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1 – 10 of 841Yang Zhou, Wenying Qu, Fan Zhou, Xinggang Li, Lijun Song and Qiang Zhu
This paper aims to understand the magnetohydrodynamics (MHD) mechanism in the molten pool under different modes of magnetic field. The comparison focuses on the Lorenz force…
Abstract
Purpose
This paper aims to understand the magnetohydrodynamics (MHD) mechanism in the molten pool under different modes of magnetic field. The comparison focuses on the Lorenz force excitation and its effect on the melt flow and solidification parameters, intending to obtain practical references for the design of magnetic field-assisted laser directed energy deposition (L-DED) equipment.
Design/methodology/approach
A three-dimensional transient multi-physical model, coupled with MHD and thermodynamic, was established. The dimension and microstructure of the molten pool under a 0T magnetic field was used as a benchmark for accuracy verification. The interaction between the melt flow and the Lorenz force is compared under a static magnetic field in the X-, Y- and Z-directions, and also an oscillating and alternating magnetic field.
Findings
The numerical results indicate that the chaotic fluctuation of melt flow trends to stable under the magnetostatic field, while a periodically oscillating melt flow could be obtained by applying a nonstatic magnetic field. The Y and Z directional applied magnetostatic field shows the effective damping effect, while the two nonstatic magnetic fields discussed in this paper have almost the same effect on melt flow. Since the heat transfer inside the molten pool is dominated by convection, the application of a magnetic field has a limited effect on the temperature gradient and solidification rate at the solidification interface due to the convection mode of melt flow is still Marangoni convection.
Originality/value
This work provided a deeper understanding of the interaction mechanism between the magnetic field and melt flow inside the molten pool, and provided practical references for magnetic field-assisted L-DED equipment design.
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Ran Sun, Aidang Shan, Chengxi Zhang and Qingxian Jia
This paper aims to investigate the feasibility of using the combination of Lorentz force and aerodynamic force as a propellantless control method for spacecraft formation.
Abstract
Purpose
This paper aims to investigate the feasibility of using the combination of Lorentz force and aerodynamic force as a propellantless control method for spacecraft formation.
Design/methodology/approach
It is assumed that each spacecraft is equipped with several large flat plates, which can rotate to produce aerodynamic force. Lorentz force can be achieved by modulating spacecraft’s electrostatic charge. An adaptive output feedback controller is designed based on a sliding mode observer to account for unknown uncertainties and the absence of relative velocity measurements. Aiming at distributing the control input, an optimal control allocation method is proposed to calculate the electrostatic charge of the Lorentz spacecraft and control commands for the atmospheric-based actuators.
Findings
Numerical examples are provided to demonstrate the effectiveness of the proposed control strategy in the presence of J2 perturbations. Simulation results show that relative motion in a formation can be precisely controlled by the proposed propellantless control method under uncertainties and unavailability of velocity measurements.
Research limitations/implications
The controllability of the system is not theoretically investigated in the current work.
Practical implications
The proposed control method introduced in this paper can be applied for small satellites formation in low Earth orbit.
Originality/value
The main contribution of the paper is the proposal of the propellantless control approach for satellite formation using the combination of Lorentz force and aerodynamic force, which can eliminate the requirement of the propulsion system.
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Jun Tu, Tao Chen, Zhi Xiong, Xiaochun Song and Songling Huang
The aim of this paper is to better understand the generation and transmission mechanism of the electromagnetic acoustic transducer (EMAT).
Abstract
Purpose
The aim of this paper is to better understand the generation and transmission mechanism of the electromagnetic acoustic transducer (EMAT).
Design/methodology/approach
A semi-analytical method was used to calculate the Lorentz force. Both the hypothetical magnetic field mirror method and the diffusion equation were adopted to solve the eddy current distribution by variables separation method in time domain. A three-dimension magnetostatic finite element model was used to calculate the static magnetic field and the relative permeability. And an experimental platform with a piezoelectric probe to generate and an EMAT to receive, the ultrasonic wave was set up to verify the distribution of the Lorentz force.
Findings
The Lorentz force at different time and in different positions of the steel plate can be easily calculated. The experimental results show a good agreement with the analytical results.
Originality/value
The accurate prediction of the Lorentz force provides an insight into the physical phenomenon of EMAT and a powerful tool to design optimum EMAT.
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Aditi, Supriyo Das and Ram Gopal
Si-based micro electro mechanical systems (MEMS) magnetometer does not require specialized magnetic materials avoiding magnetic hysteresis, ease in fabrication and low power…
Abstract
Purpose
Si-based micro electro mechanical systems (MEMS) magnetometer does not require specialized magnetic materials avoiding magnetic hysteresis, ease in fabrication and low power consumption. It can be fabricated using the same processes used for gyroscope and accelerometer fabrication. The paper reports the dicing mechanism for the released MEMS xylophone magnetic sensor fabricated using wafer bonding technology and its characterization in ambient pressure and under vacuum conditions. The purpose of this paper is to dice the wafer bonded Si-magnetometer in a cost-effective way without the use of laser dicing and test it for Lorentz force transduction.
Design/methodology/approach
A xylophone bar MEMS magnetometer using Lorentz force transduction is developed. The fabricated MEMS-based xylophone bars in literature are approximately 500 µm. The present work shows the released structure (L = 592 µm) fabricated by anodic bonding technique using conducting Si as the structural layer and tested for Lorentz force transduction. The microstructures fabricated at the wafer level are released. Dicing these released structures using conventional diamond blade dicing may damage the structures and reduce the yield. To avoid the problem, positive photoresist S1813 was filled before dicing. The dicing of the wafer, filled with photoresist and later removal of photoresist post dicing, is proposed.
Findings
The devices realized are stiction free and straight. The dynamic measurements are done using laser Doppler vibrometer to verify the released structure and test its functionality for Lorentz force transduction. The magnetic field is applied using a permanent magnet and Helmholtz coil. Two sensors with quality factors 70 and 238 are tested with resonant frequency 112.38 kHz and 114.38 kHz, respectively. The sensor D2, with Q as 238, shows a mechanical sensitivity of 500 pm/Gauss and theoretical Brownian noise-limited resolution of 53 nT/vHz.
Originality/value
The methodology and the study will help develop Lorentz force–based MEMS magnetometers such that stiction-free structures are released using wet etch after the mechanical dicing.
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Tongsheng Wang, Anna Li, Guang Xi and Zhu Huang
The purpose of this study is to investigate the enhancement and suppression of heat transfer for hybrid nanofluids (Cu–Al2O3/water) in a square enclosure containing a…
Abstract
Purpose
The purpose of this study is to investigate the enhancement and suppression of heat transfer for hybrid nanofluids (Cu–Al2O3/water) in a square enclosure containing a thermal-conductive cylinder when the Lorentz force is applied to the hybrid nanofluids.
Design/methodology/approach
Since the inner conductive cylinder in present research has a complex geometry, an in-house meshless method, namely, the local radial basis function (LRBF) method, is applied to solve the 2 dimensional (2D) incompressible Navier–Stokes equation in the fluid domain and Fourier heat conduction equation in solid domain. The solid–fluid interface remains the physical continuity of temperature and heat flux. Only the Lorentz force is considered for the presence of the magnetic field. The conjugate natural convection is assumed to be steady, thus only fully developed heat exchange from the nanofluids to solid or vice versa is comprehensively investigated.
Findings
It can be concluded that Lorentz force plays a more significant role than hybrid nanofluids in enhancing/suppressing heat transfer when the orientation of magnetic field is the same to the x direction. The thermal conductivity ratio can dramatically change the isotherms and streamlines as well as the mean value of the Nusselt number, resulting in totally different heat transfer phenomena. The included angle of magnetic field also has a significant effect on the heat transfer rate when it changes from horizontal to vertical.
Research limitations/implications
The constant thermo-physical properties of incompressible fluid and the 2D steady flow are considered in this study.
Originality/value
The conjugate MHD natural convection of hybrid nanofluids is numerically investigated by an in-house meshless LRBF method. The enhancement and suppression of heat transfer under the combined influence of the volume fraction of nanoparticles, Hartmann number and the thermal conductivity ratio are comprehensively investigated.
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Koen Delaere, Ward Heylen, Ronnie Belmans and Kay Hameyer
The magnetic and mechanical finite element systems are combined into one magnetomechanical system. Investigating the coupling terms results in a finite element expression for the…
Abstract
The magnetic and mechanical finite element systems are combined into one magnetomechanical system. Investigating the coupling terms results in a finite element expression for the magnetic forces (Lorentz force and reluctance force) for both the linear and nonlinear case. The material deformation caused by magnetostriction is represented by an equivalent set of mechanical forces, giving the same strain to the material as magnetostriction does. The resulting magnetostriction force distribution is superposed onto other force distributions (external mechanical forces, magnetic forces) before starting the mechanical deformation or vibration analysis. This procedure is incorporated into a weakly‐coupled cascade solving of the magnetomechanical problem.
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Hartmut Brauer, Konstantin Porzig, Judith Mengelkamp, Matthias Carlstedt, Marek Ziolkowski and Hannes Toepfer
The purpose of this paper is to present a novel electromagnetic non-destructive evaluation technique, so called Lorentz force eddy current testing (LET). This method can be…
Abstract
Purpose
The purpose of this paper is to present a novel electromagnetic non-destructive evaluation technique, so called Lorentz force eddy current testing (LET). This method can be applied for the detection and reconstruction of defects lying deep inside a non-magnetic conducting material.
Design/methodology/approach
In this paper the technique is described in general as well as its experimental realization. Besides that, numerical simulations are performed and compared to experimental data. Using the output data of measurements and simulations, an inverse calculation is performed in order to reconstruct the geometry of a defect by means of sophisticated optimization algorithms.
Findings
The results show that measurement data and numerical simulations are in a good agreement. The applied inverse calculation methods allow to reconstruct the dimensions of the defect in a suitable accuracy.
Originality/value
LET overcomes the frequency dependent skin-depth of traditional eddy current testing due to the use of permanent magnets and low to moderate magnetic Reynolds numbers (0.1-1). This facilitates the possibility to detect subsurface defects in conductive materials.
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Kenzo Miya, Kazuyuki Demachi and Kentaro Takase
In this study the numerical method was developed to simulate behavior of the fluxoids in NbTi and Bi‐2212. The method was named the Fluxoid Dynamics (FD) method, and is based on a…
Abstract
In this study the numerical method was developed to simulate behavior of the fluxoids in NbTi and Bi‐2212. The method was named the Fluxoid Dynamics (FD) method, and is based on a combination of two concepts : the Molecular Dynamics (MD) and Ginzburg‐Landau (G‐L) theory. Several macroscopic electromagnetic phenomena were predicted by these methods, and the results were compared with the well‐known empirical ones.
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Eva-Maria Dölker, Bojana Petković, Reinhard Schmidt, Marek Ziolkowski, Hartmut Brauer and Jens Haueisen
Lorentz force evaluation is a non-destructive evaluation method for conducting specimens. The movement of a specimen relative to a permanent magnet leads to Lorentz forces that…
Abstract
Purpose
Lorentz force evaluation is a non-destructive evaluation method for conducting specimens. The movement of a specimen relative to a permanent magnet leads to Lorentz forces that are perturbed in the presence of a defect. This defect response signal (DRS) is used for defect reconstruction. To solve a linear inverse problem for defect reconstruction, an accurate and fast forward computation method is required. As existing forward methods are either too slow or too inaccurate, the purpose of this paper is to propose the single voxel approach (SVA) as a novel method.
Design/methodology/approach
In SVA, the DRS is computed as a superposition of DRSs from single defect voxels, which are calculated in advance, by applying the boundary element source method. This research uses a setup of an isotropic conducting specimen, a spherical permanent magnet and defects of different shapes at different depths. With the help of simulations, this study compares the SVA to the previously proposed approximate forward solution (AFS) and the extended area approach (EAA) using the normalized root mean square error (NRMSE). Simulated data using the finite element method serve as the reference solution.
Findings
SVA shows across all simulations NRMSE values <2.5 per cent compared to <8 per cent for EAA and <12 per cent for AFS.
Originality/value
The superposition principle of SVA allows for the application of linear inverse methods for defect reconstruction while providing sufficient accuracy of the forward method.
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Keywords
In general, the bifurcation phenomenon of the natural convection has largely been studied. But the bifurcation of natural convection under magnetic conditions has not been studied…
Abstract
Purpose
In general, the bifurcation phenomenon of the natural convection has largely been studied. But the bifurcation of natural convection under magnetic conditions has not been studied as per the authors’ knowledge. This paper aims to investigate the changes in bifurcation phenomenon by the self-induced circular magnetic field.
Design/methodology/approach
The authors numerically solved the natural convection in an annulus. The SIMPLE algorithm was adopted for pressure-momenturm coupling. The Boussinesq approximation was used for numerical modeling of natural convection. Finally, the Lorentz force effect by the magnetic field was considered through the source terms in the momentum conservation equation.
Findings
It was determined that the heat-transfer rate changes by 17% owing to the applied magnetic effect, and the range of the Rayleigh number for flow bifurcation is changed by the magnetic effect. Moreover, under the strong magnetic condition, the flow bifurcation continues even at very high Ra. Previously, flow bifurcation has been understood as a flow instability phenomena, and the Lorentz force was regarded as a flow-damping effect; however, in this study, it was found that the magnetic field can boost the flow instability and induce flow bifurcation even in the Rayleigh number region where the bifurcation does not appear.
Originality/value
This paper is dealing with the bifurcation phenomenon in MHD natural convection problems. In the past, the electromagnetic forces were regarded as always acting to damp out the existing flows; herewith, the authors first investigated that the magnetic effect can boost the bifurcation of a kind of flow instability phenomenon.
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