Properties of superresonant systems of spherical scatterers

Systems of identical precisely spaced bubbles or similar monopole scatterers in water-e.g., inflated balloons or thin-walled shells-insonified at frequenciesomega_{SR}dose to their fundamental radial resonanceomega_{0}(bubble) frequency may themselves display resonance modes or superresonances (SR's) [1]. Ordinary single-bubble resonances magnify the local free-field pressure amplitudep_{1}by a factor(ka)^{-1},abeing the radius andkthe wavenumber in water: for air bubbles or balloons in water, this factor is of the order of 70. Under SR conditions each member of the system amplifies the local free-field amplitude by a further factor of order(ka)^{-1}. Depending upon geometry and other constraints, the pressure fieldP_{SR}on the surface and in the interior of each scatterer will then be in the range of10^{3}p_{1}to5 times 10^{3} p_{1}. This paper investigates the sensitivity of this phenomenon to small departures from the ideal model. In particular, it examines the effect of small differences in scatter positioning and volumes in the context of an SR system consisting of two bubbles/balloons close to the boundary of a thin elastic plate overlying a fluid half-space. It is found that, to observe the SR phenomenon, radii and positions should be controlled to within approximately 1/2 percent.P_{SR}is also sensitive to the angle of incidence of the plane wave train. For the simple system examined here, this sensitivity is considerable for either flexural wave trains or volume acoustic waves incident upon the bubble/ balloon pair (doublet). Practical uses of the phenomenon may range from the design of passive high-Qacoustical filter/amplifiers and acoustical lenses to improved source efficiencies.     

Dynamics of Rossby waves in an ocean with inhomogeneous currents

A model for a two-layer ocean is applied to consider, in terms of the geometrical optics approximation, the effect of mean flows propagating within the upper layer upon the dynamics of Rossby waves. The case is theoretically analysed, with the depth of the ocean's upper layer much smaller than that of the underlying layer. In this case, the flow's impact upon the baroclinic mode of Rossby waves is ubiquitous, with the exception of synchronicity. Depending on the parameters, four types of wave packets' behaviour in the vicinity of synchronicity points are singled out, namely, the elimination of the peculiarity, shadowing, and convective/absolute instability. For the mean flow profile simulating cyclonic and anticyclonic gyres, we have obtained wave packet trajectories and have studied the wave packet's interaction with the current. Specifically, it has been demonstrated that, given some modulus of the wave packet, vigorous energy exchange between the wave vector and the flow takes place. Translated by Vladimir A. Puchkin.     

Acoustic penetration of a silty sand sediment in the 1-10-kHz band

An experiment was performed to measure sediment penetrating acoustic waves to test a model of acoustic propagation, which is based on Biot's theory. Independent geophysical measurements provided model input parameters. A parametric sound source was used to project a narrow beam pulse into a silty sand sediment at a shallow grazing angle. The sediment acoustic waves were measured by an array of buried sensors and processed to measure wave directions and speeds. Two acoustic waves were observed, corresponding to the fast and slow waves predicted by Biot's theory. Discrepancies between model predictions and measured acoustic waves were examined, deficiencies in the model identified, and strategies for improvement postulated. The permeability and bulk modulus of the solid frame were of particular interest     

Wave dissipation by vegetation with layer schematization in SWAN

The energy of waves propagating through vegetation is dissipated due to the work done by the waves on the vegetation. Dalrymple et al. (1984) estimated wave dissipation by integrating the force on a cylinder over its vertical extent. This was extended by Mendez and Losada (2004) to include varying depths and the effects of wave damping due to vegetation and wave breaking for narrow-banded random waves. This paper describes the wave dissipation over a vegetation field by the implementation of the Mendez and Losada formulation in a full spectrum model SWAN, with an extension to include a vertical layer schematization for the vegetation. The present model is validated with the original equation and results from Mendez and Losada (2004). The sensitivity of the model to the shape of the frequency spectrum, directional spreading and layer schematization are investigated. The model is then applied to field measurements by using a vegetation factor. This model has the ability to calculate two-dimensional wave dissipation over a vegetation field including some important aspects such as breaking and diffraction as used in SWAN model.     

Matched-Filter Envelope Detector Deflection Performance for a Correlated Phase Channel

In this paper, the effect of channel phase coherence upon a matched filter envelope detector output is investigated for a pulsed radar or active sonar. A novel model for the correlated channel phases allows the explicit calculation of the loss in detection performance using the deflection criteria. The theoretical model yields good agreement with simulations when the phase correlation coefficients between the first and last pulses are between 0.1 and 1.0. It is shown that a 3-dB loss in performance, as compared to the optimum detector for perfect coherence, requires phase correlation between adjacent pulses ofrho_{i,i+1} = 0.91, 0.96, and 0.96 for 10, 20, and 30 pulses, respectively. On the other hand, the same performance is obtained with a noncoherent combiner of the matched filter pulse returns when correlation between adjacent pulses,rho_{i,i+1} = 0.8, 0.835, and 0.84 for 10, 20, and 30 pulses, respectively. Ifrho_{i,i+1}is smaller than these quantities, one is better off performing noncoherent detection.     

Tentative engineering approach to scour around breakwaters in random waves

An approach by which the scour depth and protection layer width around the head of vertical-wall breakwaters, the scour and deposition depths as well as the protection layer widths at the round head of rubble-mound breakwaters in random waves can be derived is presented. Here the formulas for scour depth by Sumer and Fredsøe (1997) for vertical-wall breakwaters for regular waves and Fredsøe and Sumer (1997) for rubble-mound breakwaters for irregular waves are used. They are combined with describing the waves as a stationary Gaussian narrow-band random process to derive the scour and deposition depths as well as protection layer widths in random waves. Comparisons are made between the present approach and the Fredsøe and Sumer (1997) random wave scour data for rubble-mound breakwaters.     

Modeling wave�Ccurrent bottom boundary layers beneath shoaling and breaking waves

The boundary layer characteristics beneath waves transforming on a natural beach are affected by both waves and wave-induced currents, and their predictability is more difficult and challenging than for those observed over a seabed of uniform depth. In this research, a first-order boundary layer model is developed to investigate the characteristics of bottom boundary layers in a wave–current coexisting environment beneath shoaling and breaking waves. The main difference between the present modeling approach and previous methods is in the mathematical formulation for the mean horizontal pressure gradient term in the governing equations for the cross-shore wave-induced currents. This term is obtained from the wave-averaged momentum equation, and its magnitude depends on the balance between the wave excess momentum flux gradient and the hydrostatic pressure gradient due to spatial variations in the wave field of propagating waves and mean water level fluctuations. A turbulence closure scheme is used with a modified low Reynolds number k-ε model. The model was validated with two published experimental datasets for normally incident shoaling and breaking waves over a sloping seabed. For shoaling waves, model results agree well with data for the instantaneous velocity profiles, oscillatory wave amplitudes, and mean velocity profiles. For breaking waves, a good agreement is obtained between model and data for the vertical distribution of mean shear stress. In particular, the model reproduced the local onshore mean flow near the bottom beneath shoaling waves, and the vertically decreasing pattern of mean shear stress beneath breaking waves. These successful demonstrations for wave–current bottom boundary layers are attributed to a novel formulation of the mean pressure gradient incorporated in the present model. The proposed new formulation plays an important role in modeling the boundary layer characteristics beneath shoaling and breaking waves, and ensuring that the present model is applicable to nearshore sediment transport and morphology evolution.     

风浪和海洋飞沫对海表面拖曳系数和风廓线的影响

基于埃克曼理论,本文将波致应力和飞沫应力引入到海-气边界层的界面应力中,来研究海表面风浪和海洋飞沫对海-气边界层动量交换的影响,并得到修改后的埃克曼模型的理论解。波致应力是由风浪谱和波增长函数估计,并得到在中低风速下,波致应力、飞沫应力与湍流应力相比,对海表面拖曳系数和风廓线的影响非常小。当风速高于25米/秒时,海洋飞沫通过飞沫应力对海-气界面应力的作用远高于波致应力,以至于波致应力可以忽略。海表面拖曳系数在高风速下,随着风速的增大而减小。通过采用风浪谱的不同波龄,得到海洋飞沫的产生会导致海-气边界层风速的增加。最后,理论解与现场的观察数据进行了对比。对比后的数据表明,在中高风速下,飞沫对海-气边界层的影响远大于表面风浪。     

港域波浪数学模型的改进与验证

通过物理模型对改进的港内波浪传播变形数学模型进行验证。该数学模型以推广的时变缓坡方程为控制方程,采用含松弛因子的ADI法求解,并对波浪反射和透射边界模拟方法进行改进。先通过物理模型试验确定斜向浪入射条件下抛石防波堤前的波浪反射系数,作为数学模型中部分反射边界模拟的依据。然后进行了一个典型港口内波浪折射、绕射和反射的模型试验,测量港内波浪分布。对比模型试验和数学模型计算的结果表明,数学模型可较好地模拟港内复杂地形和边界条件下规则波和不规则波的传播变形。     

Acoustic wave scattering from two solid boundaries at the oceanbottom: reflection loss

An acoustic wave scattering model is formulated and solved for three homogeneous layers consisting of a thin solid sediment layer sandwiched by semi-infinite water and solid basalt media. The model is applied to two cases to analyze both the physical parameters affecting reflection loss and the effects of interface roughness scattering. It is shown that effects of attenuation in the sediment layer, especially of S-waves, combine with conversion and scattering processes of the basalt interface to constitute the dominant mechanism of reflection loss, especially in the small grazing angle directions. The scattering process is found not only to produce the well-known acoustic energy loss from specular to nonspecular directions, but also to alter the conversion efficiency between P and S waves with a resulting loss or gain     

Locating and determining the orientation of underwater research equipment: Acoustic range and range rate data

A formulation is presented for evaluating the performance of acoustic data systems to determine the location and orientation of underwater research apparatus. The variables to describe the position of the underwater research apparatus are referenced to a surface ship, and represent the straight-line distance from the stern of the ship to a point on the apparatus (R), the angular distance down from the sea surface (Phi), and the angular distance from the direction of motion of the ship (Theta). The three orientation variables on the apparatus are the angle between thezandz'axes (i), the angle between thexandx'axes in thex'-y'plane (Omega), and the angle which locates thex'axis in thex'-y'plane (omega). A simple model for the sound velocity variation with depth is included in the range data analysis, while the Doppler data are shown not to need that further complication in the analysis. An error model is constructed and applied to three geometries which represent common underwater research devices. Accuracy goals for possible applications of these devices are discussed, and performance requirements for an acoustic system which would meet the goals are derived.     

Experimental Study of Wave Breaking on Gentle Slope

—An experimental study of regular wave and irregular wave breaking is performed on a gentleslope of 1:200.In the experiment,asymmetry of wave profile is analyzed to determine its effect on wavebreaker indices and to explain the difference between Goda and Nelson about the breaker indices of regu-lar waves on very mild slopes.The study shows that the breaker index of irregular waves is under less influ-ence of bottom slope i,relative water depth d/L_0 and the asymmetry of wave profile than that of regularwaves.The breaker index of regular waves from Goda may be used in the case of irregular waves, whilethe coefficient A should be 0.15.The ratio of irregular wavelength to the length calculated by linear wavetheory is 0.74.Analysis is also made on the waveheight damping coefficient of regular waves after break-ing and on the breaking probability of large irregular waves.     

Numerical simulation of wave-induced laminar boundary layers

The three-dimensional numerical model with σ-coordinate transformation in the vertical direction is applied to the simulation of surface water waves and wave-induced laminar boundary layers. Unlike most of the previous investigations that solved the simplified one-dimensional boundary layer equation of motion and neglected the interaction between boundary layer and outside flow, the present model solves the full Navier–Stokes equations (NSE) in the entire domain from bottom to free surface. A non-uniform mesh system is used in the vertical direction to resolve the thin boundary layer. Linear wave, Stokes wave, cnoidal wave and solitary wave are considered. The numerical results are compared to analytical solutions and available experimental data. The numerical results agree favorably to all of the experimental data. It is found that the analytical solutions are accurate for both linear wave and Stokes wave but inadequate for cnoidal wave or solitary wave. The possible reason is that the existing analytical solutions for cnoidal and solitary waves adopt the first-order approximation for free stream velocity and thus overestimate the near bottom velocity. Besides velocity, the present model also provides accurate results for wave-induced bed shear stress.     

完全非线性深水波的数值模拟

基于势流理论,并结合深水波质点运动从水面向下呈e指数衰减的特性,建立了完全非线性数值变深水槽模型,通过实时模拟活塞式造波机运动来产生波浪.采用时域高阶边界元法进行模拟,利用混合欧拉-拉格朗日方法和四阶Runge-Kutta方法追踪流体瞬时水面,应用镜像格林函数消除了水槽两个侧面的积分,在水槽末端布置人工阻尼层来消除反射...     

石岛地震台远震记录反演研究

利用石岛地震台的远震体波记录,采用旋转相关函数法和接收函数法分别反演了台站下方介质的各向异性特征和速度结构.(1)对震中距25°~35°且记录良好的5次地震的ScS震相,采用旋转相关函数法反演了岩石圈的剪切波分裂参数.对深源地震的反演结果表明,石岛地震台快波偏振方向为N94°E,这意味着西沙附近处于近东西向微偏南的拉张或地壳下方的地幔流方向为近东西微偏南,西沙地区地壳是过渡性的,其底部的驱动力主要来自与欧亚板块运动一致的物质流.快慢波时间延迟为1.3 s,估算各向异性层厚度为100 km左右.(2)对震中距20°~60°的9次远震P波波形三分向记录,采用接收函数法反演了地壳和上地幔的S波速度结构.反演结果表明,石岛地震台下方地壳分为3层:约5 km以上有一速度梯度带,S波速度从1.5 km/s逐渐增加到3.5 km/s,其间有若干小的分层;在5~16 km的平均速度为3.8 km/s左右,其间有若干小的分层;在16.0~26.5 km的速度为3.6 km/s左右,这是一个明显的低速层;莫霍面埋深为26.5 km,莫霍面以下平均速度为4.7 km/s,也有若干小的分层,尤其是在莫霍面之下有一个明显的低速层.根据转换波到时分析和速度剖面左右摆动现象,认为反演结果中的小分层可能是不真实的,但在16.0~26.5 km的低速层的真实程度还是较高的,表明下地壳具有一定的塑性.     

有限区域大气-海浪耦合模式的建立及海表粗糙度参数化试验

利用LINUX操作系统下的进程通讯(IPC)技术将中尺度大气模式MM5(V3)与第三代海浪模式WW3进行双向耦合,建立考虑大气-海浪相互作用的风浪耦合模式,在耦合模式中引入3种海表粗糙度参数化方案,通过对一次热带气旋过程的模拟,研究大气-海浪相互作用对热带气旋系统的影响及耦合模式对海表粗糙度参数化方案的敏感性。结果表明:LINUX系统下的进程通讯技术可以方便有效地实现大气和海浪模式的双向耦合,模式运行稳定;耦合模式能够较好的模拟热带气旋的发展和演变过程及其影响下海浪场的分布和演变,模拟结果对海表粗糙度参数化方案较敏感;海浪的反馈作用同时影响了海气间的动力和热力作用过程,不同的海表粗糙度参数化方案下,海浪对两种作用过程不同的影响程度决定了其对气旋系统强度的影响。     

Static and dynamic modeling of a SAR imaged ocean scene

A number of models exist that attempt to explain wave imagery obtained with a synthetic aperture radar (SAR). These models are of two types; static models that depend on instantaneous surface features and dynamic models that employ surface velocities. Radar backscatter values (sigma_{0}) were calculated from 1.3- and 9.4-GHz SAR data collected off Marineland, FL. Thesigma_{0}data (averaged over many wave trains) collected at Marineland can best be modeled by the Bragg-Rice-Phillips model which is based on roughness variation and the complex dielectric constant of oceans. This result suggests that capillaries on the surface of oceanic waves are the primary cause for the surface return observed by a SAR. Salinity and temperature of the sea at small and medium incidence angles produce little effect upon sea-surface reflection coefficients atX-band, for either of the linear polarizations. The authors' observation of moving ocean, imaged by the SAR and studied in the SAR optical correlator, support a theory that the ocean surface appears relatively stationary in the absence of currents. The reflecting surface is most likely moving slowly (i.e., capillaries) relative to the phase velocity of the large gravity waves.     

Flow under standing waves: Part 1. Shear stress distribution,energy flux and steady streaming

The conditions for energy flux, momentum flux and the resulting streaming velocity are analysed for standing waves formed in front of a fully reflecting wall. The exchange of energy between the outer wave motion and the near bed oscillatory boundary layer is considered, determining the horizontal energy flux inside and outside the boundary layer. The momentum balance, the mean shear stress and the resulting time averaged streaming velocities are determined. For a laminar bed boundary layer the analysis of the wave drift gives results similar to the original work of Longuet–Higgins from 1953. The work is extended to turbulent bed boundary layers by application of a numerical model. The similarities and differences between laminar and turbulent flow conditions are discussed, and quantitative results for the magnitude of the mean shear stress and drift velocity are presented. Full two-dimensional simulations of standing waves have also been made by application of a general purpose Navier–Stokes solver. The results agree well with those obtained by the boundary layer analysis. Wave reflection from a plane sloping wall is also investigated by using the same numerical model and by physical laboratory experiments. The phase shift of the reflected wave train is compared with theoretical and empirical models.     

Modeling wave-mud interaction on the central chenier-plain coast,western Louisiana Shelf,USA

The strong coupling between hydrodynamics and seafloors on shallow muddy shelves, and resulting bed reworking, have been extensively documented. On these shelves, spectral wave transformation is driven by a complex combination of forcing mechanisms that include nonlinear wave interactions and wave energy dissipation induced by fluid-mud at a range of frequencies. Wave-mud interaction is investigated herein by using a previously validated nonlinear spectral wave model and observations of waves and near-bed conditions on a mildly-sloping seafloor off the muddy central chenier-plain coast, western Louisiana Shelf, United States. Measurements were made along a cross-shelf transect spanning 1 km between 4 and 3 m water depths. The high-resolution observations of waves and near-bed conditions suggest presence of a fluid mud layer with thickness sometimes exceeding 10 cm under strong long wave action (1 meter wave height with 7 s peak period at 4 meter depth). Spectral wave transformation is modeled using the stochastic formulation of the nonlinear Mild Slope Equation, modified to account for wave-breaking and mud-induced dissipation. The model is used in an inverse manner in order to estimate the viscosity of the fluid mud layer, which is a key parameter controlling mud-induced wave dissipation but complicated to measure in the field during major wave events. Estimated kinematic viscosities vary between 10⁻⁴-10⁻³ m²/s. Combining these results of the wave model simulations with in-depth analysis of near-bed conditions and boundary layer modeling allows for a detailed investigation of the interaction of nonlinear wave propagation and mud characteristics. The results indicate that mud-induced dissipation is most efficient when the wave-induced resuspensions of concentrations  > 10 g/L settle due to relatively small bottom stresses to form a fluid mud layer that is not as thin and viscous as a consolidated seafloor in absence of wave action but also not as thick and soft as a near-bed high concentration layer that forms during strong wave action.     

The energy cycle of wind waves on the sea surface

The problems of wind-induced waves on the sea surface are considered. To this end, the empirical fetch laws that determine variations in the basic periods and heights of waves in relation to their fetch are used. The relation between the fetch and the physical time is found, as are the dependences of the basic characteristics of waves on the time of wind forcing. It is found that about 5% of wind energy dissipated in the near-water air layer contributes to the growth of wave heights, i.e. wave energy, although this quantity depends on the age of waves and the exponent in the fetch laws. With consideration for estimates of the probability distribution functions for the wind over the world ocean [11], it is found that the rate of wind-energy dissipation in the near-water air layer is on the order of 1 W/m². The calculations of wind waves [19] for the world ocean for 2007 have made it possible to assess the mean characteristics of the cycle of wave development and their seasonal variations. An analysis of these calculations [19] shows that about 20% of wind energy is transferred to the water surface. The remaining amount (80%) of wind energy is spent on the generation of turbulence in the near-water air layer. About 2%, i.e., one tenth of the energy transferred to water, is spent on turbulence generation due to the instability of the vertical velocity profile of the Stokes drift current and on energy dissipation in the surf zones. Of the remaining 18%, 5% is spent directly on wave growth and 13% is spent on the generation of turbulence during wave breaking and on a small-scale spectral region. These annually and globally mean estimates have a seasonal cycle with an amplitude on the order of 20% in absolute values but with a smaller amplitude in relative values. According to [19] and to the results of this study, the annually mean height of waves is estimated as 2.7 m and their age is estimated as 1.17.