Steepness Of Wave Front Research Articles

Restrike characteristics of vacuum circuit breaker in its opening process under DC voltage

AbstractThe restrike of vacuum circuit breaker during the breaking process causes overvoltage and intensifies the insulation deterioration of high‐voltage equipment. To solve this problem, the vacuum breaker aging test method and the restrike characteristics are researched. A withstand voltage test platform of 40.5‐kV vacuum circuit breaker was established to obtain the effects of the contact spacing and loop and inrush currents of vacuum circuit breaker on the number of restrike, the wavefront steepness of the breakdown voltage, and the breakdown duration. The number of restrike during the breaking of vacuum circuit breaker decreased and the duration of the repeated‐breakdown arc increased with the increased contact spacing of vacuum circuit breaker. As the loop current of the test system increased, the number of restrike during the breaking of vacuum circuit breaker, the wavefront steepness of the breakdown voltage, and the duration of the restrike arc increased. As the inrush current amplitude increased during the closing of vacuum circuit breaker, the number of restrike and the duration of the repeated‐breakdown arc during the breaking of vacuum circuit breaker increased. The results provide a theoretical basis and data support for the preventive test of 40.5‐kV vacuum circuit breaker and the optimization of the withstand voltage test.

Wave Front Steepness and Influence on Horizontal Deck Impact Loads

In design storm sea states, wave-in-deck forces need to be analysed for fixed and floating offshore platforms. Due to the complex physics of wave impact phenomena, numerical analyses should be complemented by model test data. With a large statistical variability, such experiments usually involve running many 3-h storm realisations. Efforts are being done to establish efficient procedures and still obtain improved statistical accuracy, by means of an initial simplified screening based on parameters derived from the incident wave record only. Here, we investigate the vertical rise velocity of the incident wave elevation at a fixed point in space, which indirectly measures both the local slope and the near-surface orbital velocity. A derived simple deck slamming model is also suggested, to support the check of the physical basis of the approach. Correlation against data from a GBS wave-in-deck model test is used for checking this model. The results show that, although there is a significant random scatter in the measured impact forces, especially in the local slamming forces but also in the global forces, there is a correlation to the rise velocity. Comparisons to the simple load model also show promising results when seen on background of the complex physics and random scatter of the impact problem.

Nonlinear deformation and run-up of single tsunami waves of positive polarity: numerical simulations and analytical predictions

Abstract. The estimate of an individual wave run-up is especially important for tsunami warning and risk assessment, as it allows for evaluating the inundation area. Here, as a model of tsunamis, we use the long single wave of positive polarity. The period of such a wave is rather long, which makes it different from the famous Korteweg–de Vries soliton. This wave nonlinearly deforms during its propagation in the ocean, which results in a steep wave front formation. Situations in which waves approach the coast with a steep front are often observed during large tsunamis, e.g. the 2004 Indian Ocean and 2011 Tohoku tsunamis. Here we study the nonlinear deformation and run-up of long single waves of positive polarity in the conjoined water basin, which consists of the constant depth section and a plane beach. The work is performed numerically and analytically in the framework of the nonlinear shallow-water theory. Analytically, wave propagation along the constant depth section and its run up on a beach are considered independently without taking into account wave interaction with the toe of the bottom slope. The propagation along the bottom of constant depth is described by the Riemann wave, while the wave run-up on a plane beach is calculated using rigorous analytical solutions of the nonlinear shallow-water theory following the Carrier–Greenspan approach. Numerically, we use the finite-volume method with the second-order UNO2 reconstruction in space and the third-order Runge–Kutta scheme with locally adaptive time steps. During wave propagation along the constant depth section, the wave becomes asymmetric with a steep wave front. It is shown that the maximum run-up height depends on the front steepness of the incoming wave approaching the toe of the bottom slope. The corresponding formula for maximum run-up height, which takes into account the wave front steepness, is proposed.

Wave Profiles of a Virtual Extreme Tsunami in the Northern Region of South China Sea

Tsunami wave profile is one of the important inputs for evaluating impacts of the tsunami on coastal and offshore structures. To understand the tsunami wave patterns along the northern coastal regions of South China Sea, we simulate the tsunami propagation in the South China Sea generated by a hypothetical earthquake of magnitude [Formula: see text] in Manila trench. The Boussinesq equations are adopted to account for the nonlinear and dispersive effects on the propagation and deformation of tsunami wave propagating over the continental shelf. Focusing on several locations in the northern region of South China Sea, the nested grids with gradually higher spatial resolution are used to ensure computational accuracy. Numerical results reveal that undular bores and several soliton-like waves may develop in shallow water in regions where water depth is less than 100[Formula: see text]m. The shallow continental shelf length and the steepness of wavefront are the two dominant variables that determine the development of undulation.

Freak wave forces on a vertical cylinder

This study investigates the wave forces acting on a vertical truncated cylinder induced by freak waves. A series of freak wave trains were generated based on the New Year wave prototype in a wave flume. The inline forces and pitch moments on the fixed cylinder were measured and Morison predictions on them were also performed with different wave kinematic models. Compared with the Morison results, larger force peaks and shallower force troughs are observed in the measurements. Besides, the wave front steepness is found to have a closer relationship with the non-dimensional inline force than the conventional wave steepness. Wavelet transform-based analyses were also successfully performed to reveal the local characteristics of the incident waves, inline forces and transfer functions between them.

Travelling wave analysis of a mathematical model of glioblastoma growth

In this paper we analyse a previously proposed cell-based model of glioblastoma (brain tumour) growth, which is based on the assumption that the cancer cells switch phenotypes between a proliferative and motile state (Gerlee and Nelander, 2012). The dynamics of this model can be described by a system of partial differential equations, which exhibits travelling wave solutions whose wave speed depends crucially on the rates of phenotypic switching. We show that under certain conditions on the model parameters, a closed form expression of the wave speed can be obtained, and using singular perturbation methods we also derive an approximate expression of the wave front shape. These new analytical results agree with simulations of the cell-based model, and importantly show that the inverse relationship between wave front steepness and speed observed for the Fisher equation no longer holds when phenotypic switching is considered.

Dynamic microplasticity manifestation in consolidated sandstone in the acoustical frequency range

ABSTRACTMicroplasticity manifestations caused by acoustic wave in the frequency range of about 4.5 kHz–7.0 kHz are detected in consolidated artificial sandstone. Equipment was tested by means of comparison of data obtained for a standard material (aluminium) and sandstone. Microplasticity manifestations in acoustic records are present in the form of ladder‐like changes in the amplitude course. The stress plateaus in the acoustic trace interrupt the amplitude course, transform the wavefront, and shift the arrival time along the time axis. Microplasticity contribution to the acoustic record changes with the increase in the strain amplitude value. The combined elastic–microplastic process conditions the wavefront steepness and its duration. Stress plateaus exert influence on the waveform and, accordingly, on pulse frequency response. These results confirm the earlier data obtained for weakly consolidated rock. This contribution to wave propagation physics can be useful in solving applied problems in material science, seismic prospecting, diagnostics, etc.

Modeling of lightning impulse behavior of long air gaps and insulators including predischarge current: Implications on insulation coordination of overhead transmission lines and substations

Evaluation of the dielectric strength of transmission line insulation subjected to fast-front overvoltages is of major importance for the insulation coordination of overhead lines and connected substations. Among models proposed in literature for the prediction of the dielectric behavior of long air gaps and insulators under lightning overvoltages, leader development models (also called leader progression models) have a greater physical significance. These models consider the predischarge current flowing in the gap during the leader propagation phase preceding breakdown. However, this current is often disregarded for simplicity in fast-front overvoltage simulations. In this study the effects of simulating predischarge current on voltage–time characteristics of long air gaps and insulators, critical currents of overhead transmission lines as well as fast-front overvoltages arising at substations are investigated with the aid of ATP–EMTP. Including predischarge current in simulations, affecting the flashover characteristics of air gaps and insulators, results in slightly higher estimates of the minimum backflashover current of overhead transmission lines. However, it does not affect the estimated minimum shielding failure flashover current of overhead lines. In addition, simulating predischarge current may affect the overvoltages arising at substations due to shielding failure of the connected overhead lines depending on withstand or flashover of line insulation. In the case of backflashover the wavefront steepness and amplitude of the overvoltages are lower. Thus, predischarge current effects should be considered in insulation coordination of overhead transmission lines and substations.

Run-up amplification of transient long waves

The extreme characteristics of long wave run-up are studied in this paper. First we give a brief overview of the existing theory which is mainly based on the hodograph transformation (Carrier & Greenspan, 1958). Then, using numerical simulations, we build on the work of Stefanakis et al. (2011) for an infinite sloping beach and we find that resonant run-up amplification of monochromatic waves is robust to spectral perturbations of the incoming wave and resonant regimes do exist for certain values of the frequency. In the setting of a finite beach attached to a constant depth region, resonance can only be observed when the incoming wavelength is larger than the distance from the undisturbed shoreline to the seaward boundary. Wavefront steepness is also found to play a role in wave run-up, with steeper waves reaching higher run-up values.

WAVE SLAMMING IMPACT ON OFFSHORE WIND TURBINE FOUNDATIONS

This study proposes a new parameter, the wave front steepness parameter, to better describe the wave slamming coefficient. Four cases of wave breaking were employed to study the relationship between wave front steepness and wave slamming coefficients. Based on model results with OpenFOAM, an empirical formula was derived to relate the slamming coefficient to the wave front steepness.

Attenuation and deformation characteristics of lightning impulse corona traveling along bundled transmission lines

Comprehensive study of the impulse corona characteristics is helpful to understand the transient process and guarantee a suitable insulation design of power transmission systems. In this paper, first the attenuation and deformation characteristics of lightning overvoltage traveling along a 500kV experimental transmission line are tested. Then the lightning impulse corona characteristics on different bundled conductors commonly used in the 110–1000kV power transmission lines are experimentally investigated, the impulse corona Q–V data measured in the corona cage is transformed into those on the corresponding overhead lines. The influence of bundle geometry on corona is analyzed. Attenuation and deformation effects of lightning impulse corona along transmission lines are evaluated by comparing the simulation and test results. Simulation shows the amplitude and wave-front steepness attenuate remarkably, and the deformation of the voltage mainly happens when the impulse voltage exceeds the corona inception one.

Elastic–microplastic nature of wave propagation in the weakly consolidated rock

Abstract Microplasticity effects caused by seismic wave from two sources of opposite polarity on frequencies about 300 Hz and 1000 Hz are detected in the borehole-to-borehole measurements in loam. For both sources, microplasticity manifestations on seismic records are presented in the form of the ladder-like changes in amplitude course. The stress plateaus on seismic trace interrupt the amplitude course, transform wavefront, and shift the arrival time and amplitude maximum along the time axis. Transformation character depends on the initial deformation type (compression or tension) that generates the source with positive or negative polarity. For the opposite-polarity sources, the arrival time and wave form appreciably differ from each other. Increase in strain amplitude leads to wavefront microplasticity increase and the significant arrival time difference on records with the positive and negative polarity. The combined elastic–microplastic process forms the wavefront steepness and its duration. This contribution to wave propagation physics can be useful in solving the applied problems in material science, seismic prospecting, diagnostics etc.

Evolution of Kelvin‐Helmholtz activity on the dusk flank magnetopause

Our purpose is to characterize the evolution of the magnetopause Kelvin‐Helmholtz (KH) wave activity with changes in thickness of the adjacent boundary layer, geomagnetic latitude and interplanetary magnetic field (IMF) orientation. As the IMF turns northward, wave activity may be generated at the dayside before propagating down the tail, where the boundary layer is expected to support longer wavelengths. We use two‐point observations on the dusk magnetopause at low latitudes, from Geotail on the dayside and Cluster tailward of the dusk terminator. We quantify the wavelength, power, wavefront steepness and propagation direction at Cluster. An estimate of the thickness of the low‐latitude boundary layer (LLBL) is obtained by correlating normal distances to the magnetopause, derived from two empirical solar‐wind‐driven models, with a systematic relationship (the “transition parameter”) found between the electron number density and temperature; the correlation factor is used to infer the temporal evolution of the thickness of the locally sampled layer. We find that wavelengths are controlled by the IMF clock angle, as expected when generated by the KH mechanism at the dayside, although amplitudes, wavefront steepness and propagation directions are more closely correlated with the layer thickness. A survey of parameter space provides evidence of the contribution of the KH mechanism to the widening of the electron LLBL.

Steepness and spectrum of nonlinear deformed shallow water wave

The process of the nonlinear deformation of a shallow water wave in a basin of a constant depth is studied. Characteristics of the first breaking of the wave are analyzed in detail. The Fourier spectrum and steepness of the nonlinear wave are calculated. It is shown that the spectral amplitudes can be expressed using the wave front steepness, which allows the practical estimations.

Formation of shock waves in inhomogeneous active fibres

The formation dynamics of the shock wave of a pulse envelope is considered in fibres with the gain, dispersion, and nonlinearity distributed over the fibre length. It is shown that the wave-front steepness for inhomogeneous optical fibres can be strongly increased even when the local group-velocity dispersion substantially differs from zero if its average value (over the length of the shock-wave formation) is close to zero.

Influence of wave front steepness and accumulated charge on creeping discharge

AbstractThe creeping flashover voltage–time characteristics for the composite insulating configuration by insertion of a barrier between a needle electrode and a plane electrode in N2 gas have been investigated. A maximum creeping flashover voltage of around 10 µs has been found in N2 gas for negative polarity. This anomalous voltage–time characteristic is associated with the low field relaxation effect of the accumulated charges on the barrier for an applied pulse voltage with a highly steep wave front. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(1): 1–8, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10249

Self-similarity of near-breaking short gravity wind waves

We show that the asymmetric profiles of the near-breaking wind waves observed in a large wind-wave tank do not depend on the wave scale. The experiments were performed for wind-generated waves of wavelength increasing with fetch. The wave crests just prior to breaking exhibit steep slopes exceeding 0.6 at the front and much smaller but remarkably constant slope at the rear. The statistics of wave geometric properties, such as the along-wind crest-slope distribution or the wavefront steepness, is found to be scale invariant. This implies that the shape of the near-breaking wind waves is self-similar. Similar experiments with paddle-generated waves amplified by wind reveal the same scale-independent features. These results suggest a certain universality of the underlying physical mechanisms leading to breaking for wave fields in equilibrium with wind.

N<SUB>2</SUB>ガス中負極性パルス電圧波頭部での沿面放電の進展における波頭峻度と蓄積電荷の影響

N<SUB>2</SUB>ガス中負極性パルス電圧波頭部での沿面放電の進展における波頭峻度と蓄積電荷の影響

Current and electromagnetic field associated with lightning-return strokes to tall towers

An analysis of electric and magnetic fields radiated by lightning first and subsequent return strokes to tall towers is presented. The contributions of the various components of the fields, namely, static, induction, and radiation for the electric field, and induction and radiation for the magnetic field are illustrated and discussed. It is shown in particular that the presence of a tower tends, in general, to increase substantially the electric and magnetic field peaks and their derivatives. This increase is mainly caused by the presence of two oppositely propagating current wavefronts originating from the tower top and by the very high propagation velocity of current pulses within the tower, and depends essentially on the wavefront steepness of the channel-base current. Because of the last factor, the increase of the field magnitudes is found to be significantly higher for subsequent return strokes, which are characterized by much faster risetimes compared to first return strokes. The presented results are consistent with experimental observations of current in lightning strokes to the Toronto CN Tower and of the associated electric and magnetic fields measured 2 km away. These findings partially explain the fact that subsequent return strokes characterized by lower current peaks but higher front steepnesses and return stroke speeds may result in higher field peaks. The results obtained have important implications in electromagnetic (EM) compatibility. It is found that lightning strokes to tall metallic objects lead to increased EM field disturbances. Also, subsequent return strokes are to be considered an even more important source of EM interference than first return strokes. Indeed, EM fields from subsequent strokes are characterized by faster fronts and additionally, they may reach greater peaks than first strokes. Lastly, findings of this study emphasize the difficulty of extracting reliable lightning return stroke current information from remote EM field measurements using oversimplified formulae.

Sensitivity analysis of lightning performance calculations for transmission lines and substations

Analysis has been made of the influence of various lightning parameters on the computed risk of line and substation insulation failure. The analysed lightning current parameters are the mean value and the standard deviation of the current magnitude, as well as the mean value and the standard deviation of current wavefront steepness in an assumed log-normal frequency distribution. The influence of the lightning channel surge impedance on the computer risk of insulation failure is also investigated. The whole analysis is performed for two different equivalent circuits. In the first case, the line outage rate due to the backflashes is predicted. In the second case, the number of strokes to a tower top in front of a gas-insulated substation with a cable entrance, causing one transformer insulation failure, is estimated. The purpose of the paper is to analyse the sensitivity of the lightning-outage-rate prediction method to some uncertainties of the most important lightning parameters. It was found that the mean value and the standard deviation of the lightning current magnitude have the most important influence on the computed risk of insulation failure. On the other hand, the lightning current steepness as well as lightning channel surge impedance have small influence on the computed outage rate.