The effect of a uniform vertical magnetic field on the onset of oscillatory marangoni convection in a horizontal layer of conducting fluid

Summary In this paper we use classical linear stability theory to undertake a detailed investigation of the effect of a uniform vertical magnetic field on the onset of oscillatory thermocapillary-driven Marangoni convection in a horizontal layer of quiescent, electrically conducting fluid heated from below. For simplicity we restrict our attention to the simplest case of a fluid layer with a non-deformable free surface and perfectly electrically conducting boundaries in which the onset of convection is always steady in the absence of the magnetic field. The present numerical calculations show that the presence of the magnetic field can cause the preferred mode of instability to be oscillatory rather than steady convection. Nevertheless, in all the cases investigated the effect of the magnetic field is always to stabilise the layer relative to the case of no magnetic field.     

Growth rates of Bénard-Marangoni convection in a fluid layer in the presence of a magnetic field

In this paper we analyze the effect of a uniform vertical magnetic field on the linear growth (and decay) rates of the steady Bénard-Marangoni instability in a horizontal layer of quiescent, electrically conducting fluid with a uniform vertical temperature gradient subject to a prescribed heat flux at its lower boundary. Explicit analytical expressions for the linear growth rates of long-waves instability modes are derived for the first time. The numerically-calculated linear growth (or decay) rates showing the stabilizing effect of the magnetic field are also presented.     

Thermal instability in a micropolar fluid layer subject to a magnetic field

In the present paper we have considered thermal instability in a heat conducting micropolar fluid layer under the influence of a transverse magnetic field. Assuming the bounding surfaces to be rigid the eigenvalue problem is solved using finite-difference and Wilkinson's iteration techniques. Here it is seen that the instability sets in not only for adverse temperature gradient but also for positive temperature gradient. Both the microtation and the magnetic field are seen to stabilize the fluid layer. However, the stabilizing effect of microrotation becomes less significant when the strength of the magnetic field is large. In the case of heating from below, the critical wave number is seen to be insensitive to increase in the strength of the magnetic field, while it increases significantly when the fluid is heated from above.     

Thermal relaxation effect on magnetohydrodynamic instability in a rotating micropolar fluid layer heated from below

Summary. The effect of a vertical magnetic field on the onset of convective instability in a conducting micropolar fluid (Oldroyd fluid) layer heated from below confined between two horizontal planes under the simultaneous action of the rotation of the system and a vertical temperature gradient is considered. Linear stability theory and normal mode analysis are used to derive an eigenvalue system of order twelve, and an exact eigenvalue equation for natural instability is obtained. Under somewhat artificial boundary conditions, this equation can be solved exactly to yield the required eigenvalue relationship from which various critical values are determined in detail. The effects of magnetic field, the relaxation time and micropolar parameters on the critical Rayleigh number and critical wave number are discussed and presented graphically. The analysis presented in this paper is more general than any previous investigation.     

Measurement of Effective Free Layer Magnetization Orientation of TMR Sensors

Measurements of thermal magnetization fluctuation noise spectrum in multi-directional magnetic field reveal tilt of effective free layer orientation in TMR sensors. We propose a method for effective sensor stiffness field measurement and demonstrate that deviations of stiffness from nominal values are caused by the angular dependence of the stiffness field. Ferro-magnetic resonance peak measurements indicate that the free layer magnetization orientation may be tilted from the cross-track direction. This magnetization tilt is caused by reference layer, hard bias problems and shape anisotropy effects. This reduces sensor efficiency and leads to amplitude asymmetry, multi-domain configuration and sensor instability. FMR-based method for detection of potentially unstable heads based on these observations is proposed.     

Stability of Ferrofluid Flows in a Horizontal Channel Subjected to a Longitudinal Temperature Gradient and an Oblique Magnetic Field

The stability of thermoconvective flows of a ferrofluid in a horizontal channel subjected to a longitudinal temperature gradient and an oblique strong magnetic field is studied. The flows are governed by the mass, momentum, heat balance and Maxwell equations, in the Boussinesq approximation. Strong magnetic fields are characterized by a small parameter measuring the deviation of the magnetization across the layer from the external magnetic field. An approximate solution of the stationary hydrodynamic problem was found in analytical form in Hennenberg et al. (Phys Fluids 18:093602, 2006). The flow depends on the thermal (gravitational) and magnetic Rayleigh numbers. In the present paper the linear stability of that basic flow is studied by the Galerkin method. The analysis shows that for large Prandtl numbers, typical for ferrofluids, and relatively small magnetic Rayleigh numbers, only oscillatory instability can appear. For a given magnetic Rayleigh number, the critical wavenumber does not depend on the inclination of the magnetic field, while the critical thermal Rayleigh number slightly changes. For horizontal and vertical magnetic fields, both critical numbers decrease when increasing the magnetic Rayleigh number.     

Stability of magnetohydrodynamic stratified shear flows

Summary A study is made of the stability of a stratified shear flow in a perfectly conducting fluid in the presence of an external magnetic field aligned with the flow. A semi-circle theorem for the present hydromagnetic case is proved. The magnetic field is found to have a stabilizing effect on the flow. The Rayleigh-Taylor instability problem in a stratified conducting fluid is discussed. Finally, a study is made of the absorption of wave energy by the mean flow in the hydromagnetic case by considering a shear flow with an anti-symmetric velocity profile given byU=tanhz. Unlike the hydrodynamic case, it is found that, in the critical layer atU=0, the transfer of the wave energy to the mean flow occurs for any value of the Richardson number. This result implies again the stabilizing effect of the magnetic field on the shear flow.     

Stability of magnetic fluid penetration through a porous medium with uniform magnetic field oblique to the interface

Recent work has shown that the fingering instability, which develops when a more viscous fluid is pushed through the voids of a porous medium or through a Hele-Shaw cell by a less viscous fluid, can be prevented if a magnetic field is applied tangential to a flat fluid interface separating magnetizable and non-magnetizable fluids. This earlier work is extended here by considering equilibrium magnetic field components both perpendicular and parallel to the flat interface. The tangential field component stabilizes those waves traveling along the field lines while the normal field is destabilizing. The analysis is developed through a general set of relations for perturbation field and flow interfacial variables defined for a "prototype" magnetizable fluid layer which can be used to describe the small signal stability characteristics of layered fluid systems. In a uniform tangential magnetic field geometry, experimental results of the most unstable wavelength in a Hele-Shaw cell are shown to agree well with theory.     

交变磁场对高硬熔覆层成型质量的影响

为改善激光熔覆高硬熔覆层的成型质量,外加辅助交变磁场是行之有效的方法。本工作设计了一种可调频、调幅的简易机械式类正弦交变电磁场发生装置,在激光熔覆过程中辅助施加了类正弦交变磁场,通过改变磁感应强度和交变频率,在Q235A表面制备了单道和多道高硬合金熔覆层,通过对熔覆层的宏、微观检测,研究了交变磁场对熔覆层几何特征、稀释率和微观组织的影响。研究结果表明,当磁场频率和磁感应强度变化时,熔覆层宽度波动范围很小,且平均值基本保持不变;熔覆层高度和接触角随着磁感应强度的增大而减小;稀释率的大小基本不受磁场参数变化的影响;在磁场作用下,熔覆层中气孔向熔覆层表面迁移,熔融金属对流加剧,粗大枝晶破断,熔覆层组织更加均匀致密。     

脉冲磁场激励下铁磁梁式板动力响应特征研究

基于广义变分原理得到磁力计算模型和磁场摄动技术,研究了不可移简支铁磁梁式板在矩形和锯齿形两类典型脉冲磁场作用下的磁弹性动力响应特征及动力失稳现象。数值模拟了铁磁梁式板在稳恒磁场,脉冲磁场下的振幅,频率的变化及其动力失稳特征。揭示了脉冲磁场的形状,脉冲持续时间,脉冲幅值与铁磁梁式板振幅,频率,临界失稳磁场值等特征参量之间的关系,为工程实际中铁磁结构的安全设计和可靠性评估提供依据和参考。     

Thermal instability of a rotating curved plate subjected to an applied magnetic field

The effect of a magnetic field on thermal instability in mixed convection flow on a heated rotating convex surface is studied in this paper. The onset position characterized by the Goertler number G _δ depends on the Grashof number, the rotational number, the Prandtl number, the magnetic field parameter, and the wave number. The buoyancy force, the centrifugal force, the Lorentz force, and the Coriolis force are found to significantly affect the flow structure and heat transfer of the flow. Negative rotation (clockwise) destabilizes the boundary layer flow on a convex surface. However, the Lorentz force stabilizes the flow. Numerical data in this study show the same order of magnitude like experimental data.     

磁脉冲作用下铁磁梁式板的磁弹塑性动力响应与失稳

基于等效的夹层梁模型和伽辽金方法，首先推导了不可移铁磁简支梁式板的磁弹塑性动力控制方程，在此基础上采用龙格-库塔方法数值模拟了矩形、锯齿形、三角形和正弦形四类横向磁脉冲作用下，铁磁梁式板的动力响应曲线及其动力失稳特征。数值结果表明：在磁脉冲持续时间、磁脉冲峰值相同的情形下，铁磁梁式板发生动力失稳时，矩形磁脉冲对应的临界磁场值最小，依次为锯齿形磁脉冲，三角形磁脉冲，正弦形磁脉冲。数值结果还进一步展示了磁脉冲结束后，铁磁梁式板的残余构形等力学特征。     

Resonance instabilities in the boundary-layer flow over a rotating-disk under the influence of a uniform magnetic field

Instabilities due to resonating waves in the three-dimensional incompressible viscous/inviscid rotating-disk boundary-layer flow are investigated numerically. The influence of a normal magnetic field on the resonances leading, respectively, to the direct spatial instability, the direct temporal instability and the absolute instability are worked out, in an attempt to determine the physically most significant one. It is found that the magnetic field has a stabilizing influence on all the resonance mechanisms outlined. However, the direct spatial-resonance-instability mechanism persists for low Reynolds numbers, when the flow is still in the laminar regime. Thus, attention should be directed to this specific instability mechanism, which offers a strong route to transition to turbulence by means of triggering the nonlinearity.     

强磁场下金属凝固研究进展

强磁场下金属凝固研究是一个新开辟的研究方向,有着广阔的前景。叙述了强磁场的基本效应,综述了强磁场下金属凝固的研究进展。重点介绍了磁场对凝固动力学、固/液界面稳定性、单相合金生长、共晶生长、热电磁流动及其作用、第二相颗粒运动的影响等方面的研究工作。指出了需要进一步研究的主要问题,以及所需的相关物理参数。     

基于主磁场不均匀的分数域磁共振成像方法

现有核磁共振设备面对主磁场不均匀多是采取贴磁片等补偿磁场不均匀等硬件方法,但这给成像带来图像伪影,图像模糊等不良影响。针对磁共振成像中磁场不均匀的问题,提出了一种主磁场不均匀下的分数域磁共振成像方法。首先选择待成像活体组织的某一层,在该层上选择若干个点,测量该层面上的磁场强度大小,在磁共振成像原理的基础上,建立成像区域磁场强度分布模型,然后建立磁场的多项式模型,按照测量的磁场中是否存在明显的二阶分量可以将该多项式模型分为二阶多项式模型和高阶多项式模型;之后,将这两个模型分别代入磁共振的自由感应衰减(FID)信号中,对于二阶模型可以用分数阶傅里叶变换工具进行求解成像物体某一层上的自旋密度函数,对于高阶模型需要通过求解代数方程的方法得到成像物体某一层面上的自旋密度函数,这样便建立了主磁场任意不均匀下的磁共振信号模型。实验结果表明,该方法达到与均匀主磁场下近似同样的效果。     

Effects of a high-gradient magnetic field on the migratory behavior of primary crystal silicon in hypereutectic Al–Si alloy

The migration of primary Si grains during the solidification of Al–18 wt%Si alloy under a high-gradient magnetic field has been investigated experimentally. It was found that under a gradient magnetic field, the primary Si grains migrated toward one end of the specimen, forming a Si-rich layer, and the thickness of the Si-rich layer increased with increasing magnetic flux density. No movement of Si grains was apparent under a magnetic field below 2.3 T. For magnetic fields above 6.6 T, however, the thickness of the Si-rich layer was almost constant. It was shown that the static field also played a role in impeding the movement of the grains. The primary Si grains were refined in the Si layer, even though the primary silicon grains were very dense. The effect of the magnetic flux density on the migratory behavior is discussed.     

磁流体薄片中聚焦激光束对交流磁场的相位独立响应

报导了磁流体薄片中聚焦激光束在探测50Hz交流磁场时产生与磁场相位无关响应的效应。聚焦于并通过磁流体薄片的633nm激光束的偏转角仅与交流磁场的幅度相关。为了探寻其机理，还研究了磁流体中聚焦激光束对开／关磁场或激光的瞬态响应。提出了基于磁熵守恒的磁流体中磁性粒子密度再分布正反馈模型来解释这一效应。这一模型也可以预 磁流体中一种磁－热失稳现象。     

Surface waves on viscoelastic magnetic fluid film flow down a vertical column

Long-wave perturbation method is employed to solve the nonlinear hydromagnetic stability of a thin electrically conductive viscoelastic fluid film flowing down on the outside surface of a vertical cylinder. As the viscoelastic effect and the magnetic force are introduced into governing equation, the results are more compatible with the practice situation. The modeling results display that the degree of instability in the film flow is further intensified by the lateral curvature of cylinder. It is also observed that by increasing the effect of the magnetic field and by decreasing the viscoelastic effect tends to enhance the stability as traveling down along the vertical cylinder. Moreover, the instability state could be neutralized by controlling the applied magnetic field. The optimum conditions can be found through the use of a system to alter stability of the film flow by controlling the applied magnetic field.     

Effect of rotation on a layer of micropolar ferromagnetic fluid heated from below saturating a porous medium

This paper deals with the theoretical investigation of the effect of rotation on a layer of micropolar ferromagnetic fluid heated from below saturating a porous medium subjected to a transverse uniform magnetic field. For a flat fluid layer contained between two free boundaries, an exact solution is obtained using a linear stability analysis theory and normal mode analysis method. For the case of stationary convection, the effect of various parameters like medium permeability, rotation, non-buoyancy magnetization, coupling parameter, spin diffusion parameter and micropolar heat conduction has been analyzed. The critical magnetic thermal Rayleigh number for the onset of instability are also determined numerically for sufficiently large values of magnetic parameter M₁ and results are depicted graphically. The principle of exchange of stabilities is found to hold true for the micropolar ferromagnetic fluid saturating a porous medium heated from below in the absence of micropolar viscous effect, microinertia and rotation. The oscillatory modes are introduced due to the presence of the micropolar viscous effect, microinertia and rotation, which were non-existent in their absence. The sufficient conditions for the non-existence of overstability are also obtained.     

Effect of High Magnetic Field on Growth Behavior of Compound Layers during Reactive Diffusion between Solid Cu and Liquid Al

The effect of magnetic field on the growth behavior of compound layer was examined at the interface between the solid Cu and liquid Al during reactive diffusion. It was found that the thickness of compound layer was reduced by the high magnetic field. The growth activation energy in β, γ1 and ε2 layers under a high magnetic field was larger than those in non magnetic circumstances, the increment percentage being 4.8%, 13.3% and 5.5%, respectively.