Reverse Drift Research Articles

Dynamics of rip currents associated with groynes — field measurements, modelling and implications for beach safety

Rip currents can occur around groynes and other coastal structures (e.g. breakwaters and geological headlands), which provide a boundary to the wave-induced flow field and present a hazard to water users worldwide. Shore-normal groynes are common along drift-aligned coasts. In northern Europe these are often low-energy, fetch-limited environments that are dominated by locally generated wind waves. The presence of boundary-controlled rip currents due to groynes is supported anecdotally in literature, but few field measurements of their presence and dynamics exist. This study provides new quantitative scientific understanding of the dynamics of boundary-controlled rip currents within this setting. A 10-day field experiment at Boscombe beach on the south coast of England measured rip currents and nearshore hydrodynamics around an impermeable groyne field. Observations from both fixed instruments (wave, tide and current metres) and GPS-drifters confirmed the presence and characteristics of hazardous topographic rip currents around a groyne structure during a number of oblique wave events and drift reversals. The strongest offshore-directed rip velocities of up to 1ms−1 (10-min average) and 2ms−1 (instantaneous) were measured on the updrift side of the groyne where the longshore current generated within the embayment was deflected offshore. These strong rip flows were measured under relatively small wave heights (Hs<1m) and presented a significant bathing hazard. A calibrated and validated numerical model (XBeach) was used, in support of measured data, to explore the key environmental controls on rip behaviour across a range of groyne configurations and wave conditions not observed in the field. Key outcomes were that: 1) upstream deflected rip flows were found to be strongly correlated to the alongshore thrust due to wave forcing; 2) the groyne length relative to surf zone width strongly controlled the offshore extent of rip flows, with a significant increase in surf zone exits above a relative groyne length (length/surf width) of 1.25; and 3) rip flows increase up to a relative groyne spacing (spacing/length) of around 4–6, above which alongshore currents are fully developed. These findings have been distilled into some key principals that are relevant to the assessment of boundary-controlled rip hazard on beaches.

Marine megavertebrates adrift: A framework for the interpretation of stranding data in perspective of the European Marine Strategy Framework Directive and other regional agreements

Abstract For many species of marine megafauna, strandings remain the most important source of biological samples. Because of their opportunistic nature however, strandings data have long been under- or misused in the assessment of population conservation status. Even if many national and international regulations promote the use of strandings in monitoring strategies, the interpretation of strandings remains controversial. The aim of this study is to provide a context for the interpretation of marine megafauna stranding data, in order to assess the achievement of specific objectives against Good Environmental Status criteria in the context of the EU Marine Strategy Framework Directive or other regional agreements. The first step is to construct an a priori spatial distribution under a null hypothesis H0. The a priori spatial distribution of theoretical dead animals can either be set uniformly, consistent with current knowledge on abundance of marine vertebrates, or based on management objectives. The drift prediction of these theoretical carcasses would provide a time series of strandings expected under the null hypothesis. The reverse drift of observed strandings would highlight mortality areas of stranded animals. The correction of these areas by the probability of getting stranded according to drift conditions would provide an estimated distribution of dead animals inferred from strandings. The differences between expected and observed situations constitute anomalies and highlight cases where inferred distribution departs from the a priori spatial distribution. This work proposes several population indicators that can be used anywhere in the world and can be applied for all large marine vertebrates found stranded. The integration of these indicators in MSFD and various regional agreements could provide cost-effective and relevant information on protected species. In the context of impaired ecological situations, the complementary use of several population indicators could strengthen the diagnosis made regarding conservation status and hence conservation strategies.

Seismic Strengthening of Bond-Critical Regions in Wall-Type Bridge Piers Using Active Confinement

AbstractActive confinement by means of pretensioned steel anchor rods is used for bond strengthening of lap-spliced reinforcement and for improving the seismic performance of wall-type bridge piers. Representative as-built and strengthened pier specimens with lap-spliced reinforcement within the critical hinging region were tested under large drift reversals. The test parameters included ratio of pier longitudinal reinforcement, diameter of the spliced bars, and configuration of the pretensioned steel rods. The strengthened piers developed enhanced bond resistance and low bond degradation of the spliced bars, increases in the lateral load and drift capacities, and much less pinching in the lateral load–drift response when compared with the as-built specimens. In this study, a design approach is developed for engineers to evaluate the active lateral pressure required for adequate bond strengthening and for designing the strengthening system. A design example is provided to illustrate the use of the propose...

Repair and FRP Strengthening of Earthquake-Damaged RC Shallow Beam-Column Joints

Three earthquake-resistant wide beam-column joint specimens (one interior, and two exterior) which developed beam plastic hinging mechanism and beam flexural failure under large drift reversals were repaired and FRP strengthened for restoring their structural function. The repair and strengthening technique consisted of: (i) sealing the major flexural cracks that developed within the beam plastic hinge region using epoxy injection; (ii) adding carbon fiber-reinforced polymer (CFRP) reinforcement at the top and bottom faces of the beam to control the growth of the flexural cracks, boost up the beam negative and positive flexural capacities and at the same time compensate in part for the loss in the joint lateral stiffness; and (iii) providing CFRP jacket at the potential hinging regions of the columns for confining the concrete and enhancing the bond capacity of the spliced reinforcement. The effectiveness of the upgrading technique is demonstrated by comparing the performance of the upgraded joints when subjected to lateral cyclic loading against the performance of the same joints before damage. With its simplicity in application, the upgrading technique succeeded in bringing the damaged joints back into service conditions and in restoring their flexural stiffness under lateral load which degraded due to large drift reversals.

Aging is a simple deprivation syndrome driven by a quasi-programmed preventable and reversible drift of control system set points due to inappropriate organism-environment interaction.

There are two well-known but opposing concepts of the reason for aging. The first supposes that senescence is programmed similarly to the genetic program of development from a zygote up to a mature organism. Genetically determined senile wasting is thought to be associated with the necessity to renovate the population to ensure its adaptation and survival. According to the concept of the stochastic aging (due to accumulation of occasional error and damage), there is no built-in program of aging. There is only a program of development up to the state of maturity, and then the organism should be able to maintain itself limitlessly. However, although the efficiency of repair systems is assumed to be rather high, it is less than 100%. Just this has to result in aging because of accumulation of various errors. We have continued and developed another approach that considers both programmed and stochastic concepts to be incorrect. Aging is a simple deprivation syndrome driven by preventable and even reversible drifts of control systems set points because of an inappropriate "organism-environment" interaction.

A strategy for producing refined currents in the Equatorial Atlantic in the context of the search of the AF447 wreckage

On the night of 1st June 2009, a Rio-Paris Air France flight (AF447) disappeared in a highly variable and poorly observed part of the western tropical Atlantic Ocean. The first debris was located 5 days after the accident. Several reverse drift computations were conducted in order to define the likely position of the wreckage. Unfortunately, the performance of the operational ocean analyses available in the region of interest ranges from 80 to 100 km of positioning error after 5 days of inverse drift computation. In preparation of the third phase of research of the wreckage at sea, a series of numerical experiments was performed at Meteo-France and Mercator Ocean in an attempt to better compute the surface currents in the region and for the period of the accident of the AF447 (May and June 2009). Tailored high-resolution atmosphere and ocean reanalyses were first produced respectively at Meteo-France and Mercator Ocean. Several nested experiments were then performed with a small and flexible ocean model limited to the region of interest. The date of the initial conditions and the type of atmospheric forcing fields were varied in order to produce a small ensemble from which information on the sensitivity to these changes could be derived. Probabilistic and statistical combinations between model and observations were tested and a solution was finally selected by means of a comparison of drift computations with independent surface drift observations.

ON PLASMA ROTATION AND DRIFTING SUBPULSES IN PULSARS: USING ALIGNED PULSAR B0826–34 AS A VOLTMETER

We derive the exact drift velocity of plasma in the pulsar polar cap, in contrast to the order-of-magnitude expressions presented by Ruderman & Sutherland (1975) and generally used throughout the literature. We emphasize that the drift velocity depends not on the absolute value, as is generally used, but on the variation of the accelerating potential across the polar cap. If we assume that drifting subpulses in pulsars are indeed due to this plasma drift, several observed subpulse-drift phenomena that are incompatible with the Ruderman & Sutherland family of models can now be explained: we show that variations of drift rate, outright drift reversals, and the connection between drift rates and mode changes have natural explanations within the frame of the "standard" pulsar model, when derived exactly. We apply this model for drifting subpulses to the case of PSR B0826-34, an aligned pulsar with two separate subpulse-drift regions emitted at two different colatitudes. Careful measurement of the changing and reversing drift rate in each band independently sets limits on the variation of the accelerating potential drop. The derived variation is small, ~10^{-3} times the vacuum potential drop voltage. We discuss the implications of this result for pulsar modeling.

The significance of stranding data as indicators of cetacean populations at sea: Modelling the drift of cetacean carcasses

Stranded marine mammals are an important source of information and biological samples on cetacean population. Nevertheless, collecting stranding data remains opportunistic and its representativity must be improved, both qualitatively and quantitatively. Drifts of small cetaceans found by-caught in fishery observation projects and subsequently released dead with a numbered tag fitted to the tail fluke were predicted by using the Météo-France drift model MOTHY and allowed us to assess the proportion of dead dolphins recovered by volunteers of the French stranding network. Only 8% of dolphins were recovered ashore. The spatial representativity of strandings was assessed by performing back-calculation of carcass drift with the same model in order to map the likely origin of stranded cetaceans. As a first step, external visual criteria of time-after-death (as a proxy to drift duration) were obtained from series of photographs of dead small cetaceans maintained in a floating cage for 40 days and from tagged by-caught dolphins recovered stranded after a drift in real condition. Then, pictures of 242 stranded common dolphins (Delphinus delphis) were used to establish the average distribution of dolphin time-after-death in this area. Finally, 40 days-long reverse drifts of the 829 common dolphins recorded in the winter months of 2004–2009 were weighted by the modelled distribution of time-after-death in order to map the areas of likely origin. It appeared that most stranded common dolphins recorded along the French Atlantic coast originated from the continental shelf, mostly in the south of the Bay of Biscay. These results open new perspective on the use of stranding data and biological samples as sources of indicators in monitoring strategies.

High temperature long term stability of SiC Schottky diodes

Reliability of Silicon Carbide (SiC) power devices is still an open problem, preventing a wider application of such a promising technology. Moreover, specific reliability assessment procedures must be developed for SiC devices, as they are designed to work at temperatures well beyond those of standard Silicon devices. A detailed investigation about the reliability of 600V, 6A Silicon Carbide Schottky diodes is accomplished along this paper. It is based on an extensive set of high temperature reverse bias endurance tests, performed on devices featuring different packages. Only small forward voltage drop and reverse current drifts have been recorded after a 1000h long test, confirming the parametric stability and the reliability level reached by last generation SiC Schottky diodes. Moreover, devices assembled in TO220 package without flame retardant components in the molding compound performed better than devices assembled in other TO220 packages, or assembled in hermetic TO3 package, pointing out the role played by the interface between the green molding compound and the top passivation layer in the long term parametric stability.

Very Low Reset Current for an RRAM Device Achieved in the Oxygen-Vacancy-Controlled Regime

We propose the bipolar-mode operation of resistive random access memory devices in a purely oxygen-vacancy (V0)-controlled regime, which is achieved by very low compliance capping for forming/set transitions. This regime enables us to achieve a very low reset current of 10-100 nA, in which the governing mechanism for switching only involves the reversible drift of oxygen ions to and from oxygen soluble gate electrodes. The physical analysis of a gate stack in this V0 regime confirms the absence of metallic nanofilaments. These findings pave the way for the realization of ultralow switching power RRAM devices.

Evidence for compliance controlled oxygen vacancy and metal filament based resistive switching mechanisms in RRAM

We present electrical evidence on asymmetric metal–insulator–semiconductor (MIS) based test structures in support of the presence of two different independent switching mechanisms in a resistive random access memory (RRAM) device. The valid mechanism for switching depends on the compliance capping ( I gl ) for forming/SET transition. Our results convincingly show that low compliance based switching only involves reversible oxygen ion drift to and from oxygen gettering gate electrodes, while high compliance switching involves formation and rupture of conductive metallic nanofilaments, as verified further by our physical analysis investigations. We have observed this unique dual mode switching mechanism only in NiSi-based gate electrodes, which have a moderate oxygen solubility as well as relatively low melting point.

Short‐Lived Absorptive Type III–like Microwave Bursts as a Signature of Fragmented Electron Injections

In this paper, we devote ourselves to interpreting the short-lived absorptive type III-like microwave bursts in the 2006 December 13 flare event observed with high temporal and spectral resolutions (8 ms and 10 MHz) by the Chinese Solar Broadband Radio Spectrometer (SBRS/Huairou) at 2.6-3.8 GHz. In the decimeter-centimeter wavelength range, we first present the observations of short-lived bursts represented as a number of absorptive "spikes" superposed on the type IV continuum that can be connected by fast-drifting lines. The mean drift rate, the instantaneous bandwidth, and the absorption depth of these absorptive spikes are about –12 GHz s−1, 70 MHz, and 40%, respectively. The duration at a single frequency band can be less than the instrument resolution of 8 ms. On the basis of numerical investigations of the loss-cone instability, we suggest that fragmented electron injections with durations of as short as several milliseconds into the loss cone could be the most appropriate mechanism with which to explain the bursts. The length of an electron beam is estimated to be about 400 km, on the basis of the observational results. These injections may be related to the fragmented energy release processes during the flare. We also observe some absorptive type III-like bursts accompanying ordinary type III bursts with reverse drifts. They start at the same frequency, and the starting frequency slowly drifts to the low-frequency region. This could be a signature of propagating bidirectional electron beams originating near the reconnection region.

Features of the capacitance-voltage characteristics in a MOS structure due to the oxide charge

MOS structures based on p-and n-type silicon and subjected to the effect of voltages of both polarities with a magnitude as large as 70 V are studied. In all cases, an increase in the effective positive charge at the Si/SiO2 boundary was observed. In the case of structures with p-Si, a steplike increase in the high-frequency capacitance in the inversion region at some value of the threshold voltage was observed. The increase in the capacitance and the threshold voltage were governed by the value of the effective charge and the area of the gate of the structure. The observed effect is accounted for by lateral diffusion of free electrons accumulated in the semiconductor in the vicinity of the gate contact. After completion of the effect of the voltage on the structures, the capacitance-voltage characteristics were recovered to the state close to the initial state in the relaxation time characteristic of reverse drift of ions and emission of charge carriers by tunneling from “slow” traps in the vicinity of the Si/SiO2 interface.

Advances in Multi- and Single-Wafer SiC Epitaxy for the Production and Development of Power Diodes

In this paper, we present results of epitaxial layer deposition for production needs using our hot-wall CVD multi-wafer system VP2000HW from Epigress with a capability of processing 7×3” or 6×100mm wafers per run in a new 100mm setup. Intra-wafer and wafer-to-wafer homogeneities of doping and thickness for full-loaded 6×100mm and 7×3” runs will be shown. Results on Schottky Barrier Diodes (SBD) processed in the multi-wafer system will be given. Furthermore, we show results for n- and p-type SiC homoepitaxial growth on 3”, 4° off-oriented substrates using a single-wafer hot-wall reactor VP508GFR from Epigress for the development of PiN-diodes with blocking voltages above 6.5 kV. Characteristics of n- and p-type epilayers and doping memory effects are discussed. 6.5 kV PiN-diodes were fabricated and electrically characterized. Results on reverse blocking behaviour, forward characteristics and drift stability will be presented.

Mobile Ionic Impurities in Poly(vinyl alcohol) Gate Dielectric: Possible Source of the Hysteresis in Organic Field‐Effect Transistors

Threshold voltage shifts in organic field effect transistors (OFETs) have been reported frequently. Cyclic sweeps of the gate voltage in OFETs reveal a hysteresis in the transfer characteristics (drain-source current versus gate-source voltage) thereby unfolding an electrical instability of the transistor element. On the one hand bistable transistors with a nonvolatile hysteresis in the transfer characteristics may be used in organic memory elements, whereas on the other hand hysteresis free transistors are desired in integrated organic circuits. Therefore understanding the causes of these electrical bior instabilities in organic field-effect devices is of primary interest. OFETs using charged electrets or ferroelectric-like gate dielectrics (e.g., see [7,8,12,14]) show a hysteresis which is normally attributed to the intrinsic properties of these materials. In polymer dielectrics without an intrinsic hysteresis such as poly(vinyl alcohol) (PVA), gate voltage induced hysteresis are often interpreted in terms of electrostatic screening of trapped charge carriers released under the influence of the electric field between the gate and the source/drain electrodes. Experimental results so far revealed a fairly complex picture of this “bias-stress” effect where the detailed mechanisms involved are not yet fully understood. Several publications report evidences of a charge trapping process in the organic semiconductor close to the semiconductor/dielectric interface. Lindner et al. supposed that the origin of the hysteresis in organic devices is a combination of slow transport (polarons or mobile ions) with a reaction other than trap recharging, e.g., a direct polaron-bipolaron reaction or a complex formation reaction of polarons / bipolarons with counterions. Bias-stress experiments with pentacene on various inorganic dielectrics indicate a reversible structural change of the semiconductor. A substantial number of experiments demonstrate an influence of the gate dielectric material on the formation of hysteresis effects. Transistors with a polymer gate dielectric are more likely to show a pronounced hysteresis than transistors with an inorganic gate dielectric. It has been reported that hydroxyl groups in the form of silanols at the SiO2-dielectric/semiconductor interface can work as electron traps which can be eliminated by a thin alkane interlayer. In many organic dielectrics hydroxyl groups are present in large numbers, either as integral part of the chemical structure and/or as impurities remaining from the process of synthesis. Such hydroxyl groups therefore were suspected of being responsible for the hysteresis effect in many OFET configurations. Lee et al. conclude that the increase of hydroxyl groups in polymer dielectrics equally increases electron trapping sites which in turn cause a large hysteresis in OFETs, but as they also discussed, the drain current as well as the gate leakage current increased with increasing hydroxyl density, which may be in contradiction with the proposed idea of immobilized carriers. Polymer dielectrics are often hosts for mobile ions. It has been demonstrated for example that mobile Na ions may diffuse from an underlying substrate into organic semiconductors, like pentacene or poly(3-hexylthiophene) under the influence of an applied voltage. Thereby they cause an increase of the current through the semiconductor and, additionally, a current-voltage hysteresis. The finding that mobile ions in semiconductors alter their electrical performance is not new. p-n junction devices, called flexodes, with a variable current-voltage (I–V) characteristics resulting from a reversible drift of Li ions were suggested in 1963. Two years later, small traces of mobile alkali ions in SiO2 gate dielectrics were reported to cause significant problems in semiconductor devices. In fact, the practical application of MOSFETs was delayed in the early 1960s because of severe gate bias instability problems caused by mobile ionic oxide charges like Na, C O M M U N IC A TI O N

Suitability of 4H-SiC Homoepitaxy for the Production and Development of Power Devices

ABSTRACTIn this paper, we present results of epitaxial layer deposition for production needs using our hot-wall CVD multi-wafer system VP2000HW from Epigress with a capability of processing 6×100mm wafers per run. Intra-wafer and wafer-to-wafer homogeneities of doping and thickness for full-loaded 6×100mm runs will be shown and compared to results of the former 7×3″ setup. The characteristic of the run-to-run reproducibility for the 6×100mm setup will be discussed. To demonstrate the suitability of the reactor for device production results on Schottky Barrier Diodes (SBD) processed in the multi-wafer system will be given. Furthermore, we show results for n- and p-type SiC homoepitaxial growth on 3″, 4° off-oriented substrates using a single-wafer hot-wall reactor VP508GFR from Epigress for the development of PiN-diodes with blocking voltages above 6.5 kV. Characteristics of n- and p-type epilayers and doping memory effects are discussed. 6.5 kV PiN-diodes were fabricated and electrically characterized. Results on reverse blocking behaviour, forward characteristics and drift stability will be presented.

Signatures of magnetic reconnection in solar radio observations?

Magnetic reconnection occurs during eruptive processes (flares, CMEs) in the solar corona. This leads to a change of magnetic connectivity. Nonthermal electrons propagate along the coronal magnetic field thereby exciting dm- and m-wave radio burst emission after acceleration during reconnection or other energy release processes in heights of some Mm to ⩾700 Mm. We summarize the results of some case studies which can be interpreted as radio evidence of magnetic reconnection: under certain conditions, simple spectral structures (pulsation pulses, reverse drift bursts) are formed by simultaneously acting but widely spaced radio sources. Narrowband spikes are emitted as a side-effect during large-scale coronal loop collisions. In dynamic radio spectra, the lower fast mode shock formed in the reconnection outflow appears as type II burst-like but nondrifting emission lane. It has been several times observed at the harmonic mode of the local plasma frequency between 250 and 500 MHz and at heights of ≈200 Mm.

Spatial Structure and Spectra of X-Ray Sources During the 0.8 – 4.5 GHz Reverse Drift Bursts Observations

In the years 2002 – 2005, 38 groups of the reverse drift bursts (RDBs) were observed in the 0.8 – 4.5 GHz frequency range by the Ondřejov radiospectrograph. In 21 cases, which were observed at the times of the RHESSI observations, spatial structure, positional changes, and spectra of X-ray sources during RDB observations are studied in detail. First, based on the frequency drift and the spatial structure of the associated X-ray source, the events are classified as: (a) fast drifting RDBs with a compact X-ray source, (b) fast drifting RDBs with a multiple X-ray source (FM), and slowly drifting RDBs. Then, the spectra of X-ray sources at the times of RDBs are analyzed. It is found that most fast drifting RDBs (16 of 17 cases) are associated with the spectra having a distinct power-law (non-thermal) component. In contrast, the X-ray spectra associated with the slowly drifting RDBs are predominantly purely thermal (in three out of four cases; in the 26 July 2004, case the X-ray spectrum is thermal and high temperature, with non-thermal component). Two special cases of RDBs observed during the 28 October 2003, and 23 July 2004, flares are added for comparison. The most frequent events are those with fast drifting RDBs, a compact short-lasting X-ray sources, and a power-law X-ray spectrum. The individual reverse drift bursts (∼1 s duration) do not show a clear temporal association with individual peaks of hard X-ray bursts. During slowly drifting RDBs the shape of the associated X-ray source changed or expanded. Among them the most interesting one was observed in 26 July 2004, when the very slowly drifting RDBs (+40 MHz s−1) were associated with an X-ray loop-like source continuously elongating in the southwest direction. In the most cases the model of RDBs with electron beams is compatible with the observations, but in flares on 26 July 2004, and 28 October 2003, the RDBs are probably generated by some other type of an agent; we propose here a thermal conduction front.

Use of the Extended Kalman Filter for State Dependent Drift Estimation in Weakly Nonlinear Sensors

A number of mechanisms are responsible for the generation of reversible or irreversible drift in the response of a sensor. In this letter, we discuss three approaches for the identification of reversible state dependent drift in sensors through the use of the Extended Kalman Filter. We compare their performance by simulation and demonstrate their validity by estimating the drift of an accelerometer, modeled as a weakly nonlinear system.

Controlled multiple reversals of a ratchet effect

A single particle confined in an asymmetric potential demonstrates an anticipated ratchet effect by drifting along the 'easy' ratchet direction when subjected to non-equilibrium fluctuations. This well-known effect can, however, be dramatically changed if the potential captures several interacting particles. Here we demonstrate that the inter-particle interactions in a chain of repelling particles captured by a ratchet potential can, in a controllable way, lead to multiple drift reversals, with the drift sign alternating from positive to negative as the number of particles per ratchet period changes from odd to even. To demonstrate experimentally the validity of this very general prediction, we performed transport measurements on a.c.-driven vortices trapped in a superconductor by an array of nanometre-scale asymmetric traps. We found that the direction of the vortex drift does undergo multiple reversals as the vortex density is increased, in excellent agreement with the model predictions. This drastic change in the drift behaviour between single- and multi-particle systems can shed some light on the different behaviour of ratchets and biomembranes in two drift regimes: diluted (single particles) and concentrated (interacting particles).