); (3) the effect of
Rossby and Reynolds numbers; and (4) the effect of anisotropy in the initial
conditions. Initial conditions include the Taylor-Green vortex, the
Arn'old-Beltrami-Childress flow, and random flows with large-scale energy
spectrum proportional to . The decay laws obtained in the simulations for
the energy, helicity, and enstrophy in each case can be explained with
phenomenological arguments that separate the decay of two-dimensional from
three-dimensional modes, and that take into account the role of helicity and
rotation in slowing down the energy decay. The time evolution of the energy
spectrum and development of anisotropies in the simulations are also discussed.
Finally, the effect of rotation and helicity in the skewness and kurtosis of
the flow is considered.Comment: Sections reordered to address comments by referee
Restrictive dermopathy with massive thrombosis: a previously uncreognized finding
Restrictive dermopathy (RD) is a lethal genodermatosis characterized by IUGR, tight and rigid skin, prominent superficial vasculature, epidermal hyperkeratosis, typical facial features, sparse/absent eyelashes and eyebrows, thin dysplastic clavicles, pulmonary hypoplasia and arthrogryposis. It is caused by LMNA or, more frequently, ZMPSTE24 mutations. We report 2 siblings with RD and ZMPSTE24 mutations. CASE REPORT: The mother is 28y G2P1. The couple was 1st cousin of Pakistani origin. Family history was unremarkable. The 1st pregnancy resulted in IUD at 27w, preceded by decreased fetal movement, oligohydramnios and IUGR at 24w. Autopsy was inconclusive and G-banding was not possible. Placenta showed ...postprin
Crossover from Rate-Equation to Diffusion-Controlled Kinetics in Two-Particle Coagulation
We develop an analytical diffusion-equation-type approximation scheme for the
one-dimensional coagulation reaction A+A->A with partial reaction probability
on particle encounters which are otherwise hard-core. The new approximation
describes the crossover from the mean-field rate-equation behavior at short
times to the universal, fluctuation-dominated behavior at large times. The
approximation becomes quantitatively accurate when the system is initially
close to the continuum behavior, i.e., for small initial density and fast
reaction. For large initial density and slow reaction, lattice effects are
nonnegligible for an extended initial time interval. In such cases our
approximation provides the correct description of the initial mean-field as
well as the asymptotic large-time, fluctuation-dominated behavior. However, the
intermediate-time crossover between the two regimes is described only
semiquantitatively.Comment: 21 pages, plain Te
Interpocket polarization model for magnetic structures in rare-earth hexaborides
The origin of peculiar magnetic structures in cubic rare-earth (R)
hexaborides RB_6 is traced back to their characteristic band structure. The
three sphere-like Fermi surfaces induce interpocket polarization of the
conduction band as a part of a RKKY-type interaction. It is shown for the
free-electron-like model that the interpocket polarization gives rise to a
broad maximum in the intersite interaction I(q) around q=(1/4,1/4,1/2) in the
Brillouin zone. This maximum is consistent with the superstructure observed in
R=Ce, Gd and Dy. The wave-number dependence of I(q) is independently extracted
from analysis of the spin-wave spectrum measured for NdB_6. It is found that
I(q) obtained from fitting the data has a similarly to that derived by the
interpocket polarization model, except that the absolute maximum now occurs at
(0,0,1/2) in consistency with the A-type structure. The overall shape of I(q)
gives a hint toward understanding an incommensurate structure in PrB_6 as well.Comment: 5 pages, 3 figures, submitted to J.Phys.Soc.Jp
Thermalization and free decay in Surface Quasi-Geostrophic flows
We derive statistical equilibrium solutions of the truncated inviscid surface
quasi-geostrophic (SQG) equations, and verify the validity of these solutions
at late times in numerical simulations of the truncated SQG equations. The
results indicate enstrophy thermalizes while energy can condense at the gravest
modes, in agreement with previous indications of a direct cascade of enstrophy
and an inverse cascade of energy in forced-dissipative SQG systems. At early
times, the truncated inviscid SQG simulations show a behavior reminiscent of
forced-dissipative SQG turbulence, and we identify spectral scaling laws for
the energy and enstrophy spectra. Finally, a comparison between viscous and
inviscid simulations allows us to identify free-decay similarity laws for the
enstrophy in SQG turbulence at very large Reynolds number
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