Nonuniform Currents Research Articles

Hybrid Transverse Magneto‐Thermoelectric Cooling in Artificially Tilted Multilayers

AbstractIn artificially tilted multilayers comprising two different conductors that are alternately and obliquely stacked, transverse thermoelectric conversion occurs, in which charge and heat currents are interconverted in the orthogonal direction. Although transverse thermoelectric conversion also occurs in homogeneous materials as an intrinsic transport phenomenon owing to the effects of magnetic fields, magnetization, and spins on conduction carriers, such magneto‐thermoelectric effects are investigated independently of thermoelectrics for artificially tilted multilayers. Here, this study shows that the synergy of these different principles improves the performance of transverse thermoelectric conversion. Using lock‐in thermography techniques, transverse thermoelectric conversion processes are visualized in artificially tilted multilayers and the experiments clarify how nonuniform charge currents are converted into orthogonal heat currents. Through the measurements of temperature change under magnetic fields, the contributions of the magneto‐thermoelectric effects are quantified in the artificially tilted multilayers and magnetically enhanced hybrid transverse thermoelectric cooling is demonstrated. By replacing one of the conductors in the multilayer with permanent magnets, the same functionality is obtained even in the absence of magnetic fields, paving the way for the creation of “thermoelectric permanent magnets” that exhibit efficient transverse thermoelectric conversion together with spontaneous magnetization. This study provides a new material design guideline for transverse thermoelectrics.

Compact two-port antenna with high isolation based on the defected ground for THz communication

The two-port antenna is designed for THz communication where isolation is achieved with the defective ground. The radiator of the two-port antenna is inspired by the half-cutting technology which reduces the size and makes it a compact antenna. The T-shaped stub and open-ended slot are used to modify the capacitive and inductive reactance which diminish and discontinued the surface current to improve the port isolation. The T-shaped stub improves the electric length of the ground and enhances the port isolation by up to 3.4 dB and open-ended slot in the ground is used to weaken the mutual current and create the non-uniform current which intensifications the port isolation up to 7.7 dB at 2.2 THz. The dimensions of the design is 100 µm × 105 µm and the edge distance of the antennas is only 0.20λ with maximum isolation is 30 dB at 2.2 THz. The bandwidth of the two-port antenna is 1.74 THz to 2.52 THz that can be applicable in wireless communication applications.

Field-Free Spin-Orbit Torque Driven Switching of Perpendicular Magnetic Tunnel Junction through Bending Current.

Current-induced spin-orbit torques (SOTs) enable fast and efficient manipulation of the magnetic state of magnetic tunnel junctions (MTJs), making them attractive for memory, in-memory computing, and logic applications. However, the requirement of the external magnetic field to achieve deterministic switching in perpendicularly magnetized SOT-MTJs limits its implementation for practical applications. Here, we introduce a field-free switching (FFS) solution for the SOT-MTJ device by shaping the SOT channel to create a "bend" in the SOT current. The resulting bend in the charge current creates a spatially nonuniform spin current, which translates into inhomogeneous SOT on an adjacent magnetic free layer enabling deterministic switching. We demonstrate FFS experimentally on scaled SOT-MTJs at nanosecond time scales. This proposed scheme is scalable, material-agnostic, and readily compatible with wafer-scale manufacturing, thus creating a pathway for developing purely current-driven SOT systems.

Design and Non-Uniform Current Analysis of a 0.35-THz Extended Interaction Oscillator Based on Pseudospark-Sourced Multiple Sheet Electron Beams

A novel method, which combines a multiple-beam extended interaction oscillator (EIO) with pseudospark-sourced (PS) sheet electron beams, is applied to generate high-power terahertz sources. For a multiple-beam EIO, the beam cross-section is significantly improved by replacing the commonly used pencil electron beams with sheet electron beams. The PS electron beams have the advantage of high current density and operate without a focus magnetic field. The volume of the cavity is larger when the EIO operates in the TM31-3π mode than in the conventional TM01-2π mode at the same operating frequency. The EIO operating at the terahertz frequency has a larger cavity volume, which means greater power capacity and lower manufacturing difficulty. For a PS multiple-beam EIO, the non-uniformity of electron beam currents is a common problem. In order to study this problem, an original high-order mode EIO driven by PS multiple sheet electron beams is presented with enhanced output power at 0.35 THz. The authors analyze electron beams with different currents through particle-in-cell (PIC) simulations. Simulation results show that the EIO can operate stably even in the case of non-uniform PS electron beam currents. When each current is 1.4 A, simulation results show the EIO’s output power of 4.9 kW at 0.35 THz. Considering the low conductivity of 1.1 × 107 S/m, the efficiency is still 1.42%.

High-fidelity simulation of ship and linear shear current interaction

In this paper, the hydrodynamic performance of KCS (KRISO Container Ship) in the presence of a non-uniform shear current is investigated for the first time using high-fidelity CFD (Computational Fluid Dynamics) simulations. Various shear current strength conditions Frs with different directions were considered and results were compared to the ones with no shear current. The studies were conducted for the ship advancing at Fr = 0.26, 0.367. The focus is laid on the shear current effects on the ship resistance as well as the free surface elevation, pressure distribution, vortex structures, boundary layer, and nominal wake. Results showed that the presence of the shear current affects the hydrodynamic performance of the ship profoundly such that it increases/decreases the ship resistance in heading/following shear current with the largest change for the cases with the strongest shear current. The viscous resistance is dominant at Fr = 0.26 for all shear current conditions while the wave resistance is dominant for most cases at Fr = 0.367. The wave resistance reduces as the shear current direction varies from head to following shear current, similar to the theoretical model predictions. The viscous component follows a similar trend. The lateral resistance generated by the cross flow in beam shear current increases with the increase of Frs and the reduce of Fr and it is strongly dominated by the lateral pressure resistance. The Kelvin angle reduces in the head shear current and increases in the following shear current while the beam shear current reduces the Kelvin angle on the windward side of the ship and increases on the leeward side. The shear current also altered the transverse wavelength such that it increased in head shear current and reduced in following shear current. The predicted Kelvin angles and transverse wavelengths confirmed the finding of simplified theories on ship-shear current interaction. The presence of shear current also significantly affects the boundary layer, vortical structures, and the wake field.

Numerical investigations on fluid characteristics around the bottom-fixed aquacultural farm

Offshore aquacultural farms are world widely booming in recent years. The bottom-fixed aquacultural farm gains more and more attention because of the large volume and great strength. This study numerically investigates the fluid velocity distribution characteristic around the bottom-fixed aquacultural farm, which is of great importance for fish welfare and pollutant diffusion. The numerical model is developed based on the porous media model for the nets and the rigid walls for the fixed frame. The k-omega turbulence model and the finite volume method are employed to solve the Navier-Stokes equations. The flow velocity distributions around and inside the farm in the varied current velocity are analyzed. The vertical and horizontal flow velocity distributions in various current directions with different net solidities are investigated in detail. Results demonstrated that the configurations of trusses, vertical and horizontal columns of the farm and the interaction with the nets result in complex velocity distributions inside and around the farm. The velocity attenuations at different depths are quite different in uniform and non-uniform currents. The velocity distributions in different attack angles of the flow provide valuable suggestions on the arrangement and layout of the multiple farms.

Transmission Line Modeling and Crosstalk Analysis of Multibraided Shielded TWP/Twinax Cables

In this article, the transmission line model of multibraided shielded cables is improved by including the proximity effect in the shield model. This is performed by deriving the analytical expressions of the transfer impedance and admittance of a shielded structure with nonuniform current distribution around the shield surfaces. After that, the formulations are generalized to be applicable for shielded twisted-wire pair (TWP) and twinax cables with line apertures and braided structure with nonuniform shield current. It is found that the nonuniform distribution of the currents in the outer and inner shield surfaces have high effects on the inner induced common-mode and differential-mode currents of braided shielded cables. In addition, the crosstalk between multishielded TWP and twinax cables is investigated in detail, indicating that the proximity effects between the shields have a critical impact on generating the crosstalk between the shielded cables. The proposed model is validated with the commercial software FEKO and excellent agreements are achieved.

Experimental and simulation studies of SCIF considering non-uniform critical current

As an effective and reliable method of obtaining extremely high magnetic fields, rare-earth barium copper oxide (REBCO) magnets have contributed significantly to the development of condensed matter physics, chemistry, life sciences, and materials research. However, large screening currents in REBCO magnets can significantly degrade the magnetic field quality. Recent experiments on the inner insert of a 30 T superconducting magnet at the Institute of Electrical Engineering, Chinese Academy of Sciences showed that the measured magnetic field is highly asymmetric single-peaked field instead of the designed saddle-shaped fields, and it differs significantly from the values calculated based on the commonly-used uniform and symmetric screening current model. Therefore, a simulation model based on the T–A formulation with consideration of accurate dimensions and critical currents was developed. The simulation results with this new developed model indicate that the angle dependence can cause more than an 8.3% difference in the critical currents of the symmetrically located double pancake (DP) coils of the magnet. The average relative error between the measured and calculated magnetic field values was sharply reduced by a maximum of 44.4% after considering the non-uniform critical currents. In consideration of manufacturing uncertainties and non-uniform critical currents, better magnetic field quality of REBCO magnets can be achieved by arranging the location of each DP with this model.

Current-induced out-of-plane torques in a single permalloy layer with lateral structural asymmetry

We investigated current-induced torques in a single permalloy layer connected to nonmagnetic electrodes with lateral inversion asymmetry by spin-torque ferromagnetic resonance. Considering the symmetry of the spin-torque ferromagnetic resonance spectrum with respect to the magnetic field direction, we successfully separated various types of torque. In addition to in-plane damping-like and field-like torques, we detected two additional components of out-of-plane (OOP) torques, the symmetries of which with respect to the magnetic field direction correspond to an OOP field-like torque and an OOP damping-like torque. We obtained the OOP torques in particular by breaking a lateral inversion symmetry of the nonmagnetic electrodes, and the sign of the torques is reversed by inverting the electrode structure. The microwave frequency dependence of the OOP torques indicates that the OOP torques are not attributable to a spin current but to the Oersted field generated by a nonuniform charge current, ${B}_{\mathrm{Oersted}}$, and an inductive field, ${B}_{\mathrm{induc}.}$ (which originates from ${B}_{\mathrm{Oersted}}$). We propose an external field-free and fast magnetization switching of a ferromagnetic layer with perpendicular magnetic anisotropy by using ${B}_{\mathrm{induc}.}$, which can be achieved only by fabricating the electrodes asymmetrically.

ON THE PROPAGATION OF EQUATORIAL WAVES INTERACTING WITH A NON-UNIFORM CURRENT

We consider the propagation of equatorial waves of small amplitude, in a flow with an underlying non-uniform current. Without making the too restrictive rigid-lid approximation, by exploiting the available Hamiltonian structure of the problem, we derive the dispersion relation for the propagation of coupled long-waves: a surface wave and an internal wave. Also, we investigate the above-mentioned model of wave-current interactions in the general case with arbitrary vorticities.

Ionic current magnetic fields in 3D finite-length nanopores and nanoslits.

Deoxyribonucleic acid (DNA) encodes all genetic information, and in genetic disorders, DNA sequencing is used as an effective diagnosis. Nanopore/slit is one of the recent and successful tools for DNA sequencing. Passage of DNA along the pores creates non-uniform ionic currents which creates non-uniform electric and magnetic fields, accordingly. Sensing the electric field is usually used for sequencing application. We suggest to use the magnetic field induced by pressure-driven ionic currents as a secondary signal. We systematically compared the induced magnetic field of nanopores and nanoslits with equal cross-sectional area. The 3D magnetic field is numerically obtained by solving the Poisson-Nernst-Planck, Ampere, and Navier–Stokes equations. As expected, the maximum value of the maximum magnetic flux occurs near the wall and inside the channel, and increasing the pressure gradient along the pore/slit increases the flowrate and magnetic field, consequently. At a given pressure difference across the pore/slit, nanopores are better than nanoslits in sensing the magnetic flux. For example, by applying 2 MPa across the pore/slit, the maximum magnetic flux density for nanopore, nanoslit AR = 1 and nanoslit AR = 5 are 1.10 pT, 1.08 pT and 0.45 pT, accordingly. Also, at a given flowrate across the pore/slit, nanoslits are the better choice. It should be noted the external magnetic fields as small as pico-Tesla are detectable and measurable in voltage/pressure driven electrokinetic flow slits.Graphical abstract

Hamiltonian approach to modelling interfacial internal waves over variable bottom

We study the effects of an uneven bottom on the internal wave propagation in the presence of stratification and underlying non-uniform currents. Thus, the presented models incorporate vorticity (wave–current interactions), geophysical effects (Coriolis force) and a variable bathymetry. An example of the physical situation described above is well illustrated by the equatorial internal waves in the presence of the Equatorial Undercurrent (EUC). We find that the interface (physically coinciding with the thermocline and the pycnocline) satisfies in the long wave approximation a KdV–mKdV type equation with variable coefficients. The soliton propagation over variable depth leads to effects such as soliton fission, which is analysed and studied numerically as well.

A Stochastic Spatiotemporal Model of Rat Ventricular Myocyte Calcium Dynamics Demonstrated Necessary Features for Calcium Wave Propagation.

Calcium (Ca2+) plays a central role in the excitation and contraction of cardiac myocytes. Experiments have indicated that calcium release is stochastic and regulated locally suggesting the possibility of spatially heterogeneous calcium levels in the cells. This spatial heterogeneity might be important in mediating different signaling pathways. During more than 50 years of computational cell biology, the computational models have been advanced to incorporate more ionic currents, going from deterministic models to stochastic models. While periodic increases in cytoplasmic Ca2+ concentration drive cardiac contraction, aberrant Ca2+ release can underly cardiac arrhythmia. However, the study of the spatial role of calcium ions has been limited due to the computational expense of using a three-dimensional stochastic computational model. In this paper, we introduce a three-dimensional stochastic computational model for rat ventricular myocytes at the whole-cell level that incorporate detailed calcium dynamics, with (1) non-uniform release site placement, (2) non-uniform membrane ionic currents and membrane buffers, (3) stochastic calcium-leak dynamics and (4) non-junctional or rogue ryanodine receptors. The model simulates spark-induced spark activation and spark-induced Ca2+ wave initiation and propagation that occur under conditions of calcium overload at the closed-cell condition, but not when Ca2+ levels are normal. This is considered important since the presence of Ca2+ waves contribute to the activation of arrhythmogenic currents.

Using Blade Element Momentum Theory to Predict the Effect of Wave-Current Interactions on the Performance of Tidal Stream Turbines

The non-uniformity and dynamics of the environment tidal stream turbines need to operate within will significantly influence the durability and reliability of tidal energy systems. The loadings on the turbine will increase substantially when the turbine is deployed in high magnitude waves with non-uniform tidal currents. The limitations of numerical solutions will be understood when the outcomes are verified with empirical data from system operations. In this paper, a Blade Element Momentum model is used to predict and compare the performance of a scaled turbine within a flume and a tow tank. Firstly, the numerical and experimental work is analysed for a turbine operating at flow speeds of 0.5m/s amd 1.0 m/s, wave heights of 0.2 m and 0.4 m and wave periods of 1.5 s and 1.7 s. Good agreement between the model and the experimental work was observed. However, in low TSRs the model tends to under predict the thrust, and the variation between the maximum and minimum values obtained within the experiments. Secondly, a turbine operating at flow speeds of 1.0 m/s and 4 different inflow profiles is analysed, where the wave heights for these cases were 0.09 m and 0.19 m and with wave periods of 2 s and 1.43 s. In this evaluation, the model tends to over predict the values of Ct and Cp when compared to those calculated from the experimental data. However, when investigating the values used to calculating both the thrust and torque coefficients, there is better agreement with these, which means the methodology used to determine these coefficients with inflow profiles should be revised.

The onset of instabilities and finite amplitude waves in a model of aluminum reduction cells with nonuniform cathode current

In aluminum reduction cells, the electric current density at the cathode is seldom uniform. This may be due to a variety of reasons. One of the reasons is that a mass of undissolved alumina may get enrobed by the cryolitic bath, and this mix freezes due to the decrease in temperature below the liquidus point. Thus, an electrically resistive solid layer (“mud”) is formed at a part of the cathode. This leads to horizontal electric currents in the liquid aluminum layer, and their magnetohydrodynamic (MHD) interaction with the vertical magnetic field induces a rotating flow of aluminum. This may cause undesirable MHD instabilities. A quasi-two-dimensional model for the laboratory facility has been presented. The results show that the dominating feature of the flow is a depression of the free surface of the liquid metal above the mud spot. This is due to strong rotation of the liquid metal around the mud spot, which causes the centrifugal force. Other effects may include superimposed conventional MHD instability, which manifests itself in a rotating interface, modulated waves, reflection of the waves from the corners of the domain, sloshing, etc. It has been shown that small mud spots do not affect the cell stability, while the large ones may cause such deep depressions that the centrifugal force completely removes the liquid metal above the spot.

Effect of Ship-Induced Langmuir-Type Circulations on Distribution of Surface-Active Substances and Damping of Short Wind Waves

It has recently been shown that the interaction of ship-generated nonuniform currents with ambient surface waves can lead to the generation of Langmuir-type circulations (LTCs) (Basovich 2011) and a persistent wake (Somero et al. 2018). Based on this work, it is shown here that the LTC and surface currents of the persistent wake are responsible for the redistribution of surface-active substances (SAS) and a corresponding change in the damping of short surface waves. The persistent wake is a region of the ship wake, where initial ship-generated perturbations have mostly decayed. The LTCs are similar in nature to Langmuir circulations which arise as a result of instability of wind-driven current. LTCs produce a secondary flow with velocity transverse to the direction of the ship, and width significantly larger than the ship beam. Because LTCs are generated in large scale, they persist for a long time after the passage of the ship. Transverse surface currents produced by LTCs in the ship wake redistribute the SAS films at the sea surface. These currents create strong convergence and divergence zones which in turn produce streaks with different concentrations of SAS. The change in concentration of SAS affects the film pressure and the damping effect of SAS on the short surface waves. This effect is represented by a damping factor and is a crucial parameter in determination of the spectral density of short wind waves. Therefore, the damping effect of the film, as represented by the damping factor, is responsible for sea surface roughness modification and is important for prediction of synthetic-aperture RADAR (SAR) imagery of ship wakes on the ocean surface. In this article, we present the mathematical and computational methods, along with simulation results for a naval surface combatant operating in calm, head, and following seas. The simulation results clearly show that the convergence and divergence zones strongly influence the relative SAS concentration and the spatial distribution of the damping factor, the latter of which defines the structure of SAR images of the persistent wake. Comparisons of the magnitude of the damping factor with available SAR data are shown to be in good agreement.

On solitary wave in nonuniform shear currents

In this paper, steady solutions of solitary waves in the presence of nonuniform shear currents are obtained by use of the high-level Green-Naghdi (HLGN) model. We focus on large-amplitude solitary waves in strong opposing shear currents. The linear-type currents, quadratic-type currents and cubic-type currents are considered. In particular, the wave speed, wave profile, velocity field, particle trajectories and vorticity distribution are studied. It is demonstrated that presence of the nonuniform shear current modifies the velocity field and vorticity field of the solitary wave.

Current Estimation and Path Following for an Autonomous Underwater Vehicle (AUV) by Using a High-gain Observer Based on an AUV Dynamic Model

A path following problem for autonomous underwater vehicles (AUVs) under a nonuniform current is presented in this paper. A dynamic model of an AUV in a nonuniform flow was adopted to develop a high-gain observer (HGO) for estimation of the three-dimensional current velocities along AUV trajectories. The HGO was chosen as a nonlinear estimation algorithm, and the observer gain was computed by solving a Linear Matrix Inequality (LMI) which represented the estimation error dynamics. The current velocities were determined by calculating the differences between the measured absolute velocities of the vehicle and the estimated relative velocities of the vehicle estimated by the observer. The estimation error means of the HGO using the LMI have smaller values than the state observer with a gain matrix determined by the pole-placement approach. For the path following study, the desired curved path was represented by using a Serret-Frenet frame which propagated along the curve. The path-following system includes a guidance law, an update law and a proportional and integral controller. Two cases of numerical simulations were conducted to verify the performance of the path following system combined with HGO for current compensation, and the results of both cases have shown that the AUV reached and converged to the desired path.

Particle swarm optimization algorithms with selective differential evolution for AUV path planning

Particle swarm optimization (PSO)-based algorithms are suitable for path planning of the Autonomous Underwater Vehicle (AUV) due to their high computational efficiency. However, such algorithms may produce sub-optimal paths or require higher computational load to produce an optimal path. This paper proposed a new approach that improves the ability of PSO-based algorithms to search for the optimal path while maintaining a low computational requirement. By hybridizing with differential evolution (DE), the proposed algorithms carry out the DE operator selectively to improve the search ability. The algorithms were applied in an offline AUV path planner to generate a near-optimal path that safely guides the AUV through an environment with a priori known obstacles and time-invariant non-uniform currents. The algorithm performances were benchmarked against other algorithms in an offline path planner because if the proposed algorithms can provide better computational efficiency to demonstrate the minimum capability of a path planner, then they will outperform the tested algorithms in a realistic scenario. Through Monte Carlo simulations and Kruskal-Wallis test, SDEAPSO (selective DE-hybridized PSO with adaptive factor) and SDEQPSO (selective DE-hybridized Quantum-behaved PSO) were found to be capable of generating feasible AUV path with higher efficiency than other algorithms tested, as indicated by their lower computational requirement and excellent path quality.

Effects of wave forces on knotless polyethylene and chain-link wire netting panels for marine aquaculture cages

The growth of offshore marine aquaculture requires improvements in the engineering and materials used for cage netting in order to better withstand wave forces. To clarify the hydrodynamic characteristics of knotless polyethylene (PE) and chain-link wire netting panels in non-uniform currents, we measured the wave forces acting on netting panels with varying solidity ratios and other parameters under regular wave conditions in a wave tank. The drag and added mass coefficients (CD, CM) were obtained using the Morison equation, and the behavioral trends in CD and CM for the netting panels were determined based on the Reynolds number (Re) and the Keulegan-Carpenter (KC) number. The experimentally derived coefficients were then incorporated into numerical simulations with the same conditions, and then were used to model the dynamic behavior of the netting panels. The normal CD of PE netting was similar to that in a uniform current and was positively associated with the solidity ratio, decreasing as Re increased. When the KC number was > 30, the normal CM of both the PE and chain-link wire netting remained unchanged, averaging 4.3 and 2.8, respectively. Large differences in displacement were observed for varying values of CD, suggesting that the effects of marine fouling should be considered during the design stage. Overall, these results provide a basis for further research toward a comprehensive hydrodynamic model for netting panels that can inform improved engineering for modern aquaculture.