Presence Of Horizontal Magnetic Field Research Articles

The magnetically controlled ionopause boundary observed by LPW onboard MAVEN within magnetic pile-up region of Mars

We have analysed plasma and magnetic field data obtained from 627 orbits of Mars Atmosphere and Volatile Evolution (MAVEN) that occurred during October 2014, April 2015, September 2017 and May 2018, when the spacecraft was crossing the magnetic pile-up boundary of Mars. Two broad peaks and a drop are observed in this magnetic field region. The magnetic cavity boundary lies between the ionosphere and the magnetic pile-up boundary. A steep ionopause-like boundary is observed in 24 profiles of 627 orbits, when MAVEN was passing from the magnetic pile-up region during the daytime ionosphere in presence of horizontal magnetic field of high strength, while their night side ionosphere did not show such boundary in presence of horizontal magnetic field of low strength. These profiles are unique, which have not been reported earlier. We propose that the horizontal magnetic field can form such ionopause within the magnetic pile-up boundary during the daytime if time and location of the magnetic anomaly coincide with the ion and electron density measurements. The nighttime ionosphere is thought to be produced within the magnetic pile-up boundary due to transportation of plasma from dayside to nightside across the terminator by a horizontal plasma flow velocity as has been predicted by the previous studies.

Instability of the plane parallel flow through a saturated porous medium in the presence of a longitudinal magnetic field using the Chebyshev collocation method

The onset of instability against small disturbances of the pressure driven laminar fluid flow through a saturated porous medium in the presence of horizontal magnetic field between the two parallel impermeable plates is studied numerically. Assuming the channel walls have zero conductivity. The Chebyshev collocation method is used to solve the 6th order generalized Orr–Sommerfeld equation. The critical values of the Reynolds number Rec, the wave number αc and the wave speed cc are computed for various sets of the values of the porous parameter σp, magnetic Reynolds number Rm and Alfven number A. The marginal stability curves are drawn for various values of the physical parameters present in the problem. The onset of instability is also discussed in detail using the curves of growth rate of the disturbances and the critical values for various parameters present in the problem. It is observed that the role of σp, Rm and A are to delay the onset of instability of the system. The degree of stabilization is moderate up to certain value of A but it is so rapid when the Alfven number exceeds that value is observed. It is also seen that, the role of the combined influence of porosity of the porous medium and the strength of the magnetic field together suppress the growth of the disturbances very significantly. In this study the results computed are also compared with the existing literature as a particular case for ordinary Newtonian fluids.

Transitions near the onset of low Prandtl-number rotating convection in presence of horizontal magnetic field

We investigate the transitions near the onset of thermal convection in electrically conducting low Prandtl-number (Pr) fluids in the presence of rotation about a vertical axis and external horizontal magnetic field. Three-dimensional direct numerical simulations (DNSs) and low dimensional modeling are performed with the Rayleigh–Bénard convection system in the ranges 0 < Q ≤ 1000 and 0 < Ta ≤ 500 of the Chandrasekhar number (Q) and the Taylor number (Ta), respectively, for that purpose. For larger Q(≥32.7), DNSs show substantial enhancement of convective heat transport and only finite amplitude steady two dimensional roll patterns at the onset. On the other hand, for smaller Q(<32.7), very rich dynamics involving different stationary as well as time dependent patterns, including stationary two-dimensional rolls, cross rolls, and oscillatory cross rolls, are observed at the onset of convection. Our investigation uncovers the cause of enhancement of heat transport and the origin of different flow patterns at the onset. We establish that a first order transition to convection occurring at the onset is responsible for the enhancement of the heat transport there. Furthermore, as the Rayleigh number (Ra) is increased beyond the onset, subsequent transitions near it are also explored in detail for smaller Q, and these are found to be associated with a variety of bifurcations including subcritical/supercritical pitchfork, Hopf, imperfect pitchfork, imperfect gluing, and Neimark–Sacker.

Thermal Instability of a Viscoelastic Walters'(model B') Fluid in Hydromagnetics in the Presence of Suspended Particles

The effect of suspended particles on thermal instability of a viscoelastic Walters' (model B') fluid in the presence of a uniform horizontal magnetic field is considered. The sufficient conditions for non-existence of overstability are found in the presence of horizontal magnetic field. The effect of suspended particles is to postpone the onset of convection. For stationary convection, Walters' (model B') viscoelastic fluid behaves like a Newtonian viscous fluid.

Stability of Stratified Rivlin-Ericksen Fluid in the Presence of Horizontal Magnetic Field and Uniform Horizontal Rotation in Porous Medium

The influence of viscosity, viscoelasticity and medium permeability on the stability of stratified Rivlin-Ericksen viscoelastic fluid is examined for viscoelastic polymeric solutions in the simultaneous presence of a uniform horizontal magnetic field (H, 0, 0) and uniform horizontal rotation (Ω,0,0). These solutions are known as Rivlin-Ericksen fluids and their rheology is approximated by the Rivlin-Ericksen constitutive relations, proposed by Rivlin and Ericksen [13]. The effects of Coriolis forces on the stability is chosen along the direction of the magnetic field and transverse to that of the gravitational field (o,o,-g). The system is found to be stable for all wave numbers for stable stratifications and unstable for unstable stratifications for the stratifications in density, viscosity, viscoelasticity, medium permeability and medium porosity. The system can be completely stabilized by large enough magnetic field, which was unstable in the absence of magnetic field; provided the initial configuration is top-heavy density wise. The kinematic viscosity and kinematic viscoelasticity have damping effects on the growth rates with the increase in kinematic viscosity and kinematic viscoelasticity, respectively, for a fixed wave number. The medium permeability has enhancing effects on the growth rates with its increase for a fixed wave number. The above results have also been shown graphically.

Hall effect on thermal stability of ferromagnetic fluid in porous medium in the presence of horizontal magnetic field

This paper deals with the theoretical investigation of the effect of Hall currents on the thermal stability of a ferromagnetic fluid heated from below in porous medium. For a fluid layer between two free boundaries, an exact solution is obtained using a linearized stability theory and normal mode analysis. A dispersion relation governing the effects of medium permeability, a uniform horizontal magnetic field, magnetization and Hall currents is derived. For the case of stationary convection, it is found that the magnetic field and magnetization have a stabilizing effect on the system, as such their effect is to postpone the onset of thermal instability whereas Hall currents are found to hasten the onset of thermal instability. The medium permeability hastens the onset of convection under certain conditions. The principle of exchange of stabilities is not valid for the problem under consideration whereas in the absence of Hall currents (hence magnetic field), it is valid under certain conditions.

Model III: Benard convection in the presence of horizontal magnetic field and rotation

Motivated by the need to understand better the roles of viscosity, electrical conductivity of the boundary and the interaction between all the possible wave motions in a magnetic rotating system, we study the linear stability of a simple system which can support five wave modes that can be excited in a rotating diffusive fluid under the influence of a magnetic field and gravity. This is a Benard layer rotating uniformly about a horizontal axis in the presence of a horizontal magnetic field inclined at an angle to the rotation vector, a situation previously studied succinctly under the name of model III. The stability is governed by seven dimensionless parameters: (i) the modified Rayleigh number, R, the Elsasser number, Λ, and the Ekman number, E, which, respectively, represent the ratios of the buoyancy, Lorentz and viscous forces to the Coriolis force, (ii) q and pm which represent the ratios of thermal diffusivity and viscosity to magnetic diffusivity, (iii) r denoting the ratio of electrical conductivity of boundary to that of fluid, and (iv) f which measures the angle between field and rotation vector. A comprehensive investigation of the properties of these waves is carried out in the geophysically relevant case of small Ekman numbers, although some exact solutions are presented for other values of E. The preferred mode of convection is identified for each type of convection and the overall preferred mode is discussed and regime diagrams for the preference of the different modes are constructed in the parameter space. It is shown that viscosity plays a crucial role in the identification of the preferred mode of convection, although it may be very small. The assumption that E≪1 naturally leads to the development of boundary layers, and a rich variety of them is found to exist in the system. The contribution of every boundary layer to the solution is found and the variables of the system strongly affected by the boundary layers are identified. The boundary layers are found to be affected by the dynamic and electrical properties of the boundary. Whereas free boundaries can sometimes allow exact solutions, rigid boundaries always develop boundary layers. The electrical conductivity of the boundary is found to have a quantitative effect on four modes while it leads to different mainstream solutions in the case of one mode, which can exist for a limited range of parameters and can take the form of a propagating wave only.

Natural convection of nanofluids in an enclosure between a circular and a sinusoidal cylinder in the presence of magnetic field

In this study natural convection heat transfer of Cu–water nanofluid in a cold outer circular enclosure containing a hot inner sinusoidal circular cylinder in the presence of horizontal magnetic field is investigated numerically using the Control Volume based Finite Element Method (CVFEM). Both circular enclosure and inner cylinder are maintained at constant temperature. The governing equations of fluid motion and heat transfer in their vorticity stream function form are used to simulate the fluid flow and heat transfer. The effective thermal conductivity and viscosity of nanofluid are calculated using the Maxwell–Garnetts (MG) and Brinkman models, respectively. The calculations were performed for different governing parameters such as the Hartmann number, Rayleigh number, values of the number of undulations of the inner cylinder and nanoparticle volume fraction. The results indicate that in the absence of magnetic field, enhancement ratio decreases as Rayleigh number increases while an opposite trend is observed in the presence of magnetic field. Also it is found that the average Nusselt number is an increasing function of nanoparticle volume fraction, the number of undulations and Rayleigh numbers while it is a decreasing function of Hartmann number.

Stability of stratified elastico-viscous Walters' (Model B') fluid in the presence of horizontal magnetic field and rotation in porous medium

Combined effect of magnetic field and rotation is considered on the stability of stratified visco-elastic Walters' (Model B') fluid in porous medium. In contrast to the Newtonian fluids, the system is found to be unstable at stable stratification for low values of permeability or high values of kinematic viscoelasticity. Magnetic field is found to stabilize the small wavelength perturbations for unstable stratification. Variation of growth rate in the case of unstable stratification is depicted graphically with the variation in viscosity, viscoelasticity and permeability. It has been found that the growth rate increases with the increase in kinematic viscosity and permeability, whereas it decreases with the increase in kinematic viscoelasticity.

Magnetic effect for electrochemically driven cellular convection.

Hydrodynamic instability analogous to Rayleigh-Bénard convection is observed in an electrolytic solution between two parallel copper wire electrodes. The laser interferometric technique can reveal the dissipation structure created by the motion of the fluid, which is controlled electrochemically. It is shown that under the presence of horizontal magnetic field the roll cells move horizontally along the electrodes. The electrochemically driven convection is simply controlled and monitored by setting and measuring the electrochemical parameters and forms many kinds of spatiotemporal patterns, especially under the magnetic field. The phenomenon is modeled by considering a Boussinesq fluid under a concentration gradient. The stability of the resulting equations is studied by linear stability analysis. The time dependent nonlinear system is investigated numerically and the main features of the experimental response are reproduced.

Convective instability in the presence of a nonuniform horizontal magnetic field

Newcomb’s criterion for convective stability in the presence of a horizontal magnetic field is written in a form which explicitly shows the effect of vertical variations of the magnetic field strength. It is shown that a nonuniform horizontal magnetic field can be destabilizing as well as stabilizing.