A scalar model for MHD turbulence

A model for homogeneous MHD turbulence is proposed. Nonlinear interactions acts between nearest neighbours in a discretized wavenumbers space and conservation properties (total energy and v-b correlation) are verified. The model can be truncated at will. With three modes, a bifurcation analysis is given. In the turbulent case (dissipation and kinetic forcing are present) one obtains time fluctuations at all scales and time-averaged power law spectra, the small scales exhibit intermittency effects. Typical MHD phenomena such as the dynamo effect or the increase of v-b correlation in the decaying cases are also observed.     

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采用FLUENT软件分别对外加均匀横向磁场的等截面三维充分发展液态金属管流的层流模型和低雷诺数湍流Lam/Bremhost(LB)模型进行了数值模拟,分析了外加磁场对普通方管LB模型速度分布和压降的影响。比较在相同哈特曼数下,层流和湍流模型方管截面上速度分布和管道中MHD压降。其中,对电流的计算采用磁感应方程来求得。数值模拟结果证明了用低雷诺数LB湍流模型解决方管磁流体流动的可行性。通过层流模型和湍流模型的对比可知,层流模型有较短的入口长度,但管内流体的压降却很大;而湍流模型管内速度更加平均化,管内压降较小,但管内入口长度较长。     

Exchange bias and asymmetric hysteresis loops from a microscopic model of core/shell nanoparticles

We present Monte Carlo simulations of hysteresis loops of a model of a magnetic nanoparticle with a ferromagnetic core and an antiferromagnetic shell with varying values of the core/shell interface exchange coupling which aim to clarify the microscopic origin of exchange bias observed experimentally. We have found loop shifts in the field direction as well as displacements along the magnetization axis that increase in magnitude when increasing the interfacial exchange coupling. Overlap functions computed from the spin configurations along the loops have been obtained to explain the origin and magnitude of these features microscopically.     

聚变堆液态包层准二维磁流体湍流模型研究

为研究聚变堆氚增殖包层中液态金属湍流磁流体动力学(MHD)效应,开发了一种新的准二维单方程 MHD 湍流模型,并进行了相关数值模拟程序的编制及验证。对于矩形管道中的准二维 MHD 湍流流动,三维流 动主要发生在哈德曼层中,中心的主流区呈现出二维流动。为了反映这种特殊的流动特征,新湍流模型在标准 k-ε 模型的基础之上去掉了传统的耗散项,代之以电磁耗散项来模拟湍流 MHD 效应。同时,采用 Bradshaw 假设来对 湍流涡粘系数进行模化。为验证该湍流模型是否合理,编制了相关数值模拟程序,并利用直接数值模拟(DNS)结 果对该程序进行了校正,数值模拟结果与 DNS 结果吻合较好。计算结果表明,该湍流模型可应用于聚变堆液态 包层 MHD 湍流流动的数值模拟。     

Spectral slope and Kolmogorov constant of MHD turbulence

The spectral slope of strong MHD turbulence has recently been a matter of controversy. While the Goldreich-Sridhar model predicts a -5/3 slope, shallower slopes have been observed in numerics. We argue that earlier numerics were affected by driving due to a diffuse locality of energy transfer. Our highest-resolution simulation (3072(2)×1024) exhibited the asymptotic -5/3 scaling. We also discover that the dynamic alignment, proposed in models with -3/2 slope, saturates and cannot modify the asymptotic, high Reynolds number slope. From the observed -5/3 scaling we measure the Kolmogorov constant C(KA)=3.27±0.07 for Alfvénic turbulence and C(K)=4.2±0.2 for full MHD turbulence, which is higher than the hydrodynamic value of 1.64. This larger C(K) indicates inefficient energy transfer in MHD turbulence, which is in agreement with diffuse locality.     

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通过数值模拟研究了欧姆定律中的Hall效应修正对MHD湍流的能量衰减的影响。在低或没有磁螺旋性约束的情况下推导出了 t–1能量衰减律。     

Depression of nonlinearity in decaying isotropic MHD turbulence

Spectral method simulations show that undriven magnetohydrodynamic turbulence spontaneously generates coherent spatial correlations of several types, associated with local Beltrami fields, directional alignment of velocity and magnetic fields, and antialignment of magnetic and fluid acceleration components. These correlations suppress nonlinearity to levels lower than what is obtained from Gaussian fields, and occur in spatial patches. We suggest that this rapid relaxation leads to non-Gaussian statistics and spatial intermittency.     

Self-organized criticality in MHD driven plasma edge turbulence

We analyze long-range time correlations and self-similar characteristics of the electrostatic turbulence at the plasma edge and scrape-off layer in the Tokamak Chauffage Alfvén Brésillien (TCABR), with low and high Magnetohydrodynamics (MHD) activity. We find evidence of self-organized criticality (SOC), mainly in the region near the tokamak limiter. Comparative analyses of data before and during the MHD activity reveals that during the high MHD activity the Hurst parameter decreases. Finally, we present a cellular automaton whose parameters are adjusted to simulate the analyzed turbulence SOC change with the MHD activity variation.     

Origin and evolution of MHD turbulence in the solar wind

Summary Solar wind turbulence has both solar and interplanetary sources. Outward travelling waves are generated at the Sun with a spectrum related to the turbulent processes in the solar atmosphere. Instabilities at velocity shears give a local contribution of waves with inward and outward propagation. Nonlinear interactions between the two kinds of waves lead to an energy cascade and to a Kolmogoroff spectrum. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Unusual preflare modulation of microwave radiation in coronal magnetic loops

In the analysis of low-frequency modulation of the microwave radiation of coronal magnetic loops, we detected modulation whose frequency increases from two to three times just before the beginning of the flare and then decreases to the initial values as the flare proceeds. It is shown that such an effect can be evidence for a sharp increase in the loop current just before the flare process, which leads to the plasma heating and increased energy release. The flute instability can serve as a possible reason for the sharp increase in current.     

Large deviation statistics of the energy-flux fluctuation in the shell model of turbulence

The energy-flux fluctuation in the shell model of turbulence is numerically analyzed from the large deviation statistical point of view. We first observe that the rate function defined in the inertial range is independent of the Reynolds number. The rate function derived by the cascade model of the log-Poisson statistics turns out to be in good agreement with the present numerical result in the region where strong singularity of fluctuation exits. This fact may imply the universality as well as the robustness of the large deviation statistical quantities in turbulence.     

Dynamical evolution of coherent structures in intermittenttwo-dimensional MHD turbulence

Recent satellite observations indicate that the Earth's magnetotail is generally in a state of intermittent turbulence. A model of sporadic localized merging of coherent structures has recently been proposed by Chang to describe the dynamics of the Earth's magnetotail. The authors report the results of MHD simulations regarding the development and merging of two dimensional coherent structures. With a magnetic shear, such coherent structures are generated in alignment with the imposed current sheet. The calculated fluctuation spectra suggest long-ranged correlations with power-law characteristics     

On two-dimensionalization of three-dimensional turbulence in shell models

Applying a modified version of the Gledzer-Ohkitani-Yamada (GOY) shell model, the signatures of so-called two-dimensionalization effect of three-dimensional incompressible, homogeneous, isotropic fully developed unforced turbulence have been studied and reproduced. Within the framework of shell models we have obtained the following results: (i) progressive steepening of the energy spectrum with increased strength of the rotation, and, (ii) depletion in the energy flux of the forward forward cascade, sometimes leading to an inverse cascade. The presence of extended self-similarity and self-similar PDFs for longitudinal velocity differences are also presented for the rotating 3D turbulence case.     

A hybrid Lagrangian-Eulerian particle model for reacting pollutant dispersion in non-homogeneous non-isotropic turbulence

Lagrangian stochastic models are recognized as being powerful tools for pollutant dispersion at different scales in complex terrain and at different stability conditions. One of the still unresolved problems is the difficulty of including chemical reactions when, for example, NO₂ or O₃ concentrations have to be predicted in the presence of NO_x emissions. In this work, a Lagrangian stochastic (single particle) model is modified in order to account for simple chemical reactions and tested against measured data in a wind tunnel. It is well-known that, in the single particle models the trajectories are considered independent and hence the concentration correlations and fluctuations cannot be calculated. However, these models can be simply modified to account for the segregation throughout a proper parameterisation derived from measurements. Further, in order to avoid the use of the large amount of computational resources, which would be necessary due to the release of an high number of particles filling the whole domain, needed to reproduce the ozone background concentration, we mark the particles with a deficit of ozone instead of its concentration. A numerical experiment is carried out and the results of the comparisons between calculated and measured concentrations of different species are presented and discussed.     

Inverse cascade regime in shell models of two-dimensional turbulence

We consider shell models that display an inverse energy cascade similar to two-dimensional turbulence (together with a direct cascade of an enstrophylike invariant). Previous attempts to construct such models ended negatively, stating that shell models give rise to a "quasiequilibrium" situation with equipartition of the energy among the shells. We show analytically that the quasiequilibrium state predicts its own disappearance upon changing the model parameters in favor of the establishment of an inverse cascade regime with Kolmogorov scaling. The latter regime is found where predicted, offering a useful model to study inverse cascades.