On the universality of asymptotic limits in the theory of field line diffusion and perpendicular transport of energetic particles

We discuss the random walk of magnetic field lines in astrophysical plasmas. Based on the standard theory of field line diffusion we show that there are two asymptotic limits. In these limits field line wandering is universal because in both regimes the field line diffusion coefficient depends only on fundamental length scales and absolute magnetic field strengths. As examples we discuss the field line diffusion coefficient for different prominent turbulence models namely the slab model, the two-dimensional model, and the Goldreich–Sridhar model. We show that the field line diffusion coefficient for the latter model agrees with the results obtained for slab and two-dimensional turbulence in limiting cases. We also discuss the transport of energetic particles perpendicular with respect to the mean magnetic field. Based on the unified nonlinear transport theory we consider again asymptotic limits. It is shown that one can identify four different regimes in which the transport is again universal. In all four cases perpendicular transport only depends on fundamental length scales of turbulence, magnetic field values, and the parallel diffusion coefficient.     

狭长型封闭舱室内非定常流动模拟的湍流模型比较

针对狭长型封闭舱室内非定常流动模拟的湍流模型选择问题，以飞机舱室为典型环境，使用相似准则为依据的热缩比法搭建了实验平台。将实验结果与RNG k-ε、DES、LES三种湍流模型的数值模拟结果进行对比和分析，评估狭长型封闭舱室内非定常流动特征研究中合适的湍流模型。结果显示，RNG k-ε和DES模型可以定性描述流动变化趋势，但是LES模型在流场非定常性和不稳定性捕捉更为准确，其流场结构更接近实验结果。模拟结果与实验结果对比显示，LES模型能更加真实地反映狭长型封闭舱室非定常流动的情况。     

使用GAO-YONG湍流方程组 对顶盖驱动方腔流的计算

运用SIMPLER方法、QUICK格式及交错网格技术求解GAO-YONG湍流方程组,对二维顶盖驱动方腔流进行数值模拟,得到方腔内的涡量分布和方腔中心水平剖线上的速度变化曲线,并与DNS(Directly Numerical Simulation)结果进行定性对比,有较好的一致性.结果表明,GAO-YONG湍流方程组中与湍流多尺度现象相对应的机械能方程对此湍流方程组能够合理地模拟出顶盖驱动方腔流中复杂的湍流粘性分布以及湍流能量逆转现象起到了关键作用.研究进一步证明了GAO-YONG湍流方程组能够得到复杂流动的真实粘性场.     

Magnetic Field Fluctuations in the Solar Wind, Foreshock and Magnetosheath： Cluster Data Analysis

46 magnetosheath crossing events from the two years (2001.2-2003.1) of Cluster magnetic field measurements are identified and used to investigate the characters of the magnetic field fluctuations in the regions of undisturbed solar wind, foreshock, magnetosheath. The preliminary results indicate the properties of the plasma turbulence in the magnetosheath are strongly controlled by IMF orientation with respect to the bow shock normal. The amplitude of the magnetic field magnitude and direction variations behind quasi-parallel bow shock are larger than those behind quasi-perpendicular bow shock. Almost purely compressional waves are found in quasi-perpendicular magnetosheath.     

Numerical investigation of the influence of large turbulence scales on the parallel and perpendicular transport of cosmic rays

In recent analytical investigations it has been demonstrated that the turbulence behavior at large scales has a very strong influence on the perpendicular diffusion coefficient of charged particles. In the present paper we use computer simulations to investigate numerically cross field transport and particle propagation along the mean magnetic field for different turbulence models at large scales. Our results are compared with quasilinear theory and nonlinear diffusion theories. We show that for different forms of the turbulence spectrum at large scales, the perpendicular mean free paths obtained numerically are in agreement with recent predictions made by analytical theory. It is also shown that the parallel diffusion coefficient contains always a strong nonlinear contribution which is, however, independent of the assumed spectrum at large scales.     

均匀各向同性湍流的脱体涡数值模拟

采用脱体涡模拟方法对均匀各向同性湍流进行了数值模拟,并与Comte-Bellot实验测量结果进行了对比,验证了该文的脱体涡模拟方法对均匀各向同性湍流模拟的可靠性.初始速度场的生成采用Rogallo所提出的构造方法,初始湍能谱满足Von Karman波谱分布;在对流输运项的选择方面,分别采用二阶中心型格式、四阶偏斜对称型中心格式和迎风型低耗散通量分裂格式,考察它们在均匀各向同性湍流模拟中的计算精度和适用性.同时,通过改变计算域大小以及脱体涡模拟方法中的模型常数,达到能谱截断波数的改变,考察它对各向同性湍流计算的能谱以及能谱截断处小尺度涡能量积累问题的影响.      

On the effects of dynamical turbulence on the perpendicular diffusion of low-energy cosmic ray electrons

Diffusion perpendicular to the heliospheric magnetic field plays an integral role in the transport of charged particles in the heliosphere. In this study the perpendicular diffusion coefficient of low-energy cosmic ray electrons is calculated, using an equation derived from the random ballistic decorrelation interpretation of nonlinear guiding centre theory. An observationally motivated 2D turbulence power spectrum is assumed and the effects of various turbulence inputs on the resulting perpendicular diffusion coefficient are investigated. The perpendicular diffusion coefficients are first determined at 1 AU, for both magnetostatic and dynamical turbulence conditions. These solutions are also evaluated for radial distances of 0.1 AU to 10 AU to further investigate the values of the perpendicular diffusion coefficients in the very inner heliosphere. The results of this study show that the dissipation range of the turbulence power spectrum provides a negligible contribution towards the perpendicular diffusion coefficient, and that solutions derived using only the energy containing range serve as good approximations for solutions derived assuming the full 2D turbulence power spectrum. Finally, it is shown that the effects of dynamical turbulence, as considered in the present study, do not affect the perpendicular diffusion coefficients derived from the scattering theory considered here.     

Energetic particle transport in anisotropic magnetic turbulence

We study energetic particle transport in a magnetic field configuration which models the solar wind magnetic turbulence plus the background field. A power-law Fourier amplitude is used for the fully 3D turbulence model, and in order to model anisotropic turbulence, the constant amplitude surfaces in k space are ellipsoids. The turbulence correlation lengths parallel (perpendicular) to the background magnetic field l_∥ (l_⊥) are varied in a wide range, and proton energies from 1 MeV to 10 GeV are assumed. Considering propagation on a distance corresponding to 1 AU, it is found that transport parallel and perpendicular to the background field heavily depends on the turbulence anisotropy, that is on the ratio l_∥/l_⊥. The spatial distribution of energetic particle follows the shape of magnetic flux tube up to about 10 MeV, while for larger energies the structure of the magnetic flux tube is progressively washed out. The scatterplots of particle distribution show intermittent, non Gaussian structures for l_∥ ≪ l_⊥ (quasi slab turbulence), while a more diffusive, Gaussian structure is obtained for l_∥ ≫ l_⊥ (quasi 2D turbulence). The long time behavior of transport shows that anomalous (subdiffusive perpendicular and superdiffusive parallel) transport regimes are obtained for l_∥ ≪ l_⊥, while Gaussian diffusive transport is obtained for both l_∥ ≫ l_⊥ and the isotropic turbulence case.     

行星际结构与垂直无碰撞激波的相互作用

应用一维混合模拟方法数值研究了两种行星际结构──反向磁场和高密度等离子团与垂直无碰撞激波的相互作用．结果表明，随着激波上游区磁场的反向，下游区磁场将逐渐改变符号，且等离子体密度和速度分别呈现较强的湍动．激波上游和下游的物理量依然满足Rankine-Hugonoit关系．当高密度的等离子体团通过垂直无碰撞激波时，部分质子被激波反射，部分质子被加速并进入下游区域．由于质子速度分布为非Maxwell分布，在激波下游也激发出较强的湍动．     

Von Karman模型三维大气紊流仿真理论与方法

基于三维紊流场相关函数矩阵,用蒙特卡罗法仿真生成Von Karman模型三维空间大气紊流场,实现了对复杂的Von Karman模型的精确推导和应用,仿真流场的相关特性在理论上可以任意逼近Von Karman理论模型.通过提高白噪声序列的白化程度及优化蒙特卡罗算法的方法提高了仿真精度和效率.数值仿真结果证明:所生成的空间大气紊流场符合模型特性,具有较好的统计特性和仿真精度,满足实时飞行仿真对紊流场数据的要求.     

Peculiarities of galactic cosmic ray Forbush effects during October–November 2003

Features of two successive Forbush effects of the galactic cosmic ray intensity in October–November 2003 have been studied based on the neutron monitors data. The rigidity spectrum of the galactic cosmic ray intensity in the course of the first Forbush effect (22–27 October) is gradually hardening, while the rigidity spectrum of the second Forbush effect (28 October–10 November) from the starting moment is very hard. As far, the energy range of the turbulence of the interplanetary magnetic field is in general responsible for the diffusion of galactic cosmic ray particles of the energy 5–50 GeV (to which neutron monitors are sensitive), we postulate that the gradually hardening (from day to day) of the rigidity spectrum of the first Forbush effect is associated with the enhancement of the power spectral density in the energy range of the interplanetary magnetic field turbulence caused by the large scale irregularities generated due to the interaction of the extending high speed disturbances with the background solar wind. The very hard rigidity spectrum (from the starting moment) of the second Forbush effect is generally associated with the well established new structure of the energy range of the interplanetary magnetic field turbulence enriched by the already created large scale irregularities. The gradually softening of the rigidity spectrum during the recovery phase of the second Forbush effect confirms that the disturbed interplanetary magnetic field turbulence step by step returns to the initial state.     

On the mechanisms of cooling of the nightside thermosphere of venus

The plausible mechanisms of cooling of the nightside Venus' thermosphere are analysed with the aid of the model of the atmospheric heat budget that incorporates, in addition to thermal conduction and IR radiation in the 15 μ band of CO₂, heating and cooling due to global scale winds, eddy turbulence, and IR radiation in the rotational bands of H₂O and CO, as well as the 63 μ line of atomic oxygen. The H₂O mixing ratio and parameters of turbulence required for cooling of the thermosphere down to the observed low temperatures are evaluated.     

微重力量值对气液两相流相分布及液相湍流统计量影响的数值研究

气液两相流在空间领域具有广阔的应用前景, 深入理解微重力量值(微重力大小)对相分布和液相湍流的影响十分必要. 采用欧拉elax-elax拉格朗日双向耦合模型深入研究了不同微重力量值对相分布和液相湍流的影响. 液相速度场通过直接数值模拟求解, 气泡的运动轨迹由牛顿运动方程跟踪. 研究表明, 气泡分布和液相湍流与微重力量值均具有直接联系. 在低微重力量值下, 气泡近似均匀分布在槽道内, 且对液相湍流统计量几乎没有影响; 然而当微重力量值较高时, 大量气泡聚集在壁面附近, 液相湍流由于气泡的注入受到极大调制.      

Statistics of the Interplanetary Magnetic Fields Observed at 0.72 AU And 1 AU

This paper presents the Interplanetary Magnetic Field (IMF) observations at 0.72 AU measured by Venus Express (VEX) and 1 AU by Advanced Composition Explorer (ACE) in 2007. The distributions of daily averages of $B$ are lognormal in both locations. The multiscale structure of the magnetic field fluctuations was described by studying the increments of $B$ over a range scales from 10 min to 21.3 hours. All the Probability Distribution Functions (PDFs) can be described quantitatively by Tsallis distribution function. On the ecliptic plane from 0.72 AU to 1 AU, the entropy index q increases with distance over all scales, indicating the intermittency of turbulence is growing. The widths of the PDFs at 0.72 AU are larger than those at 1 AU at all scales, which indicating the turbulence at 0.72 AU is more intense than that at 1 AU. This helps us understand the nature and development of the magnetic field fluctuations.      

贮箱增压孔板流量系数仿真研究

数值模拟方法是研究流体流场特性的有效手段之一,采用流体仿真手段,根据仿真实验数据,找出适合计算孔板流量系数的湍流模型,通过仿真等效直径、组合工况下同一孔板的流场分布,分析其实验数据并拟合其流量系数变化曲线,对实际流量试验有一定的参考价值。     

基于涡方法生成大涡模拟进口条件的数值计算

探索改进涡方法来生成大涡模拟的非定常进口条件.改进方法中,为了避免局部漩涡数量过多或者过少,采用密度分布方式放置漩涡场.并用Langevin方程控制漩涡运动,模拟实验方法中的蜂窝器,使改进后的涡方法生成的脉动速度场更加符合湍流的特征.在已知雷诺平均的流场结果下,利用涡方法产生漩涡场,进而生成能满足大涡模拟所需要的非定常进口流场.为了检验改进后的涡方法在生成脉动速度过程中的效果,在槽道中进行了对比数值试验,并借助于直接数值模拟数据做对比,对比分析槽道进出口的平均速度、涡量以及雷诺应力统计,证明改进后的涡方法在生成大涡模拟进口条件下是非常有效的.      

高超声速二方程湍流模型的数值模拟对比

对基于Reynolds和Favré(密度加权)混合平均的二方程湍流模型进行了修正,同时根据数值模拟高超声速流动时必须具有高分辨率捕捉间断面与在边界层内抑制数值粘性的能力的要求,提出了新的总变差减小(TVD)格式熵修正函数.在此基础上,通过对压缩拐角的高超声速湍流的数值模拟,对基于Reynolds和Favré混合平均的二方程湍流模型,以及其它不可压缩模型及可压缩性修正模型进行了对比,显示了不同湍流模型及可压缩性修正在计算壁面压力分布和热流分布上的特点,说明了对高超声速压缩拐角型流动,湍流模型可压缩修正的必要性,得到了基于Reynolds和Favré混合平均的二方程湍流模型的计算结果最接近实验结果.      

倒V电子沉降通量能谱的理论计算

沿极光区磁力线大约在2000公里到8000公里的高度范围内,存在着一个等离子体湍流和大尺度平行电场的加速区。沿磁力线运动的等离子体片中的电子通过此加速区时,受到等离子体湍流和平行电场的共同作用,形成电子沉降的倒V结构。从一维准线性的动力学方程出发,导出了沉降电子通量的能谱方程,得出了电子通量能谱的理论公式。对等离子体湍流和平行电场对沉降电子能谱的影响作了分析和讨论。本文所提出的理论可以解释目前观测到的某些基本现象。      

Energy dependence of the rigidity spectrum of Forbush decrease of galactic cosmic ray intensity

We show that rigidity spectrum of Forbush decrease (Fd) of galactic cosmic ray (GCR) intensity in September 9–23, 2005 clearly depends on energy. We calculated rigidity spectrum of the Fd based on the neutron monitors and Nagoya muon telescope channels’ data divided in three groups according to their cut off rigidities. We found that temporal changes of rigidity spectrum exponent γ are approximately similar for all cut off rigidity groups, but γ values are the larger the higher are cut off rigidities. We conclude that rigidity spectrum of Fd is hard for lower energy range and is soft for the higher energy range. We believe that an energy dependence of the power law rigidity spectrum of Fd is observed owing to the preferential convection–diffusion mechanism during Fd in September 9–23, 2005. It is a reflection of an influence of the temporal changes of the structure of the interplanetary magnetic field (IMF) turbulence in different range of frequency f during Fd. Particularly, a decisive role in formation of the character of the rigidity spectrum belongs to the changes of the exponent ν of the power spectral density (PSD) of the IMF turbulence (PSD ∝ f^−ν). The exponent ν is greater for high frequency region of the IMF turbulence (responsible for scattering of low rigidity particles of GCR), than for low frequency region of the IMF turbulence (being responsible for scattering of higher rigidity particles). Also, we challenge to estimate an existence of slab/2D structure of solar wind turbulence during the Fd in September 9–23, 2005 based on the distribution of average turbulence energy among the IMF’s components.     

大气紊流下飞艇动力学建模与姿态控制

大气紊流是影响高空飞艇水平面内运动的重要原因,紊流扰动对飞艇的安全性能和定点悬停有严重影响,能否克服扰动,直接决定飞艇的载荷能力和续航时间.根据高空大气紊流的特点以及紊流风速对飞艇姿态调整的影响建立风场的传递函数模型,根据风速三角形关系建立扰动下的飞艇水平面内动力学模型,利用内模控制原理设计出内模控制器,将扰动信号反馈到系统的输入端,从而对扰动进行直接控制.仿真结果表明所设计的控制器可以有效抑制大气紊流对飞艇飞行性能的影响.