VELOCITY SHEAR INSTABILITY IN DUSTY PLASMAS OF COMET

The velocity shear instability in a magnetized, three-component dusty plasma is investigated for both positively and negatively charged dust particles. The critical shears as a function of the relative charge of dust grains for both positively and negatively charged dust grains are in the same form. The instability excitation is easier in colder proton’s environment for positively charged dust, and so also in colder electron’s environment for negatively charged dust. For a certain flow pattern, the instability excites in different direction for different sign of charged dust grains. This conclusion may be helpful in interpreting the helical structures and streamer splitted phenomena in a cometary tail.

Stability analysis of viscous Z-pinch plasma with a sheared axial flow

Within the magnetohydrodynamics （MHD） frame, we analyse the effect of viscosity on magneto-Rayleigh Taylor （MRT） instability in a Z-pinch configuration by using an exact method and an approximate method separately. It is demonstrated that the plasma viscosity indeed has a stabilization effect on the MRT mode in the whole wavenumber region, and its influence increases with the perturbation wavenumber increasing. After the characteristics and feasibility of the approximate method have been investigated, we apply it to the stability analysis of viscous plasma where a sheared axial flow （SAF） is involved, and we attain an analytical dispersion relation. It is suggested that the viscosity and the SAF are complemental with each other, and a wide wavenumber range of perturbation is possible to be restrained if the SAF and the viscosity are large enough. Finally, we calculate the possible value of viscosity parameter according to the current experimental conditions, and the results show that since the value of viscosity is much less than the threshold value, its mitigation effect is small enough to be neglected. The role of the viscosity in the stabilization becomes considerable only if special techniques are so developed that the Z-pinch plasma viscosity can be increased greatly.

Numerical Simulation of Tripolar Vortex in Dusty Plasma with Sheared Flow and Sheared Magnetic Field

This article presents a study we have made of one class of coherent structures of the tripolar vortex. Considering the sheared flow and sheared magnetic field which are common in the thermonuclear plasma and space plasma, we have simulated the dynamics of the tripolar vortex. The results show that the tripolar vortex is largely stable in most cases, but a strongly sheared magnetic field will make the structure less stable, and lead it to decays into single vortices with the large space scale. These results are consistent with findings from former research about the dipolar vortex.

Schamel equation in an inhomogeneous magnetized sheared flow plasma with q-nonextensive trapped electrons

An investigation is carried out for understanding the properties of ion–acoustic（IA） solitary waves in an inhomogeneous magnetized electron-ion plasma with field-aligned sheared flow under the impact of q-nonextensive trapped electrons. The Schamel equation and its stationary solution in the form of solitary waves are obtained for this inhomogeneous plasma. It is shown that the amplitude of IA solitary waves increases with higher trapping efficiency（β）, while the width remains almost the same. Further, it is found that the amplitude of the solitary waves decreases with enhanced normalized drift speed, shear flow parameter and the population of the energetic particles. The size of the nonlinear solitary structures is calculated to be a few hundred meters and it is pointed out that the present results are useful to understand the solar wind plasma.

Dynamics of oscillatory plasma flows prior to the H-mode in the HL-2A tokamak

This paper discusses edge oscillatory plasma flows, geodesic acoustic mode （GAM） and limit cycle oscillations （LCOs）, which have been measured by Doppler reflectometry prior to the high confinement mode （H-mode） in the HL-2A tokamak. The complex relations between the flows and background turbulence have been analyzed. It was observed that the GAM and LCO coexist, and these two flows and turbulence have strong nonlinear interactions during the intermediate confinement phase （I-phase）. Dynamics of the shear flows and turbulence prior to the H-mode shows that the oscillatory flows quench the turbulence along with the increase of the mean E x B flow at the early stage of the I-phase, then the oscillatory flows are damped and the further increased mean flow takes over the role in turbulence suppression. The reduced turbulent transport results in the formation of a steep edge transport barrier. It suggests that the oscillatory flows can initiate the L-H transition through providing a positive feedback for the increase of the mean E × B flow strength.

Effect of shear equilibrium flow in Tokamak plasma on resistive wall modes

A code named LARWM with non-ideal magnetohydrodynamic equations in cylindrical model is used to describe the instability in Tokamak plasma surrounded by a conducting wall with finite resistivity. We mainly take three factors related to the shear equilibrium plasma flow into consideration to study the stabilizing effect of the shear flow on the resistive wall modes （RWMs）. The three factors are the velocity amplitude of flow, the shear rate of flow on plasma surface, and the inertial energy of equilibrium plasma flow. In addition, a local shear plasma flow is also calculated by the LARWM code. Consequently, it is found that the inertial energy of the shear equilibrium plasma flow has an important role in the stabilization of the RWMs.

The Excitation of the Low-Frequency Electromagnetic Wave in Dusty Plasma with Vortex Flows

The Excitation of Alfven wave in dusty plasma with vortex flows is investigated. The coupled equations for density and electromagnetic potential of dusty plasma with vortex-flow were obtained. The stability and amplitude behavior were studied both analytically and numerically. Using a non-modal method, it was found that the presence of dust can suppress the growth of the instability which can also be affected by the vortex eccentricity.

Stability of Rayleigh-Taylor Vortices in Dusty Plasma

The evolution of Rayleigh-Taylor mode in dusty plasma with vortex-flow is investigated. Based on fluid theory and Bayly＇s method, we derive the coupling equations describing the Rayleigh-Taylor mode in the core of vortex, and research the evolution characteristics of the perturbation amplitude with time numerically. It is shown that the eccentric of vortex and the content of dust have considerable effects on the amplitude evolutions.

Comparison Between Mitigation Effects of the Finite Larmor Radius and Sheared Axial Flow on Rayleigh-Taylor Instability in Z-Pinch Implosions

A magnetohydrodynamic (MHD) formulation is derived to investigate and compare the mitigation effects of both the sheared axial flow and finite Larmor radius (FLR) on the Rayleigh-Taylor (RT) instability in Z-pinch implosions. The sheared axial flow is introduced into MHD equations in a conventional way and the FLR effect into the equations via /t → -i(w+ik⊥2pi2Ωi,), as proposed in our previous paper [Chin. Phys. Lett. 2002, 19:217] , where k⊥2 pi2 is referred to FLR effect from the general kinetic theory of magnetized plasma. Therefore the linearized continuity and momentum equations for the perturbed mass-density and velocity include both the sheared axial flow and the FLR effect. It is found that the effect of sheared axial flow with a lower peak velocity can mitigate RT instability in the whole wavenumber region and the effect of sheared axial flow with a higher one can mitigate RT instability only in the large wavenumber region (for normalized wavenumber k】2.4); The effect of FLR can mitigate RT instability in the whole wavenumber region and the mitigation effect is stronger than that of the sheared axial flow with a lower peak velocity in the almost whole wavenumber region.

Linear Stability of Taylor-Couette Flows with Axial Heat Buoyancy

The linear stability is studied of flows confined between two concentric cylinders, in which the radial temperature gradient and axial gravity are considered for an incompressible Newtonian fluid. Numerical method based on the Petrov-Galerkin scheme is developed to deal with the buoyancy term in momentum equations and an additional temperature perturbation equation. Computations of the neutral stability curves are performed for different rotation cases. It is found that the flow instability is influenced by both centrifugal and axial shear instabilities, and the two instability mechanisms interact with each other. The outer cylinder rotation plays dual roles of stabilizer and destabilizer under different rotating stages with the inner cylinder at rest. For the heat buoyancyinduced axial flow, spiral structures are found in the instability modes.

Gravity-capillary waves modulated by linear shear flow in arbitrary water depth

Considering that the fluid is inviscid and incompressible and the flow is irrotational in a fixed frame of reference and using the multiple scale analysis method, we derive a nonlinear Schrodinger equation(NLSE) describing the evolution dynamics of gravity-capillary wavetrains in arbitrary constant depth. The gravity-capillary waves(GCWs) are influenced by a linear shear flow(LSF) which consists of a uniform flow and a shear flow with constant vorticity. The modulational instability(MI) of GCWs with the LSF is analyzed using the NLSE. The MI is effectively modified by the LSF. In infinite depth, there are four asymptotes which are the boundaries between MI and modulational stability(MS) in the instability diagram. In addition, the dimensionless free surface elevation as a function of time for different dimensionless water depth,surface tension, uniform flow and vorticity is exhibited. It is found that the decay of free surface elevation and the steepness of free surface amplitude change over time, which are greatly affected by the water depth, surface tension, uniform flow and vorticity.

A nonlinear Schrodinger equation for gravity waves slowly modulated by linear shear flow

Assume that a fluid is inviscid, incompressible, and irrotational. A nonlinear Schr?dinger equation(NLSE) describing the evolution of gravity waves in finite water depth is derived using the multiple-scale analysis method. The gravity waves are influenced by a linear shear flow, which is composed of a uniform flow and a shear flow with constant vorticity. The modulational instability(MI) of the NLSE is analyzed, and the region of the MI for gravity waves(the necessary condition for existence of freak waves) is identified. In this work, the uniform background flows along or against wave propagation are referred to as down-flow and up-flow, respectively. Uniform up-flow enhances the MI, whereas uniform down-flow reduces it. Positive vorticity enhances the MI, while negative vorticity reduces it. Hence, the influence of positive(negative)vorticity on MI can be balanced out by that of uniform down(up) flow. Furthermore, the Peregrine breather solution of the NLSE is applied to freak waves. Uniform up-flow increases the steepness of the free surface elevation, while uniform down-flow decreases it. Positive vorticity increases the steepness of the free surface elevation, whereas negative vorticity decreases it.

Dynamics of the outflow and its effect on the hydraulics of two-layer exchange flows in a channel

This paper reports that an experimental study is conducted to examine the dynamics of the outflow in two-layer exchange flows in a channel connecting between two water bodies with a small density difference. The experiments reveal the generation of Kelvin-Helmholtz (KH) instabilities within the hydraulically sub-critical flow region of the channel. During maximal exchange, those KH instabilities develops into large-amplitude KH waves as they escape the channel exit into the reservoir. The propagation speed of those waves, their generation frequency and their amplitudes are studied. The dynamics of the outflow and these waves are directly linked to the hydraulic conditions of the exchange flow within the channel.

非晶合金中剪切温升的研究进展

区别于传统的晶态金属材料,非晶合金(BMGs)不具备长程有序结构,其塑性变形载体为剪切带。剪切带一旦形成,便很快发展成为裂纹,引发材料的灾难性断裂。剪切不稳定性的研究有助于非晶合金塑性变形机理的理解,并可为非晶合金塑性变形能力的提高提供设计思路。近年来,基于非晶合金的结构特点,科研工作者努力探究非晶合金的剪切不稳定性,主要提出了结构软化诱导的剪切不稳定性和热软化引发的剪切不稳定性两种机制。本文重点总结了非晶合金中剪切温升的研究进展,介绍了测试应变速率、外部约束、试验机刚度和测试温度对剪切温升的影响,指明非晶合金中剪切引入的热远低于玻璃转变温度,暗示热软化对剪切不稳定性的影响是微弱的。本文最后对非晶合金中剪切不稳定性机制的研究方向进行了展望。

基于Kelvin-Helmholtz不稳定性和界面剪切作用的扰动波高预测模型

环雾状流广泛存在于天然气等工业环境中,深入探究扰动波特性对了解环雾状流的演化规律有着重要意义。本文在内径为15mm的垂直管路中进行了不同工况条件下的环雾状流实验,分别利用电导环传感器和液膜收集系统测量了液膜厚度和夹带率,并通过双阈值方法从液膜时序信号中提取了扰动波高数据。探究了扰动波高和夹带率随气、液相流量和工况压力的变化规律,发现两者均随着液相流量的增大而增大,而随着气相流量和工况压力的增大,扰动波高呈下降趋势,夹带率呈上升趋势,验证了两者之间存在密切关联。然后分析了影响扰动波高的尺度参数,建立了基于Kelvin-Helmholtz不稳定性和界面剪切作用的扰动波高预测模型,相对均方根误差rRMSE为4.05%,且98.7%的数据点都在±10.0%的误差以内,预测效果良好。最后,将新模型与现有的扰动波高关系式进行比较,预测精度和可扩展性都有了较大提高。

垂直切变基流中非地转涡旋波的不稳定

利用Ｂｏｕｓｓｉｎｅｓｑ方程研究了扰动传播方向与垂直切变基流有－夹角时的不稳定问题，即斜交型不稳定问题。当Ｒｉｃｈａｒｄｓｏｎ数不大时，在α中尺度波段其增长率最大，此时该斜交型不稳定的性质既不同于重力惯性波的对称不稳定，也与Ｒｏｓｓｂｙ波的斜压不稳定有差异，而是非地转涡旋波的不稳定。

亚暴膨胀相近磁尾位形不稳定性模型Ⅱ.中磁尾等离子体流与近磁尾亚暴活动的关系

在单流体MHD近似下，研究了有背景等离子体流存在时的近磁尾位形不稳定性．分析表明，等离子体地向流使两支漂移气球模DBM1和DBM2的不稳定增长更快．与卫星观测资料对比显示，诸多的观测观象与本模型的预言一致．这个结果对中磁尾磁场重联及高速等离子体流与亚暴膨胀相在近地触发之间的关系给出了新的解释．

高纬磁层顶尾部边界层的离子流剪切不稳定性

研究了高纬磁层顶尾部边界层在非均匀磁场条件下的离子流剪切不稳定性．由于考虑了尾向电流，我们发现离子流剪切不稳定性的激发与扰动波长有关，并得到了激发该不稳定性的准临界波长的表达式．对于磁层顶存在的波长为地球半径量级的表面波扰动，离子流剪切不稳定性倾向于发生在磁层顶边界层的内边界．沿磁层顶外边界传播的稳定的表面波和其内边界的离子流剪切不稳定性同时存在，这将有助于解释磁鞘太阳风动量持续不断地向地磁层的传输．

2007年3月3-5日辽宁省暴雪和大风天气的中尺度分析

使用中尺度数值模式WRFV2.2.1对辽宁省2007年3月3—5日的暴雪和大风天气过程进行了数值模拟,结合10min一次的地面自动观测站资料和数字化多普勒天气雷达探测资料,研究了中尺度重力波的结构及其环境场特征,探讨了波动的激发机制。对流层上层中尺度重力波生成在350—250hPa(约9—11km),周期为2—3h,水平波长30—40km,波动沿水平方向传播约9h。地面气压扰动振幅约为2hPa,周期为2—3h,波动由西南向东北方向传播,方向与地面风向相反。沿波的传播方向,地面观测的逐时降水量呈波动特征,周期约为2h。对流层上层中尺度重力波减弱后,雷达降水回波强度出现显著的波动特征。对流层上层中尺度重力波生成在朝向脊区传播的高空急流出口区下方,300hPa环境场具有显著的切变不稳定特征。波动生成在理查逊数小于0.25的地区,在中尺度重力波生成的高度上,暖平流强,风速低,风切变大。中尺度重力波生成地区出现显著的不平衡气流,拉格朗日罗斯贝数大于0.7,水平散度倾向出现明显的大值,其中-▽w·V/z的量级明显大于其他各项,表明对流层上层重力波的生成及发展与环境场的显著风切变有关。

轴向剪切流对Z箍缩等离子体瑞利-泰勒不稳定性的抑制

利用理想磁流体力学模型对有轴向剪切流的Z箍缩等离子体不稳定性进行了分析。给出了可压缩模型的色散关系,分别对可压缩及不可压缩模型中轴向剪切流对Z箍缩等离子体瑞利 泰勒不稳定性的抑制作用进行了比较,讨论了可压缩性对含有轴向剪切流系统不稳定性的影响。结果表明,可压缩性能够减缓瑞利 泰勒 凯尔文 亥姆霍兹(RT KH)模扰动的增长,因而使得轴向剪切流对系统不稳定性的抑制作用表现得更为突出。计算结果还说明,在RT不稳定性线性增长阶段,等离子体温度较低,使用可压缩模型能够更真实地描述系统的状态。