510 research outputs found
Intermittency in the large N-limit of a spherical shell model for turbulence
A spherical shell model for turbulence, obtained by coupling replicas of
the Gledzer, Okhitani and Yamada shell model, is considered. Conservation of
energy and of an helicity-like invariant is imposed in the inviscid limit. In
the limit this model is analytically soluble and is remarkably
similar to the random coupling model version of shell dynamics. We have studied
numerically the convergence of the scaling exponents toward the value predicted
by Kolmogorov theory (K41). We have found that the rate of convergence to the
K41 solution is linear in 1/N. The restoring of Kolmogorov law has been related
to the behaviour of the probability distribution functions of the instantaneous
scaling exponent.Comment: 10 pages, Latex, 3 Postscript figures, to be published on Europhys.
Let
Helicity cascades in rotating turbulence
The effect of helicity (velocity-vorticity correlations) is studied in direct
numerical simulations of rotating turbulence down to Rossby numbers of 0.02.
The results suggest that the presence of net helicity plays an important role
in the dynamics of the flow. In particular, at small Rossby number, the energy
cascades to large scales, as expected, but helicity then can dominate the
cascade to small scales. A phenomenological interpretation in terms of a direct
cascade of helicity slowed down by wave-eddy interactions leads to the
prediction of new inertial indices for the small-scale energy and helicity
spectra.Comment: 7 pages, 8 figure
On the universality of small scale turbulence
The proposed universality of small scale turbulence is investigated for a set
of measurements in a cryogenic free jet with a variation of the Reynolds number
(Re) from 8500 to 10^6. The traditional analysis of the statistics of velocity
increments by means of structure functions or probability density functions is
replaced by a new method which is based on the theory of stochastic Markovian
processes. It gives access to a more complete characterization by means of
joint probabilities of finding velocity increments at several scales. Based on
this more precise method our results call in question the concept of
universality.Comment: 4 pages, 4 figure
Classical and quantum regimes of two-dimensional turbulence in trapped Bose-Einstein condensates
We investigate two-dimensional turbulence in finite-temperature trapped
Bose-Einstein condensates within damped Gross-Pitaevskii theory. Turbulence is
produced via circular motion of a Gaussian potential barrier stirring the
condensate. We systematically explore a range of stirring parameters and
identify three regimes, characterized by the injection of distinct quantum
vortex structures into the condensate: (A) periodic vortex dipole injection,
(B) irregular injection of a mixture of vortex dipoles and co-rotating vortex
clusters, and (C) continuous injection of oblique solitons that decay into
vortex dipoles. Spectral analysis of the kinetic energy associated with
vortices reveals that regime (B) can intermittently exhibit a Kolmogorov
power law over almost a decade of length or wavenumber () scales.
The kinetic energy spectrum of regime (C) exhibits a clear power law
associated with an inertial range for weak-wave turbulence, and a
power law for high wavenumbers. We thus identify distinct regimes of forcing
for generating either two-dimensional quantum turbulence or classical weak-wave
turbulence that may be realizable experimentally.Comment: 11 pages, 10 figures. Minor updates to text and figures 1, 2 and
Large scale flow effects, energy transfer, and self-similarity on turbulence
The effect of large scales on the statistics and dynamics of turbulent
fluctuations is studied using data from high resolution direct numerical
simulations. Three different kinds of forcing, and spatial resolutions ranging
from 256^3 to 1024^3, are being used. The study is carried out by investigating
the nonlinear triadic interactions in Fourier space, transfer functions,
structure functions, and probability density functions. Our results show that
the large scale flow plays an important role in the development and the
statistical properties of the small scale turbulence. The role of helicity is
also investigated. We discuss the link between these findings and
intermittency, deviations from universality, and possible origins of the
bottleneck effect. Finally, we briefly describe the consequences of our results
for the subgrid modeling of turbulent flows
Intermittency in the Joint Cascade of Energy and Helicity
The statistics of the energy and helicity fluxes in isotropic turbulence are
studied using high resolution direct numerical simulation. The scaling
exponents of the energy flux agree with those of the transverse velocity
structure functions through refined similarity hypothesis, consistent with
Kraichnan's prediction \cite{Kr74}. The helicity flux is even more intermittent
than the energy flux and its scaling exponents are closer to those of the
passive scalar. Using Waleffe's helical decomposition, we demonstrate that the
existence of positive mean helicity flux inhibits the energy transfer in the
negative helical modes, a non-passive effect
Spectral energy dynamics in magnetohydrodynamic turbulence
Spectral direct numerical simulations of incompressible MHD turbulence at a
resolution of up to collocation points are presented for a
statistically isotropic system as well as for a setup with an imposed strong
mean magnetic field. The spectra of residual energy,
, and total energy,
, are observed to scale self-similarly in
the inertial range as ,
(isotropic case) and ,
(anisotropic case, perpendicular to the mean
field direction). A model of dynamic equilibrium between kinetic and magnetic
energy, based on the corresponding evolution equations of the eddy-damped
quasi-normal Markovian (EDQNM) closure approximation, explains the findings.
The assumed interplay of turbulent dynamo and Alfv\'en effect yields
which is confirmed by the simulations.Comment: accepted for publication by PR
Energy spectrum of buoyancy-driven turbulence
Using high-resolution direct numerical simulation and arguments based on the
kinetic energy flux , we demonstrate that for stably stratified flows,
the kinetic energy spectrum , the entropy spectrum
, and , consistent with the
Bolgiano-Obukhov scaling. This scaling arises due to the conversion of kinetic
energy to the potential energy by buoyancy. For weaker buoyancy, this
conversion is weak, hence follows Kolmogorov's spectrum with a
constant energy flux. For Rayleigh B\'{e}nard convection, we show that the
energy supply rate by buoyancy is positive, which leads to an increasing
with , thus ruling out Bolgiano-Obukhov scaling for the
convective turbulence. Our numerical results show that convective turbulence
for unit Prandt number exhibits a constant and for a narrow band of wavenumbers.Comment: arXiv admin note: text overlap with arXiv:1404.214
CMB anisotropies due to cosmological magnetosonic waves
We study scalar mode perturbations (magnetosonic waves) induced by a helical
stochastic cosmological magnetic field and derive analytically the
corresponding cosmic microwave background (CMB) temperature and polarization
anisotropy angular power spectra. We show that the presence of a stochastic
magnetic field, or an homogeneous magnetic field, influences the acoustic
oscillation pattern of the CMB anisotropy power spectrum, effectively acting as
a reduction of the baryon fraction. We find that the scalar magnetic energy
density perturbation contribution to the CMB temperature anisotropy is small
compared to the contribution to the CMB -polarization anisotropy.Comment: 17 pages, references added, version accepted for publication in Phys.
Rev.
- …