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.

Analytical studies on the evolution processes of rarefied deuterium plasma shell Z-pinch by PIC and MHD simulations

In this paper, we analytically explore the magnetic field and mass density evolutions obtained in particle-in-cell（PIC）and magnetohydrodynamics（MHD） simulations of a rarefied deuterium shell Z-pinch and compare those results, and also we study the effects of artificially increased Spitzer resistivity on the magnetic field evolution and Z-pinch dynamic process in the MHD simulation. There are significant differences between the profiles of mass density in the PIC and MHD simulations before 45 ns of the Z-pinch in this study. However, after the shock formation in the PIC simulation,the mass density profile is similar to that in the MHD simulation in the case of using multiplier 2 to modify the Spitzer resistivity. Compared with the magnetic field profiles of the PIC simulation of the shell, the magnetic field diffusion has still not been sufficiently revealed in the MHD simulation even though their convergence ratios become the same by using larger multipliers in the resistivity. The MHD simulation results suggest that the magnetic field diffusion is greatly enhanced by increasing the Spitzer resistivity used, which, however, causes the implosion characteristic to change from shock compression to weak shock, even shockless evolution, and expedites the expansion of the shell. Too large a multiplier is not suggested to be used to modify the resistivity in some Z-pinch applications, such as the Z-pinch driven inertial confinement fusion（ICF） in a dynamic hohlraum. Two-fluid or Hall MHD model, even the PIC/fluid hybrid simulation would be considered as a suitable physical model when there exist the plasma regions with very low density in the simulated domain.

An Unconventional Divergence Preserving Finite-Volume Discretization of Lagrangian Ideal MHD

We construct an unconventional divergence preserving discretization of updated Lagrangian ideal magnetohydrodynamics(MHD)over simplicial grids.The cell-centered finite-volume(FV)method employed to discretize the conservation laws of volume,momentum,and total energy is rigorously the same as the one developed to simulate hyperelasticity equations.By construction this moving mesh method ensures the compatibility between the mesh displacement and the approximation of the volume flux by means of the nodal velocity and the attached unit corner normal vector which is nothing but the partial derivative of the cell volume with respect to the node coordinate under consideration.This is precisely the definition of the compatibility with the Geometrical Conservation Law which is the cornerstone of any proper multi-dimensional moving mesh FV discretization.The momentum and the total energy fluxes are approximated utilizing the partition of cell faces into sub-faces and the concept of sub-face force which is the traction force attached to each sub-face impinging at a node.We observe that the time evolution of the magnetic field might be simply expressed in terms of the deformation gradient which characterizes the Lagrange-to-Euler mapping.In this framework,the divergence of the magnetic field is conserved with respect to time thanks to the Piola formula.Therefore,we solve the fully compatible updated Lagrangian discretization of the deformation gradient tensor for updating in a simple manner the cell-centered value of the magnetic field.Finally,the sub-face traction force is expressed in terms of the nodal velocity to ensure a semi-discrete entropy inequality within each cell.The conservation of momentum and total energy is recovered prescribing the balance of all the sub-face forces attached to the sub-faces impinging at a given node.This balance corresponds to a vectorial system satisfied by the nodal velocity.It always admits a unique solution which provides the nodal velocity.The robustness and the accuracy of this unconventional FV scheme have been demonstrated by employing various representative test cases.Finally,it is worth emphasizing that once you have an updated Lagrangian code for solving hyperelasticity you also get an almost free updated Lagrangian code for solving ideal MHD ensuring exactly the compatibility with the involution constraint for the magnetic field at the discrete level.

Cross-Diffusion Effects on an MHD Williamson Nanofluid Flow Past a Nonlinear Stretching Sheet Immersed in a Permeable Medium

The primary aim of this research endeavor is to examine the characteristics of magnetohydrodynamicWilliamson nanofluid flow past a nonlinear stretching surface that is immersed in a permeable medium.In the current analysis,the impacts of Soret and Dufour(cross-diffusion effects)have been attentively taken into consideration.Using appropriate similarity variable transformations,the governing nonlinear partial differential equations were altered into nonlinear ordinary differential equations and then solved numerically using the Runge Kutta Fehlberg-45 method along with the shooting technique.Numerical simulations were then perceived to show the consequence of various physical parameters on the plots of velocity,temperature,and concentration of the nanofluid flow.Boosting the magnetic,Williamson,porosity,and stretching sheet index parameters,the velocity of the fluid flow decreases.The temperature is enhanced as theWilliamson and Brownian motion parameters upsurge,but it decreases as the Prandtl,thermophoresis,stretching sheet index,and Dufour parameters escalate.The concentration distribution decreases as the thermophoresis andmagnetic parameters upsurge,but it escalates as the Soret,Schmidt,Brownian motion,and stretching sheet index parameters increase.Skin friction coefficient boosted as the stretching sheet index and magnetic parameters enhanced against the Williamson parameter.The findings from this study have been contrasted with earlier findings on local Nusselt numbers,which show substantial support and endorse the existing approach’s validity.The numerical values of the local Sherwood number gradually increase as the Schmidt,Soret,stretching sheet index,and thermophoresis parameters are upsurged.

Effects of Viscous Dissipation and Periodic Heat Flux on MHD Free Convection Channel Flow with Heat Generation

This study investigates the influence of periodic heat flux and viscous dissipation on magnetohydrodynamic(MHD)flow through a vertical channel with heat generation.A theoretical approach is employed.The channel is exposed to a perpendicular magnetic field,while one side experiences a periodic heat flow,and the other side undergoes a periodic temperature variation.Numerical solutions for the governing partial differential equations are obtained using a finite difference approach,complemented by an eigenfunction expansion method for analytical solutions.Visualizations and discussions illustrate how different variables affect the flow velocity and temperature fields.This offers comprehensive insights into MHD flow behavior and its interactions with the magnetic field,heat flux,viscous dissipation,and heat generation.The findings hold significance for engineering applications concerning fluid dynamics and heat transfer,offering valuable knowledge in this field.The study concludes that the transient velocity and temperature profiles exhibit periodic patterns under periodic heat flow conditions.A temperature reduction is observed with an increase in the wall temperature phase angle.In contrast,an increase in the heat flux phase angle values raises the temperature values.

Artificial Intelligence-Driven FVM-ANNModel for Entropy Analysis ofMHD Natural Bioconvection in Nanofluid-Filled Porous Cavities

The research examines fluid behavior in a porous box-shaped enclosure.The fluid contains nanoscale particles and swimming microbes and is subject to magnetic forces at an angle.Natural circulation driven by biological factors is investigated.The analysis combines a traditional numerical approach with machine learning techniques.Mathematical equations describing the system are transformed into a dimensionless form and then solved using computational methods.The artificial neural network(ANN)model,trained with the Levenberg-Marquardt method,accurately predicts(Nu)values,showing high correlation(R=1),low mean squared error(MSE),and minimal error clustering.Parametric analysis reveals significant effects of parameters,length and location of source(B),(D),heat generation/absorption coefficient(Q),and porosity parameter(ε).Increasing the cooling area length(B)reduces streamline intensity and local Nusselt and Sherwood numbers,while decreasing isotherms,isoconcentrations,and micro-rotation.The Bejan number(Be+)decreases with increasing(B),whereas(Be+++),and global entropy(e+++)increase.Variations in(Q)slightly affect streamlines but reduce isotherm intensity and average Nusselt numbers.Higher(D)significantly impacts isotherms,iso-concentrations,andmicro-rotation,altering streamline contours and local Bejan number distribution.Increased(ε)enhances streamline strength and local Nusselt number profiles but has mixed effects on average Nusselt numbers.These findings highlight the complex interactions between cooling area length,fluid flow,and heat transfer properties.By combining finite volume method(FVM)with machine learning technique,this study provides valuable insights into the complex interactions between key parameters and heat transfer,contributing to the development of more efficient designs in applications such as cooling systems,energy storage,and bioengineering.

Mixed convectional and chemical reactive flow of nanofluid with slanted MHD on moving permeable stretching/shrinking sheet through nonlinear radiation,energy omission

Hybrid nanofluids are remarkable functioning liquids that are intended to reduce the energy loss while maximizing the heat transmission.In the involvement of suction and nonlinear thermal radiation effects,this study attempted to explore the energy transmission features of the inclined magnetohydrodynamic(MHD)stagnation flow of CNTs-hybrid nanofluid across the nonlinear permeable stretching or shrinking sheet.This work also included some noteworthy features like chemical reactions,variable molecular diffusivity,quadratic convection,viscous dissipation,velocity slip and heat omission assessment.Employing appropriate similarity components,the model equations were modified to ODEs and computed by using the HAM technique.The impact of various relevant flow characteristics on movement,heat and concentration profiles was investigated and plotted on a graph.Considering various model factors,the significance of drag friction,heat and mass transfer rate were also computed in tabular and graphical form.This leads to the conclusion that such factors have a considerable impact on the dynamics of fluid as well as other engineering measurements of interest.Furthermore,viscous forces are dominated by increasing the values ofλ_(p),δ_(m)andδ_(q),and as a result,F(ξ)accelerates while the opposite trend is observed for M andφ.The drag friction is boosted by the augmentation M,λ_(p)andφ,but the rate of heat transfer declined.According to our findings,hybrid nanoliquid effects dominate that of ordinary nanofluid in terms of F(ξ),Θ(ξ)andφ(ξ)profiles.The HAM and the numerical technique(shooting method)were found to be in good agreement.

磁驱动固体套筒实验模拟中的电流系数

采用不可压缩理论模型,对FP-2装置上开展的磁驱动固体套筒实验进行了模拟分析。模拟结果表明,无论是二维磁流体力学理论模型,还是其他不可压缩理论模型,回流罩结构磁驱动固体套筒的边界磁感应强度公式中都包含一个小于1的套筒电流系数。对不同套筒厚度、不同套筒半径条件下磁驱动固体套筒实验的电流系数进行了模拟,发现电流系数不仅与套筒内半径有关,还与套筒厚度有关;套筒内半径越大,套筒电流系数越小;套筒厚度越大,套筒电流系数越小。准确掌握磁驱动固体套筒电流系数的变化规律,可使磁流体程序从磁驱动固体套筒实验的后验模拟发展为精确预测,使磁流体力学模型真正具备正确设计和指导磁驱动固体套筒相关实验的理论能力。

MHD控制微电离等离子体射流

为了研究磁流体动力学(MHD)控制低温微电离等离子体射流,实现推力矢量的可行性,提出了一种基于MHD控制等离子体流动理论的试验方法,建立了种子(碱金属盐)诱导燃气电离的MHD流动控制试验台,研究了不同温度和不同磁场方向条件下射流偏转向量角,并用用户自定义函数(UDF)加载MHD模型求解三维N-S方程,探究了数值研究MHD控制的可行性.结果表明:向燃烧室内注入低电离能种子能够诱导燃气电离,形成磁流体,在磁场作用下实现推力矢量控制;等离子体射流偏转的数值模拟结果与试验结果在一定程度上是一致的,说明数值模拟MHD流动控制具有一定的可信度.

MHD能量旁路超燃冲压发动机可行性分析

使用准一维模型对磁流体能量旁路超燃冲压发动机MPCE(Magneto-Plasma-Chem ical Engine)进行了性能计算.考察了理想和非理想的超燃冲压发动机应用磁流体MHD(magnetohydrodynam ics)能量旁路的效果,对于理想的超燃冲压发动机应用MHD反而会使发动机的性能降低;对于非理想的超燃冲压发动机,MHD的作用使发动机的比冲增加.考虑发动机工作的工程限制条件对超燃冲压发动机和MPCE的性能进行了比较,结果表明应用MHD可以扩大超燃冲压发动机的工作范围,在非设计马赫数下提高发动机的性能.计算了负载系数、通道压力系数等重要的设计参数对MPCE性能的影响,结果显示优化参数设置可以使发动机比冲增加,但是同时又会受到工程条件的限制.

喷口电弧二维磁流体动力学(MHD)数学模型

为了揭示电弧在开断过程中的复杂物理现象，建立的喷口电弧二维磁流体动力学数学模型，强调了电弧、气流、电磁场三者的相互作用，消除了因忽略Ｌｏｒｅｎｔｚ力和电流密度径向分量引起的物理意义失真．应用有限元法对模型中Ｎ－Ｓ（ＮａｖｉｅｒＳｔｏｋｅｓ）方程和二维电磁场方程进行数值求解．数值计算结果反映了Ｌｏｒｅｎｔｚ力以及电流密度径向分量对电弧特性的影响，同时揭示了电弧热边界区的存在．

日冕物质抛射的理想MHD模型研究

概括了日冕物质抛射的一些观测结果和它们与其它太阳活动现象的相关性。简要回顾了较早期日冕物质抛射的理论研究，着重介绍了最近研究得较多的理论机制，即能量储存机制，以及其中的磁通量绳突变模型与其它理论模型的MHD数值和解析研究以及相应的重要应用．

不考虑磁层顶磁重联的全球三维MHD模型

本文介绍了一个新的磁层全球三维MHD模型 ,该模型可通过把IMF和地球磁场分开处理的方法“关闭”磁层顶的磁重联 ,从而可直观地显示不同IMF条件下磁层顶的IMF与地磁场的反平行区域 ,即磁重联最可能发生的地方 .结果表明 ,IMFBx 分量对磁层顶磁重联有重要影响 .由于可关闭磁重联 ,该模型还可有效地研究有无磁重联时 ,太阳风对磁层位形 (如晨 昏不对称性 )、粒子输运等重要问题的影响 ,有助于揭示磁层物理现象的基本特性 .

基于MHD的小间隙真空电弧仿真研究

真空开关电弧电流过零前的电弧等离子体状态对真空开关能否顺利开断具有十分重要的影响。本文在建立小间隙真空电弧磁流体动力学(MHD)模型的基础上,采用通用计算流体力学仿真软件对电弧电流过零前0.5 ms时的电弧等离子体的特性进行数值仿真研究。计算得到了真空电弧等离子体压力、等离子体密度、离子纵向速度、阳极表面纵向电流密度和电弧电压等的空间分布。仿真结果表明:从阴极到阳极,真空电弧等离子体压力和密度逐渐增大,而等离子体速度逐渐减小;阳极表面电流密度存在较大径向梯度,最大值低于形成阳极斑点的极限收缩电流,阳极仍不活跃。仿真得到的等离子体密度分布与高速摄像机拍摄得到的CMOS图像光强基本吻合,计算得到的电弧电压与测得的电弧电压是一致的,从而验证了本模型的可行性。

MHD数值模拟中清除伪磁场散度方法

针对全MHD(Magnetohydrodynamics)数值模拟中存在伪磁场散度的问题,发展了如下计算方法:基本格式基于八波对称形式方程组,补充相关源项以保持方程组守恒性,并采用投影方法辅助清除伪散度.投影方法中,基于有限体积方法求解三维Poisson方程.算例显示,对于光滑解析磁场,伪磁场散度得到有效清除;对于带激波高超声速MHD流动,全局投影下自由来流区域误差增大.提出一种局部投影方法,在高磁场散度区域进行投影.结果表明,最终流场收敛稳定,高磁场散度得到有效清除,而低散度区域散度不受影响.

一种高效的霍尔电场数值模拟方法

针对高超声速飞行器电磁调控分析中霍尔电场高效数值模拟方法的需求,以磁流体动力学基本原理为基础,构造了双曲形式的霍尔电场控制方程.基于自行开发的计算流体力学数值求解框架,在非结构混合网格上通过格心有限体积方法建立了新的霍尔电场数值模拟方法.迎风分裂格式基于Rusanov格式,时间推进基于LU-SGS隐式算法.以立方体区域泊松方程算例和分段电极流动通道霍尔电势算例为例对文章数值方法的准确性、收敛特性以及计算效率进行了考核和验证.数值模拟结果表明,当前数值方法计算结果与传统方法相同,且具有良好的收敛特性,对于立方体区域泊松方程算例,当前方法所需墙上时间为传统方法的28.76%,而对于分段电极流动通道霍尔电势算例,当前方法所需墙上时间仅为传统方法的0.61%.文章提出的新的霍尔电场数值模拟方法具有较高的计算效率.

一种确定模拟的行星际MHD激波局地参数的新方法

简要阐述了分析模拟的行星际磁流体力学（ＭＨＤ）激波的局部性质时，采用无厚度局部平面激波这一假设的合理性，说明了在激波未扰动区域（激波上游），物理量在几个小时内的变化很小这一事实．利用平面激波的分析方法，提出了分析模拟的行星际ＭＨＤ 激波的新方法，包括激波位置的确定，上下游状态参数的选择，激波局部参数的计算以及激波的分类，最后应用这种方法对一个二维的ＭＨＤ模拟结果进行了分析．结果证实了过去文献关于磁流体力学混合激波空间连接和时间演化的链式规则，而且说明位于太阳赤道附近的慢激波和中间激波最终会发展为快激波．

液态金属自由表面膜流MHD效应的数值模拟

对液态金属自由表面膜流在强磁场下的磁流体力学效应进行了数值模拟研究,获得了液态金属自由表面的形状、截面流速分布及截面上的电动势分布,从而能对膜流的一些磁流体动力学行为作出解释。数值计算结果与理论分析和实验结果符合较好。由实验和数值模拟结果可以得出,液态金属膜流通过强磁场时,磁场会阻碍膜流的运动。

矩形管道中液态金属MHD流动数值模拟研究

基于感应电磁场方程发展了低磁雷诺数条件下充分发展液态金属管道流的数值模拟程序。为了校正程序,分别计算了两种工况:液态金属在全绝缘管道和部分绝缘管道中的流动,数值结果与Hunt和Shercliff的解析解吻合的很好,表明该程序具有很高的精度。最后利用发展的程序对液态金属钠钾合金在管壁材料为304不锈钢的全导电管道中的流动进行了数值模拟,并对流速分布和MHD压降结果与实验结果进行了比较,结果表明数值与实验结果吻合较好。

壁面催化对高温非平衡流场磁控效果影响分析

以轨道再入实验飞行器为研究对象,采用热化学非平衡磁流体动力学模型对高超声速飞行器的表面热流进行数值模拟,分析了不同飞行工况下壁面催化条件对气动热环境影响规律,研究了外加磁场条件对热化学非平衡流场影响机制.结果表明:再入过程中,表面热流随催化复合系数的增加呈单调递增分布,壁面催化条件显著影响磁流体动力学控制效果,总热流密度与壁面附近原子组分堆积量、扩散梯度及温度梯度密切相关.外加磁场作用下,壁面附近氧原子、氮原子组分堆积量减少;洛伦兹力导致激波脱体距离增大,组分扩散梯度、壁面温度梯度降低.磁控热防护系统“电磁冷却”能力从大到小依次为全催化、有限催化、非催化壁面.