1,065 research outputs found
On the emergence of helicity in rotating stratified turbulence
We perform numerical simulations of decaying rotating stratified turbulence
and show, in the Boussinesq framework, that helicity (velocity-vorticity
correlation), as observed in super-cell storms and hurricanes, is spontaneously
created due to an interplay between buoyancy and rotation common to large-scale
atmospheric and oceanic flows. Helicity emerges from the joint action of eddies
and of inertia-gravity waves (with inertia and gravity with respective
associated frequencies and ), and it occurs when the waves are
sufficiently strong. For the amount of helicity produced is correctly
predicted by a quasi-linear balance equation. Outside this regime, and up to
the highest Reynolds number obtained in this study, namely ,
helicity production is found to be persistent for as large as , and for and respectively as large as and
.Comment: 10 pages, 5 figure
A hybrid MPI-OpenMP scheme for scalable parallel pseudospectral computations for fluid turbulence
A hybrid scheme that utilizes MPI for distributed memory parallelism and
OpenMP for shared memory parallelism is presented. The work is motivated by the
desire to achieve exceptionally high Reynolds numbers in pseudospectral
computations of fluid turbulence on emerging petascale, high core-count,
massively parallel processing systems. The hybrid implementation derives from
and augments a well-tested scalable MPI-parallelized pseudospectral code. The
hybrid paradigm leads to a new picture for the domain decomposition of the
pseudospectral grids, which is helpful in understanding, among other things,
the 3D transpose of the global data that is necessary for the parallel fast
Fourier transforms that are the central component of the numerical
discretizations. Details of the hybrid implementation are provided, and
performance tests illustrate the utility of the method. It is shown that the
hybrid scheme achieves near ideal scalability up to ~20000 compute cores with a
maximum mean efficiency of 83%. Data are presented that demonstrate how to
choose the optimal number of MPI processes and OpenMP threads in order to
optimize code performance on two different platforms.Comment: Submitted to Parallel Computin
Cancellation exponent and multifractal structure in two-dimensional magnetohydrodynamics: direct numerical simulations and Lagrangian averaged modeling
We present direct numerical simulations and Lagrangian averaged (also known
as alpha-model) simulations of forced and free decaying magnetohydrodynamic
turbulence in two dimensions. The statistics of sign cancellations of the
current at small scales is studied using both the cancellation exponent and the
fractal dimension of the structures. The alpha-model is found to have the same
scaling behavior between positive and negative contributions as the direct
numerical simulations. The alpha-model is also able to reproduce the time
evolution of these quantities in free decaying turbulence. At large Reynolds
numbers, an independence of the cancellation exponent with the Reynolds numbers
is observed.Comment: Finite size box effects have been taken into account in the
definition of the partition function. This has resulted in a more clear
scaling in all figures. Several points are clarified in the tex
On the inverse cascade of magnetic helicity
We study the inverse cascade of magnetic helicity in conducting fluids by
investigating the detailed transfer of helicity between different spherical
shells in Fourier space in direct numerical simulations of three-dimensional
magnetohydrodynamics (MHD). Two different numerical simulations are used, one
where the system is forced with an electromotive force in the induction
equation, and one in which the system is forced mechanically with an ABC flow
and the magnetic field is solely sustained by a dynamo action. The magnetic
helicity cascade at the initial stages of both simulations is observed to be
inverse and local (in scale space) in the large scales, and direct and local in
the small scales. When saturation is approached most of the helicity is
concentrated in the large scales and the cascade is non-local. Helicity is
transfered directly from the forced scales to the largest scales. At the same
time, a smaller in amplitude direct cascade is observed from the largest scale
to small scales.Comment: Submitted to PR
A numerical study of the alpha model for two-dimensional magnetohydrodynamic turbulent flows
We explore some consequences of the ``alpha model,'' also called the
``Lagrangian-averaged'' model, for two-dimensional incompressible
magnetohydrodynamic (MHD) turbulence. This model is an extension of the
smoothing procedure in fluid dynamics which filters velocity fields locally
while leaving their associated vorticities unsmoothed, and has proved useful
for high Reynolds number turbulence computations. We consider several known
effects (selective decay, dynamic alignment, inverse cascades, and the
probability distribution functions of fluctuating turbulent quantities) in
magnetofluid turbulence and compare the results of numerical solutions of the
primitive MHD equations with their alpha-model counterparts' performance for
the same flows, in regimes where available resolution is adequate to explore
both. The hope is to justify the use of the alpha model in regimes that lie
outside currently available resolution, as will be the case in particular in
three-dimensional geometry or for magnetic Prandtl numbers differing
significantly from unity. We focus our investigation, using direct numerical
simulations with a standard and fully parallelized pseudo-spectral method and
periodic boundary conditions in two space dimensions, on the role that such a
modeling of the small scales using the Lagrangian-averaged framework plays in
the large-scale dynamics of MHD turbulence. Several flows are examined, and for
all of them one can conclude that the statistical properties of the large-scale
spectra are recovered, whereas small-scale detailed phase information (such as
e.g. the location of structures) is lost.Comment: 22 pages, 20 figure
Energy transfer in Hall-MHD turbulence: cascades, backscatter, and dynamo action
Scale interactions in Hall MHD are studied using both the mean field theory
derivation of transport coefficients, and direct numerical simulations in three
space dimensions. In the magnetically dominated regime, the eddy resistivity is
found to be negative definite, leading to large scale instabilities. A direct
cascade of the total energy is observed, although as the amplitude of the Hall
effect is increased, backscatter of magnetic energy to large scales is found, a
feature not present in MHD flows. The coupling between the magnetic and
velocity fields is different than in the MHD case, and backscatter of energy
from small scale magnetic fields to large scale flows is also observed. For the
magnetic helicity, a strong quenching of its transfer is found. We also discuss
non-helical magnetically forced Hall-MHD simulations where growth of a large
scale magnetic field is observed.Comment: 25 pages, 16 figure
Numerical solutions of the three-dimensional magnetohydrodynamic alpha-model
We present direct numerical simulations and alpha-model simulations of four
familiar three-dimensional magnetohydrodynamic (MHD) turbulence effects:
selective decay, dynamic alignment, inverse cascade of magnetic helicity, and
the helical dynamo effect. The MHD alpha-model is shown to capture the
long-wavelength spectra in all these problems, allowing for a significant
reduction of computer time and memory at the same kinetic and magnetic Reynolds
numbers. In the helical dynamo, not only does the alpha-model correctly
reproduce the growth rate of magnetic energy during the kinematic regime, but
it also captures the nonlinear saturation level and the late generation of a
large scale magnetic field by the helical turbulence.Comment: 12 pages, 19 figure
Economic activities and deforestation in Brazil's Carajás region. Examining production deforestation linkages
Esse artigo avalia os impactos das atividades produtivas nas condições macroeconômicas e no desmatamento usando um modelo CGE para duas regiões. O objeto de estudo e a interação entre os setores produtivos (especialmente agricultura, pecuária, madeira e ferro gusa) e desmatamento na região leste da Amazônia brasileira. Essa região, também conhecida como o Corredor de Carajás, experimentou um rápido crescimento e povoamento nas últimas três décadas. Nesse mesmo perÃodo, uma parcela significativa de seus recursos naturais, florestas em particular, foi exaurida. As simulações realizadas revelam que o desenvolvimento da metalurgia só indiretamente conduz a um nÃvel mais elevado de desmatamento ao estimular a expansão da agricultura e da pecuária. A expansão das atividades agrÃcolas gera como sub-produto madeira suficiente para atender a demanda crescente da produção de carvão causada pelo crescimento da metalurgia.This paper assesses the impacts of productive activities on macroeconomic conditions and deforestation using a two-region CGE model. The object ofstudy is the interaction between productive sectors (especially agriculture, livestock, timber and pig iron) and deforestation in the Eastern Amazon region ofBrazil. This region, also known as the Carajas Comdor, has experienced rapid growth and settlement over the last three decades. Over this same period a significant portion of its natural resources (particularly forest) has been depleted. The growth simulations reveal that expansion and development of the metals sector only indirectly result in higher levels of deforestation by stimulating the expansion of agriculture and livestock production. The expansion of agricultural activities results in sufficient wood by-products to meet the greater demand for charcoal production caused by development of the metals sector
Numerical Solutions of the Three-Dimensional Magnetohydrodynamic Α Model
We present direct numerical simulations and α-model simulations of four familiar three-dimensional magnetohydrodynamic (MHD) turbulence effects: selective decay, dynamic alignment, inverse cascade of magnetic helicity, and the helical dynamo effect. The MHD α model is shown to capture the long-wavelength spectra in all these problems, allowing for a significant reduction of computer time and memory at the same kinetic and magnetic Reynolds numbers. In the helical dynamo, not only does the α model correctly reproduce the growth rate of magnetic energy during the kinematic regime, it also captures the nonlinear saturation level and the late generation of a large scale magnetic field by the helical turbulence
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