Bble Size Distribution for Waves Propagating over A Submerged Breakwater

Experiments are carried out to study the characteristics of active bubbles entrained by breaking waves as these propagate over an abruptly topographical change or a submerged breakwater. Underwater sounds generated by the entrained air bubbles are detected by a hydrophone connected to a charge amplifier and a data acquisition system. The size distribution of the bubbles is then determined inversely from the received sound frequencies. The sound signals are converted from time domain to time-frequency domain by applying Gabor transform. The number of bubbles with different sizes are counted from the signal peaks in the time-frequency domain. The characteristics of the bubbles are in terms of bubble size spectra, which account for the variation in bubble probability density related to the bubble radius r. The experimental data demonstrate that the bubble probability density function shows a-2.39 power-law scaling with radius for r＞0.8 mm, and a-1.11 power law for r＜0.8 mm.     

Spectral spreading from surface bubble motion

At low frequencies, surface bubbles contribute to acoustic backscattering in aggregate, and the motion of these bubble masses causes spectral spreading of the acoustic signals. This motion of the bubbles entrained in the surface waves is used to obtain the power spectrum of a low-frequency surface-scattered signal at a low grazing angle. A spectral distribution of the deterministic surface drift, augmented by breaking wave crests, is developed for the wave frequency components that are actively breaking. This motion is combined with the random motion in a wave cycle to predict the spectral widths of low-angle backscattered sound. To permit comparisons with measured data, convolutions of these spectra with simple square pulses of various durations are performed     

Acoustic Propagation Through Clustered Bubble Clouds

One of the underlying assumptions in the effective medium theory describing the propagation of acoustic waves through bubble clouds is that the probability of an individual bubble being located at some position in space is independent of the locations of other bubbles. However, bubbles within naturally occurring clouds may be influenced by the dynamics of the fluids in which they are entrained so that they become preferentially concentrated, or clustered, leading to statistical dependence in their positions. For bubble clouds in which the important scattering terms include those with interactions between at least two bubbles, statistical dependence between bubble positions leads to a reduction in the attenuation of the coherent acoustic pressure field from that which would be predicted for a nonclustered bubble cloud. Bubble clustering can be accommodated in effective medium theories using correlation functions describing the relationship between the positions of the bubbles. For double scattering, the two-bubble correlation (i.e., the pair correlation function) must be used, for triple scattering, the three bubble correlation must be used, and so on. In contrast to the three attenuation of the coherent field, making the assumption of independent bubble positions leads to an underestimate of the incoherent field. Both the coherent and incoherent acoustic fields for bubble clouds exhibiting correlated bubble positions are explored in this paper with the use of numerical simulations.     

气泵法生成气泡帷幕的特性研究

气泡帷幕在水下噪声抑制方面有非常广泛的应用。为了更好地利用气泡帷幕的衰减特性,在实验室水槽设备中采用气泵法产生气泡帷幕,通过对不同深度、气流量接收信号的时频分析,利用共振谱法对气泡分布进行反演。研究表明:(1)气泡的分布可以利用高斯分布近似描述。(2)随着深度增加,气泡含量也有略微增加。(3)气泵法产生的气泡帷幕的气流量大小对衰减的峰值所在的频段基本没有影响,但气流量的改变会影响声波衰减的强弱,气流量大的情况下,气泡帷幕对声波衰减效果更强。通过掌握气泵法生成气泡帷幕中气泡的分布规律,可以有效指导气泡帷幕的设计。     

Experiments investigating the use of fiber-optic probes formeasuring bubble-size distributions

An experimental investigation was conducted in the laboratory in order to evaluate the use of fiber-optic probes for measuring the size distribution of large transient bubbles entrained beneath breaking ocean waves. Measurements were made in a unidirectional flow cell using two-fiber Optoflow fiber-optic probes (Photonetics Inc.). It was found that the rise times of the signal pulses created when bubbles crossed the probe tip were closely correlated with the bubble velocities. Therefore, bubble velocities and hence bubble sizes could be estimated using a single probe. It was observed that the correlation between the rise time and the bubble velocity varied significantly between probes but was only a weak function of water type (i.e., freshwater or seawater) and the bubble impaction angle. Estimates of the bubble-size distributions and void fraction made using the fiber-optic probes and video image data were compared. It was found that the fiber-optic probes could be used to obtain fairly accurate estimates of the void fraction. The bubble-size distributions estimated from the probe signals compared favorably with those estimated from video images. It was concluded that the probes could be used to make reasonably accurate measurements of the bubble-size distributions beneath breaking ocean waves     

表面波破碎泡径谱的理论研究

采用新的无量纲变量组合推导出波浪破碎形成的气泡总数与气体体积卷入率、湍流能谱密度以及表面张力之间的关系;利用观测结果提出两点假设,并据此将泡径谱N(a)推广为随泡径和深度的分布函数N(a,z)。     

Study on the Bubble Growth and Departure with A Lattice Boltzmann Method

For the growth and departure of bubbles from an orifice, a free energy lattice Boltzmann model is adopted to deal with this complex multiphase flow phenomenon. A virtual layer is set at the boundary of the flow domain to deal with the no-slip boundary condition. Effects of the viscosity, surface tension, gas inertial force and buoyancy on the characteristics of bubbles when they grow and departure from an orifice in quiescent liquid are studied. The releasing period and departure diameter of the bubble are influenced by the residual gas at the orifice, and the interaction between bubbles is taken into consideration. The relations between the releasing period or departure diameter and the gravity acceleration show fair agreements with previous numerical and theoretical results. And the influence of the gas outflow velocity on bubble formation is discussed as well. For the bubbles growing in cross-flow field, effects of the cross-flow speed and the gas outflow velocity on the bubble formation are discussed, which is related to the application in ship resistance reduction. And optimal choice of the ship speed and gas outflow velocity is studied. Cases in this paper also prove that this high density ratio LBM model has its flexibility and effectiveness on multiphase flow simulations.     

Formation of seep bubble plumes in the Coal Oil Point seep field

The fate of marine seep gases (transport to the atmosphere or dissolution, and either bacterial oxidation or diffusion to the atmosphere) is intimately connected with bubble and bubble-plume processes, which are strongly size-dependent. Based on measurements with a video bubble measurement system in the Coal Oil Point seep field in the Santa Barbara Channel, California, which recorded the bubble-emission size distribution (Φ) for a range of seep vents, three distinct plume types were identified, termed minor, major, and mixed. Minor plumes generally emitted bubbles with a lower emission flux, Q, and had narrow, peaked Φ that were well described by a Gaussian function. Major plumes showed broad Φ spanning very small to very large bubbles, and were well described by a power law function. Mixed plumes showed characteristics of both major and minor plume classes, i.e., they were described by a combination of Gaussian and power law functions, albeit poorly. To understand the underlying formation mechanism, laboratory bubble plumes were created from fixed capillary tubes, and by percolating air through sediment beds of four different grain sizes for a range of Q. Capillary tubes produced a Φ that was Gaussian for low Q. The peak radius of the Gaussian function describing Φ increased with capillary diameter. At high Q, they produced a broad distribution, which was primarily described by a power law. Sediment-bed bubble plumes were mixed plumes for low Q, and major plumes for high Q. For low-Q sediment-bed Φ, the peak radius decreased with increasing grain size. For high Q, sediment-bed Φ exhibited a decreased sensitivity to grain size, and Φ tended toward a power law, similar to that for major seep plumes.     

A new optical instrument for the study of bubbles at high voidfractions within breaking waves

An optical imaging system (BubbleCam) has been tested for the quantification of bubble distributions at high void fractions formed beneath breaking waves. The instrument consists of a CCD video camera, stroboscopic light source, and optics allowing adjustable magnification, a fixed imaging volume, and the resolution of bubbles 3 pixels in radius and larger (equivalent to a minimum bubble radius of about 200 μm in the test configuration). BubbleCam has been deployed in a shore-based configuration (data and power supplied via shore-connected cables) as well as an autonomous device in the open sea with its own power supply and data storage. The resulting images are processed using a variant of the Hough transform which allows computer-automated counting and measurement of the bubbles within the video frames. In addition, images can be qualitatively examined to provide insights into bubble plume evolution and creation mechanisms     

近岸破碎波水体掺气及气泡输运过程研究

为准确探讨破碎波作用下气体如何卷入以及气泡的形成与输运特性, 文章结合粒子图像测速技术(particle image velocimetry, PIV)、高速相机和气泡测量系统, 以及基于Navier-Stokes方程的三维数值模型对气泡形成及其运动过程进行研究。研究结果表明: 文章建立的数值模型能合理地捕捉到破碎波作用下气体的卷入及其输运过程; 波浪的破碎会形成较大的气腔, 其破裂过程又将产生大量的气体微团; 气泡会增加水体的紊动, 造成水体与空气交界面附近形成大量的漩涡以及水体的飞溅; 气泡的破裂会消耗大量的水体能量, 同时发现较大的紊动动能与气泡的生成有关, 且气泡数随平均紊动动能的增加呈线性增长关系。     

Movement of large gas bubbles in unsaturated fine‐grained sediments

Abstract

Where undissolved gas occurs within fine‐grained marine sediments it usually takes the form of discrete bubbles that are much larger than the normal void spaces. The possibility of buoyancy‐induced movement of these relatively large bubbles must be included when considering the transport of gas through marine sediments. A theoretical analysis shows that, under static loading conditions, bubbles larger than a critical size should have sufficient buoyancy to move upward through a fine‐grained sediment stratum, whereas bubbles smaller than the critical size should remain fixed in position. The critical radius is directly proportional to sediment shear strength, and bubbles of a realistic size should move upward only in extremely weak sediments. Further theoretical analysis shows that the critical bubble size is reduced under cyclic loading conditions, but movement of typical‐sized bubbles should still be restricted to sediments of low shear strength. A simple laboratory experiment provides support for the conclusions of the theoretical analysis. The results indicate that buoyancy‐induced movement of relatively large gas bubbles in fine‐grained sediments is most likely to occur under storm loading conditions and is unlikely to occur at depths greater than a few meters below the seabed.     

Unsteady air bubble entrainment and detrainment at a plunging breaker: dominant time scales and similarity of water level variations

At plunging breakers, air bubbles are entrained at the impingement of the water jet, formed at the top of the wave, with the water free surface in front. During the present study, air bubble entrainment at a pseudo-plunging breaker was investigated at near full-scale and further experimental work studied the bubble detrainment process. Experimental observations included the generation and propagation of waves downstream of the plunge point. Experimental results highlighted a number of unsteady air–water flow patterns and emphasise high levels of aeration: i.e., depth-averaged void fraction of more than 10% next to jet impact in shallow waters. Unsteady bubble injection experiments showed a strong vortical motion induced by the rising bubbles. Altogether, the results suggest that a dominant time scale is the bubble rise time d₁/u_r, which cannot be scaled properly with an undistorted Froude model. The study contributes to a better understanding of unsteady bubble entrainment at a pseudo-plunging breaker and the associated vortical circulation.     

海洋表层气泡运动规律的研究

建立了海洋表层气泡运动和半径变化的数学模型,通过计算发现一定深度的气泡存在一个临界半径;在一定初始深度气泡的半径小于临界半径时不能到达水面,随着时间的增长,气泡半径逐渐变小,直至最终溶解;气泡的半径大于临界半径时,气泡随着时间的增长,半径逐渐增大,直至达到水面破碎;气泡的初始深度不同,其临界半径也不同;半径在临界点附近的气泡的存活时间最长.对海洋表层气泡运动规律的研究对了解海洋表层气泡的分布规律有重要意义.     

Void fraction measurements and scale effects in breaking waves in freshwater and seawater

This paper follows from the work of Blenkinsopp and Chaplin (2007) and describes detailed measurements of the time-varying distribution of void fractions generated by breaking waves in freshwater, artificial seawater and natural seawater under laboratory conditions, along with flow visualisation of the entrainment process. The measurements were made with highly sensitive optical fibre phase detection probes and the results demonstrate that although an additional population of fine (d < 0.3 mm) bubbles existed in the seawater cases, the total volume and distribution of entrained air, and the spatial and temporal evolution of the bubble plumes were very similar in all three water types. The influence of water type may be relatively insignificant, but a numerical bubble tracking model shows that the effect of scale is an important consideration when modelling the post-entrainment evolution of breaker-entrained bubble plumes. Consequently the results suggest that while the use of freshwater in laboratory models of oceanic processes can be considered valid in most situations, the effect of scale may impact interpretation of the results.     

Assessment of acoustic iterative inverse method for bubble sizing to experimental data

Comparative study was carried out for an acoustic iterative inverse method to estimate bubble size distributions in water. Conventional bubble sizing methods consider only sound attenuation for sizing. Choi and Yoon [IEEE, 26(1), 125–130 (2001)] reported an acoustic iterative inverse method, which extracts the sound speed component from the measured sound attenuation. It can more accurately estimate the bubble size distributions in water than do the conventional methods. The estimation results of acoustic iterative inverse method were compared with other experimental data. The experimental data show good agreement with the estimation from the acoustic iterative inverse method. This iterative technique can be utilized for bubble sizing in the ocean.     

Oceanic bubble population measurements using a buoy-deployedcombination frequency technique

This paper presents the results of using a combination frequency acoustic technique to measure the near-surface bubble population in the open sea. The combination frequency technique monitors the appearance of sum-and-difference signals generated by the nonlinear interaction of two sound fields: one, a high-frequency signal, scatters geometrically from the bubble, and the other, of much lower frequency, is used to excite the bubble into resonant pulsation. The text details the calibration of the apparatus necessary to relate the measured heights of the sum-and-difference terms to actual numbers of bubbles and describes the experimental procedure for the collection of the oceanic data. In total, six trials were performed over a one-day period, each comprising ten “snapshots” of the local bubble population at ten discrete radii. This data was augmented with simultaneous video, slide, and dictaphone records of the state of the sea around the measurement position     

冷泉活动区气泡羽状流数值模拟的进一步研究

以南海气泡羽状流赋存特征为参考,本文继续研究冷泉活动区气泡羽状流的地震响应。为使羽状流建模方案更合理,根据水中气泡在上升过程时其半径的变化情况,修改原建模方案,重新建立了羽状流水体模型。通过正演模拟得到了羽状流炮集地震记录,单炮记录上显示出明显的散射波场特征和模型的周期特点。通过叠前深度偏移处理炮集地震记录,得到边界收敛效果较好的成像剖面,且成像精度较高。以上研究又一次奠定了羽状流地震响应进一步研究的基础,也逐步探索出更适合羽状流地震资料的处理方法,为识别羽状流提供了理论指导。     

Acoustic bubble counting technique using sound speed extracted fromsound attenuation

Sound attenuation has been solely used to estimate bubble size distributions of bubbly water in the conventional acoustic bubble sizing methods. These conventional methods are useful for the void fraction around 10^-6 or lower. However, the change of compressibility in the bubbly water also should be considered in bubble sizing for the void fraction around 10^-5 or higher. Recently the sound speed as well as sound attenuation was considered for acoustic bubble size estimation in bubbly water. In this paper, the sound speed estimated from sound attenuation in bubbly water by an iterative method is used for a bubble counting. This new iterative inverse bubble sizing technique is numerically tested for bubble distributions of single-size Gaussian, and power-law functions. The numerical simulation results are in agreement with the given bubble distributions even for the high void fractions of 10^-4-10^-3. It suggests that the iterative inverse technique can be a very powerful tool for practical use in acoustic bubble counting in the ocean     

A two-frequency hydroacoustic scatterometer for bubble scatteringinvestigations

High-frequency bubble layer scattering investigations require the measurement of the intensity of backscattered sound and the corresponding depth of the scatterers below the moving surface. Especially at high sea state conditions and high acoustic frequencies, bubbles acoustically mask the surface, i.e., the surface return cannot be detected. However, this environmental condition is the most interesting one in bubble scattering investigations and a reliable method is required to determine the range of the scatterers to the surface displacement. A method for the determination of the vertical profiling of acoustic scattering in the presence of bubbles at high sea state conditions is presented. It is based on the transmission of a low-frequency signal alternately to the high-frequency signal at which the scattering investigations are performed. The only information that is extracted from the low-frequency echo is the onset of the surface return. It is used to compute the true depth of scatterers at the high frequency. Experiments were conducted to determine the optimum low frequency at which the detection of the surface onset in the presence of a high bubble concentration is ensured. A screening ratio is defined to give a measure of the acoustic masking of the sea surface. It is depicted for an extreme wind condition (20 m/s) for the frequency range of 5-25 kHz and as a function of wind speed for 50 kHz measurements. Selected results of subsurface bubble scattering at 50 kHz from experiments under open sea conditions are presented for the wind speed regime from 9 to 22 m/s. Additionally, the two-frequency scatterometer is used to measure sea state characteristics simultaneously to the scattering investigations by remote sensing techniques     

INFLUENCE OF SEDIMENT SOUND VELOCITY ON TIME-FREQUENCY CHARACTERISTICS OF THE SIGNAL IN THE DEEP SEA

This paper uses acoustical method to obtain the sound speed structure information of the deep sea sediment. The time-frequency characteristics of the explosive sound signals in the deep sea are analysed by using the narrow-band filters, and the bottom-reflecting path and bottom-refracting path are separated in time domain. Based on the two kinds of selected bottom models the travel-time difference between the two paths mentioned above is calculated according to the ray theory, and simple analytical formulae are obtained. The bottom model, which is more suitable for the experimental area, is identified by comparing the theoretical with experimental travel-time differences versus range.