Effect of variable gravitational field on thermal instability of a rotating fluid layer with magnetic field in porous medium

The effect of variable gravitational field on thermal instability of a rotating fluid layer in the presence of magnetic field in porous medium is investigated. It is found that the system is stable when gravity is decreasing upwards. The principle of exchange of stability is valid in the absence of rotation and magnetic field when gravity increases upwards. In the stationary convection, rotation has stabilizing or destabilizing effect depending upon whether gravity is increasing or decreasing upwards. The medium permeability and magnetic field have stabilizing or destabilizing effect depending upon condition.     

MHD flow of two immiscible visco-elastic Rivlin-Ericksen fluids through a non-conducting rectangular channel in presence of transient pressure gradient

The aim of this paper is to investigate the problem of MHD flow of two immiscible viscoelastic Rivlin-Ericksen fluids through a non-conducting rectangular channel in presence of transient pressure gradient. Appropriate to the boundary conditions of the problem the solution is derived by variable separation technique. Using this solution the interface velocity, flux, skin friction and mean velocity are derived. In absence of the magnetic field and the elastic behaviour the corresponding classical problem can be derived.     

Stability of stratified fluid in porous medium in the presence of suspended particles and variable magnetic field

General equations governing the stability of stratified fluid in a stratified porous medium in the presence of suspended particles and variable horizontal magnetic field, separately, have been derived. Assuming stratifications in density, viscosity, suspended particles number density, medium porosity, medium permeability and a magnetic field of exponential form the dispersion relations have been obtained. Systems have been found to be stable for stable stratifications and unstable for unstable stratifications. A system which was unstable in the absence of magnetic field can be completely stabilized by a magnetic field for a certain wave-number range. The behaviour of growth rates with respect to fluid viscosity, medium permeability, suspended particles number density and magnetic field has been examined analytically.     

Effects of finite larmor radius and variable gravitational field on thermal instability of fluid layer under rotation in porous medium

The effect of finite Larmor radius, magnetic field, rotation and variable gravitational field on thermal instability of fluid layer in porous medium is investigated. It is found that the principle of exchange of stability is valid in the absence of magnetic field and rotation. The system is stable/unstable depending upon certain conditions in the presence of rotation, magnetic field and medium permeability. The system is stable in presence of finite Larmor radius. The above work has been carried out under research project financed by University Grants Commission New Delhi (India) and the authors are grateful to University Grants Commission for their financial support.     

Effect of a variable magnetic field on the instability of a stratified rotating fluid layer in porous medium

The instability of a stratified rotating fluid layer through porous medium in the presence of an inhomogeneous magnetic field is investigated. For exponentially varying density and magnetic field variations, an eigenvalue solution has been obtained. The dispersion relation is obtained and discussed for both the stable and unstable stratifications separately. It is found, for non-porous medium, that for the stable mode of disturbance, the system is always stable, and for the unstable mode of disturbance, it is stabilized only under a certain condition for the Alfvèn velocity, rotation and the stratification parameter. In the latter case, both rotation and magnetic field are found to have a stabilizing effect on the growth rate. In the presence of porous medium, it is found, for real growth rate n, that the inhomogeneous magnetic field has always a stabilizing effect on the considered system. It is found also, for complex growth rate n, that the system is stable for the stable stratification case, while it is stable or unstable for the unstable case under a certain wavenumbers range depending on the Alfvèn velocity and the stratification parameter. The presence of the magnetic field is found to stabilize a certain wavenumbers band, whereas the system was unstable for all wavenumbers in the absence of the magnetic field. Also, the presence of porous medium is found to hide the stabilizing effect played by rotation on the considered system for non-porous medium, i.e., rotation does not have any significant effect on the stability criterion in this case.     

Flows in a rotating elastico-viscous fluid induced by torsional oscillations of a plate acting for a finite time

Unsteady flows in an electrically conducting rotating elastico-viscous liquid in the presence of a uniform magnetic field and induced by small amplitude torsional oscillations, acting for a finite time, of an infinite, rigid, non-conducting plate, are discussed. This analysis indicates the general features of the steady and unsteady velocity field, and the structure of the associated boundary layers on the plate including the effect of rotation, hydromagnetic and elastic parameters involved in the problem. The velocity field related to small elastic parameter is calculated with physical significance. It is further shown that the Ekman suction velocity, which is responsible for the generation of an axial inflow toward the bounadry layer, does not appear unless the plate is subject to oscillations for an indefinite period. Several limiting results are found to follow as special cases of this analysis.     

On the folding instability of an elastico-viscous layer under compression

In this paper the problem of folding instability of a uniformly thick elastico-viscous layer sandwiched in another elastico-viscous medium of infinite extent is discussed. The layer is under lateral compression and the time-dependence of strain for viscosity has been taken as varying exponentially. It is found that elastic failure may occur after certain intervals of time depending on definite material constants.     

On couple-stress fluid heated from below in porous medium in hydromagnetics

A layer of electrically conducting couple-stress fluid heated from below in porous medium in presence of magnetic field is considered. For stationary convection, the couplestress and magnetic field postpone the onset of convection whereas the medium permeability hastens the onset of convection. The magnetic field introduces oscillatory modes in the system which were non-existent in its absence. A sufficient condition for the non-existence of overstability is obtained.     

Thermosolutal convection in a Maxwellian viscoelastic fluid in porous medium

The thermosolutal convection in a layer of Maxwellian viscoelastic fluid heated and soluted from below in porous medium is considered. The effects of uniform magnetic field and uniform rotation on the thermosolutal convection are also considered. For stationary convection, the Maxwellian viscoelastic fluid behaves like a Newtonian fluid. The sufficient conditions for the nonexistence of overstability are obtained. The critical Rayleigh number is found to increase with the increase in magnetic field, rotation and stable solute gradient.     

Heat transfer in fluctuating flow of an elastico-viscous fluid past an infinite plate with constant suction

An analysis of heat transfer in a two dimensional flow of an elastico-viscous fluid (Walters liquid B) past an infinite porous plate has been carried out under the following conditions: (1) constant suction, (2) free stream oscillates in time about a constant mean, (3) the plate is thermally insulated. Approximate solutions to the temperature field have been derived on taking into consideration viscous dissipative terms. The mean wall temperature has been shown graphically and it is observed that it decreases with increasing frequency.     

The instability of streaming Walters' viscoelastic fluid B′ in porous medium

The instability of streaming Walters' elastico-viscous fluid B in porous medium is considered. The case of two uniform streaming fluids separated by a horizontal boundary is considered. It is found that for the special case when perturbations in the direction of streaming are ignored, the system can be stable or unstable, depending upon kinematic viscoelasticity, medium porosity and medium permeability, for both potential unstable and potentially stable configurations. In every other direction, a minimum value of wave-number has been found and the system is unstable for all wave-numbers greater than this minimum wave number.     

磁性液体在磁场中产生光的双折射效应机理

磁性液体是一种特殊的高分子稳定胶体,在磁场中会产生光的双折射效应,对磁性液体在胶体学科方面展开研究,发现磁性液体在磁场中的弱絮凝行为表现异常明显,显示出特有的方向性,且又不至胶体系统失稳,证明了磁性液体中的磁性微粒在磁场中聚集成方向性的链状而又不失稳的临界状态存在。从而揭示了方向性弱絮凝是磁性液体在磁场中产生光的双折射效应的机理。     

Thermosolutal instability in porous medium in a partially ionized plasma

The thermosolutal instability in porous medium in a partially ionized plasma in the presence of a uniform vertical magnetic field is considered. The presence of each, magnetic field and stable solute gradient, brings oscillatory modes which were nonexistent in their absence. The collisional effects may also bring in oscillatory modes. The stable solute gradient and magnetic field are found to have stabilizing effect whereas the medium permeability and collisional frequency have destabilizing effect on the thermosolutal instability in porous medium.     

On the resonance effect in a rotating magnetic fluid

Data for the low-frequency magnetic susceptibility of a rotating magnetic fluid measured in a permanent bias field are presented. It is found that the susceptibility of the medium resonantly grows when the rotation frequency coincides with the measuring field frequency. Reasons for and mechanisms behind this phenomenon are discussed.     

Rayleigh-taylor instability of viscoelastic fluids with suspended particles in porous medium in hydromagnetics

The instability of the plane interface between two viscoelastic (Oldroydian) superposed conducting fluids permeated with suspended particles in porous medium is studied when the whole system is immersed in a uniform magnetic field. The dispersion relation for the Oldroydian viscoelastic fluid is obtained which also yields dispersion relations for Maxwellian and Newtonian fluids in special cases, in the presence of suspended particles in porous medium in hydromagnelics. The system is found to be stable for potentially stable case. The presence of magnetic field stabilizes certain wave number band whereas the system was unstable for all wave numbers in the absence of magnetic field, for the potentially unstable configuration. The growth rates increase (for certain wave numbers) and decrease (for other wave numbers) with the increase in stress relaxation time, strain retardation time, suspended particles number density and medium permeability.     

Analysis of liquid sloshing of a tuned magnetic fluid damper for single and co-axial cylindrical containers

A tuned magnetic fluid damper (TMFD) is a dynamic absorber using a magnetic fluid. A characteristic of the TMFD is changing natural frequency of a magnetic fluid sloshing under a magnetic field. The magnetic fluid sloshing in a coaxial cylindrical container was analyzed theoretically under the axisymmetrical magnetic field. The theoretical results showed that a radial component of the magnetic field also changed the natural sloshing frequency. A policy of the suitable design of the TMFD was presented and the effect of the radial component of the magnetic field was verified experimentally.     

Gravitational Instability of Fluid of Finite Electrical and Thermal Conductivity Flowing through Porous Medium

In view of the importance of porosity in astrophysical context the problem of gravitational instability of fluid of finite electrical and thermal conductivity flowing through porous medium is studied. Equations of the problem are stated and from them linearized perturbation equations are derived. Dispersion relations are obtained and Jeans' criterion of instability is discussed taking properties of the medium in different combinations for parallel and perpendicular directions to the magnetic field. Thermal conductivity modifies the Jeans' expression. Magnetic field and porosity of the medium also modify the Jeans' expression in case of wave propagation in perpendicular direction to the magnetic field but finite electrical conductivity neutralizes the effect of porosity on Jeans' criterion of instability.     

Oscillations of a bubble separated from an air cavity under compression caused by magnetic field in a magnetic fluid

Based on the concept of mapping the free surface geometry of a weakly magnetic medium by the topography of isolines of the magnetic field strength magnitude, the shape of the free surface of a magnetic fluid is studied in the static state at two stages: the initial stage, where an annular magnet is moving toward the surface of the magnetic fluid column in a tube, and at the stage where the air cavity is pressed to the bottom. It is shown that the separation of bubbles from the air cavity occurs at the magnet axis in the immediate vicinity of the magnet symmetry plane. A method and an experimental setup are proposed to investigate possible electromagnetic indication of the size of air bubbles formed in the magnetic fluid. The results of an experimental study of bubble separation from an air cavity held in the magnetic fluid and compressed by the pondermotive forces of the magnetic field are discussed. The results may be of importance for designing a new technique for metering small gas shots to a reactor.     

Influential Factors of Internal Magnetic Field Gradient in Reservoir Rock and Its Effects on NMR Response

Nuclear magnetic resonance (NMR) plays a significant role in porous media analysis and petroleum exploration, but its response is significantly influenced by the internal magnetic field gradient in fluid saturated porous medium, which obviously limits the accuracy of rock core analysis and logging interpretation. The influential factors of the internal magnetic field gradient in formation and its influences on NMR response are studied in this paper, based on NMR mechanism through one- and two-dimensional core NMR experiments. The results indicate that the internal magnetic field gradient is positively correlated with the static magnetic field strength and the magnetic susceptibility difference between pore fluid and solid grains, while it presents negative correlation with pore radius. The internal magnetic field gradient produces an additional diffusion relaxation in hydrogen relaxation system and accelerates the attenuation of magnetization vector. As a result, T₂ spectrum shifts to the left and NMR porosity and diffusion coefficient of the fluid could be inaccurate. This research sets a foundation for the NMR porosity correction and fluid distribution on T₂-G maps based on the internal magnetic field gradient correction.     

The dynamic behavior of magnetic fluid adsorbed to small permanent magnet in alternating magnetic field

The dynamic behavior of a magnetic fluid adsorbed to a small NdFeB permanent magnet subjected to an alternating magnetic field was studied with a high speed video camera system. The directions of alternating magnetic field are parallel and opposite to that of the permanent magnet. It was found that the surface of magnetic fluid responds to the external alternating magnetic field in elongation and contraction with a lot of spikes. Generation of a capillary magnetic fluid jet was observed in the neighbourhood of a specific frequency of alternating field. The effect of gravitational force on surface phenomena of magnetic fluid adsorbed to the permanent magnet was revealed.