Magnetopause properties at the dusk magnetospheric flank from global magnetohydrodynamic simulations,the kinetic Vlasov equilibrium,and in situ observations--Potential implications for SMILE

We derived the properties of the terrestrial magnetopause(MP)from two modeling approaches,one global–fluid,the other local–kinetic,and compared the results with data collected in situ by the Magnetospheric Multiscale 2(MMS2)spacecraft.We used global magnetohydrodynamic(MHD)simulations of the Earth’s magnetosphere(publicly available from the NASA-CCMC[National Aeronautics and Space Administration–Community Coordinated Modeling Center])and local Vlasov equilibrium models(based on kinetic models for tangential discontinuities)to extract spatial profiles of the plasma and field variables at the Earth’s MP.The global MHD simulations used initial solar wind conditions extracted from the OMNI database at the time epoch when the MMS2 observes the MP.The kinetic Vlasov model used asymptotic boundary conditions derived from the same in situ MMS measurements upstream or downstream of the MP.The global MHD simulations provide a three-dimensional image of the magnetosphere at the time when the MMS2 crosses the MP.The Vlasov model provides a one-dimensional local view of the MP derived from first principles of kinetic theory.The MMS2 experimental data also serve as a reference for comparing and validating the numerical simulations and modeling.We found that the MP transition layer formed in global MHD simulations was generally localized closer to the Earth(roughly by one Earth radius)from the position of the real MP observed by the MMS.We also found that the global MHD simulations overestimated the thickness of the MP transition by one order of magnitude for three analyzed variables:magnetic field,density,and tangential speed.The MP thickness derived from the local Vlasov equilibrium was consistent with observations for all three of these variables.The overestimation of density in the Vlasov equilibrium was reduced compared with the global MHD solutions.We discuss our results in the context of future SMILE(Solar wind Magnetosphere Ionosphere Link Explorer)campaigns for observing the Earth’s MP.

Detailed analysis of fan-shaped jets in three dimensional numerical simulation

We performed three dimensional resistive magnetohydrodynamic simulations to study the magnetic reconnection using an initially shearing magnetic field configuration（force free field with a current sheet in the middle of the computational box）.It is shown that there are two types of reconnection jets：the ordinary reconnection jets and fan-shaped jets,which are formed along the guide magnetic field.The fan-shaped jets are significantly different from the ordinary reconnection jets which are ejected by magnetic tension force.There are two driving forces for accelerating the fan-shaped jets.One is the Lorentz force which initially dominates the motion of fluid elements,and then the gas pressure gradient force accelerates the fluid elements in the later stage.The dependence on magnetic reconnection angle and resistivity value has also been studied.The formation and evolution of these jets provide a new understanding of dynamic magnetohydrodynamic jets.

Numerical simulation of busbar configuration in large aluminum electrolysis cell

Various busbar configurations were built and modeled by the custom code based on the commercial package ANSYS for the 500 kA aluminum electrolysis cell.The configuration parameters,such as side riser entry ratio,number of cathode bars connected to each riser,vertical location of side cathode busbar and short side cathode busbar,distance between rows of cells in potline,the number of neighboring cells,ratio of compensation busbar carried passing under cell and its horizontal location under cell along with large magnetohydrodynamic(MHD) computation based on the custom evaluation function were simulated and discussed.The results show that a cell with riser entry ratio of 11:9:8:9:11 and cathode busbar located at the level of aluminum solution,50% upstream cathode current passing under cell for magnetic field compensation,the distance between rows of 50 m is more stable.

Numerical investigation of Dufour and Soret effects on unsteadyMHD natural convection flow past vertical plate embedded innon-Darcy porous medium

The Dufour and Soret effects on the unsteady twodimensional magnetonyaro dynamics （MHD） doublediffusive free convective flow of an electrically conducting fluid past a vertical plate embedded in a nonDarcy porous medium are investigated numeri cally. The governing nonlinear dimensionless equations are solved by an implicit finite difference scheme of the CrankNicolson type with a tridiagonal matrix manipulation. The effects of various parameters entering into the problem on the unsteady dimension less velocity, temperature, and concentration profiles are studied in detail. Furthermore, the time variation of the skin friction coefficient, the Nusselt number, and the Sherwood number is presented and analyzed. The results show that the unsteady velocity, tem perature, and concentration profiles are substantially influenced by the Dufour and Soret effects. When the Dufour number increases or the Soret number decreases, both the skin friction and the Sherwood number decrease, while the Nusselt number increases. It is found that, when the magnetic parameter increases, the velocity and the temperature decrease in the boundary layer.

Numerical experiments on the evolution in coronal magnetic configurations including a filament in response to the change in the photosphere

We investigate equilibrium height of a flux rope, and its internal equilib- rium in a realistic plasma environment by carrying out numerical simulations of the evolution of systems including a current-carrying flux rope. We find that the equilib- rium height of a flux rope is approximately described by a power-law function of the relative strength of the background field. Our simulations indicate that the flux rope can escape more easily from a weaker background field. This further confirms that a catastrophe in the magnetic configuration of interest can be triggered by a decrease in strength of the background field. Our results show that it takes some time to reach internal equilibrium depending on the initial state of the flux rope. The plasma flow inside the flux rope due to the adjustment for the internal equilibrium of the flux rope remains small and does not last very long when the initial state of the flux rope com- mences from the stable branch of the theoretical equilibrium curve. This work also confirms the influence of the initial radius of the flux rope in its evolution; the results indicate that a flux rope with a larger initial radius erupts more easily. In addition, by using a realistic plasma environment and a much higher resolution in our simulations, we notice some different characteristics compared to previous studies in Forbes.

Gas-Kinetic BGK Scheme for Three Dimensional Magnetohydrodynamics

The gas-kinetic theory based flux splitting method has been successfully proposed for solving one-and two-dimensional ideal magnetohydrodynamics by Xu et al. [J.Comput.Phys.,1999;2000],respectively.This paper extends the kinetic method to solve three-dimensional ideal magnetohydrodynamics equations,where an adaptive parameter 17 is used to control the numerical dissipation in the flux splitting method. Several numerical examples are given to demonstrate that the proposed method can achieve high numerical accuracy and resolve strong discontinuous waves in three dimensional ideal MHD problems.

Numerical simulation on magnetohydrodynamic power generation channel of scramjet

The reverse magnetohydrodynamic(MHD)energy bypass technology is a promising energy redis⁃tribution technology in the scramjet system,in augmented with a power generation equipment to supply the neces⁃sary long-distance flight airframe power.In this paper,a computational model of the scramjet magnetohydrody⁃namic channel is developed and verified by using the commercial software Fluent.It is found that when the mag⁃netic induction intensity is 1,2,3,4 T,the power generation efficiency is 22.5%,22.3%,22.0%,21.5%,and decreases with the increase of the magnetic induction intensity,and the enthalpy extraction rate is 0.026%,0.1%,0.21%,0.34%,and increases with the increase of the magnetic induction intensity.The deceleration ef⁃fect of electromagnetic action on the airflow in the power channel increases with the increase of magnetic induc⁃tion intensity.The stronger the magnetic field intensity,the more obvious the decreasing effect of fluid Mach num⁃ber in the channel.The power generation efficiency decreases as the magnetic induction intensity increases and the enthalpy extraction rate is reversed.As the local currents gathering at inlet and outlet of the power generation area,total temperature and enthalpy along the flow direction do not vary linearly,and there are maximum and minimum values at inlet and outlet.Increasing the number of electrodes can effectively regulate the percentage of Joule heat dissipation,which can improve the power generation efficiency.

高空核爆炸磁流体动力学电磁脉冲

高空核爆炸产生的磁流体动力学(晚期)电磁脉冲对电力系统等国家重要基础设施具有严重影响。由于晚期电磁脉冲产生的机理复杂,依赖因素众多,包括爆炸当量、爆高、爆炸方位、爆炸时间、观察点位置以及土壤电导率等,因此,目前还没有成熟的代码可以模拟整个晚期电磁脉冲的产生过程。介绍晚期电磁脉冲的产生机理,讨论晚期电磁脉冲电场随核装置爆炸当量、爆高和大气状况的变化关系。E3A电场峰值随爆炸当量线性增加,而E3B电场峰值则随爆炸当量增加出现明显的饱和效应。分析了当前晚期电磁脉冲模拟代码现状,为进一步研究晚期电磁脉冲数值模拟方法和代码研发提供参考。

高超声速飞行器流场电磁场松耦合数值模拟

在高超声速再入式飞行器通信黑障问题众多解决方案中,电磁调控方法是最具可行性的方法之一.一个细致可靠的电磁调控方案不仅需要准确考虑外加电磁场对导电流体作用,还需要考虑因流体高速运动产生的感生电磁场对导电流体的作用.对于高超声速飞行器典型飞行状态,全磁流体力学方法因数值刚性问题很难直接用于流场和电磁场耦合模拟,而低磁雷诺数近似方法无法考虑感生磁场对导电流体作用.针对磁流体力学两种经典方法应用于高超声速飞行器电磁调控方案设计时存在的困难和不足,文章在数值求解流动控制方程N-S方程和电磁控制方程双曲Maxwell方程的基础上,将两者结合起来松耦合迭代求解,建立适用于高超声速飞行器流场和电磁耦合的数值模拟方法.通过磁流体激波反射和二维超声速磁流体喷管两个典型算例,对文章的数值方法进行验证.基于数值方法对外加偶极子磁场作用下某高超声速飞行器典型飞行状态下流场电磁耦合问题进行了研究,数值模拟结果表明,磁雷诺数为0.7557时,不考虑感生磁场对流体运动影响时激波脱体距离相对于考虑感生磁场作用时大11.11%,此误差已不容忽视,对于该飞行状态,应完整考虑流场和磁场相互作用而不是仅考虑外加磁场对流场运动影响.

磁等离子体推进器阳极半径对阳极功率沉降的影响

随着人类航天事业的发展,需要研发适用于不同空间任务的推进系统,与磁约束核聚变原理类似的磁等离子体动力推进器(MegnetoPlasmaDynamic Thruster,MPDT)在推力功率比和比冲方面具有优越性能。阳极功率沉降是MPDT运行过程中等离子体与壁面相互作用的结果,占总功率的40%~90%,严重降低推进器效率。针对该问题,本文从阳极功率沉降的角度探究了阳极半径对推进器效率的影响。基于磁流体力学方程(MagnetoHydroDynamic,MHD),利用数值计算方法建立径向放电参数数值模型和阳极功率沉降物理模型,研究计算了阳极半径对放电参数和阳极功率沉降的影响规律,并建立热仿真模型对阳极水冷散热结构进行了热仿真。研究结果表明:增大阳极半径,电子密度及离子速度得到提高,阳极功率沉降增大,但阳极功率沉降分数降低,推进器效率得到提升。热仿真结果显示,水冷结构阳极在输入阳极功率沉降约为3 kW时,对应的阳极冷却水温差为5 K。本研究验证了阳极功率沉降物理模型的可靠性,并指出增大阳极半径是提升推进器效率的有效手段。

Feedback of Efficient Shock Acceleration on Magnetic-field Structure Inside Young Type Ia Supernova Remnants

Using an effective adiabatic index γ_(eff) to mimic the feedback of efficient shock acceleration,we simulate the temporal evolution of a young type Ia supernova remnant (SNR) with two different background magnetic field(BMF) topologies:a uniform and a turbulent BMF.The density distribution and magnetic-field characteristics of our benchmark SNR are studied with two-dimensional cylindrical magnetohydrodynamic simulations.When γ_(eff)is considered,we find that:(1) the two-shock structure shrinks and the downstream magnetic-field orientation is dominated by the Rayleigh–Taylor instability structures;(2) there exists more quasi-radial magnetic fields inside the shocked region;and (3) inside the intershock region,both the quasi-radial magnetic energy density and the total magnetic energy density are enhanced:in the radial direction,with γ_(eff)=1.1,they are amplified about 10–26 times more than those with γ_(eff)=5/3.While in the angular direction,the total magnetic energy densities could be amplified about 350 times more than those with γ_(eff)=5/3,and there are more grid cells within the intershock region where the magnetic energy density is amplified by a factor greater than 100.

Magnetohydrodynamic process in solar activity

Magnetohydrodynamics is one of the major disciplines in solar physics. Vigorous magnetohydrodynamic process is taking place in the solar convection zone and atmosphere. It controls the generating and structuring of the solar mag- netic fields, causes the accumulation of magnetic non-potential energy in the solar atmosphere and triggers the explosive magnetic energy release, manifested as vi- olent solar flares and coronal mass ejections. Nowadays detailed observations in solar astrophysics from space and on the ground urge a great need for the studies of magnetohydrodynamics and plasma physics to achieve better understanding of the mechanism or mechanisms of solar activity. On the other hand, the spectac- ular solar activity always serves as a great laboratory of magnetohydrodynamics. In this article, we reviewed a few key unresolved problems in solar activity studies and discussed the relevant issues in solar magnetohydrodynamics.

Mathematical modeling of fractional derivatives for magnetohydrodynamic fluid flow between two parallel plates by the radial basis function method

Investigations into the magnetohydrodynamics of viscous fluids have become more important in recent years,owing to their practical significance and numerous applications in astro-physical and geo-physical phenomena.In this paper,the radial base function was utilized to answer fractional equation associated with fluid flow passing through two parallel flat plates with a magnetic field.The magnetohydrodynamics coupled stress fluid flows between two parallel plates,with the bottom plate being stationary and the top plate moving at a persistent velocity.We compared the radial basis function approach to the numerical method(fourth-order Range-Kutta)in order to verify its validity.The findings demonstrated that the discrepancy between these two techniques is quite negligible,indicating that this method is very reliable.The impact of the magnetic field parameter and Reynolds number on the velocity distribution perpendicular to the fluid flow direction is illustrated.Eventually,the velocity parameter is compared for diverse conditionsα,Reynolds and position(y),the maximum of which occurs atα=0.4.Also,the maximum velocity values occur inα=0.4 and Re=1000 and the concavity of the graph is less forα=0.8.

Magnetohydrodynamic Effect on Free Convection of Three Dimensional Laminar Flow in Porous Annulus

A numerical study has been carried out to investigate heat transfer by free convection under the effect of MHD （magnetohydrodynamic） for steady state three-dimensional laminar flow in horizontal and vertical cylindrical annulus filled with saturated porous media （sand silica） with fins attached to the inner cylinder. A single electric coil placed around the inner cylinder to generate a magnetic field. The governing equations which used are continuity, momentum （using Darcy＇s law） and energy equations which are transformed to dimensionless equations. The finite difference approach is used to obtain all the computational results using Fortran 90 program. The parameters affected on the system are Rayleigh number ranging within （102 ~ Ra＊ 〈 104）, and MHD （Mn） （0 〈_ Mn 〈_ 100） and radius ratio Rr （0.225, 0.338 and 0.435）. The results obtained are presented graphically in the form of streamline and isotherm contour plots and the results show that heat transfer decrease with the increase of magnetohydrodynamic. It was found that the average Nusselt number increase with Ra＊ and decrease with H~ Mn and Rr. A correlation for the average Nusselt number in terms of Ra＊ and Mn, has been developed for the inner cylinder.

太阳与内日球立体探测卫星计划科学目标建议

面向认知太阳风暴和保障航天安全的国家战略需求,中国近年来提出太阳与内日球立体探测卫星计划,拟适时部署环黄道面的拉格朗日点L3-L4-L5定点卫星星座和脱离黄道面的太阳极轨对偶卫星。通过该卫星计划的初步科学论证分析,建议将其科学目标凝练为太阳磁场、太阳风暴、太阳风,将其应用目标定位于日地空间天气预报,建议配置成像类、粒子类、波场类的综合科学载荷。此外,利用日地空间的三维磁流体数值模拟和计算机图形学的艺术可视化,形象地表达环黄道面卫星“对日凝视”和太阳极轨卫星“鸟瞰全景”的探测概念。中国太阳与内日球立体探测卫星计划将揭示太阳磁场的起源规律及其爆发机理、日地耦合系统的空间天气变化机理,并能够为日地空间天气三维数值建模提供观测数据驱动的初边值条件,必将极大推动空间天气的立体监测、前沿研究、精准预报的一体化进程。

Numerical Simulation of 2D Supersonic Magnetohydrodynamic Channel and Study on Hall Effect

In this research effort,numerical simulation of two-dimensional magnetohydrodynamic（MHD） channel is performed and Hall effect is studied.The computational model consists of the Navier-Stokes（N-S） equations coupled with electrical-magnetic source terms,Maxwell equations and the generalized Ohm＇s law.Boundary conditions for the electrical potential equation considering Hall effect are derived.To start with,the MHD channel with single-pair electrodes is studied and flow of the electric current is in accordance with physical principle.Then the MHD channel with five-pair electrodes is numerically simulated.The results show that the electrical current concentrates on the downstream of the anode and the upstream of the cathode due to Hall effect,and the flow field becomes asymmetrical.At the current value of the magnetic interaction parameter,the electrical-magnetic force affects the flow remarkably,decreasing the outlet Mach number and increasing the outlet pressure;what＇s more,the flow structure in the channel becomes extremely complex.Performances of MHD channels with continual electrodes and segmented electrodes are compared.The results show that performance of the MHD channel with segmented electrodes is better than that with continual electrodes with the increase of Hall parameter.

Shape Effect of Nanoparticles on Nanofluid Flow Containing Gyrotactic Microorganisms

In this paper,we discussed the effect of nanoparticles shape on bioconvection nanofluid flow over the vertical cone in a permeable medium.The nanofluid contains water,Al2O3 nanoparticles with sphere(spherical)and lamina(non-spherical)shapes and motile microorganisms.The phenomena of heat absorption/generation,Joule heating and thermal radiation with chemical reactions have been incorporated.The similarity transformations technique is used to transform a governing system of partial differential equations into ordinary differential equations.The numerical bvp4c MATLAB program is used to find the solution of ordinary differential equations.The interesting aspects of pertinent parameters on mass transfer,energy,concentration,and density of themotilemicroorganisms’profiles are computed and discussed.Our analysis depicts that the performance of sphere shape nanoparticles in the form of velocity distribution,temperature distribution,skin friction,Sherwood number and Motile density number is better than lamina(non-spherical)shapes nanoparticles.

辐射磁流体力学数值实验研究(代前言)

辐射磁流体力学(RMHD)是磁流体力学和等离子体物理学一个新的分支,它研究与辐射有显著能量和/或动量交换的磁流体动力学行为。天体辐射磁流体力学描述天体等离子体在宏观尺度上的电磁相互作用、结构、辐射、动力学和爆发现象。"天体辐射磁流体力学"是中国科学院数理学部2015—2016年度所支持的一个学科发展战略研究项目,其目的是评估这一生长中的学科分支的发展态势、国内外研究现状、适用的主要科学对象和发展战略,重点设定在三维数值模拟研究,或广义而言,数值实验研究。为了推动RMHD三维数值实验研究,这一专卷收入了天体物理学、太阳和空间物理学、受控等离子体实验等领域关于RMHD研究的部分调研和评述报告。

高超声速磁流体力学控制霍尔效应影响

针对霍尔效应对高超声速磁流体力学控制的影响问题,考虑高超声速流动过程中高温化学反应、气体分子热力学温度激发(即平动、转动、振动以及电子温度能量模态之间的激发与松弛过程)及多电离组分等离子体霍尔系数分布,通过耦合求解各向异性霍尔电场泊松方程和带电磁源项的高温热化学非平衡流动控制方程组,建立了高超声速流动磁流体力学控制霍尔效应数值模拟方法,开展了多种条件下高超声速流动磁流体力学控制数值模拟,分析了霍尔效应"漏电"与"聚集"现象原理及其对气动力/热特性的影响机制,详细探讨了不同空域、速域和飞行器特征尺度条件下霍尔效应的作用机理和影响规律.研究表明:1)霍尔效应改变了流场等离子体洛伦兹力分布,削弱了整体的力学效果,使整体的磁阻特性降低;2)霍尔效应对高超声速磁流体力学控制的影响,与壁面导电性和壁面附近漏电层的"漏电"现象紧密相关,要增强磁控效果,必须抑制壁面附近的"漏电"现象;3)霍尔效应对磁控热防护效果的影响较为复杂,受"漏电"现象和电流"聚集"现象共同作用;4)基于本文基准状态,当高度高于67 km或速度高于5.7 km/s或特征尺度大于0.5 m时,霍尔效应使磁控热防护效果增强,电流"聚集"现象对气动热环境的影响占主导;反之,则霍尔效应使磁控热防护效果减弱,"漏电"现象对气动热环境的影响占主导.

热化学模型对高超声速磁流体控制数值模拟影响分析

针对等离子体流场的模拟准确性问题及其对高超声速磁流体控制的影响,通过数值求解三维非平衡Navier-Stokes流场控制方程和Maxwell电磁场控制方程,建立了三维低磁雷诺数磁流体数值模拟方法及程序,分析了不同空气组分化学反应模型和壁面有限催化效率等因素对高超声速磁流体控制的影响.研究表明:不同空气组分化学反应模型对高超声速磁流体流场结构、气动力/热特性控制的影响不容忽视;对于本文计算条件,Park化学反应模型在组分模型一致性、等离子体模拟准确性等方面具有一定优势;磁控热防护效果,受壁面有限催化复合系数影响较大,两者呈非线性关系,不同表面区域差异较大;磁场对磁阻力伞及其磁阻力特性影响,受壁面催化效应的影响相对较小.