A model for the horizontal directionality of breaking wave noise in the surf zone

A model is presented for the horizontal directionality of the noise generated by individual breaking waves in the surf zone. The model is based on the interaction between sound radiated by ringing bubbles created in a breaking wave crest and the population of acoustically quiescent bubbles left on the seaward side of the wave in the shallow water waveguide. The effect of the quiescent bubbles is to absorb sound from all but the very ends of the breaking crest, resulting in the formation of "acoustic hot-spots." The model calculations are in good agreement with observations of the horizontal directionality of noise from individual breaking waves in the surf zone.     

Breaking wind waves as a source of ambient noise

A theoretical model for the prediction of ambient noise level due to collective oscillations of air bubbles under breaking wind waves is presented. The model uses a budget of the energy flux from the breaking waves to quantify acoustic power radiation by a bubble cloud. A shift of the noise spectra to lower frequency due to collective bubble oscillation is assumed. The model derives good estimates of the magnitude, slope, and frequency range of the noise spectra using the wind speed or height of breaking waves.     

Long time-base observations of surf noise

A year of surf noise observations in the very near shore region of La Jolla Shores beach are presented. Ambient sound levels and surface wave height were recorded for 9 min every hour from July 1997 through June 1998 at a monitoring station located 360 m seaward of the beach in 8-m deep water. Sound segments that were dominated by the noise from breaking surf formed the basis of a correlation analysis between surf noise level and wave height, wave period, wind speed, and mean water depth. The analysis shows that surf noise is primarily determined by wave height, and scales approximately with the wave height squared. The surface wave energy flux onto the beach also scales with wave height squared, leading to the conclusion that the conversion of the mechanical energy of the surface wave field into noise energy is approximately constant. In fact, the ratio of noise energy to surface wave energy flux varies by up to a factor of 3 over the range of energy fluxes considered (100-3000 W per m).     

Energy partitioning in laser-induced millimeter-sized spherical cavitation up to the fourth oscillation

To investigate the energy partitioning up to the fourth oscillation of a millimeter-scale spherical cavitation bubble induced by laser, we used nanosecond laser pulses to generate highly spherical cavitation bubbles and shadowgraphs to measure the radius-time curve. Using the extended Gilmore model and considering the continuous condensation of the vapor in the bubble, the time evolution of the bubble radius, bubble wall velocity, and pressure in the bubble is calculated till the 4th oscillation. Using Kirkwood-Bethe hypothesis, the evolution of velocity and pressure of shock wave at the optical breakdown, the first and second collapses are calculated. The shock wave energy at the breakdown and bubble collapse is directly calculated by numerical method. We found the simulated radius-time curve fits well with experimental data for the first four oscillations. The energy partition at the breakdown is the same as that in previous studies, the ratio of shock wave energy to bubble energy is about 2:1. In the first collapse and the second collapse, the ratio of shock wave energy to bubble energy is 14.54:1 and 2.81:1 respectively. In the third and fourth collapses, the ratio is less, namely than 1.5:1 and 0.42:1 respectively. The formation mechanism of the shock wave at the collapse is analyzed. The breakdown shock wave is mainly driven by the expansion of the supercritical liquid resulting from the thermalization of the energy of the free electrons in the plasma, and the collapse shock wave is mainly driven by the compressed liquid around the bubble.     

Acoustic phase conjugation in a three-phase medium

Acoustic phase conjugation is studied in a sandy marine sediment that contains air bubbles in its fluid fraction. The considered phase conjugation is a four-wave nonlinear parametric sound interaction caused by nonlinear bubble oscillations which are known to be dominant in acoustic nonlinear interactions in three-phase marine sediments. Two various mechanisms of phase conjugation are studied. One of them is based on the stimulated Raman-type sound scattering on resonance bubble oscillations. The other is associated with sound interactions with bubble oscillations whose frequencies are far from resonance bubble frequencies. Nonlinear equations to solve the phase conjugation problem are derived, expressions for acoustic wave amplitudes with a conjugate wave front are obtained and compared for various frequencies of the excited bubble oscillations.     

Air-borne sound generated by sea waves

This paper describes a semi-empiric model and measurements of air-borne sound generated by breaking sea waves. Measurements have been performed at the Baltic Sea. Shores with different slopes and sediment types have been investigated. Results showed that the sound pressure level increased from 60 dB at 0.4 m wave height to 78 dB at 2.0 m wave height. The 1/3 octave spectrum was dependent on the surf type. A scaling model based on the dissipated wave power and a surf similarity parameter is proposed and compared to measurements. The predictions show satisfactory agreement to the measurements.     

The characteristic of the bubble generated by underwater high-voltage discharge

When underwater high-voltage discharges, the electric energy is converted to mechanical energy, including the shock wave and the bubble wave. A simplified model is built based on Rayleigh model and the relationship between the pulsation period and the maximum radius of the bubble. Then, the analytical results are compared with the experimental results. During the experiments, we find that: (1) the generation of the bubble presents a high relation with the types of the electrodes; (2) A liquid jet during the bubble’s rebound is the main reason of the wave generated by the bubble.     

声波在水-含气沉积物界面的反射

含气泡海洋沉积物的声学特性是海底探测的重要问题。为了研究气泡存在对水-含气沉积物界面声反射系数的影响,本文基于气泡振动修正的Biot波动方程推导了气泡存在修正的Biot弹性模量,并结合水-沉积物界面的“开孔”边界条件推导了声波从水入射到水-含气沉积物界面的反射系数。数值分析表明气泡的振动导致反射系数呈现显著的频率特性。在气泡共振频率附近,由于气泡的共振引发的强散射和强衰减,使得反射系数很大,无论以何种角度入射,声波都很难进入含气泡的沉积物。本文研究结果表明,气泡半径、含量、声波频率以及入射角度都是影响水-含气沉积物界面反射系数的主要因素。      

次声波在平流层波导中传播的数值模拟

对次声波在大气中传播进行了建模。通过结合保色散关系空间差分格式和Runge-Kutta时间格式的数值方法,建立了次声波传播模型。应用该次声波模型,研究了在耗散的重力分层大气中次声波的平流层导行传播。数值模拟结果表明,当次声波波包在平流层高度上被反射时,反射区域存在焦散现象,在声波的声压下降的同时,声波的能量得到聚焦。通过数值模拟结果与射线计算结果的对比表明,大气中声波传播的轨迹的精确描述需要应用全波解。      

波浪破碎气体的卷入过程及相关统计量的估计

基于实验观测,导出了波浪破碎能量耗散ε_ed、气泡云卷入深度z_b、气体卷入速率Q(z)和湍流动能耗散率ε_T(z)的表达式,在此基础上建立了一种简单、实用的气泡粒径谱参数化模式N(a,z),揭示了波浪破碎气泡云卷入过程能量耗散、气泡破碎临界Hinze特征尺度和气泡粒径谱在不同海况下的变化. 研究表明：气泡云卷入过程能量耗 关键词： 波浪破碎能量耗散 气泡云卷入深度 气泡粒径谱     

Nonlinear non-spherical oscillations of gas bubble under a periodic variation of ambient liquid pressure

A mathematical model is constructed for the bubble dynamics, in which the interphase surface variation is presented in the form of a series in spherical harmonics, and the equations are written with the accuracy up to the squared amplitude of the distortion of the spherical shape of the bubble. In the oscillation regimes close to periodic sonoluminescence of a single bubble in a standing acoustic wave, the character of air bubble oscillations in water was studied depending on the bubble initial radius and the amplitude of the liquid pressure variation. It was found that non-spherical oscillations of bounded amplitude can take place outside the region of linearly stable spherical oscillations. Both the oscillations with a period equal to one or several periods of the liquid pressure variation and aperiodic oscillations are observed. It is shown that neglecting the distortions in the form of spherical harmonics with large numbers (i > 3) may lead to a change of oscillation regimes. The influence of distortions on the bubble surface shape for the harmonics with i > 8 is insignificant.     

Anomalous sea surface reverberation scale model experiments

Low frequency sea surface sound backscattering from approximately 100 Hz to a few kHz observed from the 1960s broadband measurements using explosive charges to the Critical Sea Test measurements conducted in the 1990 s is substantially higher than explained by rough sea surface scattering theory. Alternative theories for explaining this difference range from scattering by bubble plumes/clouds formed by breaking waves to stochastic scattering from fluctuating bubble layers near the sea surface. In each case, theories focus on reverberation in the absence of the large-scale surface wave height fluctuations that are characteristic of a sea that produces bubble clouds and plumes. At shallow grazing angles, shadowing of bubble plumes and clouds caused by surface wave height fluctuations may induce first order changes in the backscattered signal strength. To understand the magnitude of shadowing effects under controlled and repeatable conditions, scale model experiments were performed in a 3 m x 1.5 m x 1.5 m tank at the Technical University of Denmark. The experiments used a 1 MHz transducer as the source and receiver, a computer controlled data acquisition system, a scale model target, and a surface wave generator. The scattered signal strength fluctuations observed at shallow angles are characteristic of the predicted ocean environment. These experiments demonstrate that shadowing has a first order impact on bubble plume and cloud scattering strength and emphasize the usefulness of model scale experiments for studying underwater acoustic events under controlled conditions.     

Mechanical effects of laser-induced cavitation bubble on different geometrical confinements for laser propulsion in water

Laser propulsions of six kinds of propelled objects in water and air are studied in this paper. The kinetic energy and momentum coupling coefficients gained by the objects after implementation of single laser pulses are investigated experimentally. It is shown that the propulsion effects are better in water than in air. Both in water and in air, the propulsion effects are better if there is a cavity on the laser irradiated surface of the object, and a hemispherical cavity works better than a 90°-conical cavity. A concept of equivalent reference pressure is proposed in this paper. It means that the asymmetry in the liquid induced by a rigid boundary near a spherical or nonspherical oscillating bubble can be approximated as the perturbation induced by a compressive stress wave passing through a bubble in the infinite static liquid. Thus, the collapse time and the pressure surrounding the nonspherical collapsing bubble can be estimated based on the maximum velocity of the liquid jet tip. Experiments also show that cavitation with oscillations and collapse can be induced at the object-water interface on the outside surface of the object head by the penetrating intensive stress wave and the elastic deformation of the object head. The bulging velocity of the object surface is calculated based on the propagation theory of stress waves at medium interfaces.     

A two-frequency acoustic technique for bubble resonant oscillation studies

A two-frequency acoustic apparatus has been developed to study the dynamics of a single gas or vapor bubble in water. An advantage of the apparatus is its capability of trapping a bubble by an ultrasonic standing wave while independently driving it into oscillations by a second lower frequency acoustic wave. For a preliminary application, the apparatus is used to study resonant oscillations. First, near-resonant coupling between the volume and the n = 3 shape oscillation modes of air bubbles at room temperature is studied, where n is the mode number. The stability boundary, amplitude versus frequency, of the volume oscillation forms a wedge centered at the resonant frequency, which qualitatively agrees with a theoretical prediction based on a phase-space analysis. Next, the resonant volume oscillations of vapor bubbles are studied. The resonant radius of vapor bubbles at 80 degrees C driven at 1682 Hz is determined to be 0.7 mm, in agreement with a prediction obtained by numerical simulation.     

大气声传播通道的声源当量估计方法

提出了一种基于大气声传播通道的爆炸声源能量估计方法,通过将计算大气声学的传播能量分布结果与大气中传播的声压幅度衰减模型相结合,使用平流层通道与热层通道传播损失能量比例作为修正量,提高了对爆炸声源能量的估计精度。在多次地面爆炸实验得到的数据中,使用观测距离800 km以上且同时存在平流层通道与热层通道的次声接收信号,对比了平流层顶风速修正的能量估计方法与该文提出的基于大气声传播通道的能量估计方法。实验结果验证了相对于传统风速修正的能量估计方法,该方法可显著降低估计误差。     

Measuring and modeling the bubble population produced by an underwater explosion

Underwater explosions have been studied intensively in the United States since 1941 [e.g., R. H. Cole, Underwater Explosions (Princeton University Press, Princeton, NJ, 1945), pp. 3-13]. Research to date has primarily focused on the initial shock and subsequent pressure waves caused by the oscillations of the "gas-globe" resulting from charge detonation. These phenomena have relatively short timescales (typically less than 2 s). However, after the gas-globe rises through the water column and breaks the surface, there remains behind a cloud of bubbles and perhaps debris from the explosion container which has been markedly less studied. A recent experiment measured the spatial and temporal acoustic response of the bubble cloud resulting from a 13.6 kg PBXN-111 charge detonated at 15.2 m (50 ft) depth. A directional projector was used to propagate linear frequency-modulated (5-65 kHz) and 40 kHz tonal pulses through the bubble cloud. Two hydrophone arrays were positioned so as to measure the energy lost in propagating through the bubble cloud. Three methods have been utilized to invert measurements and estimate the bubble population. The bubble population estimates have been used to develop a model for the bubble population resulting from an underwater explosion.     

弹性微管内气泡的非线性受迫振动

将弹性管壁视为膜弹性结构, 探索在外部声场作用下弹性微管内液柱-气泡-管壁构成耦合振动系统的非线性特征. 利用逐级近似法对系统非线性共振频率、基频和三倍频振动幅值响应、 分频激励共振机理等进行了理论分析. 基频和三倍频振动的幅-频响应数值结果表明: 气泡的轴向共振和管壁共振不能同时出现; 两垂直方向的振动均表现出幅值响应多值性, 进而可能引起系统的不稳定声响应; 三倍频振动在低频区的声响应强于高频区. 关键词： 弹性微管 受迫振动 非线性振动 气泡声响应     

Bubble dynamics and size distributions during focused ultrasound insonation

The deposition of ultrasonic energy in tissue can cause tissue damage due to local heating. For pressures above a critical threshold, cavitation will occur, inducing a much larger thermal energy deposition in a local region. The present work develops a nonlinear bubble dynamics model to numerically investigate bubble oscillations and bubble-enhanced heating during focused ultrasound (HIFU) insonation. The model is applied to calculate two threshold-dependent phenomena occurring for nonlinearly oscillating bubbles: Shape instability and growth by rectified diffusion. These instabilities in turn are shown to place physical boundaries on the time-dependent bubble size distribution, and thus the thermal energy deposition.     

Shock-wave model of acoustic cavitation

Shock-wave model of liquid cavitation due to an acoustic wave was developed, showing how the primary energy of an acoustic radiator is absorbed in the cavitation region owing to the formation of spherical shock-waves inside each gas bubble. The model is based on the concept of a hypothetical spatial wave moving through the cavitation region. It permits using the classical system of Rankine-Hugoniot equations to calculate the total energy absorbed in the cavitation region. Additionally, the model makes it possible to explain some newly discovered properties of acoustic cavitation that occur at extremely high oscillatory velocities of the radiators, at which the mode of bubble oscillation changes and the bubble behavior approaches that of an empty Rayleigh cavity. Experimental verification of the proposed model was conducted using an acoustic calorimeter with a set of barbell horns. The maximum amplitude of the oscillatory velocity of the horns' radiating surfaces was 17 m/s. Static pressure in the calorimeter was varied in the range from 1 to 5 bars. The experimental data and the results of the calculations according to the proposed model were in good agreement. Simple algebraic expressions that follow from the model can be used for engineering calculations of the energy parameters of the ultrasonic radiators used in sonochemical reactors.