Multipolar Structure of Equilibrium Shear Flow Field in Toroidal Plasmas

The multipolar velocity field structures are investigated by 2D momentum conservation equation with 3D equilibrium sheared flows in the full toroidal system. Numerical results show that the non-existence of radial velocity field in equilibrium surfaces is suitable only for the zero-order term of our 2D simulation. The non-zero-order radial velocity field is still preserved, even when converted to conventional magnetic surface coordinates. The distribution of velocity field vectors of the order of 1, 2, and 3 are presented respectively in 2, 4, and 6 polar fields with the local vortex structure. The excitation mechanisms of these velocity vortices are the coupling effects of the magneto-fluid structure patterns and the toroidal effects. These results can help us understand the complexity of core physics, the transverse transport across magnetic field by the radial plasma flow and the formation of velocity vortices.

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.

Stationary Flow of Blood in a Rigid Vessel in the Presence of an External Magnetic Field: Considerations about the Forces and Wall Shear Stresses

The magnetohydrodynamics laws govern the motion of a conducting fluid, such as blood, in an externally applied static magnetic field B0. When an artery is exposed to a magnetic field, the blood charged particles are deviated by the Lorentz force thus inducing electrical currents and voltages along the vessel walls and in the neighboring tissues. Such a situation may occur in several biomedical applications: magnetic resonance imaging (MRI), magnetic drug transport and targeting, tissue engineering… In this paper, we consider the steady unidirectional blood flow in a straight circular rigid vessel with non-conducting walls, in the presence of an exterior static magnetic field. The exact solution of Gold (1962) (with the induced fields not neglected) is revisited. It is shown that the integration over a cross section of the vessel of the longitudinal projection of the Lorentz force is zero, and that this result is related to the existence of current return paths, whose contributions compensate each other over the section. It is also demonstrated that the classical definition of the shear stresses cannot apply in this situation of magnetohydrodynamic flow, because, due to the existence of the Lorentz force, the axisymmetry is broken.

Impact of an External Magnetic Field on the Shear Stresses Exerted by Blood Flowing in a Large Vessel

The aim of this paper is to provide an advanced analysis of the shear stresses exerted on vessel walls by the flowing blood, when a limb or the whole body, or a vessel prosthesis, a scaffold… is placed in an external static magnetic field B0. This type of situation could occur in several biomedical applications, such as magnetic resonance imaging (MRI), magnetic drug transport and targeting, tissue engineering, mechanotransduction studies… Since blood is a conducting fluid, its charged particles are deviated by the Hall effect, and the equations of motion include the Lorentz force. Consequently, the velocity profile is no longer axisymmetric, and the velocity gradients at the wall vary all around the vessel. To illustrate this idea, we expand the exact solution given by Gold (1962) for the stationary flow of blood in a rigid vessel with an insulating wall in the presence of an external static magnetic field: the analytical expressions for the velocity gradients are provided and evaluated near the wall. We demonstrate that the derivative of the longitudinal velocity with respect to the radial coordinate is preponderant when compared to the θ-derivative, and that elevated values of B0 would be required to induce some noteworthy influence on the shear stresses at the vessel wall.

Out-of-plane shear flow effects on fast magnetic reconnection in a two-dimensional hybrid simulation model

The effects of out-of-plane shear flows on fast magnetic reconnection are numerically investigated by a two- dimensional （2D） hybrid model in an initial Harris sheet equilibrium with flows. The equilibrium and driven shear flows out of the 2D reconnection plane with symmetric and antisymmetric profiles respectively are used in the simulation. It is found that the out-of-plane flows with shears in-plane can change the quadrupolar structure of the out-of-plane magnetic field and, therefore, modify the growth rate of magnetic reconnection. Furthermore, the driven flow varying along the anti-parallel magnetic field can either enhance or reduce the reconnection rate as the direction of flow changes. Secondary islands are also generated in the process with converting the initial X-point into an O-point.

Generation of Zonal Flow and Magnetic Field by Planetary Waves in the Earth’s Ionosphere

Possibility of generation of large-scale sheared zonal flow and magnetic field by coupled under the typical ionospheric conditions short-scale planetary low-frequency waves is shown. Propagation of coupled internal-gravity-Alfven, Rossby-Khantadze, Rossby-Alfven-Khantadze and collision-less electron skin depth order drift-Alfven waves is revealed and investigated in detail. To describe the nonlinear interaction of such coupled waves with sheared zonal flow the corresponding nonlinear equations are deduced. The instability mechanism is based on the nonlinear parametric triple interaction of the finite amplitude short-scale planetary waves leading to the inverse energy cascade toward the longer wavelengths. It is shown that under such interaction intense sheared magnetic fields can be generated. Appropriate growth rates are discussed in detail.

Study on Fluid Shear Flow on Osteoblast-like Cells in Their Tensile Loading Experiments

In vitro cell loading experiments are used to investigate stimulation of strain to cellular proliferation. As the flowing conditions of culture fluid in loading systems has been little known, strictly people can not detect the influence of strain to cellular proliferation exactly because shear flow can enhance cell proliferation either. Based on the working principle and cyclic loading parameters, we simplify Navier-Stokes equation to describe the flow of culture fluid on substrates of uniaxial and equi-biaxial flat tensile loading systems and four point bending system. With approximate solutions, the distributions of velocity field and wall shear flow to cells are gained. Results show: shear flows are zero in the middle (or fixed point or line) of substrate for all systems, and they get larger proportionally to distance from middle and substrate elongate; the shear flow on the substrate of four point bending system is much greater than those of others. This shear flow in four point bending system, confirmed by Owan, I., et al with OPN mRNA increase in their experiment, could cause more influence to osteoblast-like cells than that caused by strain. We estimate the average magnitude of shear stress in Owan’s device, the results are consistent with other experimental data about shear flow. In conclusion our study makes it possible to differentiate the influence of strain on cellular proliferation to that of shear flow in loading experiments with the devices mentioned above quantitatively.

Exact solutions for the flow of second grade fluid in annulus between torsionally oscillating cylinders

The velocity field and the associated shear stress corresponding to the torsional oscillatory flow of a second grade fluid, between two infinite coaxial circular cylinders, are determined by means of the Laplace and Hankel transforms. At time t = 0, the fluid and both the cylinders are at rest and at t = 0 ＋ , cylinders suddenly begin to oscillate around their common axis in a simple harmonic way having angular frequencies ω 1 and ω 2 . The obtained solutions satisfy the governing differential equation and all imposed initial and boundary conditions. The solutions for the motion between the cylinders, when one of them is at rest, can be obtained from our general solutions. Furthermore, the corresponding solutions for Newtonian fluid are also obtained as limiting cases of our general solutions.

Exact analytical solutions for axial flow of a fractional second grade fluid between two coaxial cylinders

The velocity field and the adequate shear stress corresponding to the longitudinal flow of a fractional second grade fluid,between two infinite coaxial circular cylinders,are determined by applying the Laplace and finite Hankel transforms.Initially the fluid is at rest,and at time t=0^+, the inner cylinder suddenly begins to translate along the common axis with constant acceleration. The solutions that have been obtained are presented in terms of generalized G functions.Moreover, these solutions satisfy both the governing differential equations and all imposed initial and boundary conditions.The corresponding solutions for ordinary second grade and Newtonian fluids are obtained as limiting cases of the general solutions.Finally,some characteristics of the motion,as well as the influences of the material and fractional parameters on the fluid motion and a comparison between models,are underlined by graphical illustrations.

基于四波前横向剪切干涉的波前传感技术与应用(特邀)

高科技的发展对精密干涉成像提出了更高的要求。在现代光学和生物医学领域,无标记成像技术不依赖于传统的染料或荧光标记,进行3D活细胞原位观察和分析,促进定量相位显微术的发展。在光学检测技术领域,对于干涉系统的现场化、实时化的应用具有迫切需求,如激光波前的瞬态检测分析、高速流场检测、自适应光学的检测和控制、高精度光学系统像差分析等都迫切需要一个紧凑型、抗环境干扰、瞬态成像的干涉系统。为此,针对针对这些需求,全面介绍了四波前横向剪切干涉相位成像技术的原理、发展历程、波前重构方法以及其广泛的应用。四波前横向剪切干涉仪能够通过在一个单一的干涉图中获取两个正交剪切方向的四个剪切波前来实现瞬态相位成像,由随机编码光栅和相位棋盘组成新颖的四波干涉传感器(Four-wave Interferometric Sensor,FIS4)。FIS4干涉传感器凭借其独特的优势,如紧凑性、鲁棒性、高时间分辨率以及与现有显微系统的兼容性,在生物医学、光学测量、材料表征等众多领域展现出广阔的应用前景。这一技术的发展不仅为相关领域提供了新的研究工具,也为跨学科的创新和发现开辟了新的可能性。

基于详细化学反应机理的加力燃烧室湍流燃烧数值研究

为了深入理解加力燃烧室的湍流燃烧特性,开展了燃油配置和内涵进气参数对燃烧性能影响的二维数值研究.基于燃油喷嘴和火焰稳定器附近涡系结构和涡量探究了油气掺混过程,通过燃烧室压力、温度、流速、湍动能等分布情况揭示了流场和温度场的组织状况,阐述了燃油、氧气的分布情况以及主要燃烧产物的生成机制,揭示了燃烧室典型截面温度和流速的分布特征.结果表明:燃油总流量增加整体上增强了三圈的燃烧性能,内涵进气参数下降减弱了内圈和中圈的燃烧性能,较高内涵进气压力是保障高效燃烧最关键的运行措施.

基于风速概率分布与风切变指数的直流输电线路离子流场数值模拟方法

特高压直流(UHVDC)输电线路下方的离子流场是电磁环境的重要评估指标之一。风速作为常见也是影响较大的因素之一,有着明显的地域特点。为了避免资源浪费,在对地高度设计时,应考虑到不同地区的风速分布差异,而不是用过大裕度换取安全度。为此,该文提出一种基于风速概率分布与风切变指数的输电线路离子流场数值模拟方法。该方法通过待计算地区的风速数据构建Weibull风速概率分布函数模型,选择风速数据在95%以内的最大临界风速作为参考值,考虑风速传感器的安装高度,根据风切变指数计算场域不同高度处的风速分布,作为离子流场计算中不同高度节点的风速输入量。结果表明,引入风切变指数后的离子流密度与合成电场强度误差分别下降了16.2百分点和3.8百分点,明显优于传统固定风速模型。并以昆明地区某±800kV直流输电线路为例,采用该方法进行了计算。该方法能够有针对性地考虑输电线路所处区域的风速影响,特别是在高海拔高风速地区,可为输电线路在合成电场安全限值下的对地高度设计提供参考依据。

环下润滑高速滚动轴承环间流场及温度场分析

以三点角接触球轴承为研究对象,采用多重参考系(MRF)描述轴承钢球的自转与公转以及各部件的运动,应用流体体积(VOF)理论,建立轴承油气两相热流耦合计算模型,分析轴承腔内流场及温度场分布情况及影响机制。结果表明:轴承钢球自转存在接触角时,钢球自转会影响环间及内外圈油液分布状态,两侧出口处油液分布不均匀导致轴承两侧出口处温升不同;供油量的增加明显提高轴承腔内滑油体积分数,内外滚道油液附着显著提升,腔内及内外滚道温升不断下降;轴承内圈滑油体积分数明显高于轴承外圈,轴承内圈温升小于轴承外圈;轴承钢球滑动摩擦较滚动摩擦产生的热量大,相比钢球有自转时,钢球无自转时轴承内外圈的温升较高;随着供油量增加,轴承腔内滑油体积分数逐渐增加,使得轴承腔内平均温升及滑油黏性剪切温升和摩擦温升减小,但滑油黏性剪切温升占比会逐渐增加,摩擦温升占比逐渐下降;钢球有自转较无自转时摩擦温升占比小,黏性温升占比大。

高炉炉缸铁水流场影响因素的数值模拟研究

对高炉炉缸铁水流场影响因素开展数值模拟研究。结果表明:铁水剪切应力较高区域主要集中在炉缸出铁口下方;提高高炉利用系数会增大炉缸壁面上的剪切应力;在高炉正常生产情况下,死料柱孔隙率和铁口直径变化对炉缸侧壁铁水剪切应力影响较小;增加铁口深度和减小铁口倾角可以有效降低铁水对炉缸侧壁的剪切应力。实际生产中,保持良好原燃料条件,适度增加铁口深度,采取小铁口倾角出铁有利于减缓铁水对炉缸内衬的冲刷,从而实现炉缸长寿的目标。

海底电缆覆盖物清理的移动射流过程仿真分析

在实际工况中,高压喷嘴存在横向移动的速度,使得射流对土体的冲埋深度难以预测,导致海缆表面覆盖物清理不完全。针对此问题,建立了优化后的流场与冲埋深度预测理论模型,利用数值仿真对模型进行了验证。首先,建立了一种喷嘴流场优化模型,阐述了冲埋深度的预测方法;其次,建立了水射流破土数值仿真模型,选取射流压强、喷嘴直径、喷嘴横移速度以及土壤剪切强度4种控制参数,分析了不同参数对冲埋深度的影响。结果表明,随着射流压强、喷嘴直径的增大,冲坑深度逐渐增大;随着喷嘴横移速度和土体剪切强度的增加,冲坑深度逐渐减小。最后,理论结果和仿真结果进行对比,两者误差在允许范围之内。该模型可在一定程度上解决冲埋深度难以预测的问题,可为工程应用提供指导。

高原低涡移出高原后持续活动的典型个例分析

利用1°×1°NCEP/NCAR再分析资料,选取500 hPa分别为西风槽、切变线和切变流场影响背景下、移出高原后维持48 h以上的3次典型高原低涡个例,分析了低涡维持期间500 hPa环境场、主要影响系统、涡度与温度平流、200 hPa形势场及垂直动力结构,并利用涡度方程对总涡源和各强迫项进行了诊断分析,结果表明:(1)西风槽影响时高原低涡移动路径受槽前西南气流引导,切变线影响时低涡沿切变线自西向东移动,切变流场影响时低涡移动主要受西太平洋副热带高压(下称西太副高)进退的影响,当西太副高出现明显西伸时,可导致低涡折向西退,3次个例均持续有正涡度平流和冷平流向涡区输送;(2)西风槽和切变线影响时南亚高压为东西带状分布,切变流场影响时南亚高压为北拱形;(3)高原低涡东移发展达到最强时,3次个例在200hPa均有低槽或低压叠加,从而形成深厚的正涡度柱;(4)500 hPa存在正涡度变率中心,低涡沿正涡度变率中心方向移动,高空槽和切变流场影响时正涡度变率主要来自水平输送项,切变线影响时主要来自辐合辐散项。

动脉血流旋动原理在人造血管研制和血管移植术中的应用

目的将血液流动的旋动流原理用于心血管介入治疗,以期解决小口径人造血管的急性血栓堵塞问题和搭桥手术后下游处血管内膜增生引起的血管再狭窄问题。方法使用计算流体力学(CFD)分别研究具有旋动流特性的新型小口径、S型搭桥和偏心搭桥模型中的流场以及壁面剪切力的分布。同时研究在旋动流下血小板的黏附情况和不同角度的S型搭桥下的血管内膜增生。结果旋动流能明显提高壁面剪切力,抑制血小板的黏附以及血管内膜增生。结论在心血管介入治疗和器械设计中引入旋动流确实可明显改善这些器械中的血流流场,达到抑制小口径人造血管的急性血栓形成和搭桥手术后血管内膜增生的目的。

血液三元件阻抗动态测量方法

根据血液的三元件阻抗模型 ,以双频率复阻抗信息为已知量 ,建立了全信息动态计算三元件的实用公式 .为实现上述目标 ,研制了相应的检测系统 ,其组成分两部分 :双频率复阻抗检测装置 ,用于检测幅值及相位的基础量和搏动量 ;可控均匀切变流场发生装置 。

剪切流场对塑料发泡成核行为的影响

本文着重讨论了剪切流场对成核行为的影响,对宏观规律进行微观分析。通过剪切流场对高聚物熔体中分子取向排列的影响,在熔体中形成大量均匀分布的成核点,这不仅为开发优质泡体提供了理论依据,也是为国外正在加紧开发的微孔发泡技术提供参考。

泡沫塑料加工过程中的气泡成核理论(Ⅱ)——剪切能成核理论及其发展

介绍了剪切流场中泡沫塑料加工的气泡成核理论,分析评述了剪切流场中泡核拉伸模型、空穴成核模型对气泡成核机理的解释及存在的不足;对静态熔体中气泡成核过程进行了能量描述;介绍了最新的气泡成核中的剪切能成核理论,该理论较完善地解释了剪切流场中气泡成核过程;最后指出了气泡成核理论研究的发展方向。