Space-frequency distribution of sound field intensity in the vicinity of the temperature front in shallow water

The sound field produced by a low-frequency (100–1000 Hz) point source in a shallow water region in the presence of the temperature front is considered. It is shown that the latter anisotropic inhomogeinity leads to a substantial horizontal refraction and to a number of related effects: a redistribution of the sound field in the horizontal plane, changes in the spectral and mode contents of a pulse in the course of its propagation, and changes in the arrival times at the receiver. A theoretical analysis and a numerical modeling are carried out for the Polar Front of the Barents Sea.     

A numerical halynometry experiment on an acoustic path in the Arctic ocean

It is shown that, in the Arctic-type deep-water waveguide, the first water mode can be used for acoustic halynometry at frequencies of about 40 Hz and higher. For this task, the stochastic sound scattering by the ice cover and the frequency dispersion of modes are the interfering factors. The data of numerical modeling are presented for the levels of the regular and stochastic components of the pseudo-noise signal and for the impulse response of the waveguide to narrow-band and broadband transmissions. To suppress the stochastically scattered component of the sound field, one can use a horizontal extended array. However, choosing the experimental site in a region with a mainly smooth ice cover may be more advantageous. It is shown that the frequency band of the received signal can be broadened by introducing a frequency-time correction of the propagation time of the first mode as a function of the sound frequency.     

Analytical solution based on the wavenumber integration method for the acoustic field in a Pekeris waveguide

An exact solution based on the wavenumber integration method is proposed and implemented in a numerical model for the acoustic field in a Pekeris waveguide excited by either a point source in cylindrical geometry or a line source in plane geometry. Besides, an unconditionally stable numerical solution is also presented, which entirely resolves the stability problem in previous methods. Generally the branch line integral contributes to the total field only at short ranges, and hence is usually ignored in traditional normal mode models. However, for the special case where a mode lies near the branch cut, the branch line integral can contribute to the total field significantly at all ranges. The wavenumber integration method is well-suited for such problems. Numerical results are also provided, which show that the present model can serve as a benchmark for sound propagation in a Pekeris waveguide.     

Analysis of inelastic x-ray scattering spectra of low-temperature water

We analyze a set of high-resolution inelastic x-ray scattering (IXS) spectra from H2O measured at T=259, 273, and 294 K using two different phenomenological models. Model I, called the "dynamic cage model," combines the short time in-cage dynamics described by a generalized Enskog kinetic theory with a long-time cage relaxation dynamics described by an alpha relaxation. This model is appropriate for supercooled water where the cage effect is dominant and the existence of an alpha relaxation is evident from molecular-dynamics (MD) simulation data of extended simple point charge (SPC/E) model water. Model II is essentially a generalized hydrodynamic theory called the "three effective eigenmode theory" by de Schepper et al. 11. This model is appropriate for normal liquid water where the cage effect is less prominent and there is no evidence of the alpha relaxation from the MD data. We use the model I to analyze IXS data at T=259 K (supercooled water). We successfully extract the Debye-Waller factor, the cage relaxation time from the long-time dynamics, and the dispersion relation of high-frequency sound from the short time dynamics. We then use the model II to analyze IXS data at all three temperatures, from which we are able to extract the relaxation rate of the central mode and the damping of the sound mode as well as the dispersion relation for the high-frequency sound. It turns out that the dispersion relations extracted from the two models at their respective temperatures agree with each other giving the high-frequency sound speed of 2900+/-300 m/s. This is to be compared with a slightly higher value reported previously, 3200+/-320 m/s, by analyzing similar IXS data with a phenomenological-damped harmonic oscillator model 22. This latter model has traditionally been used exclusively for the analysis of inelastic scattering spectra of water. The k-dependent sound damping and central mode relaxation rate extracted from our model analyses are compared with the known values in the hydrodynamic limit.     

Comparison of methods for calculating the sound field due to a rotating monopole

A spherical harmonic expansion for the sound field due to a rotating oscillating point source has recently been derived. This paper provides further confirmation of the expansion results by comparing it with two known numerical approaches to determining the sound field. In the advanced time approach-applicable for Mach numbers below 1-the sound at transmission time determines the field at an observation point from the distance from source to observation point at the transmission time. In the retarded time approach the field at the observation point at the observation time is determined by solving for the retarded transmission times. The results from all three approaches are shown to be in good agreement. Expressions for the far-field instantaneous frequency are also derived and shown to agree with previous work.     

Prediction of the spatial characteristics of higher mode sound transmission through a periodic slit screen

Periodic slit screens are often installed in rectangular apertures either to mask the opening or to reduce transmitted noise. This investigation considers the higher-order scattered and transmitted sound from a point source through a periodic screen mounted in a rigid baffle. In particular, this paper considers the spatial characteristics, or directivity, of the scattered field for a particular higher order mode. Uncoupled higher-order mode analysis is used to estimate the scattered and transmitted sound fields for the selected mode of the aperture without the presence of the screen. Transmission coefficients for the screens are then calculated using the well established equivalent mass layer effect and applied to the calculated higher-order mode scattered and transmitted sound pressures. Using an anechoic chamber, measurements were made over a small arc of the scattered sound field through a range of screens of different aspect ratios but the same porosity. The screens consisted of equally spaced open slits and solid laths and the number of slits was reduced to change the aspect ratio. In each case the point source was positioned on the impinging side of the aperture so as to drive one particular scattered higher-order aperture mode at or near cut-on. Comparison of the measured and predicted sound pressures indicate that good estimates of the sound field can be obtained through the approach of applying simple corrections for the presence of the screens to the estimates for the open aperture established using uncoupled calculations for the higher-order modes. It is also shown that the results for a specific mode excited at one particular frequency are applicable at other excitation frequencies provided that the correct non-dimensionalisation is applied.     

Charge Composition of Ions in a Cluster Plasma Formed under the Action of a High-Power Laser Pulse

A model describing the ionization of atoms and ions in a cluster under irradiation by a short laser pulse has been constructed. It is shown that the electron-impact ionization weakly affects the final charge composition, and the main mechanism of ion formation in the cluster is the over-barrier ionization by an electric field. The electric field acting on atoms and ions is the result of combined action of the external laser field and the intrinsic electric field of the cluster. The key parameters of the cluster beam and the laser pulse, which determine the properties of the charge composition of the cluster plasma, have been established.     

Relaxation dynamics of a broadband nanosecond acoustic pulse in a bubbly medium

The first experimental observation of the propagation dynamics of a short broadband acoustic pulse in a resonance medium with gas bubbles is carried out. The probing pulse is generated using the optoacoustic effect. It is shown that the theory of short pulse propagation in media with generalized resonance relaxation adequately and accurately describes the dynamics of short pulse dispersion. A possibility to determine the relaxation and resonance parameters of media by the pulsed testing technique is demonstrated.     

Selection of modes in a shallow sea by using a vertical antenna array

The experimental data on selecting the modes in a shallow sea (the Barents Sea) on 17-and 8-km-long paths are presented. The data are obtained with the use of a 93-m-long vertical receiving array of 32 hydrophones and a point sound source transmitting a pulsed signal with linear frequency modulation in a frequency band of 100–300 Hz. The experimental selection of modes is based on the structure of normal waves in a waveguide with a perfectly reflecting impedance bottom. The bottom impedance for different modes is determined from the experiment. A pressure-release bottom and a bottom with an impedance that is intermediate between the pressure-release and rigid cases correspond to the first mode and the higher modes, respectively. The amplitudes of the modes and their directivity are determined. On the basis of the mode dispersion data and the comparison of the mode contents observed at distances of 8 and 17 km, it is concluded that higher modes are generated at the distances from 8 to 17 km.     

利用气枪声源数据的地声参数反演*

本文通过对东海采集的气枪声源数据进行处理,基于地声参数对不同声场观测量的敏感性差异,利用分步反演的策略进行分析。首先选择距离接收阵七个不同的距离点数据,通过warping变换准确提取第三阶至第八阶简正波的频散曲线;利用海底声速对频散曲线敏感的特性来反演声速;由Hamilton经验公式求得海底密度;通过传播损失拟合获得海底衰减。不同距离点数据反演的海底声速与密度一致性较高。实验提取的频散曲线和和反演参数仿真结果、实验获取的传播损失与反演参数获得传播损失均拟合较好。     

各向异性焊缝中超声相控阵瞬态声场仿真及缺陷定位方法

针对多层各向异性奥氏体不锈钢焊缝中超声相控阵瞬态声场的仿真问题,提出应用高斯声束等效点源模型计算宽带离散化的多个单频稳态声场,通过傅里叶变换将其拓展为瞬态声场,并分析了声场转换过程的主要影响参数.该方法可快速计算焊缝内部超声相控阵聚焦声场的瞬态能量分布和任意一点的时域波形信号.在此基础上针对多层奥氏体不锈钢焊缝内部缺陷...     

Slow light propagation via quantum coherence in a semiconductor quantum well

With the Schrödinger equations, we investigate the low-intensity light pulse propagation through a semiconductor quantum wells. Through studying the dispersion and absorption properties of the weak probe field, it is shown that slow light propagation is observed in this system. From the view point of practical purpose, it is more advantageous than its corresponding atomic system. Such investigation of slow light propagation may lead to important practical applications in semiconductor quantum information.     

Billet expansion as a mechanism for noise production in impact forming machines

In this paper a new mechanism for the production of impulsive noise in impact forming machines is proposed; namely the sudden radial expansion of the billet at the instant of impact. First of all a mathematical model of a forging machine is described which, though simplified in order to render it amenable to calculation, contains the essential features necessary for a realistic calculation of the sound pulse. A theory then is developed which enables the sound pulse to be computed given the rate of expansion of the billet surface.It is shown that the main part of the pulse from this source is produced during the first few microseconds of impact, the remaining few milliseconds of impact time producing a relatively small pulse. It being assumed, then, that the acceleration of the billet surface is essentially a delta function, the sound pulse produced by such an acceleration is computed and shown to constitute a significant part of the peak pressure measured. Finally the variation of pulse height with billet dimensions and impact velocity are given.     

The use of effective phase velocity to decrease the direction finding error for a low-frequency signal in a waveguide

The mode, interference, and phase structures of the low-frequency sound field and the spatial responses of the extended linear array in a waveguide are studied. The wide-angle parabolic approximation and the normal mode method are used to perform calculations. A possibility of approximating the field in the zone of interference maxima by the equivalent plane wave model with the real amplitude and averaged effective phase velocity (EPV) calculated or measured from the phase gradient at the array aperture is investigated. The use of EPV in the zone of interference maxima is shown to decrease substantially the direction finding error. The conclusion is drawn that the array and the transfer function of the waveguide can be approximately matched if the speed of sound in water is substituted with the EPV in the bearing algorithm.     

Amplification of ultimately-short pulses in graphene in the presence of a high-frequency field

The Maxwell equations for an electromagnetic field propagating in graphene are considered taking into account strong Coulomb repulsion between electrons of the same site possessing opposite spin projections. The derived effective equation has the form of a classical 2D sine-Gordon equation. Electrons are treated in terms of quantum formalism with allowance for the dispersion law in the presence of Coulomb interaction. The effective equation is analyzed numerically and the effect of Coulomb repulsion is revealed. It is shown that the system in an external homogeneous electromagnetic field, with its period much shorter than the characteristic pulse length, may show the amplification of an ultimately short pulse.     

Low-frequency horizontal acoustic refraction caused by internal wave solitons in a shallow sea

The effect of internal wave solitons on the sound field generated by a point source in a shallow sea is considered. In the framework of the theory of “horizontal rays and vertical modes,” the sound field pattern governed by the aforementioned hydrodynamic effect is investigated. It is shown that solitons can induce time-periodic focusing and defocusing of horizontal rays propagating at shallow angles to the internal wave front. This may result in the formation of “dynamical” horizontal sound channels, which, in its turn, results in considerable temporal fluctuations of the field along the acoustic track oriented along the internal wave front. For the sound field calculations, an approach is developed on the basis of the parabolic approximation in the horizontal plane and the mode representation in the vertical direction. The results obtained can be used for remote monitoring of internal wave packets in a shallow sea.     

Achromatic doublets using group indices of refraction

One main function of short pulses is to concentrate energy in time and space [1]. The use of refractive lenses allows us to concentrate energy in a small volume of focusing around the focal point of the lens. When using refractive lenses, there are three effects that affect the concentration of energy around the focal point of the lens. These are the group velocity dispersion (GVD), the propagation time difference (PTD), and the aberrations of the lens. In this paper, we study lenses which are diffraction limited so that the monochromatic aberrations are negligible; the group velocity dispersion and the propagation time difference are the main effects affecting the spreading of the pulse at the focus. We will show that for 100-fs pulses the spatial spreading is larger than the temporal spreading of the pulse. It is already known that the effect of spatial spreading of the pulse due to PTD can be reduced by using achromatic optics. We use the theory proposed by A. Vaughan to analyze simple lenses and normal achromatic doublets, where normal means doublets that we can buy from catalogs. We then use the Vaughan theory to design achromatic doublets in phase and group, which produce no spatial spreading of the pulse, i.e., PTD = 0, when the doublet is designed for the carrier of the pulse. We compare these phase and group achromatic doublets with normal achromatic doublets. Finally, we show that apochromatic optics can give a much better correction of PTD than using normal achromatic doublets.     

快速数值差分递推算法在偏振模色散作用下光脉冲传输中的应用研究

利用快速数值差分递推算法对非线性薛定谔方程进行推导分析,得到一个同时考虑群速色散效应、非线性效应以及偏振模色散效应的时域快速数值差分递推关系,并将该方法应用于研究偏振模色散作用下的光脉冲传输,得到的计算结果分别与解析结果和传统的分步傅里叶算法所得结果进行对比,表明这种快速数值差分递推算法拥有很高的计算准确度,并且物理模型更加合理,完全适合分析计算偏振模色散作用下的光脉冲传输问题.利用该方法研究了光脉冲的畸变以及脉冲展宽问题,获得的计算结果为光纤传输系统的设计提供了参考.     

Generation of a magnetic field in a weakly inhomogeneous plasma interacting with a short laser pulse

The generation of a quasistationary magnetic field in a plasma interacting with a weakly focused low-intensity short laser pulse has been studied. It has been shown that the magnetic field changes direction at times comparable with the free path time of effective electrons. Generation also occurs after the switching off of the short pulse and the maximum field is proportional to the duration of the pulse and is reached at times larger than the free path time of the suprathermal electrons.     

Propagation of a short electromagnetic pulse in a linear medium with frequency and spatial dispersion: Direct integration of the Maxwell equations by the finite difference method

The specific features of the propagation of a short (on the order of ten oscillations) electromagnetic pulse in an isotropic linear medium with frequency and spatial dispersion, which were established via integration of a system of Maxwell equations by the finite-difference time-domain method using an auxiliary differential equation, are discussed. The variation of the direction of the electric field-strength vector oscillations that occurs in an optically active medium upon propagation of a linearly polarized ultrashort pulse incident on it is shown to be significantly different from the rotation of the polarization plane (proportional to the propagation coordinate) in the case of the incidence of a long pulse.