Field Flux Research Articles

Homogeneous Venturi-effect concentrators for creeping flows: Magnifying flow velocities and heat fluxes simultaneously

Although thermal metamaterials have received considerable attention for approximately a decade, previous studies have so far focused on heat conduction or thermal convection in porous media, hampering applications related to moving in media. Based on transformation heat transfer, here we have analytically designed Venturi-effect concentrators by simultaneously manipulating the dynamic viscosity and the thermal conductivity. Further based on this theory and integral median theorem, inhomogeneous concentrators metamaterials simplified to homogeneous counterparts are proposed. Noteworthily, the proposed method can be employed to simplify not only thermal metamaterials, but also other inhomogeneous metamaterials, such as optical metamaterials, electromagnetic metamaterials, acoustic metamaterials, among others. Numerical simulations demonstrate that these Venturi-effect concentrators can not only amplify both flow velocities and heat fluxes in thermal-flow fields, but also achieve cloaking effects in thermal-flow systems. Since these Venturi-effect concentrators do not interfere with the thermal-flow field outside the concentrators, they secure advantages over conventional Venturi tubes and will facilitate potential applications related to Venturi effects. • Homogeneous Venturi-effect concentrators for creeping flows are fabricated. • Analytic solutions of concentrators are obtained by transformation heat transfer. • A general way to simplify inhomogeneous metamaterials to homogeneous ones is built. • Our study paves a way to further explore applications related to Venturi effects.

Coupling effects of hydrogen and Dy/Nb on magnetic properties of sintered NdFeB magnets

The magnetic-property changes in sintered NdFeB magnets with different content of Dy and Nb after electrochemical hydrogen charging (EHC) were investigated. The phases of Dy2Fe14B, NbFeB, and Fe2Nb are confirmed by energy-dispersive spectrometry (EDS) and selected area electron diffraction. The electron back-scatter diffraction (EBSD) and electrochemical test show that the amount of Nd-rich grain boundary phases as hydrogen diffusion channels and the concentration of hydrogen near the surface of the magnets increase with the addition of Dy and Nb. Before and after EHC, the surface roughness and magnetic properties were measured using a laser scanning confocal microscopy, pulsed field magnetometer and flux meter. The results revealed that the surface roughness of magnets increases, owing to the absorption of hydrogen. During EHC, the remanence (Br) remains unchanged, whereas the magnitudes of the coercivity, magnetic moment (Mt), and maximum energy density ((BH)max) decrease by different degrees because of the presence of Dy and Nb compounds. Hydrogen decreases the stability of the magnet in demagnetization field by reducing the critical energy required for magnetic moment rotation, nucleation of reverse domains, and expansion of domain walls, and the presence of Dy and Nb compounds exacerbates the stability.

Direct data-driven forecast of local turbulent heat flux in Rayleigh–Bénard convection

A combined convolutional autoencoder–recurrent neural network machine learning model is presented to directly analyze and forecast the dynamics and low-order statistics of the local convective heat flux field in a two-dimensional turbulent Rayleigh–Bénard convection flow at Prandtl number Pr=7 and Rayleigh number Ra=107. Two recurrent neural networks are applied for the temporal advancement of turbulent heat transfer data in the reduced latent data space, an echo state network, and a recurrent gated unit. Thereby, our work exploits the modular combination of three different machine learning algorithms to build a fully data-driven and reduced model for the dynamics of the turbulent heat transfer in a complex thermally driven flow. The convolutional autoencoder with 12 hidden layers is able to reduce the dimensionality of the turbulence data to about 0.2% of their original size. Our results indicate a fairly good accuracy in the first- and second-order statistics of the convective heat flux. The algorithm is also able to reproduce the intermittent plume-mixing dynamics at the upper edges of the thermal boundary layers with some deviations. The same holds for the probability density function of the local convective heat flux with differences in the far tails. Furthermore, we demonstrate the noise resilience of the framework. This suggests that the present model might be applicable as a reduced dynamical model that delivers transport fluxes and their variations to coarse grids of larger-scale computational models, such as global circulation models for atmosphere and ocean.

Characterisation of the Tyre Spray Ejected Downstream of a Bluff Automotive Body

<div class="section abstract"><div class="htmlview paragraph">Considerations of surface contamination and airborne spray are becoming increasingly significant throughout the automotive design process. Advanced driver assistance systems, such as autonomous cruise control, are growing in popularity. These systems rely on external sensors, the performance of which may be impaired by both direct obstruction and spray. Existing experimental methods of assessing front-end surface contamination and wiper performance have typically utilised fixed spray-grids positioned upstream of the vehicle. The resulting spray is largely steady in nature, in contrast to the unsteady flow-field and tyre spray that would be produced by preceding vehicles. This paper presents the numerical analysis of the spray ejected downstream of a simplified automotive body. The continuous phase (air) is solved using a DDES-based approach coupled with a Lagrangian representation of the dispersed phase (water). Two configurations are examined, a square-back configuration and a variation employing 20° rear-end side tapers. The inclusion of side tapering results in a significant change in wake topology and the resulting spray cloud. Good agreement is achieved between initial single-phase predictions of the continuous phase and existing experimental data. The spray cloud of both configurations is found to be highly unsteady, driven by vortical structures in the near- and far-wake, and is altered significantly by what are relatively minor changes to the geometry. Proper Orthogonal Decomposition reveals comparable spatial modes in the mass flux field of the dispersed phase and the continuous phase velocity field downstream of the vehicle. However, the correlation between the temporal coefficients of these modes is relatively weak. This highlights the presence of slip effects between the two phases, coming as a result of particle inertia, and the need to consider both phases simultaneously in future studies of spray dynamics.</div></div>

PDM Klein–Gordon oscillators in cosmic string spacetime in magnetic and Aharonov–Bohm flux fields within the Kaluza–Klein theory

In the cosmic string spacetime and within Kaluza–Klein theory (KKT) backgrounds (indulging magnetic and Aharonov-Bohm flux fields), we introduce and study position-dependent mass (PDM) Klein–Gordon (KG) oscillators. The effective PDM is introduced as a deformation/defect in the momentum operator. We show that there are four different ways to obtain KG-oscillator. Two of which are readily known and the other two are obtained as byproducts of PDM settings. Next, we provide a thorough analysis on the corresponding spectra under different parametric effects, including the curvature parameter’s effect. Such analysis is used as a reference/lead model which is used in the discussion of different PDM KG-oscillators models: a mixed power-law and exponential type PDM model that yields a pseudo-confined PDM KG-oscillator in cosmic string spacetime within KKT (i.e., the PDM KG-oscillators are confined in their own PDM manifested Cornell-type confinement), and a PDM KG-oscillator confined in a Cornell-type potential. Moreover, we extend our study and discuss a non-Hermitian PT-symmetric PDM-Coulombic-type KG-particle model in cosmic string spacetime within KKT.

Euler-Lagrange numerical simulation of improved magnetic drug delivery in a three-dimensional CT-based carotid artery bifurcation

Background and objectiveMagnetic drug targeting (MDT) is a promising method to improve the therapy efficiency for cardiovascular diseases (CVDs) and cancers. In MDT, therapeutic agents are bonded to superparamagnetic iron oxide nanoparticle (SPION) cores and then are guided toward the damaged tissue through a magnetic field. Fundamentally, it's vital to steer the SPIONs to the desired location to increase the capture efficiency at the target lesion. Hence, the present study aims to enhance the drug delivery to the desired branch in a carotid bifurcation. Besides, it is tried to decrement the particles' entry to the unwanted outlet by using four different magnet configurations (with a maximum magnetic flux density of 0.4 T) implanted adjacent to the artery wall. Also, the effect of particles’ diameter –ranging from 200 nm to 2 µm– on the drug delivery performance is studied in the four cases. MethodsThe Eulerian-Lagrangian approach with one-way coupling is employed for numerical simulation of the problem using the finite element method (FEM). The dominant forces acting on particles are drag and magnetophoretic. A computed tomography (CT) model of the carotid bifurcation is adopted to have a 3D realistic geometry. The blood flow is considered to be laminar, incompressible, pulsatile, and non-Newtonian. Boundary conditions are applied using the three-element Windkessel equation. ResultsResults are presented in terms of velocity, pressure, magnetic field flux density, wall shear stress, and streamlines. Also, the number of particles in each direction is presented for the four studied cases. The results show that using proper magnets configurations makes it possible to guide more particles to the desired branch (up to 4%) while preventing particles from entering the unwanted branch (up to 13%). By defining connectivity between oscillatory shear index (OSI) value and magnetic drug delivery efficacy, it becomes clear that places with lower OSI values are more proper to place the magnets than areas with higher OSI values. ConclusionsIt was observed that increasing the diameter of particles does not necessarily result in a higher drug delivery efficiency. The configuration of the magnets and the size of particles are the main affecting parameters that should be chosen precisely to meet the most efficient drug delivery performance.Magnetic drug targeting (MDT) is a promising method to improve the therapy efficiency for cardiovascular diseases (CVDs) and cancers. Fundamentally, it's vital to steer the superparamagnetic iron oxide nanoparticles (SPIONs) to the target lesion location to increase the capture efficiency. Hence, the present study aims to enhance the drug delivery to the desired branch in a 3D carotid bifurcation. Besides, it is tried to decrement the particles' entry to the unwanted outlet by using four different magnet configurations implanted adjacent to the artery wall.The Eulerian-Lagrangian approach with one-way coupling is employed for numerical simulation of the problem using the finite element method (FEM). The dominant forces acting on particles are drag and magnetophoretic. The blood flow is laminar, incompressible, pulsatile, and non-Newtonian.The results show that it is possible to guide more particles to the desired branch (up to 4%) while preventing particles from entering the unwanted branch (up to 13%). By defining connectivity between oscillatory shear index (OSI) value and magnetic drug delivery efficacy, it becomes clear that places with lower OSI values are more proper to place the magnets than areas with higher OSI values.

Smoothed embedded finite-volume method (sEFVM) for modeling contact mechanics in deformable faulted and fractured porous media

A smoothed embedded finite-volume modeling (sEFVM) method is presented for faulted and fractured heterogeneous poroelastic media. The method casts a fully coupled strategy to treat the coupling between fault slip mechanics, deformation mechanics, and fluid flow equations. This ensures the stability and consistency of the simulation results, especially, as the fault slip is implicitly found through an iterative prediction-correction procedure. The computational grid is generated independently for embedded faults and rock matrix. The efficiency is further enhanced by extending the finite-volume discrete space by introducing only one degree of freedom per fault element. The embedded approach can lead to an oscillatory stress field at the fault, which damages the robustness of the implicit slip detection strategy. To resolve this challenge, a smoothed embedded strategy is devised, in which the stress and slip profiles are post processed within the iterative loops by fitting the best curve based on a least-square error criterion. The sEFVM provides locally conservative mass flux and stress fields, on staggered grid. Its performance is further investigated for several proof-of-the-concept test cases, including a multiple fault system in a heterogeneous domain. Results indicate that the method develops a promising approach for field-scale relevant simulation of induced seismicity.

Vanishing flux perturbation, pressure, and magnetic field limit in a Chaplygin magnetogasdynamics

The Riemann problem for a Chaplygin magnetogasdynamics system of compressible fluid flow with a triple parameter perturbation containing flux, pressure, and magnetic field is first solved. Second, it is shown that as the triple parameter flux perturbation, pressure, and magnetic field vanish, any Riemann solution containing two shock waves tends to a delta-shock solution to the transport equations and any Riemann solution containing two rarefaction waves tends to a vacuum solution to the transport equations. Third, it is also proved that as the double parameter flux perturbation and magnetic field vanish, any Riemann solution containing two shock waves approaches a delta-shock solution to the Chaplygin gas equations and any Riemann solution containing two rarefaction waves tends to a solution containing two contact discontinuities of the Chaplygin gas equations. Finally, some numerical results are presented to be consistent with the theoretical analysis.

The Importance of Horizontal Poynting Flux in the Solar Photosphere

Abstract The electromagnetic energy flux in the lower atmosphere of the Sun is a key tool to describe the energy balance of the solar atmosphere. Current investigations on energy flux in the solar atmosphere focus primarily on the vertical electromagnetic flux through the photosphere, ignoring the Poynting flux in other directions and its possible contributions to local heating. Based on a realistic Bifrost simulation of a quiet-Sun (coronal hole) atmosphere, we find that the total electromagnetic energy flux in the photosphere occurs mainly parallel to the photosphere, concentrating in small regions along intergranular lanes. Thereby, it was possible to define a proxy for this energy flux based on only variables that can be promptly retrieved from observations, namely, horizontal velocities of the small-scale magnetic elements and their longitudinal magnetic flux. Our proxy accurately describes the actual Poynting flux distribution in the simulations, with the electromagnetic energy flux reaching 1010 erg cm−2 s−1. To validate our findings, we extended the analysis to Sunrise/IMaX data. First, we show that Bifrost realistically describes photospheric quiet-Sun regions, as the simulation presents similar distributions for line-of-sight magnetic flux and horizontal velocity field. Second, we found very similar horizontal Poynting flux proxy distributions for the simulated photosphere and observational data. Our results also indicate that the horizontal Poynting flux in the observations is considerably larger than the vertical electromagnetic flux from previous observational estimates. Therefore, our analysis confirms that the electromagnetic energy flux in the photosphere is mainly horizontal and is most intense in localized regions along intergranular lanes.

Berry-phase interpretation of thin-film micromagnetism

Magnetic flux densities (B-fields) and field intensities (H-fields) in thin films are investigated from the viewpoints of Berry phase and topological Hall effect. The well-known origin of the topological Hall effect is an emergent B-field originating from the Berry phase of conduction electrons, but Maxwell’s equations predict the relevant perpendicular component Bz to be zero. This paradox is solved by treating the electrons as point-like objects in Lorentz cavities. These cavities can also be used to interpret magnetization measurements in the present and other contexts, but structural and magnetic inhomogeneities lead to major modifications of the Lorentz-hole picture.

Magnetohydrodynamic natural convection of second-grade hybrid nanofluid on variable heat flux surface

The natural convection flow of a second-grade hybrid nanofluid along a vertical plate is investigated. The effects of variable heat flux and magnetic field are considered. The governing equations for the momentum and energy transport are reduced to dimensionless equations. The finite difference method is used to solve the equations obtained by the stream-function formulation. A comparison between the present results and the relevant published results gives a good agreement. For a higher volume fraction of copper and magnetite nanoparticles, the index parameter of variable heat flux, and the Deborah number, the coefficient of skin friction decreases; however, the heat transfer increases. The converse is observed for the increasing Eckert number. The velocity and temperature increase for a larger Eckert number and decrease for a higher volume fraction of nanoparticles, the Deborah number, and the magnetic parameter. Moreover, the larger volume fraction of nanoparticles, index parameter, and the Deborah number augment the thicknesses of the momentum and thermal boundary layers.

Longitudinally Modulated Dynamo Action in Simulated M-dwarf Stars

Abstract M-dwarf stars are well known for the intense magnetic activity that many of them exhibit. In cool stars with near-surface convection zones, this magnetic activity is thought to be driven largely by the interplay of convection and the large-scale differential rotation and circulations it establishes. The highly nonlinear nature of these flows yields a fascinatingly sensitive and diverse parameter space, with a wide range of possible dynamics. We report here on a set of three global MHD simulations of rapidly rotating M2 (0.4 M ⊙) stars. Each of these three models established nests of vigorous convection that were highly modulated in longitude at low latitudes. Slight differences in their magnetic parameters led each model to disparate dynamo states, but the effect of the convective nest was a unifying feature. In each case, the action of longitudinally modulated convection led to localized (and in one case, global) reversals of the toroidal magnetic field, as well as the formation of an active longitude, with enhanced poloidal field amplitudes and flux emergence.

Influence of External Magnetic and Aharanov-Bohm (AB) Fields on the Energy Spectra of the Modified Kratzer plus Screened Coulomb Potential

In this article, the modified Kratzer plus screened Coulomb potential is scrutinized taking into consideration the effects of magnetic and AB flux fields within the non-relativistic regime using the Nikiforov-Uvarov-Functional Analysis method (NUFA) method. The energy equation and wave function of the system are obtained in close form. We find that the totality of effects these fields result in removal of degeneracy and raising the bound state energy of the system. More so, it could be concluded that to control the energy spectra of this system, the AB-flux and magnetic field will do so greatly. The results from this study can be applied in condensed matter physics, atomic and molecular physics.

Magnetic Properties and Biocompatibility of Different Thickness (Pd/Fe)n Coatings Deposited on Pure Ti Surface via Multi Arc Ion Plating.

The different thickness (Fe/Pd)n coatings were prepared by vacuum ion plating technology on a pure Ti substrate. The (Fe/Pd)n coatings were magnetized using an MC-4000 high-pressure magnetizing machine. Then, the effect of the (Fe/Pd)n coating thickness on the magnetic properties was studied. The surface and section morphology, composition, phase structure, magnetic properties, and biocompatibility of the (Fe/Pd)n coatings were studied by scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and CCTC-1 digital flux field measurement. The results showed that the (Fe/Pd)n coatings were granular, smooth, and compact, without cracks. In addition the (Fe/Pd)n coatings formed an L10 phase with a magnetic face-centered tetragonal-ordered structure after heat treatment. With the increase in the thickness of (FePd)n coatings, the content of L10 FePd phase increased and the remanence increased. The remanence values of the Fe/Pd, (Fe/Pd)5, (Fe/Pd)10, and (Fe/Pd)15 magnetic coatings were 0.83 Gs, 5.52 Gs, 7.14 Gs, and 7.94 Gs, respectively. Additionally, the (Fe/Pd)n magnetic coatings showed good blood compatibility and histocompatibility.

Ionospheric Topside Diffusive Flux and the Formation of Summer Nighttime Ionospheric Electron Density Enhancement Over Millstone Hill

AbstractIonospheric F‐region electron density is anomalously higher in the evening than during the daytime on many occasions in the summer in geomagnetic mid‐latitude regions. This unexpected ionospheric diurnal variation has been studied for several decades. The underlying processes have been suggested to be related to meridional winds, topside influx arising from sunset ionospheric collapse, and other factors. However, substantial controversies remain unresolved. Using a numerical model driven by the statistical topside O+ diffusive flux from the Millstone Hill incoherent scatter radar data, we provide new insight into the competing roles of topside diffusive flux, neutral winds, and electric fields in forming the evening density peak. Simulations indicate that while meridional winds, which turn equatorward before sunset, are essential to sustain the daytime ionization near dusk, the topside diffusive flux is critically important for the formation and timing of the summer evening density peak.

The study of heat flux and external electric field effects on carbon nanotube behavior as an atomic nano-pump

The study of heat flux and external electric field effects on carbon nanotube behavior as an atomic nano-pump

Spectral properties and time decay of the wave functions of Pauli and Dirac operators in dimension two

We consider two-dimensional Pauli and Dirac operators with a polynomially vanishing magnetic field. The main results of the paper provide resolvent expansions of these operators in the vicinity of their thresholds. It is proved that the nature of these expansions is fully determined by the flux of the magnetic field. The most important novelty of the proof is a comparison between the spatial asymptotics of the zero modes obtained in two different manners. The result of this matching allows us to compute explicitly all the singular terms in the associated resolvent expansions.As an application we show how the magnetic field influences the time decay of the associated wave-functions quantifying thereby the paramagnetic and diamagnetic effects of the spin.

Thermodynamical interactions in a rotating magneto-thermoelastic diffusive medium with microconcentrations

Current research is dedicated to study the thermo-mechanical interactions in a rotating magneto-thermoelastic diffusive medium with microconcentrations. The entire thermoelastic medium is rotating with a uniform angular velocity. The formulation is subjected to a mechanical load. Exact solution of the problem is procured by applying normal mode technique. Numerical computations for stresses, displacement components, temperature field, concentration, microconcentration and mass diffusion flux moment are performed for a suitable material and are depicted graphically. Some comparisons are made to show the effects of magnetic field, angular velocity, microconcentration parameter and time on the resulting quantities. Certain special cases of interest have been derived from the current report.

Heat transfer characteristics of single-layer and two-layer corium pool in elliptical lower head

In this study, we investigated the characteristics of natural convection heat transfer of single-layer and two-layer corium pool in the elliptical lower head by the numerical method. Numerical simulations were performed using FLUENT software with the WMLES (Wall-Modeled Large Eddy Simulation) turbulence model, solid-liquid phase change model and the VOF (Volume of Fluid) model. The heat transfer characteristics such as pool temperature field, flow velocity field and heat flux distribution were obtained. For the single-layer corium pool, different degrees of melting occurred on the wall surface from about 1712 mm arc length inside the inner wall to the liquid level. The transient velocity field of single-layer corium pool showed that the natural convection first occurs near the wall surface and then gradually forms in the center of the corium pool as the temperature rises. When the quasi-steady state is reached, the upper part of the pool forms strong turbulence, while the lower part has lower velocity and forms transverse flow. For the two-layer corium pool, the lower head wall is more severely melted than the single-layer configuration. Whereas, the crust at the wall near the stratified interface is only slightly melted due to the lower flow rate and weaker heat transfer capacity. The research revealed the flow and heat transfer mechanisms of corium pool, and provided reference for the technology design, system optimization and safety evaluation of the IVR (In-Vessel Retention) – ERVC (External Reactor Vessel Cooling) severe accident mitigation strategy for the large-scale advanced pressurized water reactor.

Analysis of Linear Hybrid Excited Flux Switching Machines with Low-Cost Ferrite Magnets

Linear hybrid excited flux switching machines (LHEFSM) combine the features of permanent magnet flux switching machines (PMFSM) and field excited flux switching machines (FEFSM). Because of the widespread usage of rare-earth PM materials, their costs are steadily rising. This study proposes an LHEFSM, a dual stator LHEFSM (DSLHEFSM), and a dual mover LHEFSM (DMLHEFSM) to solve this issue. The employment of ferrite magnets rather than rare-earth PM in these suggested designs is significant. Compared to traditional designs, the proposed designs feature greater thrust force, power density, reduced normal force, and a 25% decrease in PM volume. A yokeless primary structure was used in a DSLHEFSM to minimize the volume of the mover, increasing the thrust force density. In DMLHELFSM, on the other hand, a yokeless secondary structure was used to lower the secondary volume and the machine’s total cost. Single variable optimization was used to optimize all of the proposed designs. By completing a 3D study, the electromagnetic performances acquired from the 2D analysis were confirmed. Compared to conventional designs, the average thrust force in LHEFSM, DSLHEFSM, and DMLHEFSM was enhanced by 15%, 16.8%, and 15.6%, respectively. Overall, the presented machines had a high thrust force density, a high-power density, a high no-load electromotive force, and a low normal force, allowing them to be used in long-stroke applications.