Residual Flux Research Articles

Nonlinear turbulent parallel momentum transport due to blobs

The nonlinear flux term induced by blobs is derived by using an eddy damped quasi-normal Markovian method in a three dimensional Hasegawa–Wakatani system with a strong non-adiabatic assumption. It is shown that the nonlinear flux is comparable to the quasilinear flux in the case of mixing level turbulence and can become dominant in strong turbulence regimes such as blobs. Moreover, when strong non-adiabatic or strong parallel flow shear conditions are considered, for radially elongated blobs or streamers, nonlinear residual flux can provide the opposite parallel momentum flux to the quasilinear flux and negative nonlinear diffusivity, which introduces inward momentum flux, so that rotation in the edge region is possibly reversed and momentum is convected into the core region.

Dynamics of a Ferromagnetic Particle Levitated over a Superconductor

Under conditions where the angular momentum of a ferromagnetic particle is dominated by intrinsic spin, applied torque is predicted to cause gyroscopic precession of the particle. If the particle is sufficiently isolated from the environment, a measurement of spin precession can potentially yield sensitivity to torque beyond the standard quantum limit. Levitation of a micron-scale ferromagnetic particle above a superconductor is a possible method of near frictionless suspension enabling observation of ferromagnetic particle precession and ultrasensitive torque measurements. We experimentally investigate the dynamics of a micron-scale ferromagnetic particle levitated above a superconducting niobium surface. We find that the levitating particles are trapped in potential minima associated with residual magnetic flux pinned by the superconductor and, using an optical technique, characterize the quasiperiodic motion of the particles in these traps.

The time delay between star formation quenching and morphological transformation of galaxies in clusters: a phase–space view of EDisCS

AbstractWe explore the possible effect of cluster environments on the structure and star formation histories of galaxies by analysing the projected phase–space (PPS) of intermediate-redshift clusters (0.4 ≤ z ≤ 0.8). HST I−band imaging data from the ESO Distant Cluster Survey (EDisCS) allow us to measure deviations of the galaxies’ light distributions from symmetric and smooth profiles using two parameters, Ares (‘asymmetry’) and RFF (residual flux fraction or ‘roughness’). Combining these structural parameters with age-sensitive spectral indicators ($H_{\delta \rm {A}}$, $H_{\gamma \rm {A}}$, and Dn4000), we establish that in all environments younger star-forming galaxies of all morphologies are ‘rougher’ and more asymmetric than older, more quiescent ones. Combining a subset of the EDisCS clusters, we construct a stacked PPS diagram and find a significant correlation between the position of the galaxies on the PPS and their stellar ages, irrespective of their morphology. We also observe an increasing fraction of galaxies with older stellar populations towards the cluster core, while the galaxies’ structural parameters (Ares and RFF) do not seem to segregate strongly with PPS. These results may imply that, under the possible influence of their immediate cluster environment, galaxies have their star formation suppressed earlier, while their structural transformation happens on a longer time-scale as they accumulate and age in the cluster cores.

Promotion of lateral growth of GaN crystals on point seeds by extraction of substrates from melt in the Na-flux method

In a previous study, we successfully obtained large-diameter, low-dislocation-density GaN wafer using the Na-flux multi-point seed (MPS) technique. However, the lattice constants of the GaN wafer grown by this technique expanded due to oxygen concentration in pyramidal facets. We here invented a breakthrough technique for the promotion of lateral growth, and succeed in suppressing pyramidal facet growth by residual flux formed after extraction of the MPS-GaN substrate from the Na-Ga melt in a crucible. The surface of the grown wafer was fully composed of the c-plane and showed low oxygen concentration, so expansion of lattice constants could be successfully prevented.

Research on the corrosion resistance of Cu-Al joints brazed with flux-cored Zn-2Al filler metal

In this paper, the corrosion resistance of the Cu-Al joint brazed with flux-cored Zn-2Al filler metal was investigated, comparing with that of the joint brazed with traditional Zn-2Al filler metal. Results showed that Zn-Al eutectic and α-Al were susceptible to the corrosion attack due to lower galvanic potential than other phases. The corrosion product of the Zn-Al eutectic was η-Zn and the α-Al dissolved after immersing for 48 h due to the galvanic corrosion. The corrosion resistance of the Cu-Al joint was improved by decreasing the area of phases, which was susceptible to the corrosion, when replacing traditional filler metal by flux-cored filler metal. The excessive Al dissolution resulted in lattice distortion and numerous microvoids, which in turn led to the formation and propagation of cracks. Residual oxides and flux in Cu-Al joints could also be the crack sources due to the generation of stress concentration in the process of corrosion. The shear strength of the joint brazed with flux-cored filler metal reached 34.5 MPa after immersing in 3.5 wt% NaCl solution for 18 days, which is higher than that of the joint brazed with traditional filler metal because of two aspects of decreasing brazing time and controlling brazing flux content.

Harnessing Magnets to Design Tunable Architected Bistable Material

A new type of tunable bistable metamaterial (TBM) is proposed which is made by embedding magnets into the flexible cellular bistable materials with double tilted beams. To find out the effect of magnets on the mechanics of TBM, several samples are fabricated and magnets with different residual flux density, which is used to describe the magnetic field strength, are embedded. Quasi‐static uniaxial compression tests are performed and the results show that the bistability of the TBM is successfully adjusted by embedding magnets. Furthermore, both numerical and analytical simulations are carried out to further understand the bistable property and energy absorption ability of the proposed metamaterial.

Research of controlled switching strategy to restrain inrush current during the energisation of converter transformer

When an unloaded converter transformer is being energised, large inrush current may flow in the voltage jump side depending upon the residual flux in the core and the closing voltage angle of the power grid due to the hysteresis characteristics of the core, causing false operation of protective relays and bringing many adverse effects to the grid and primary electrical devices. Based on the analysis of the elimination of inrush current using controlled switching, the phase-to-phase electromagnetic coupling relationship which would be influenced by the core structure and winding connection mode was taken into account when considering the controlled switching strategy of three-phase transformer. With the analysis of core residual flux after de-energisation and the dynamic magnetisation of core during the energisation, a controlled switching strategy for inrush current restraining when energising converter transformer is presented in this study. Only the current arc quenching sequence during circuit breaker opening and the voltage extinguishing phase angle are asked for this strategy, and the simulation by electromagnetic transients programme and the experiment results indicate that this strategy could restrain the inrush transient efficiently.

Analysis of the residual flux influence on inrush current and electromagnetic force in large power transformer

Analysis of the residual flux influence on inrush current and electromagnetic force in large power transformer

Voltage Control of DC Generators Using Load Characteristic

The DC generators find much utilization in the devices that are used in application systems. This article describes the new voltage control of DC generators based on load characteristic. The DC generator load characteristic is important to identify the DC generator ability to provide the suitable load current. This paper proposed the performance of load characteristic of self-excited DC generator with the condition of voltage build up. The proposal allows for improvement of the more important factor in generation process which results in the voltage drop due to the losses and armature reaction. The field resistance, critical speed, and residual flux are very important factors in voltage build up process.

Micro-arcsecond structure of Sagittarius A∗ revealed by high-sensitivity 86 GHz VLBI observations

Context. The compact radio source Sagittarius A∗ (Sgr A∗) in the Galactic centre is the primary supermassive black hole candidate. General relativistic magnetohydrodynamical (GRMHD) simulations of the accretion flow around Sgr A∗ predict the presence of sub-structure at observing wavelengths of ∼3 mm and below (frequencies of 86 GHz and above). For very long baseline interferometry (VLBI) observations of Sgr A∗ at this frequency the blurring effect of interstellar scattering becomes sub-dominant, and arrays such as the high sensitivity array (HSA) and the global mm-VLBI array (GMVA) are now capable of resolving potential sub-structure in the source. Such investigations help to improve our understanding of the emission geometry of the mm-wave emission of Sgr A∗, which is crucial for constraining theoretical models and for providing a background to interpret 1 mm VLBI data from the Event Horizon Telescope (EHT). Aims. Following the closure phase analysis in our first paper, which indicates asymmetry in the 3 mm emission of Sgr A∗, here we have used the full visibility information to check for possible sub-structure. We extracted source size information from closure amplitude analysis, and investigate how this constrains a combined fit of the size-frequency relation and the scattering law for Sgr A∗. Methods. We performed high-sensitivity VLBI observations of Sgr A∗ at 3 mm using the Very Long Baseline Array (VLBA) and the Large Millimeter Telescope (LMT) in Mexico on two consecutive days in May 2015, with the second epoch including the Greenbank Telescope (GBT). Results. We confirm the asymmetry for the experiment including GBT. Modelling the emission with an elliptical Gaussian results in significant residual flux of ∼10 mJy in south-eastern direction. The analysis of closure amplitudes allows us to precisely constrain the major and minor axis size of the main emission component. We discuss systematic effects which need to be taken into account. We consider our results in the context of the existing body of size measurements over a range of observing frequencies and investigate how well-constrained the size-frequency relation is by performing a simultaneous fit to the scattering law and the size-frequency relation. Conclusions. We find an overall source geometry that matches previous findings very closely, showing a deviation in fitted model parameters less than 3% over a time scale of weeks and suggesting a highly stable global source geometry over time. The reported sub-structure in the 3 mm emission of Sgr A∗ is consistent with theoretical expectations of refractive noise on long baselines. However, comparing our findings with recent results from 1 mm and 7 mm VLBI observations, which also show evidence for east-west asymmetry, we cannot exclude an intrinsic origin. Confirmation of persistent intrinsic substructure will require further VLBI observations spread out over multiple epochs.

Modelling and measurement of water productivity and total evaporation in a dryland soybean crop

Simpler crop models simulating evaporation are needed to provide information to farmers, policy makers and decision makers on how to maximise crop yield responses to water. This is becoming important as the frequency and severity of droughts in South Africa is increasing. In this regard, prediction of yield, determination of water productivity and total evaporation (ET) are increasingly becoming essential in water resource management. The overall objective of the study was to compare the FAO AquaCrop daily model output of ET to the residual ET for non-stressed dryland soybean in a sub-humid climate. Energy balance residual ET estimates using an eddy covariance (EC) system and modelled ET using AquaCrop obtained from Glycine max (L.) Merrill grown in the midlands of KwaZulu-Natal, South Africa during the 2012/13 growing season are compared. The modelled and observed yield showed good agreement, while the residual ET was 21.6% less than the modelled. The energy balance closure computed using the daily sums of sensible heat and latent energy fluxes against daily available energy flux for unstable atmospheric conditions was 0.77. A closure of 0.99 was achieved when the EC latent energy flux was replaced with residual latent energy flux. A good fit between the modelled and observed percentage green canopy cover was observed (slope = 0.86, intercept = 15.46%, root mean square error = 10.50% and R2 = 0.83). Season-long daily residual ET values were consistently low for most of the growth stages compared to the modelled, except for the maturity stage. However, the residual ET comparisons with the AquaCrop model improved after gap-filling was applied to discarded data and for when the EC system failed.

The satellite layout optimization design approach for minimizing the residual magnetic flux density of micro- and nano-satellites

With the rapid development of space technology, more and more complex micro- and nano-satellite missions have been proposed for more sophisticated objectives such as remote sensing and space astronomy. To meet these mission requirements, the satellites need to maintain a highly precise attitude control capability. However, the satellite residual magnetic moment, as the dominant disturbance torque, greatly limits the realization of the accurate attitude determination and control system. To minimize the influence of the residual magnetic field on satellite attitude determination, in this paper, a novel satellite layout optimization design (SLOD) approach is proposed. The determination method of satellite residual magnetic field is elaborated firstly and then the corresponding SLOD model concerning complex performance constraints is established. Moreover, the improved accelerated particle swarm optimization (IAPSO) algorithm is developed to search for the optimal layout solution. Finally, the feasibility and effectiveness of the proposed methodology are validated by a numerical layout example and a simplified nano-satellite layout case.

NDT of spot welds by imaging analysis of the residual magnetic flux density – Investigation on the influence of electrode indentation on the measurement results

Nondestructive testing (NDT) of spot welds still poses a great challenge for industrial mass production like automotive body manufacturing or rail vehicle constructions. In these production areas, ultrasonic-based NDT methods with pulse-echo technology have become established. Hitherto, these methods require a lot of experience from the staff, which often makes the results subjective. Furthermore, these methods reach their limits when testing spot welds with deep electrode indentations or huge differences between sheet metal thicknesses. Imaging analysis of the residual magnetic flux density shows great potential especially when addressing the restrictions of the ultrasonic-based NDT methods. Typical spot welds show a characteristic electrode indentation. This indentation influences the measureable magnetic flux density, which depends on the distance between the surface of the spot weld and the sensor. This paper will show the influence of electrode indentation on the test result and presents the resulting restrictions for imaging analysis of the residual magnetic flux density when testing spot welds.

Influence of Contact Plate Parameters and Eddy Currents on Residual Flux Decay in AMF-Type Vacuum Interrupters

The axial magnetic field (AMF)-type contact is utilized in vacuum interrupters (VIs) due to its effectiveness in current interruption. This paper deals with the study of the eddy current phenomenon occurring in the AMF VIs. The residual AMF beyond current zero is directly the result of eddy currents in the contact plate and, thus, is dependent on the contact plate geometry. Considering that the contact plate is analogous to a disk winding, an analytical formulation is developed to compute the eddy current decay constant as a function of contact plate parameters. The analytical predictions are verified using finite-element method (FEM) simulations of the step response of a circular plate placed in a uniform magnetic field. A linear relationship is established between the contact plate parameters and the eddy current decay constant. The analytical formulation is then used to predict the residual flux decay constant in the context of AMF contact model simulations. A coil-type AMF contact is analyzed using FEM simulations, and parametric studies are carried out to study the effect of the contact opening velocity and radial slots on distribution of the contact gap AMF. The results reported in this work will enable VI engineers to improve the contact design by better control of contact plate parameters.

The Fornax Deep Survey with the VST

Context. The Fornax Deep Survey (FDS), an imaging survey in the u′, g′, r′, and i′-bands, has a supreme resolution and image depth compared to the previous spatially complete Fornax Cluster Catalog (FCC). Our new data allows us to study the galaxies down to r′-band magnitude mr′ ≈ 21 mag (Mr′ ≈ −10.5 mag), which opens a new parameter regime to investigate the evolution of dwarf galaxies in the cluster environment. After the Virgo cluster, Fornax is the second nearest galaxy cluster to us, and with its different mass and evolutionary state, it provides a valuable comparison that makes it possible to understand the various evolutionary effects on galaxies and galaxy clusters. These data provide an important legacy dataset to study the Fornax cluster. Aims. We aim to present the Fornax Deep Survey (FDS) dwarf galaxy catalog, focusing on explaining the data reduction and calibrations, assessing the quality of the data, and describing the methods used for defining the cluster memberships and first order morphological classifications for the catalog objects. We also describe the main scientific questions that will be addressed based on the catalog. This catalog will also be invaluable for future follow-up studies of the Fornax cluster dwarf galaxies. Methods. As a first step we used the SExtractor fine-tuned for dwarf galaxy detection, to find galaxies from the FDS data, covering a 26 deg2 area of the main cluster up to its virial radius, and the area around the Fornax A substructure. We made 2D-decompositions of the identified galaxies using GALFIT, measure the aperture colors, and the basic morphological parameters like concentration and residual flux fraction. We used color–magnitude, luminosity–radius and luminosity–concentration relations to separate the cluster galaxies from the background galaxies. We then divided the cluster galaxies into early- and late-type galaxies according to their morphology and gave first order morphological classifications using a combination of visual and parametric classifications. Results. Our final catalog includes 14 095 galaxies. We classify 590 galaxies as being likely Fornax cluster galaxies, of which 564 are dwarfs (Mr′ > −18.5 mag) consisting our Fornax dwarf catalog. Of the cluster dwarfs we classify 470 as early-types, and 94 as late-type galaxies. Our final catalog reaches its 50% completeness limit at magnitude Mr′ = −10.5 mag and surface brightness μ¯e,r′ = 26 mag arcsec−2, which is approximately three magnitudes deeper than the FCC. Based on previous works and comparison with a spectroscopically confirmed subsample, we estimate that our final Fornax dwarf galaxy catalog has ≲10% contamination from the background objects.

The Loss of Self-Excitation Capability in Stand-Alone Synchronous Reluctance Generators

This paper analyzes the capability of synchronous reluctance generators (SynRGs) to achieve successful self-excitation when operating in stand-alone systems. To initiate the self-excitation process, the machine steel core has to retain sufficient residual flux during the generator startup. This residual flux depends on the magnetic properties of the machine core as well as the previous machine operating condition. A hysteresis model is developed to simulate the flux linkage–current characteristics of the SynRG for different operating conditions. The model then calculates the machine residual flux linkage after each operation, which indicates the machine capability for self-excitation during the next startup. The simulation results show that the rotor steel core of the synchronous reluctance machine (SynRM) can be demagnetized when the generator is subjected to a short-circuit condition or when the generator shaft is subjected to abrupt deceleration. These simulation results are also validated by experimental measurements on a 5 hp SynRM.

Asymptotic homogenization of magnetic composite for controllable permanent magnet

A periodic composite microstructure consisting of permanent magnet and ferromagnetic material can be utilized to realize a controllable permanent magnet. For the microstructure design of the permanent magnet composite, its effective material properties should be estimated accurately and efficiently. For this, an asymptotic homogenization method is developed in this work. Specifically, the formulations for homogenized magnetic permeability and residual flux density are mathematically derived for both scalar and vector potential magnetostatic analysis approaches. Using the asymptotic expansion, cell problem equations are first derived, which will be solved by finite element method in microscopic coordinate. Then, the integral form formulations are derived for the calculation of homogenized effective properties. To show the design possibility of the composite, four numerical examples are investigated in this work. In each example, the accuracy of the obtained properties is numerically validated by comparing the magnetic field and energy produced by a homogeneous composite with the obtained effective properties and those produced by a heterogeneous model with actual periodic composite structures. In addition, the computation time of the homogeneous and heterogeneous models are compared to confirm the computational benefit of the developed homogenization method.

Ozone production in a maritime pine forest in water-stressed conditions

During two growing seasons of a maritime pine stand, in 2014 and 2015, ozone (O3) fluxes have been determined using the eddy covariance (EC) method and compared to the outputs of a big-leaf O3 deposition model including stomatal, cuticular and soil pathways. The model developed in this study generally allowed to properly reproduce the measured ozone deposition. Ozone fluxes showed a strong reduction during two water stressed periods in September 2014 and July 2015. The model partly explain this fall due to the reduction of stomatal deposition. Despite this stomatal closure, measured O3 fluxes presented systematically lower negative values than the model outputs, and sometimes even positive values around midday during periods marked by strong water stress. In other words, the difference between observed and modelled O3 fluxes (hereinafter referred to as the residual O3 flux) is systematically positive on daytime during these water-stressed periods.This positive residual flux traduced the existence of an O3 source below the flux measurement level, responsible for positive fluxes that counterbalance deposition fluxes. We developed an O3 production module based on a terpene emission algorithm and an OH concentration proxy, to try to explain the observed ozone production. As this parametrisation allowed us to reproduce well the daily and inter-daily dynamics of the residual O3 flux, it confirms that the latter actually resulted from O3 production processes. This ozone production is here highlighted for the first time using O3 fluxes measurements by the EC method. The chemical reactions possibly involved in O3 production processes in this maritime pine forest have been discussed and different mechanisms are proposed, based on peroxy radicals chemistry or stress-induced BVOCs.

Mechanics of hard-magnetic soft materials

Soft materials that can undergo rapid and large deformation through the remote and wireless action of external stimuli offer a range of tantalizing applications such as soft robots, flexible electronics, and biomedical devices. A natural and simple embodiment of such materials is to embed magnetic particles in soft polymers. Unfortunately, existing magnetically responsive soft materials such as magnetorheological elastomers and ferrogels typically use magnetically-soft particles such as iron and iron oxides, which are characterized by the low coercivity and hence lack the capability to retain remnant magnetism. Accordingly, their deformation is limited to simple elongation or shortening, rendering these materials substantially unsuited for the complex transformations required in many applications. To introduce shape-programmability, magnetically-hard particles with high coercivity have been incorporated in mechanically soft materials. In addition, recent works aimed at ameliorating this situation have developed fabrication techniques and facile routes to engineer rapid and complex transformations in a programmable manner by introducing intricate patterns of magnetic polarities in soft materials. The resulting structures, when properly designed, have been shown to exhibit a diverse and rich array of actuation behavior. In this work, we develop a suitable theoretical framework to analyze these so-called hard-magnetic soft materials to facilitate the rational design of magnetically activated functional structures and devices based on a quantitative prediction of complex shape changes. We adopt a nonlinear field theory to describe the finite deformation coupled with magnetic fields and argue that the macroscopic behavior of the fabricated materials requires a new constitutive classification — ideal hard-magnetic soft material — which assumes that (i) the material has a residual magnetic flux density, and (ii) the induced magnetic flux density exhibits a linear relation with the applied actuating magnetic field. We implement the theory and constitutive law in a finite-element framework and find remarkable agreement between the simulation and experimental results on various deformation modes of hard-magnetic soft materials. Using the developed (and validated) model, we present a set of illustrative examples to highlight the use of our model-based simulation to guide the design of experimentally realizable complex shape-morphing structures based on hard-magnetic soft materials.

Influence of electromagnetic forces in the gaps of a protective CT

Two of the main problems studied in protective current transformers (CTs) are the saturation and residual flux in the ferromagnetic core. To minimise the effects of high saturation levels and residual flux, a gapped core CT can be employed. Although solving both issues, research reports that the introduction of gaps in the ferromagnetic core can develop into a mechanical fragility when compared with a closed core. However, the fragility is not proven in the research. To investigate the mechanical fragility of a gapped core CT, this research aims to verify the influence of the electromagnetic forces in the gap borders of a protective CT under a short-circuit condition. The influence was verified in three different gap lengths: 8, 16 and 24 mm. On the basis of the results, the electromagnetic forces in the gap borders of a protective CT did not influence the CT response.