Magnetic Buoyancy Research Articles

Numerical study of motile microorganism in Williamson MHD nanofluid flow over an elastic slender surface of an irregular thickness

The energy transmission through bioconvective MHD Williamson nanofluid (NF) flow has been numerically computed over a slender stretching surface of irregular thickness in this analysis. The consequences of a uniform magnetic field, heat generation and absorption, chemical reaction, variable thermal conductivity and viscosity depending on temperature have been also considered. The scenario has been represented as a system of PDEs. By using the similarity substitutions, the system of PDEs is simplified to ODEs. The obtained set of first-order differential equations is then evaluated using the numerical methodology parametric continuation method (PCM). The results are compared for consistency and validity purposes with the bvp4c package and existing literature. It has been noticed that the velocity distribution reduces with the variation of magnetic field and buoyancy ratio factor, while boosts with the rising values of velocity power index and mixed convection constraint. Furthermore, the motile microorganism profile enhances with the influence of Rayleigh number, while declines with the action of Lewis number, Peclet number and mixed convection parameters.

Magnetic buoyancy instability and the anelastic approximation: regime of validity and relationship with compressible and Boussinesq descriptions

Magnetic buoyancy instability, which is of astrophysical importance, results from the influence of magnetic pressure variations on the density of a fluid in a gravitational field. It is inherently a compressible phenomenon and is, as such, fully described by the equations of compressible magnetohydrodynamics (MHD). For analytical and computational reasons, it is often convenient to study compressible MHD within simpler, asymptotically consistent reduced systems; the two most widely used result from the Boussinesq and anelastic approximations. Within the standard Boussinesq approximation of MHD, leading to the equations of Boussinesq magnetoconvection, magnetic buoyancy is excluded. It can, however, be included by a rescaling of the basic-state variables and by making further assumptions about the perturbation length scales. Within the anelastic approximation, no special measures are taken to incorporate magnetic buoyancy. It is, however, a priori unclear as to whether this neglect is justified, particularly in the light of the Boussinesq results. Our aims here are thus twofold. The first is to formulate the relationship between descriptions of magnetic buoyancy in the compressible, anelastic and Boussinesq systems. In so doing, we show that, under both the anelastic and Boussinesq approximations, magnetic buoyancy can be included either through a combination of a weak field and strong gradient, or, conversely, a strong field and weak gradient. Each has its own asymptotically consistent reduction, with dedicated governing equations. Our second aim is to address, through a linear stability analysis, under which conditions the standard anelastic system provides a faithful representation of magnetic buoyancy instability. For completeness, we also formulate the energy principle of ideal MHD within the anelastic framework, and demonstrate the relation with its fully compressible counterpart.

On the effect of surface bipolar magnetic regions on the convection zone dynamo

ABSTRACTWe investigate the effect of surface bipolar magnetic regions (BMRs) on the large-scale dynamo distributed in the bulk of the convection zone. The study employs the non-linear three-dimensional mean-field dynamo model. We model the emergence of the BMRs on the surface through the non-axisymmetric magnetic buoyancy effect, which acts on the large-scale toroidal magnetic field in the upper half of the convection zone. The non-axisymmetric magnetic field that results from this mechanism is shallow. On the surface, the effect of the BMRs on the magnetic field generation is dominant. However, because of the shallow distribution of BMRs, its effect on the global dynamo is less compared with the effect on the convective zone dynamo. We find that the mean-field α-effect, which acts on the non-axisymmetric magnetic field of the BMRs, provides the greater contribution to the dynamo process than the tilt of the BMRs. Even so, the fluctuations of the tilt of the BMRs lead to parity braking in the global dynamo. At the surface, the non-axisymmetric magnetic fields, which are generated because of the activity of the BMRs, show a tendency for the bihelical spectrum with positive sign for the low ℓ modes during the maximum of the magnetic activity cycle.

Effect of Throughflow on Marangoni Convection in Ferromagnetic Liquids with Variable Viscosity

The effect of surface tension and vertical throughflow in magnetic field and temperature sensitive liquid of Newtonian ferromagnetic type is examined. Combining shooting and Galerkin techniques the stationary critical eigenvalues are computed. On the ferroconvection onset, M3, non-buoyancy magnetic parameter and Pr (>1), Prandtl number have no influence. For upward or downward directions dual effect of destabilizing or stabilizing the system is observed in the presence of vertical throughflow. Throughflow effect studied through Pe, Peclet number delays ferroconvection onset and is observed to be independent to flow direction. M1, the magnetic buoyancy number, and V, the variable viscosity parameters have destabilizing influences on the system. Ferrofluids being fascinating materials are used as constant agents for magnetic resonance imaging (MRI). The great variety of magnetic fluids are tailored for technological, biological, medical and material science applications. Key words: Ferroconvection, throughflow, surface tension, temperature dependent viscosity, magnetic field dependent viscosity.

Investigation on magnetic force and buoyancy force on flow and heat transfer of Al2O3-water nanofluid at various temperature-dependent thermophysical models by a novel PLBM

A novel preconditioned lattice Boltzmann method (PLBM) is applied to investigate the thermo-hydrodynamic characteristics of Al2O3-water nanofluid with various temperature-dependent thermophysical properties and slip boundary conditions in a microchannel. Also, the effects of a magnetic field force, buoyancy force, and volume fraction of nanoparticles are analyzed, respectively. The theoretical level confirms that the convergence performance of the PLBM is greatly improved at large Reynolds numbers compared to the original LBM. Numerical results show that the Koo & Kleinstreuer model has the largest average shear stress and average Nusselt number. Among all the physical models considered, these two values are approximately 1.05 times and 1.2 times of the values by the Maxwell & Brinkmann model, respectively. Once the magnetic field is considered, the average shear stress is increased twice and the average Nusselt number increases by about 10% and further by about 20% at higher buoyancy forces. The growth of these two values gradually slows down when the volume fraction of nanoparticles exceeds 2%. The heat transfer process makes the largest contribution to entropy generation when the magnetic field force is not considered. Accordingly, it will dominate the irreversible loss due to the strong shear stress once the magnetic field force is involved in the heat transfer process.

Time-dependent squeezing bio-thermal MHD convection flow of a micropolar nanofluid between two parallel disks with multiple slip effects

We performed numerical analysis on fluid model for incompressible, time dependent electrically conducting squeezing flow of micropolar fluid. The non-Newtonian fluid is confined across two disks in which lower disk is fixed while upper disk moves upward and downward. A uniform suction is made at the lower fixed disk. The Buorngiorno nanofluid theory is implemented to discuss the thermal performance by thermopherosis and Brownian motion phenomenon. Microorganism theory is applied to sustain the suspended nanoparticles by bioconvection induced through combined influence of magnetic field and buoyancy forces. The nonlinear system is reduced to a set of ordinary differential equation by similarity variables. The dimensionless quantities are discussed in pictorial way on velocity, temperature, concentration and microorganism fields. It is perceived that micropolar and magnetic parameters have opposite trend on micro-rotation field. Brownian motion and Thermopheresis parameters have similar kind of influence on temperature profiles and the thermal fields is reduced by suction parameter. Radial velocity curves declined by the squeezing Reynolds number. Numerically values of the physical parameters are also calculated and described through tabular format. It is calculated that slip parameters are reduced the Nusselt and Sherwood numbers values at the both disks respectively. At the center of the plane between two disks, the radial velocity curves are declined, whereas such profiles are enhanced at the lower and upper disks against the magnetic parameter. The increased suction parameter reduces the radial field of f′η. Moreover the radial profiles are of parabolic nature.Microrotation and magnetic parameters tend to rotate the microrotation field in opposite way. In the left region of the central plane, the profiles are enhanced, while in the right half plane the micorotation profiles are reduced. Furthermore, the microrotation curves are intersecting at η = 0.5, where the profiles turn into opposite way.

Impact of nanoparticle shape on thermo-solutal buoyancy induced lid-driven-cavity with inclined magnetic-field

The aim of this paper is to numerically investigate the influence of nanoparticles shape on heat and mass transport phenomena in a moving lid cavity under the combined effect of thermo-solutal buoyancy force and magnetic force. The governing equations are transformed into velocity-vorticity form of equations and solved using Galerkin's weighted residual finite-element-technique. The analysis has been carried out with parameters like buoyancy ratio (−5 ≤ N ≤ 5), magnetic field inclination angle (0° ≤ ϕ ≤ 90°) with four shapes of Al2O3 nanoparticle like bricks, blades, platelets and cylinders. The results revealed that the shape factor on Nusselt number is significant depending upon the inclined magnetic field and buoyancy ratio whereas on mass transfer the shape effect is negligible. The diffusion mode of transport process is stronger than the convection mode at higher inclination angle of magnetic field. Based on a given value of N and ϕ, blade and cylinder shows the best performance in Nusselt and Sherwood number respectively except the platelet shape that shows maximum frictional loss in terms of wall shear stress.

Numerical simulation of bioconvective Darcy Forchhemier nanofluid flow with energy transition over a permeable vertical plate

In biological systems, the MHD boundary layer bioconvection flow through permeable surface has several applications, including electronic gadgets, heating systems, building thermal insulation, geological systems, renewable energy, electromagnetism and nuclear waste. The bioconvection caused by the hydromagnetic flow of a special form of water-based nanoliquid including motile microorganisms and nanoparticles across a porous upright moving surface is investigated in this report. The combination of motile microbes and nanoparticles causes nanofluid bioconvection is studied under the cumulative impact of magnetic fields and buoyancy forces. The Brownian motion, thermophoresis effects, heat absorption/generation, chemical reaction and Darcy Forchhemier impact are also unified into the nonlinear model of differential equations. The modeled boundary value problem is numerically computed by employing a suitable similarity operation and the parametric continuation procedure. The parametric study of the flow physical parameters is evaluated versus the velocity, energy, volume fraction of nanoparticles, motile microorganisms’ density, skin friction, Sherwood number and Nusselt number. It has been observed that the velocity profile reduces with the effect of porosity parameter k1, inertial parameter k2, Hartmann number and buoyancy ratio. While the energy transition profile significantly enhances with the flourishing values of Eckert number Ec, heat absorption/generation Q and Hartmann number respectively.

Mixed convective flow of Casson and Oldroyd-B fluids through a stratified stretching sheet with nonlinear thermal radiation and chemical reaction

Non-Newtonian fluids flow with mixed convection has got the researcher’s attention due to its widespread application in engineering and manufacturing systems. Furthermore, the thermal radiation in convective thermal transmission plays a key role in regulating the thermal transmission. That’s why the authors’ interest comes to the point to present a detailed analysis of the mixed convective flow of Casson and Oldroyd-B fluids through a linearly stratified stretching sheet. Also, the flow of Casson and Oldroyd-B fluids are considered as chemically reactive, thermally radiative, and magnetized by a strong magnetic field. The mathematical model of the present analysis is presented in PDEs (partial differential equations) format which is transformed into ODEs (ordinary differential equations) by employing suitable variables. The transformed ODEs are treated with an analytical technique called HAM (Homotopy analysis method) which provides an analytical solution for both linear and nonlinear differential equations. Through the use of diagrams, the converging exploration of HAM is depicted. An interesting result has been introduced that, the streamlines of Casson liquid are highly affected by a strong magnetic field as compared to Oldroyd-B liquid. The strong magnetic field and buoyancy ratio parameter moderate the velocity profile, while an opposite trend is found in the motion of liquid by using mixed convectional factors. Additionally, the impacts of magnetic and buoyancy ratio parameters are greater for Oldroyd-B fluid as associated with Casson liquid. On the other hand, the mixed convection parameter is stronger for Casson liquid as equated to Oldroyd-B fluid. The present results have a great agreement with previously published results.

MHD convective non-Darcy flow of a nanofluid through a porous stretching sheet with thermal buoyancy and chemical reaction

In this article, magnetohydrodynamic (MHD) the two-dimensional stagnation-point convection flow over a nanofluid through a porous medium due to a solar energy stretching sheet is investigated theoretically. Heat transfer subject to thermal radiation, viscous dissipation, an applied magnetic field, thermal buoyancy, and convective boundary conditions is considered. The governing boundary layer equations are reduced into ordinary differential equations by a similarity transformation. The transformed equations are solved numerically by using an efficient numerical shooting technique with a fourth-fifth order Runge–Kutta method scheme. The effects of the external parameters as jets flow, thermal convective, and mass convective on the flow field and heat transfer characteristics were obtained and discussed. Finally, the results obtained agree with the practical results and applicable in the related fields, especially, medicine, biochemistry, pharmacology, … , etc.

Inclined magnetic dipole and nanoscale energy exchange with infinite shear rate viscosity of 3D radiative cross nanofluid

AbstractWorking nanoliquids are used in bionomical concerns and to enhance the energy in the refrigeration system. The unique thermophysical characteristics of nanoliquids made nanoscience interesting and beneficial for fields of engineering, medical, and industry with real applications, like, food science, employing nanoparticles to deliver drugs, food microbiology, detection of foodborne pathogens, detection of foodborne pathogens, electronic device cooling, controlling fusion, magnetic cell separation, cancer therapeutics, nanocryosurgery, and so forth. The aim of this study is to deliver a broad analysis on nanoscale energy exchange and inclined magnetic dipole impact with the mathematical model of cross nanofluid. Scrutiny of movement of fluid is made by placing the magnetic dipole and buoyancy force. Transport of energy is investigated by thermal radiation, nonuniform heat sink–source while checking of mass transfer facts of activation energy and the chemical process is taken into consideration. Moreover, the nanostructured concepts of thermophoresis and Brownian movement along with zero‐mass flux constraint are also utilized for the comprehensiveness of this study. Mathematical relations of cross nanofluid are processed with similarity transformation, shooting methodology, and bvp4c Matlab procedure is rehearsed to get the numerical results of this attempt.

Triple Solutions with Stability Analysis of MHD Mixed Convection Flow of Micropolar Nanofluid with Radiation Effect

This paper deals with two‐dimensional steady boundary layer flow, heat, and mass transfer characteristics of micropolar nanofluid past on exponentially stretching/shrinking surface. The effect of different physical parameters like magnetic field, buoyancy, thermal radiation, and connective heat transfer are examined. Furthermore, similarity solutions are obtained by similarity transformation on the governing system of partial differential equations. The shooting method with help of the Maple software is used to achieve the numerical solutions of the equations. For the different ranges of the applied parameters, triple solutions are obtained for both cases of the surface. In view of the triple solutions, stability analysis is performed by bvp4c in the MATLAB software, where only first solution is found feasible which is discussed. The main findings of the first solution indicate the skin friction, drag force, heat, and mass transfer rates are increasing for the λ > 0 and decreasing for λ < 0 as the K is enhanced. The velocity profiles decrease with increase in magnetic, slip velocity, and suction parameters. The temperature profiles increase with increase in magnetic, thermophoresis, thermal radiation, and Brownian motion parameters, whereas concentration profiles reduce with increase in Schmitt number and Brownian motion.

Fluorine nucleosynthesis and s-processing in AGB stars driven by magnetic-buoyancy mixing

Asymptotic giant branch (AGB) stars are thought to be among the most important sources of fluorine in our Galaxy. While observations and theory agree at close-to-solar metallicity, stellar models overestimate fluorine production in comparison to heavy elements at lower metallicities. We present predictions for 19F abundance for a set of AGB models with various masses and metallicities, in which magnetic buoyancy induces the formation of the 13C neutron source (the so-called 13C pocket). In our new models, fluorine is mostly created as a consequence of secondary 14N nucleosynthesis during convective thermal pulses, with a minor contribution from the 14N existing in the 13C pocket zone. As a result, AGB stellar models with magnetic-buoyancyinduced mixing show low 19F surface abundances which agree with fluorine spectroscopic observations at both low and near-solar metallicity.

Double-diffusive Magnetic Layering

Abstract Double-diffusive systems, such as thermosolutal convection, in which the density depends on two components that diffuse at different rates, are prone to both steady and oscillatory instabilities. Such systems can evolve into layered states, in which both components, and also the density, adopt a “staircase” profile. Turbulent transport is enhanced significantly in the layered state. Here we exploit an analogy between magnetic buoyancy and thermosolutal convection in order to demonstrate the phenomenon of magnetic layering. We examine the long-term nonlinear evolution of a vertically stratified horizontal magnetic field in the so-called “diffusive regime,” where an oscillatory linear instability operates. Motivated astrophysically, we consider the case where the viscous and magnetic diffusivities are much smaller than the thermal diffusivity. We demonstrate that diffusive layering can occur even for subadiabatic temperature gradients. Magnetic layering may be relevant for stellar radiative zones, with implications for the turbulent transport of heat, magnetic field, and chemical elements.

Role of rotational radial magnetic advection in possible explaining a cycle with two peaks

• Double maximum of the 11-year solar cycle. • Turbulent effects of macroscopic magnetohydrodynamics. • Negative magnetic buoyancy. • Turbulent pumping of a large-scale magnetic field. • Reconstructing of the toroidal magnetic field of the Sun. To explain the observed phenomenon of 11-year sunspot cycle with two peaks, we draw attention to the possible role of the turbulent magnetic pumping effects in the solar convection zone (SCZ) at the reconstructing of the toroidal magnetic field that generates the sunspots. Turbulent radial pumping of magnetic field has both kinetic (diamagnetism) and magnetic (vertical advection) contributions. We take into account five physical processes for the combined magnetic reconstructing in deep layers of the SCZ. They are the Ω effect near the tachocline, magnetic buoyancy of the smoothed field, macroscopic diamagnetism, rotational magnetic advection, and deep equatorward meridional circulation. It is the effect of advection with taking into account Sun's rotation that plays a key role in the proposed scenario of magnetic reconstructing. We found that reconstructing of deep toroidal field develops differently in the near-polar and equatorial domains of the SCZ. In the lower half of the near-polar domain, two downward pumping effects (diamagnetism and advection) act against magnetic buoyancy and, as a result, they neutralize magnetic buoyancy and thereby block the toroidal field near the tachocline. At the same time, in the deep layers of the equatorial domain, the rotational advection due to the latitudinal convection anisotropy changes its direction to the opposite (from downward to upward), thereby helping magnetic buoyancy. As a result, this upward magnetic pumping here contributes to the transport of the toroidal field to the solar surface. We call this transport as first upward magnetic advection surge. It ensures a transfer of the deep strong fields to the surface in the latitudes belt of sunspots. Meanwhile, a deep equatorward meridional flow transports the blocked near pole toroidal field in the tachocline from high latitudes to low ones where are favourable conditions for floating up of the magnetic field. Here this belated field is transported upward to the solar surface (the second upward magnetic advection surge). Ultimately, two time-delayed upward magnetic surges may cause on the surface in the “royal zone” the first and second peaks of the sunspots cycle.

Floating of dia-, para-, and superparamagnetic bodies in magnetic fluids: Analysis of wall effects in the framework of inductive approach

Laboratory experiments and numerical simulations were carried out to investigate physical patterns and main characteristics of the magnetic ponderomotive buoyancy force acting on solid dia-, para-, and superparamagnetic bodies immersed in cylindrical containers filled with magnetic fluids and magnetized by a uniform magnetic field. The experimental measurements of the magnetic buoyancy force validated with numerical simulations are reliable enough for use in the benchmark testing of various applications (ferrofluid sensors, measuring devices, separators, etc.). It is shown that the superparamagnetic bodies have one (stable) equilibrium position in the center of the container in virtue of the fact that the ponderomotive force is monotonous. The non-magnetic bodies have an unstable equilibrium position in the center of the container and the force is non-monotonous with two extrema due to the competition between two mechanisms: the inhomogeneous demagnetizing field inside the cylindrical container, and the interaction of the non-magnetic body dipole with its own mirror image. Due to this competition, the force can change its direction depending on the geometry of the container, the applied magnetic field, and the nonlinear ferrofluid magnetization law. The results demonstrate that the non-magnetic bodies may be in stable equilibrium near the container walls only if two independent conditions are met: the gradient demagnetizing field is weak (the container is wide or quasi-ellipsoidal), and the applied field is weak (the ferrofluid magnetization curve is quasi-linear). If any of these conditions is abandoned, then the non-magnetic body is unable to levitate and falls on the container wall.

Presolar Grain Isotopic Ratios as Constraints to Nuclear and Stellar Parameters of Asymptotic Giant Branch Star Nucleosynthesis

Abstract Recent models for evolved low-mass stars (with M ≲ 3 M ⊙), undergoing the asymptotic giant branch (AGB) phase assume that magnetic flux-tube buoyancy drives the formation of 13C reservoirs in He-rich layers. We illustrate their crucial properties, showing how the low abundance of 13C generated below the convective envelope hampers the formation of primary 14N and the ensuing synthesis of intermediate-mass nuclei, like 19F and 22Ne. In the mentioned models, their production is therefore of a purely secondary nature. Shortage of primary 22Ne has also important effects in reducing the neutron density. Another property concerns AGB winds, which are likely to preserve C-rich subcomponents, isolated by magnetic tension, even when the envelope composition is O-rich. Conditions for the formation of C-rich compounds are therefore found in stages earlier than previously envisaged. These issues, together with the uncertainties related to several nuclear physics quantities, are discussed in the light of the isotopic admixtures of s-process elements in presolar SiC grains of stellar origin, which provide important and precise constraints to the otherwise uncertain parameters. By comparing nucleosynthesis results with measured SiC data, it is argued that such a detailed series of constraints indicates the need for new measurements of weak-interaction rates in ionized plasmas, as well as of neutron-capture cross sections, especially near the N = 50 and N = 82 neutron magic numbers. Nonetheless, the peculiarity of our models allows us to achieve fits to the presolar grain data of a quality so far never obtained in previously published attempts.

Magnetic-buoyancy-induced mixing in AGB stars: Fluorine nucleosynthesis at different metallicities

Asymptotic giant branch (AGB) stars are considered to be among the most significant contributors to the fluorine budget in our Galaxy. While observations and theory agree at close-to-solar metallicity, stellar models at lower metallicities overestimate the fluorine production with respect to that of heavy elements. We present 19F nucleosynthesis results for a set of AGB models with different masses and metallicities in which magnetic buoyancy acts as the driving process for the formation of the 13C neutron source (the so-called 13C pocket). We find that 19F is mainly produced as a result of nucleosynthesis involving secondary 14N during convective thermal pulses, with a negligible contribution from the 14N present in the 13C pocket region. A large 19F production is thus prevented, resulting in lower fluorine surface abundances. As a consequence, AGB stellar models with mixing induced by magnetic buoyancy at the base of the convective envelope agree well with available fluorine spectroscopic measurements at low and close-to-solar metallicity.

Effect of Non-linear Radiation on MHD Mixed Convection Flow of a Micropolar fluid Over an Unsteady Stretching Sheet

The current analysis explores the results of non-linear radiation on an unsteady stretching mixed Micropolar fluid. Issue modelling leads to a framework of PDE, reworked by transformations into non-linear ODEs. The Homotopy analysis methodology is applied to resolve the matter. The diagrams address the impact of the Magnetic parameter, unsteadiness parameter, Eckert number, Micropolar parameters, buoyancy parameters, Radiation parameter, Temperature ratio and Schmidt number on velocity, temperature, and concentration gradients. Amount of Nusselt number will increase in massive Prandtl number values, within the case of the high values of the unsteadiness parameter skin friction, wall couple stress and Sherwood decrease.

Magnetohydrodynamic stability of magnetars in the ultrastrong field regime I: the core

ABSTRACT We study magnetohydrodynamic stability of neutron star core matter composed of neutrons, protons, and leptons threaded by a magnetar-strength magnetic field 1014–1017 G, where quantum electrodynamical effects and Landau quantization of fermions are important. Stability is determined using the Friedman–Schutz formalism for the canonical energy of fluid perturbations, which we calculate for a magnetizable fluid with H ≠ B. Using this and the Euler–Heisenberg–Fermi–Dirac Lagrangian for a strongly magnetized fluid of Landau-quantized charged fermions, we calculate the local stability criteria for a neutron star core with a spherical axisymmetric geometry threaded by a toroidal field, accounting for magnetic and composition gradient buoyancy. We find that, for sufficiently strong fields B ≳ 1015 G, the magnetized fluid is unstable to a magnetosonic-type instability with growth times of the order of 10−3 s. The instability is triggered by sharp changes in the second-order field derivative of the Euler–Heisenberg–Fermi–Dirac Lagrangian that occur where additional Landau levels start being populated. These sharp changes are divergent at zero temperature, but are finite for non-zero temperature, so realistic neutron star core temperatures 5 × 107 K < T < 5 × 108 K are used. We conjecture that this mechanism could promote the formation of magnetic domains as predicted by Blandford and Hernquist and Suh and Mathews.