Nonuniform Currents Research Articles

Proposal for a Domain Wall Nano-Oscillator driven by Non-uniform Spin Currents.

We propose a new mechanism and a related device concept for a robust, magnetic field tunable radio-frequency (rf) oscillator using the self oscillation of a magnetic domain wall subject to a uniform static magnetic field and a spatially non-uniform vertical dc spin current. The self oscillation of the domain wall is created as it translates periodically between two unstable positions, one being in the region where both the dc spin current and the magnetic field are present, and the other, being where only the magnetic field is present. The vertical dc spin current pushes it away from one unstable position while the magnetic field pushes it away from the other. We show that such oscillations are stable under noise and can exhibit a quality factor of over 1000. A domain wall under dynamic translation, not only being a source for rich physics, is also a promising candidate for advancements in nanoelectronics with the actively researched racetrack memory architecture, digital and analog switching paradigms as candidate examples. Devising a stable rf oscillator using a domain wall is hence another step towards the realization of an all domain wall logic scheme.

A numerical treatment of a non-dimensional form of a water quality model in the Rama-nine reservoir

The purposes of this research are to develop mathematical models and numerical methods for approximating water flow directions and pollution levels in a Rama-nine reservoir, Pathumthani, Thailand with non-uniform current. The Rama-nine reservoir is opened with two parallel canals. The pollution levels in a reservoir are assessing via data collection at the real field. It is quite complex and the results obtained tentatively deviate from one point of time and position to another. There are many research works applied a mathematical model called the dispersion model to estimate the water pollutant concentration. The approximation accuracy received is seemingly unsatisfied, especially, when the water flow is not uniformly distributed.The research begins with revising a mathematical model that combines two existing mathematical models: a non-dimensional form of hydrodynamic model and a dispersion model. The model is to make suitable to the Rama-nine reservoir.The Lax- Wendroff method is subsequently used in a non-dimensional form of a shallow water equation to approximate the water velocity and elevation. Next, we use the forward differences in time and backward difference in space in advection-diffusion equation. Combined the equation with the calculated velocity is thus used in the dispersion model to approximate the concentration levels of the pollutants. The result of this research showed that the proposed concentration of the pollutants in a Rama-nine reservoir at any various time and position. The accuracy of approximation is within the units of centimeters and seconds. In addition, this research has shown that the proposed model can be applied to other water sources having non-uniformly distributed water flows.

Optimum pattern synthesis of non‐uniform spherical arrays using the Euler rotation

Method of least squares is employed for synthesis of non-uniform spherical arrays using Euler rotation of coordinate axes for reduction of side-lobe levels, controllability of 3-db beam-width of main-lobe etc. It is useful for synthesis of conformal arrays, where pattern multiplication is not applicable and array synthesis is involved. This method for synthesis of spherical arrays is general that determines radius of sphere, longitude and latitude angles of elements and their current excitations (amplitudes and phases). The authors investigate several examples to demonstrate its capability. Studies on conformal arrays deal with non-uniform excitation currents, leading to complicated input networks. Therefore the authors suggest non-uniform spacing, which shows that array synthesis by variable element spacing will realise any specifications, same as non-uniform excitations. An error function is constructed for pattern synthesis, which is a function of element spacing and excitations. The authors use combination of global and local minimisation algorithms. In examples of spherical array synthesis, 2.5 and 5.5 dB reduction of side-lobe levels have been achieved in the E- and H-plane patterns, respectively, relative to the available design data. In addition, reduction of beam-width of main-beams in E- and H-planes has been obtained by 39% and 34%, respectively.

An analytical model of the evolution of a Stokes wave and its two Benjamin–Feir sidebands on nonuniform unidirectional current

Abstract. An analytical weakly nonlinear model of the Benjamin–Feir instability of a Stokes wave on nonuniform unidirectional current is presented. The model describes evolution of a Stokes wave and its two main sidebands propagating on a slowly varying steady current. In contrast to the models based on versions of the cubic Schrödinger equation, the current variations could be strong, which allows us to examine the blockage and consider substantial variations of the wave numbers and frequencies of interacting waves. The spatial scale of the current variation is assumed to have the same order as the spatial scale of the Benjamin–Feir (BF) instability. The model includes wave action conservation law and nonlinear dispersion relation for each of the wave's triad. The effect of nonuniform current, apart from linear transformation, is in the detuning of the resonant interactions, which strongly affects the nonlinear evolution of the system. The modulation instability of Stokes waves in nonuniform moving media has special properties. Interaction with countercurrent accelerates the growth of sideband modes on a short spatial scale. An increase in initial wave steepness intensifies the wave energy exchange accompanied by wave breaking dissipation, resulting in asymmetry of sideband modes and a frequency downshift with an energy transfer jump to the lower sideband mode, and depresses the higher sideband and carrier wave. Nonlinear waves may even overpass the blocking barrier produced by strong adverse current. The frequency downshift of the energy peak is permanent and the system does not revert to its initial state. We find reasonable correspondence between the results of model simulations and available experimental results for wave interaction with blocking opposing current. Large transient or freak waves with amplitude and steepness several times those of normal waves may form during temporal nonlinear focusing of the waves accompanied by energy income from sufficiently strong opposing current. We employ the model for the estimation of the maximum amplification of wave amplitudes as a function of opposing current value and compare the result obtained with recently published experimental results and modeling results obtained with the nonlinear Schrödinger equation.

Neural-Network- and L2-Gain-Based Cascaded Control of Underwater Robot Thrust

This paper presents a robust cascaded control strategy to underwater robot thrust. The dynamics of surge motion, of propeller axial flow, of propeller shaft, and of electrically driven circuit in the motor constitute a cascaded system with respect to propeller thrust. Instead of the usual parameter perturbation, generalized modeling errors are considered in the plant, which may be parametric errors, ignored high-order modes, or some unmodeled dynamics in the underwater thrust system. External disturbances are also taken into account, which may be the random noises from mechanical or electrical equipment, or the environmental forces possibly induced by nonuniform currents, ocean internal wave, or cable tension. Combined with state feedback control, an online neural network (NN) compensator is introduced to identify the modeling errors, while L2-gain design is used to suppress the externally continuous or instantaneous disturbances. The Lyapunov's second method is applied to instruct the controller design, which guarantees the uniformly ultimately bounded (UUB) stability of the error system. By analyzing the tracking errors, it is recommended how to properly select the controller parameters. Good tracking performance and reasonable control inputs are illustrated by numerical simulations.

Nonlinear motion of non–uniform current–vortex sheets in MHD Richtmyer–Meshkov instability

Nonlinear motion of a vortex sheet initially applied a homogeneous magnetic field is investigated using the vortex blob method. We show that the existence of a vortex sheet induces a strong magnetic field and non-uniform current at the fluid interface. This system can be a model of incompressible magnetohydrodynamic Richtmyer–Meshkov instability (MHD-RMI). We present some interfacial profiles in MHD-RMI and discuss the difference between the pure vortex sheet in standard RMI and the current–vortex sheet in MHD-RMI, in which it is shown that the magnetic field suppresses the roll-up of the vortex sheet. We also find that the strong magnetic field amplification is caused by the stretch of the interface.

Nonlinear motion of non-uniform current-vortex sheets in magnetohydrodynamic flows

Nonlinear motion of vortex sheets with non-uniform current is investigated using the vortex blob method. We show that the current induced on a vortex sheet leads to a strong amplification of the magnetic field when the Lorentz force term in the governing equation is sufficiently small. When the Lorentz force term is large, an oscillation due to the Alfvén wave appears and the nonlinear growth is suppressed. We present various interfacial profiles depending on the magnitude of the Atwood number (density ratio) and Lorentz force, and discuss the complicated motion of non-uniform current-vortex sheets.

Dynamics of underwater gliders in currents

Although underwater vehicles often operate in time-varying, nonuniform currents, the effect of flow gradients on vehicle dynamics is typically ignored in motion models used for control and estimation. Forces and moments due to a nonuniform flow field are strongest when apparent mass effects are important and flight paths are most sensitive to these disturbances when flow-relative velocities are small – precisely the situation for underwater gliders. This paper presents a nonlinear multi-body dynamic model for an underwater glider operating in an unsteady, nonuniform flow. The eight degree of freedom model incorporates a cylindrically actuated moving mass, a common glider actuation scheme. To illustrate the utility of this full dynamic model, numerical motion predictions are compared with those of simpler models for a variety of nonuniform flow fields. We also demonstrate the use of the full dynamic model for flow estimation using parameter adaptive filtering.

Prediction and analysis of fatigue damage due to cross-flow and in-line VIV for marine risers in non-uniform current

Fatigue damage due to vortex induced vibrations (VIV) is one of the most significant failure modes for marine risers. Previous studies of fatigue prediction and analysis for marine risers typically focus on cross-flow VIV due to its larger response amplitude than in-line vibrations. In this study, a prediction model of VIV fatigue damage for riser accounting for both cross-flow and in-line vibrations is proposed based on the energy equilibrium theory and the forced vibration experimental data of a rigid cylinder. The coupling effect of CF- and IL-VIV is taken into account and the response amplitude is corrected by a weighted factor according to the input power within the overlap excitation region for each vibration mode. The experimental models of flexible riser under stepped and sheared current are simulated to validate the proposed approach and the results show reasonable agreement. Moreover, based on the prediction model, the top velocity of sheared current, top tension, internal fluid density and the material of structures are invested to discuss their effects on the fatigue damage. The study may be worthwhile for fatigue assessment and broaden the understanding of VIV fatigue damage characteristics for deepwater risers.

Photo-induced tunneling currents in MOS structures with various HfO2/SiO2 stacking dielectrics

In this study, the current conduction mechanisms of structures with tandem high-k dielectric in illumination are discussed. Samples of Al/SiO2/Si (S), Al/HfO2/SiO2/Si (H), and Al/3HfO2/SiO2/Si (3H) were examined. The significant observation of electron traps of sample H compares to sample S is found under the double bias capacitance-voltage (C-V) measurements in illumination. Moreover, the photo absorption sensitivity of sample H is higher than S due to the formation of HfO2 dielectric layer, which leads to larger numbers of carriers crowded through the sweep of VG before the domination of tunneling current. Additionally, the HfO2 dielectric layer would block the electrons passing through oxide from valance band, which would result in less electron-hole (e−-h+) pairs recombination effect. Also, it was found that both of the samples S and H show perimeter dependency of positive bias currents due to strong fringing field effect in dark and illumination; while sample 3H shows area dependency of positive bias currents in strong illumination. The non-uniform tunneling current through thin dielectric and through HfO2 stacking layers are importance to MOS(p) tunneling photo diodes.

Current Sharing and Redistribution in Superconducting DC Cable

Uniform current distribution is shown to be an important requirement in superconducting dc cables. The role of nonuniform critical currents of individual tape sections on current sharing in a multistrand cable was investigated using a controlled parallel network of superconducting tapes with deliberately unequal critical current values. It was shown that if there is a significant variation in the critical currents of individual tape sections used for fabricating the cable, there is a potential that the tapes with lowest critical current carry current close to their critical current even when the cable as a whole is operating below its critical current. The results presented suggest that selection of tape sections with equal critical current prevents hotspots and potential catastrophic damage to the cables. The effect of nonuniform current sharing is particularly important in cables cooled with gaseous helium because of the limited cooling capacity.

Simulation of Series Arrays of Superconducting Quantum Interference Devices

The voltage as a function of applied magnetic field (V-B) was calculated for arrays of superconducting quantum interference devices (SQUIDs) connected in series. Comparisons were made between arrays of equal area SQUIDs and superconducting quantum interference filters (SQIFs). The areas for the SQIFs were varied exponentially, so that the V-B had a sharp minimum at zero field. We used equations for the dc SQUID based on resistively shunted junctions, with typical parameters for YBa2Cu3O7 - δ ion damage Josephson junctions. The maximum transfer coefficient of the central minimum VB = (∂V/∂B)max of the SQIF decreases as the area range increases. We find that the equal area array is more robust to the effects of non-uniform junction critical currents than the SQIF, for the junction parameters and SQUID area distributions chosen. Furthermore, we find that slight variations (~5%) to the area due to fabrication irregularities have little effect on the central minimum of V-B for either device.

Critical Current Density and Temperature Dependence of Nb-Al Oxide-Nb Junction Resistance and Implications for Room Temperature Characterization

Room temperature junction resistance measurements are commonly used for screening Josephson-based circuits because testing is much easier than at cryogenic temperatures and can even be carried out at the wafer level. The value of ambient testing depends on the existence of a strong correspondence between the measured resistance at room temperature and the resistance and critical current obtained at the ultimate operating temperature. We have systematically studied the temperature dependence of junction resistance in order to quantify the emergence, with increasing critical current density, of parasitic contributions from non-uniform currents flowing in the Nb films, which tend to limit the value of room temperature screening. We will describe our measurements and our approach to correcting for these parasitic effects.

Fringe waves on an impedance cylinder

Surface diffraction of the plane waves from an impedance cylinder was investigated by the method of physical theory of diffraction (PTD). Physical optics (PO) method was used to gain the uniform scattered fields which are produced by the induced surface currents. Nonuniform currents and scattered fields were obtained by the method of PTD with the assistance of the exact solutions.

On the nonlinear Schrödinger equation for waves on a nonuniform current

A nonlinear Schrödinger equation with variable coefficients for surface waves on a large-scale steady nonuniform current has been derived without the assumption of a relative smallness of the velocity of the current. This equation can describe with good accuracy the loss of modulation stability of a wave coming to a counter current, leading to the formation of so-called rogue waves. Some theoretical estimates are compared to the numerical simulation with the exact equations for a two-dimensional potential motion of an ideal fluid with a free boundary over a nonuniform bottom at a nonzero average horizontal velocity.

Heat conduction and thermal stabilization in YBCO tape

Yttrium barium copper oxide (YBCO) coated conductors are widely used in the conduction-cooled superconducting magnets with rapid development in refrigeration technologies at present. ‘Quench’ is a state that refers to the irreversible and uncontrolled superconductor to resistive transitions in the superconductor. The propagation of ‘quench’ or ‘normal zone’ has different characteristics in these high temperature superconductors (HTS) compared to low temperature superconductors. The superconductor to normal index, known as ‘n’ is much flatter in HTS. The hot spot emerging in local region due to quench and non-uniform critical current may cause permanent damage to whole HTS tape and hence the magnet winding pack. Thus it is necessary to determine the temperature profile along the length of HTS tape under a given energy (joule heating) such that propagation of the hot spot developed locally can be prevented early. In this study, a one dimensional, time dependent heat diffusion equation with appropriate boundary conditions are used to describe the consequences of the normal zone propagation resulting in the temperature diffusion in a HTS tape. The results demonstrate the necessity of adequate cooling of the edges of the flat HTS tapes to prevent irreversible normal zone transitions.

Irreversible Electrokinetic Repulsion at Zero-Reynolds-Number Sedimentation

Stokes-flow reversibility is violated in electrolyte solutions by a streaming-potential mechanism, where nonuniform convective currents within Debye layers surrounding charged particles induce electric fields in the electroneutral Ohmic bulk. We demonstrate the irreversibility consequences of this phenomenon for the problem of particle-pair sedimentation, where the two particles experience a repulsive force driven by bulk Maxwell stresses. At small separations the force scales inversely with the third power of separation distance. This singular behavior is associated with the counterrotation of the two torque-free particles, which leads through a lubrication mechanism to an intense electric field in the narrow gap between them. At large separations the force follows an inverse dependence upon the fourth power of separation, now associated with rectilinear particle motion.

Laboratory study of two-dimensional nonuniform surface currents over a sphere moving in a fluid column

This paper presents the results of laboratory measurements of surface currents over a sphere moving in the fluid column based on the particle tracing velocimetry. A theoretical model of the fluid flowing around the sphere is presented. The experimental data are compared with the results of theoretical calculations. It is shown that, at low tow speeds of the sphere, the experimentally measured current velocities exceed the velocities calculated from the theoretical model. At greater tow velocities when generation of surface waves by the sphere was observed in the experiment, the experimental data agree well with the results of the theoretical calculations.

Fringe waves in wedge diffraction

Explicit expressions for the non-uniform currents of the physical theory of diffraction are derived in terms of Fresnel functions for wedge diffraction by taking into account the surface integrals of the modified theory of physical optics. The obtained fringe waves are compared numerically by the asymptotic representations, found in the literature.

Interplay of recombination and selection in the genomes of Chlamydia trachomatis

BackgroundChlamydia trachomatis is an obligate intracellular bacterial parasite, which causes several severe and debilitating diseases in humans. This study uses comparative genomic analyses of 12 complete published C. trachomatis genomes to assess the contribution of recombination and selection in this pathogen and to understand the major evolutionary forces acting on the genome of this bacterium.ResultsThe conserved core genes of C. trachomatis are a large proportion of the pan-genome: we identified 836 core genes in C. trachomatis out of a range of 874-927 total genes in each genome. The ratio of recombination events compared to mutation (ρ/θ) was 0.07 based on ancestral reconstructions using the ClonalFrame tool, but recombination had a significant effect on genetic diversification (r/m = 0.71). The distance-dependent decay of linkage disequilibrium also indicated that C. trachomatis populations behaved intermediately between sexual and clonal extremes. Fifty-five genes were identified as having a history of recombination and 92 were under positive selection based on statistical tests. Twenty-three genes showed evidence of being under both positive selection and recombination, which included genes with a known role in virulence and pathogencity (e.g., ompA, pmps, tarp). Analysis of inter-clade recombination flux indicated non-uniform currents of recombination between clades, which suggests the possibility of spatial population structure in C. trachomatis infections.ConclusionsC. trachomatis is the archetype of a bacterial species where recombination is relatively frequent yet gene gains by horizontal gene transfer (HGT) and losses (by deletion) are rare. Gene conversion occurs at sites across the whole C. trachomatis genome but may be more often fixed in genes that are under diversifying selection. Furthermore, genome sequencing will reveal patterns of serotype specific gene exchange and selection that will generate important research questions for understanding C. trachomatis pathogenesis.ReviewersThis article was reviewed by Dr. Jeremy Selengut, Dr. Lee S. Katz (nominated by Dr. I. King Jordan) and Dr. Arcady Mushegian.