Nonuniform Currents Research Articles

Circulations caused by interaction of underwater currents and surface waves

The interaction between non-uniform near-surface currents and long surface waves is shown to produce large-scale secondary circulations. The circulations are caused by the Craik–Leibovich vortex force imposed on the existing non-uniform current by the surface waves. The current could be produced by different types of sources, such as by ship wakes or by river and sewer outflows. In this paper the circulations are considered for three representative types of currents: a near-surface jet, a shear current, and an underwater jet. A model similar to the model of Langmuir circulations is formulated and studied numerically. The general model takes into account the effect of viscosity on the main current as well as the effect of the circulation-related advection on the main current and secondary flow itself. A simplified model that describes the initial stage of the development of circulations was used in order to demonstrate the strength of the phenomenon and its dependence on some parameters of the problem. At this initial stage, the effect of viscosity on the main current as well as the effect of advection caused by the circulations was neglected (under assumption that the perturbation velocity is small). The effect of the viscosity on the circulations was included in the solution, and it was shown that initial development of the circulations is practically independent of the viscosity. This fact simplifies the solution of the problem and removes the uncertainty related to the value of the turbulent viscosity at the initial stage of the circulations. The results obtained demonstrate that strong circulations are generated under very realistic assumptions regarding the parameters of the current and the surface waves. The maximum velocity at the surface produced by such circulations can easily reach several centimeters per second. A circulatory flow with this magnitude of velocity at the surface can significantly affect short surface waves and, correspondingly, radar and optical signatures produced by the initial currents on the sea surface. Some important conclusions about the nature of these signatures are made based on numerical results and simple qualitative arguments. Theoretical predictions include, for example, the asymmetry of centerline ship wakes and the difference in the width and length between images of two wakes of similar ships moving in opposite directions when ambient surface waves are present.

Characteristic Analysis of VIV-Induced Fatigue Damage of Top Tensioned Risers Based on Simplified Model

In this study, a simplified empirical model for fatigue analysis of top-tensioned risers due to vortex-induced vibration (VIV) in a nonuniform current is presented. Compared with previous studies on this subject, this model is able to take into account the main intrinsic nature of VIV for low mass ratio structures in lock-in regions, added mass, and nonlinear fluid damping. Based on the simplified model, a closed-form solution of fatigue damage is suggested for the case of a riser with uniform mass and cross-section oscillating in a uniform flow. Fatigue damage characteristics of the top-tensioned risers due to VIV are investigated to determine the effects of various ocean conditions and riser dimensions on fatigue damage.

Intensity dynamics in a waveguide array laser

We consider experimentally and theoretically the optical field dynamics of a five-emitter laser array subject to a ramped injection current. We have achieved experimentally an array that produces a robust oscillatory power output with a nearly constant π phase shift between the oscillations from each waveguide. The output power also decreases linearly as a function of waveguide number. Those behaviors persisted for pump currents varying between 380 and 500 mA with only a slight change in phase. Of note is the fact that the fundamental frequency of oscillation increases with injection current, and higher harmonics are produced above a threshold current of approximately 380 mA. Experimental observations and theoretical predictions are in agreement. A low dimensional model was also developed and the impact of the nonuniform injection current studied. A nonuniform injection current is capable of shifting the bifurcations of the waveguide array providing a valuable method of array tuning without additional gain or structural alterations to the array.

Output from a Josephson stimulated terahertz amplified radiation emitter

The angular dependence of the radiation-zone output power and electric polarizationof stimulated terahertz amplified radiation (STAR) emitted from a dc voltageapplied across cylindrical and rectangular stacks of intrinsic Josephson junctions iscalculated. The boundary conditions are obtained from Love’s equivalence principles.During coherent emission, a spatially uniform ac Josephson current density inthe stack acts as a surface electric current density antenna source, leading toa harmonic radiation frequency spectrum, as in experiment, but absent in allcavity models of cylindrical mesas. Spatial fluctuations of the ac Josephson currentallow its fundamental mode to lock onto the lowest finite energy cylindrical cavitymode, causing it to resonate, leading to a non-uniform magnetic surface currentdensity radiation source, and a non-trivial combined fundamental frequency outputpower with linear polarization for general radiation directions, which may be fullyor partially coherent. The higher ac Josephson harmonics do not excite othercylindrical cavity modes. For rectangular mesas, the lowest energy modes areempirically not excited, but the non-uniform ac Josephson current can excitethe harmonic sequence of modes with spatial variation across the rectangularwidths, leading to combined radiation outputs both for the fundamental andthe higher harmonics, which combinations also may be either fully or partiallycoherent. The superconducting substrate is modeled as a perfect magnetic conductor,greatly reducing the STAR emitter power and modifying its angular dependence,especially parallel to the substrate. Based upon this substrate model, existingBi2Sr2CaCu2O8 + δ crystals atop perfect electric conductors could have STAR emitter power in excess of 5 mW,acceptable for many device applications.

Consistent asymptotic expansion of Mott's solution for oxide growth

Many relatively thick metal oxide films grow according to what is called the parabolic law L=2At+…. Mott explained this for monovalent carriers by assuming that monovalent ions and electrons are the bulk charge carriers, and that their number fluxes vary as t−1/2 at sufficiently long t. In this theory no bulk charge was present, and surface charges were not discussed. However, it can be analyzed in terms of a discharging capacitor, with the oxide surfaces as the plates. The theory is inconsistent because the field decreases, corresponding to discharge, but there is no net current to cause discharge. The present work, which also includes multi-valent carriers, systematically extends the theory and obtains the discharge current. Because the Planck–Nernst equations are nonlinear (although Gauss's Law and the continuity equations are linear) this leads to a systematic order-by-order expansion in powers of t−1/2 for the number currents, concentrations, and electric field during oxide growth. At higher order the bulk develops a non-zero charge density, with a corresponding non-uniform net current, and there are corrections to the electric field and the charge-carrier currents. The second order correction to the ion current implies a logarithmic term in the thickness of the oxide layer: L=2At+Blnt+…. It would be of interest to verify this result with high-precision measurements.

Simple Pixel Structure Using Video Data Correction Method for Nonuniform Electrical Characteristics of Polycrystalline Silicon Thin-Film Transistors and Differential Aging Phenomenon of Organic Light-Emitting Diodes

A simple pixel structure using a video data correction method is proposed to compensate for electrical characteristic variations of driving thin-film transistors (TFTs) and the degradation of organic light-emitting diodes (OLEDs) in active-matrix OLED (AMOLED) displays. The proposed method senses the electrical characteristic variations of TFTs and OLEDs and stores them in external memory. The nonuniform emission current of TFTs and the aging of OLEDs are corrected by modulating video data using the stored data. Experimental results show that the emission current error due to electrical characteristic variation of driving TFTs is in the range from -63.1 to 61.4% without compensation, but is decreased to the range from -1.9 to 1.9% with the proposed correction method. The luminance error due to the degradation of an OLED is less than 1.8% when the proposed correction method is used for a 50% degraded OLED.

Mechanisms of Mechanically Induced Spontaneous Arrhythmias in Acute Regional Ischemia

Although ventricular premature beats (VPBs) during acute regional ischemia have been linked to mechanical stretch of ischemic tissue, whether and how ischemia-induced mechanical dysfunction can induce VPBs and facilitate their degradation into reentrant arrhythmias has not been yet addressed. This study used a novel multiscale electromechanical model of the rabbit ventricles to investigate the origin of and the substrate for spontaneous arrhythmias arising from ischemia-induced electrophysiological and mechanical changes. Two stages of ischemia were simulated. Dynamic mechanoelectrical feedback was modeled as spatially and temporally nonuniform membrane currents through mechanosensitive channels, the conductances of which depended on local strain rate. Our results reveal that both strains and strain rates were significantly larger in the central ischemic zone than in the border zone. However, in both ischemia stages, a VPB originated from the ischemic border in the left ventricular apical endocardium because of mechanically induced suprathreshold depolarizations. It then traveled fully intramurally until emerging from the ischemic border on the anterior epicardium. Reentry was formed only in the advanced ischemia stage as the result of a widened temporal excitable gap. Mechanically induced delayed afterdepolarization-like events contributed to the formation of reentry by further decreasing the already reduced-by-hyperkalemia local excitability, causing extended conduction block lines and slowed conduction in the ischemic region. Mechanically induced membrane depolarizations in the ischemic region are the mechanism by which mechanical activity contributes to both the origin of and substrate for spontaneous arrhythmias under the conditions of acute regional ischemia.

Simplified model for evaluation of VIV-induced fatigue damage of deepwater marine risers

A simplified empirical model for fatigue analysis of deepwater marine risers due to vortex-induced vibration (VIV) in non-uniform current is presented. A simplified modal vibration equation is employed according to the characteristics of deepwater top tensioned risers. The response amplitude of each mode is determined by a balance between the energy feeding into the riser over the lock-in regions and the energy dissipated by the fluid damping over the remainder based on the data from self-excited oscillation and forced oscillation experiments of rigid cylinders. Multi-modal VIV fatigue loading is obtained by the square root of the sum of squares approach. Compared with previous works, this model can take fully account of the main intrinsic natures of VIV for low mass ratio structures on lock-in regions, added mass and nonlinear fluid damping. In addition, a closed form solution of fatigue damage is presented for the case of a riser with uniform mass and cross-section oscillating in a uniform flow. Fatigue analysis of a typical deepwater riser operating in Gulf of Mexico and West Africa shows that the current velocity profiles affect the riser’s fatigue life significantly and the most dangerous locations of the riser are also pointed out.

EMPIRIC AND DYNAMIC DETECTION OF THE SEA BOTTOM TOPOGRAPHY FROM SYNTHETIC APERTURE RADAR IMAGE

In this study, we develop empiric and dynamic methods to detect the underlying sea bottom topography from synthetic aperture radar (SAR) images. The ocean bottom features can be seen from SAR images due to the modulation of the surface waves by the nonuniform currents, which, by motion equations, should be a function of depth. We first derive the SAR image equation at a given time from Valenzuela's formula and the expression of micro-scale wave spectrum. By integrating the SAR image equation and the governing equations for ocean currents, we establish the "forward and inverse problems" for dynamic detection of topography. To utilize the SAR images, we separate the current modulation scale from the surface waves by a two-dimensional empirical mode decomposition method based on the Delaunay triangulation with the most protruding principle and the Berstein–Bezier fitting and interpolation with the most optimum principle. Examples of bottom topography detection from SAR images are presented.

Consequences of non-uniform active currents in dendrites

Thalamocortical (TC) neurons lead double lives: In a depolarized state near threshold excitatory synaptic input evokes regular spiking that faithfully “relays” information to the cortex, but when the cell is hyperpolarized, the same excitatory input can lead to a low-threshold spike (LTS), triggered by low the threshold calcium T-currents (Figure ​(Figure1).1). LTSs in TC cells occur during sleep, when the membrane potential is hyperpolarized as a consequence of reduced levels of neuromodulatory inputs and the opening of potassium currents. The LTSs are prominent features of sleep spindles, but can also appear in states of alertness. We have known that TC cell have this dual function since it was first reported by Jahnsen and Llinas (1984), but only recently has the distribution of T-currents in the TC cells been explored. Figure 1 Sleep, epilepsy and thalamic reticular inhibitory neurons. Steriade M. Cortical, reticular and thalamocortical neurons showing connectivity (+ excitation, − inhibition), intracellular recordings and anatomical reconstructions from cats. Cortical ... T-current recorded from the somas of dissociated TC cells lacking dendrites have much smaller amplitudes than in cells recorded from thalamic slices, which suggests that the T-channels are mainly dendritic (Destexhe et al., 1998). Are the T-currents distributed uniformly in the dendrites, or are they concentrated more proximally or more distally? In this issue, Zomorrodi et al. (2008) have modeled these possibilities and concluded that the lowest threshold LTS occurs when the T-current are located proximally. Most of their simulations were based on a 3 compartment model, consisting of a soma, a proximal dendritic segment and a distal dendritic segment. They also reconstructed a complete TC cell and used simulations of the more accurate morphology to show that the results were even more robust than for the 3 compartment model. Confirmation of the prediction that T-current is concentrated in the proximal dendrites of TC cells awaits high-resolution electron microscopic imaging of the Cav3.1 and Cav3.3 T-channels. In neurons of the reticular nucleus of the thalamus, which also exhibit an LTS, the T-current is concentrated in the distal dendrites (Kovacs et al., 2009), where they generate a prolonged LTS (Destexhe et al. 1996). Although the proximal localization of T-currents in the dendrites of TC cells may require the least current to trigger an LTS when current is injected in the soma, it is not clear whether this is also true for synaptic conductance changes in the dendrites. Another open issue is whether single dendrites behave like functional units, as occurs in hippocampal cells (Gasparini et al., 2004)? If so, input from a cluster of synapses on a single dendrite could be sufficient to trigger an LTS. Or is the TC cell electrically compact? In a previous compartmental model of the TC cell (Destexhe et al., 1998), reconstructed from a 200 μm slice, the dendrites were much shorter and more electrically compact than the reconstructed TC cell in the current study, which had dendrites extending 400 μm from the soma (Zomorrodi et al., 2008). The H-current, a non-specific cation current that is activated by membrane hyperpolarization, also participates in the LTS in TC cells, particularly during sleep spindles. The effects of its distribution within the TC cell could also be studied with compartmental models, in conjunction with the distribution of the T-currents. The optimal distributions of these currents may not be independent of each other, so they need to be jointly varied. The calcium that enters the neuron during an LTS must be internally bound or extruded to maintain the equilibrium of free calcium in the cell in the long term. This is accomplished by Ca2+/Na+ metabolic exchangers in the plasma membrane. Reducing the number of T-channels needed to trigger an LTS would reduce the number of calcium ions that need to be extruded later, and hence would reduce the energy that the TC cell must expend to function. In the hippocampus, nonmyelinated axons have a fast sodium current and delayed potassium current, which reduces the overlap of the currents and minimizes the cost of an action potential (Alle et al., 2009). Similarly, pyramidal neurons and fast-spiking interneurons in the cerebral cortex also minimize energy expenditure for the patterns of action potentials they generate in vivo (Hasenstaub et al. 2009). This may be a general principle for neural information processing systems (Laughlin et al., 1998; Laughlin and Sejnowski, 2003).

Flux-flow dynamics in a long Josephson junction with nonuniform bias current

The flux-flow dynamics in a long Josephson junction is investigated for a spatiallynonuniform bias current density. Starting from the linearization about a rotatingbackground, a simple approximate solution of the modified sine–Gordon equation ispresented, making possible to derive the current–voltage characteristic of the junction. Asan example of nonuniform driving, an overlap structure with an unbiased tail is discussed.In particular, it is shown how a nonuniform distribution of the bias current density mayaffect the dynamical resistance of a flux-flow oscillator. Analytical results are comparedwith direct numerical simulations and good agreement is found for a wide range of junctiondimensions.

Steady state analysis of towed marine cables

Efficient numerical schemes were presented for the steady state solutions of towed marine cables. For most of towed systems, the steady state problem can be resolved into two-point boundary-value problem, or initial value problem in some special cases where the initial values are available directly. A new technique was proposed and attempted to solve the two-point boundary-value problem rather than the conventional shooting method due to its algorithm complexity and low efficiency. First, the boundary conditions are transformed into a set of nonlinear governing equations about the initial values, then bisection method is employed to solve these nonlinear equations with the aid of 4th order Runge-Kutta method. In common sense, non-uniform (sheared) current is assumed, which varies in magnitude and direction with depth. The schemes are validated through the DE Zoysa’s example, then several numerical examples are also presented to illustrate the numerical schemes.

Electron acoustic vortices in the presence of inhomogeneous current

Linear and nonlinear dynamics of an electron acoustic wave in an inhomogeneous magnetized plasma are studied in the presence of non-uniform background current. The modified Rayleigh instability condition is found due to shear in the magnetic field and the current. A nonlinear stationary solution is also obtained in the form of tripolar vortices. The relevance of the present study to auroral and magnetotail plasmas is pointed out.

Simulating Wave Reflection Using Radiation Boundary

A finite element wave propagation model for linear periodic waves in a coastal sea region is developed. The model includes refraction, diffraction, and reflection of gravity waves on water over arbitrary bathymetry. The authors discuss the use of the computer model in simulating waves using a number of classical examples involving a circular pile, submerged shoal, and breakwater. The method of solution involves complex potential theory. The equation used by Berkhoff (1976; Mathematical Models for Simple Harmonic Linear Water Waves—Wave Diffraction and Refraction. Delft: Delft Hydraulics Laboratory, Delft University of Technology, p. 111) and the authors within the domain is elliptic in type allowing wave trains to cross, thereby producing amphidromic points. An amphidromic point is the two-dimensional version of a node in a one-dimensional standing wave caused by imperfect reflection or wave train interference. Radiating and partially reflecting boundaries are modelled by the authors, using a parabolic equation developed in different ways by Radder (1979; On the parabolic equation method for water-wave propagation, Journal of Fluid Mechanics, 95, 159–176) and Booij (1981; Gravity waves on water with non-uniform depth and current. Delft: Delft Hydraulics Laboratory, Delft University of Technology, p. 127), allowing the passage of energy through a boundary over arbitrary bathymetry. Radder (1979) and Booij (1981) develop this equation in the domain as an alternative to the elliptic equation. Berkhoff (1976) uses a downstream radiation boundary condition based on Hankel functions for the shoal problem, valid only in constant depth. The upstream boundary condition of Berkhoff (1976) for the same shoal problem is derived using the wave ray method. The limitation of the wave ray method is that for general purposes the rays frequently cross, resulting in no solution. The method used by the authors has the advantage of simplicity in that the boundary conditions are very simple to implement but none of the physical features are lost.

Structural and electrical properties of Pb(Zr,Ti)O3 films grown by molecular beam epitaxy

Single-crystal, single-phase Pb(ZrxTi1−x)O3 films (x=0–0.4) were grown on (001) SrTiO3 and SrTiO3:Nb substrates by molecular beam epitaxy. Layer-by-layer growth of the Pb(Zr,Ti)O3 films was achieved by using PbTiO3 buffer layers between the SrTiO3 substrates and the Pb(Zr,Ti)O3 films. The layers with low Zr content showed high crystallinity with full width at half maximum of ω-rocking curves as low as 4arcmin, whereas increase in Zr concentration led to pronounced angular broadening. The PbZr0.07Ti0.93O3 films exhibited remanent polarization as high as 83μC∕cm2, but local areas suffered from nonuniform leakage current.

Output current variation and polarity effect by electric field and ion-pair non-uniformity inside thimble-type ionization chamber

A new analytic approach considering both electric field and ion-pair non-uniformity has been proposed to accurately analyze the design characteristics of an ionization chamber and to interpret measurements. It is commonly assumed that ion-pairs are generated uniformly in the air volume, but such an assumption ignores various source and geometry conditions. The new approach was applied to angular dependence analysis and to polarity effect assessment in an ionization chamber. For the angular dependence analysis, whole, uniform, and non-uniform output currents were calculated as a function of the irradiation angle for an 241Am gamma-ray source. The non-uniform output current proposed in this paper was found to be closer to the measured one. This is because the non-uniform output current takes into account the ion-pair distribution in the air volume as well as the active volume determined by the electric field. For the polarity effect assessment, the amount of field distortion due to potential difference and actual current difference was calculated. Previous methods cannot appropriately estimate the variation of polarity effect because they ignore the influence of the ion-pair distribution. The polarity effect assessment using the non-uniform output current can be more useful for obtaining the practical current difference, because this assessment considers both the variation of active volume and the ion-pair non-uniformity according to source conditions such as the irradiation angle and the distance. It is important to precisely calculate not only the active volume, but also the variation in the ion-pair distribution.

On the modeling of wave propagation on non-uniform currents and depth

By transforming two different time-dependent hyperbolic mild slope equations with dissipation term for wave propagation on non-uniform currents into wave-action conservation equation and eikonal equation, respectively, shown are the different effects of dissipation term on the eikonal equation in the two different mild slope equations. The performances of intrinsic frequency and wave number are also discussed. Thus the suitable mathematical model is chosen in which the wave number vector and intrinsic frequency are expressed both more rigorously and completely. By using the perturbation method, an extended evolution equation, which is of time-dependent parabolic type, is developed from the time-dependent hyperbolic mild slope equation which exists in the suitable mathematical model, and solved by using the alternating direction implicit (ADI) method. Presented is the numerical model for wave propagation and transformation on non-uniform currents in water of slowly varying topography. From the comparisons of the numerical solutions with the theoretical solutions of two examples of wave propagation, respectively, the results show that the numerical solutions are in good agreement with the exact ones. Calculating the interactions between incident wave and current on a sloping beach [Arthur, R.S., 1950. Refraction of shallow water waves. The combined effects of currents and underwater topography. EOS Transactions, August 31, 549–552], the differences of wave number vector between refraction and combined refraction–diffraction of waves are discussed quantitatively, while the effects of different methods of calculating wave number vector on numerical results are shown.

Three reasons for freak wave generation in the non-uniform current

Three reasons for freak wave generation due to interaction of wave with spatially non-uniform current are considered in the paper. They are as follows: wave energy amplification due to wave-current interaction; wave height amplification around caustic due to refraction wave in non-uniform current; non-linear wave interaction in shallow water due to their intersection described by the Kadomtsev–Petviashvili equation. These mechanisms can generate a large wave amplification producing a dangerous natural phenomenon.

The Effect of Inter-bundle Resistive Barriers on Coupling Loss, Current Distribution and DC Performance in ITER Conductors

The role of inter-bundle resistive barriers (metal sheet wraps), introduced to reduce the inter-bundle coupling loss in multistage cabled Cable-In-Conduit Conductors (CICC) for the International Thermonuclear Experimental Reactor (ITER) is evaluated, based on results gained recently on short sample experiments in the Twente Cable Press and SULTAN. The obvious benefit of limiting the inter bundle coupling loss unavoidably goes together with impeding the redistribution of nonuniform currents in the coil winding introduced at the terminations, as well as reduction of the heat exchange between the bundles. Six-element numerical electromagnetic code simulations are presented that qualitatively explain the effect of wraps on the DC performance, strongly depending on the testing geometry. The computations illustrate that wraps can reduce the DC performance in short sample tests. At the same time simulations of the Poloidal Field Coil Insert (PFCI), with a winding length of 50 m, have shown that omitting sub-stage wraps, can even degrade the DC performance of coils due to the short current transfer length in combination with current nonuniformity causing peak voltages in the most overloaded petals

Current redistribution and stability of superconducting triplex cable without electrical insulation carrying non-uniform current

When a small normal zone is produced in a strand in a superconducting cable by a local disturbance, current redistribution occurs and can help the recovery of its superconducting state. This effect of the current redistribution depends on the initial current distribution. In this paper, the initial current distribution in a triplex cable is controlled artificially to study its influence on the stability against local disturbances. A heat pulse is applied to a strand by a carbon paste heater to initiate the quench. Transient current distributions during quench or recovery process are measured by sets of Hall sensors placed at several locations along the cable axis. When the transport current is less than a threshold value, the stability is improved by the current redistribution from the heated strand to the others, even if the initial current distribution is not uniform. This threshold is related to the current margin of the heated strand, rather than to the current margin of the whole cable. Above this threshold current, the MQE against local disturbances is smaller in the cable with the non-uniform current distribution than in the cable with the uniform current distribution, when the total transport current is identical. If the current of the heated strand is identical and above the threshold, the MQE against local disturbances does not depend on the current distribution and agrees with the MQE of the single isolated strand. It means that the quench of one strand leads to the quench of the cable, and that the stability of one strand in the cable is not influenced by interaction with the others.