79 research outputs found
Burgers Turbulence with Large-scale Forcing
Burgers turbulence supported by white-in-time random forcing at low
wavenumbers is studied analytically and by computer simulation. It is concluded
that the probability density Q of velocity gradient displays four asymptotic
regimes at very large Reynolds number: (A) a region of large positive gradient
where Q decays rapidly (reduction of gradient by stretching); (B) an
intermediate region of negative gradient where Q falls off as the inverse third
power of gradient (transient inviscid steepening of negative gradient); (C) an
outer power-law region of negative gradient where Q falls off as the reciprocal
of gradient (shoulders of mature shocks); (D) a final region of large gradient
where Q decays very rapidly (interior of mature shocks). The probability
density of velocity difference across an interval r, divided by r, lies on Q
throughout regions A and B and into the middle of C, for small enough
inertial-range r.Comment: Revtex (8 pages) with 11 postscript figures (separate file
Phase transitions of barotropic flow coupled to a massive rotating sphere - derivation of a fixed point equation by the Bragg method
The kinetic energy of barotropic flow coupled to an infnitely massive
rotating sphere by an unresolved complex torque mechanism is approximated by a
discrete spin-lattice model of fluid vorticity on a rotating sphere, analogous
to a one-step renormalized Ising model on a sphere with global interactions.
The constrained energy functional is a function of spin-spin coupling and spin
coupling with the rotation of the sphere. A mean field approximation similar to
the Curie-Weiss theory, modeled after that used by Bragg and Williams to treat
a two dimensional Ising model of ferromagnetism, is used to find the barotropic
vorticity states at thermal equilibrium for given temperature and rotational
frequency of the sphere. A fixed point equation for the most probable
barotropic flow state is one of the main results.Comment: 31 pages, 6 figure
Reduction of mean-square advection in turbulent passive scalar mixing
Direct numerical simulation data show that the variance of the coupling term
in passive scalar advection by a random velocity field is smaller than it would
be if the velocity and scalar fields were statistically independent. This
effect is analogous to the "depression of nonlinearity" in hydrodynamic
turbulence. We show that the trends observed in the numerical data are
qualitatively consistent with the predictions of closure theories related to
Kraichnan's direct interaction approximation. The phenomenon is demonstrated
over a range of Prandtl numbers. In the inertial-convective range the depletion
is approximately constant with respect to wavenumber. The effect is weaker in
the Batchelor range
Passive Scalar: Scaling Exponents and Realizability
An isotropic passive scalar field advected by a rapidly-varying velocity
field is studied. The tail of the probability distribution for
the difference in across an inertial-range distance is found
to be Gaussian. Scaling exponents of moments of increase as
or faster at large order , if a mean dissipation conditioned on is
a nondecreasing function of . The computed numerically
under the so-called linear ansatz is found to be realizable. Some classes of
gentle modifications of the linear ansatz are not realizable.Comment: Substantially revised to conform with published version. Revtex (4
pages) with 2 postscript figures. Send email to [email protected]
Cascades, backscatter and conservation in numerical models of twoâdimensional turbulence
The equations governing atmospheric flow imply transfers of energy and potential enstrophy between scales. Accurate simulation of turbulent flow requires that numerical models, which have finite resolution and truncation errors, adequately capture these interscale transfers, particularly between resolved and unresolved scales. It is therefore important to understand how accurately these transfers are modelled in the presence of scaleâselective dissipation or other forms of subgrid model. Here, the energy and enstrophy cascades in numerical models of twoâdimensional turbulence are investigated using the barotropic vorticity equation. Energy and enstrophy transfers in spectral space due to truncated scales are calculated for a highâresolution reference solution and for several explicit and implicit subgrid models at coarser resolution. The reference solution shows that enstrophy and energy are removed from scales very close to the truncation scale and energy is transferred (backscattered) into the large scales. Some subgrid models are able to capture the removal of enstrophy from small scales, though none are scaleâselective enough; however, none are able to capture accurately the energy backscatter. We propose a scheme that perturbs the vorticity field at each time step by the addition of a particular vorticity pattern derived by filtering the predicted vorticity field. Although originally conceived as a parametrization of energy backscatter, this scheme is best interpreted as an energy âfixerâ that attempts to repair the damage to the energy spectrum caused by numerical truncation error and an imperfect subgrid model. The proposed scheme improves the energy and enstrophy behaviour of the solution and, in most cases, slightly reduces the root mean square vorticity errors.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106663/1/2166_ftp.pd
A systematic correlation between two-dimensional flow topology and the abstract statistics of turbulence
Velocity differences in the direct enstrophy cascade of two-dimensional
turbulence are correlated with the underlying flow topology. The statistics of
the transverse and longitudinal velocity differences are found to be governed
by different structures. The wings of the transverse distribution are dominated
by strong vortex centers, whereas, the tails of the longitudinal differences
are dominated by saddles. Viewed in the framework of earlier theoretical work
this result suggests that the transfer of enstrophy to smaller scales is
accomplished in regions of the flow dominated by saddles.Comment: 4 pages, 4 figure
Oceanic rings and jets as statistical equilibrium states
Equilibrium statistical mechanics of two-dimensional flows provides an
explanation and a prediction for the self-organization of large scale coherent
structures. This theory is applied in this paper to the description of oceanic
rings and jets, in the framework of a 1.5 layer quasi-geostrophic model. The
theory predicts the spontaneous formation of regions where the potential
vorticity is homogenized, with strong and localized jets at their interface.
Mesoscale rings are shown to be close to a statistical equilibrium: the theory
accounts for their shape, their drift, and their ubiquity in the ocean,
independently of the underlying generation mechanism. At basin scale, inertial
states presenting mid basin eastward jets (and then different from the
classical Fofonoff solution) are described as marginally unstable states. These
states are shown to be marginally unstable for the equilibrium statistical
theory. In that case, considering a purely inertial limit is a first step
toward more comprehensive out of equilibrium studies that would take into
account other essential aspects, such as wind forcing.Comment: 15 pages, submitted to Journal of Physical Oceanograph
Statistical mechanics of Fofonoff flows in an oceanic basin
We study the minimization of potential enstrophy at fixed circulation and
energy in an oceanic basin with arbitrary topography. For illustration, we
consider a rectangular basin and a linear topography h=by which represents
either a real bottom topography or the beta-effect appropriate to oceanic
situations. Our minimum enstrophy principle is motivated by different arguments
of statistical mechanics reviewed in the article. It leads to steady states of
the quasigeostrophic (QG) equations characterized by a linear relationship
between potential vorticity q and stream function psi. For low values of the
energy, we recover Fofonoff flows [J. Mar. Res. 13, 254 (1954)] that display a
strong westward jet. For large values of the energy, we obtain geometry induced
phase transitions between monopoles and dipoles similar to those found by
Chavanis and Sommeria [J. Fluid Mech. 314, 267 (1996)] in the absence of
topography. In the presence of topography, we recover and confirm the results
obtained by Venaille and Bouchet [Phys. Rev. Lett. 102, 104501 (2009)] using a
different formalism. In addition, we introduce relaxation equations towards
minimum potential enstrophy states and perform numerical simulations to
illustrate the phase transitions in a rectangular oceanic basin with linear
topography (or beta-effect).Comment: 26 pages, 28 figure
Relaxation equations for two-dimensional turbulent flows with a prior vorticity distribution
Using a Maximum Entropy Production Principle (MEPP), we derive a new type of
relaxation equations for two-dimensional turbulent flows in the case where a
prior vorticity distribution is prescribed instead of the Casimir constraints
[Ellis, Haven, Turkington, Nonlin., 15, 239 (2002)]. The particular case of a
Gaussian prior is specifically treated in connection to minimum enstrophy
states and Fofonoff flows. These relaxation equations are compared with other
relaxation equations proposed by Robert and Sommeria [Phys. Rev. Lett. 69, 2776
(1992)] and Chavanis [Physica D, 237, 1998 (2008)]. They can provide a
small-scale parametrization of 2D turbulence or serve as numerical algorithms
to compute maximum entropy states with appropriate constraints. We perform
numerical simulations of these relaxation equations in order to illustrate
geometry induced phase transitions in geophysical flows.Comment: 21 pages, 9 figure
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