Sound generation on bubble coalescence following detachment

A system in which bubbles coalesced on formation was used to probe one mechanism by which bubbles create sound. The aim was to determine in which situations sound is produced and to predict its amplitude. A set of carefully co-ordinated high-speed video and acoustic timeseries showed that needle-formed bubbles generated loud bubble-acoustic emissions at the instant of coalescence of secondary bubbles with the primary bubble. As the air flow rate increased, the size and number of secondary bubbles increased, and the sound amplitude also increased. On coalescence, the sound pressure always rose initially. A dimensionless scaling found that the sound amplitude emitted scaled with the volume of the secondary bubble. This scaling was shown to be consistent with the sound-emission mechanism being the equalization of pressures in the coalescing bubbles. The trend in amplitude with bubble production rate was well predicted by the scaling.     

Bubbly flow undergoing a steep pressure gradient

The effects of a steep pressure gradient on bubbly flow were studied to determine the cause of noise emanating from components of a piping system. We used an orifice to generate a local pressure difference. The behavior of bubbles passing through the orifice was observed by using a video camera, and the noise was measured by a condenser microphone outside the pipe. It was found that the sound pressure level of noise generated by a bubbly flow was proportional to the pressure difference. An empirical formula for estimating noise level is proposed. The changes in size and number of bubbles passing through an orifice were found related to the breakup, which is affected by pressure difference rather than airflow rate. The breakup of a single bubble undergoing a steep pressure difference was observed to determine the mechanism of sound generation. It was found that a bubble was broken by impingement of an inward protrusion in the bubble. The growth rate of the protrusion depended on the pressure difference.     

Evolution of a Small Distortion of the Spherical Shape of a Gas Bubble under Strong Expansion-Compression

The evolution of a small distortion of the spherical shape of a gas bubble which undergoes strong radial expansion-compression upon a single oscillation of the ambient liquid pressure under a harmonic law are analyzed by numerical experiments. It is assumed that the distortions of the spherical bubble shape are axisymmetric and have the form of individual spherical surface harmonics with numbers of 2–5. Bubble-shape oscillations prior to the beginning of expansion are taken into account. Generally, the distortion value during bubble expansion-compression depends on the phase of bubble-shape oscillation at the beginning of the expansion (initial phase). Emphasis is placed on the dependence of the maximum distortions in the initial phase at certain characteristic times of bubble expansion-compression on the amplitude of the external excitation, liquid viscosity, and distortion mode (harmonic number). The parameters of the problem are typical of the stable periodic sonolumiescence of an individual air bubble in water at room temperature. An exception is the liquid pressure oscillation amplitude, which is varied up to values that are five times the static pressure. That large excitation amplitudes are beyond the stability threshold of periodic oscillations of spherical bubbles. Their consideration is of interest from the point of view of increasing the compression ratio of the bubble gas, i. e., increasing the maximum temperature and density achievable in the final compression stage.__________Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 4, pp. 17–28, July–August, 2005.     

封闭圆柱腔内旋转点声源声压的近场频域解

在圆柱腔壁面为声学硬壁面的假定下，利用圆柱腔体空间内的格林函数导出了圆柱腔内旋转运动点声源空间声压的近场频域解；进而利用该频域解计算分析了旋转简谐单极子点源的声学特性。计算分析表明：由于考虑了壁面的影响，在圆柱腔体内，点声源旋转频率和源频率的变化将改变声压的空间指向性；源频率和旋转频率的增加伴随明显的多普勒效应，同时基频声压沿圆柱腔体轴线的分布从比较平缓变为明显波动；沿半径方向基频声压也波动变化。     

The effect of rigid-body motions on the whipping response of a ship hull subjected to an underwater bubble

The dynamic elastic response of a floating ship hull girder to an underwater explosion bubble is normally composed of two parts: rigid-body motion and elastic deformation. However, the effects of rigid-body motion have consistently been neglected in the current literature based on the assumption that they are small. In this paper, our focus is on the study of rigid-body motion effects on the hull girder's elastic deformation, also known as the ‘whipping response’. A theory of interaction between a gas bubble and a hull girder is presented. The bubble dynamic equations combined with the bubble migration, free surface effect and drag force considerations are solved numerically using the Runge–Kutta method. The rigid-body and elastic responses of the hull that are induced by the impulsive pressure of a bubble are calculated using the methods presented herein. Two different examples of real ships are given to demonstrate the effect of rigid-body motion on whipping responses. The numerical results show that rigid-body motions reduce the amplitudes and vibration natural periods of the bending moments of the hull girder. These effects can be ignored for slender hulls, but must be taken into consideration for shorter/wider hulls so as not to underestimate the longitudinal strength.     

Numerical simulations of the primary Bjerknes force experienced by bubbles in a standing ultrasonic field: Nonlinear vs. linear

The primary Bjerknes force experienced by a population of multiple bubbles in a liquid set in a nonlinear ultrasonic standing field and their translation are calculated and analyzed by numerical simulations. The force field is evaluated by considering the nonlinear bubble oscillations as well as the nonlinear character of the ultrasonic pressure field (both variables are unknown in the coupled nonlinear differential system). The results at small amplitudes agree with the classical theory on bubble translation, depending on the driving frequency in relation to the bubble resonance. It is shown that, when amplitudes are raised, the force field exhibits important modifications that strongly affect the motion of the bubbles and the way they form agglomerates. An analysis is performed on the importance of the terms in the differential system that provoke (a) the nonlinearity of the bubble oscillations and (b) the nonlinearity of the acoustic wave. This study reveals that both features should be considered to better approximate the primary Bjerknes force field. Simulations of the nonlinear ultrasonic field after the bubbles form agglomerates under the influence of this force are also performed.     

可渗透壁对翼型声涡相互作用下流场及声场的影响

由仿生学原理构建的可渗透翼型对湍流气动噪声抑制作用已展现良好的应用前景。对NACA 0012可渗透翼型和实体翼型进行了数值计算，得到了声涡相互作用下气动噪声声场和流场，分析了可渗透壁对翼型流场和声场的影响。研究表明，相对实体翼型，可渗透壁通过减小声源强度降低了主纯音噪声声压级幅值和远场总声压级，消除了高阶离散纯音，但对噪声的指向性没有较大改变。进一步的流场分析表明，可渗透壁对翼型气动性能影响不大的情况下能够降低边界层扰动和翼型后缘大尺度涡旋强度，并推迟分离泡转捩和再附位置。     

水下爆炸气泡脉动测量及分析

水下爆炸气泡破坏效应是水中兵器的重要毁伤模式之一。为研究水下爆炸气泡脉动现象,建立了小当量水下爆炸实验系统,并进行了爆炸当量分别为0.125g、1.0g、3.375g和8g TNT的水下爆炸实验。采用球形PETN装药并中心起爆,产生球形对称的气泡和冲击波载荷,并利用高速摄像系统记录水下爆炸气泡脉动过程,以及布置压力传感器测量水中冲击波压力。实验获得了清晰的水下爆炸气泡脉动过程图像,得到了冲击波和气泡脉动压力曲线。对数字化图像进行判读,得到气泡脉动直径和周期。另外根据冲击波曲线测量了气泡脉动周期,对比分析了气泡脉动相关参数。结果表明,高速摄像数据测量的气泡直径与经验公式较接近,高速摄像测量的气泡周期与冲击波曲线测量的气泡脉动周期以及经验公式结果具有较好的一致性。本文提出的实验技术安全、经济、可靠,气泡脉动参数判读精确,满足水下爆炸气泡脉动研究需求。     

金属导爆索的爆炸水声特性

为了研究金属导爆索的水声特性,进行了水下爆炸压力测试和气泡脉动实验,获得了金属导爆索水下爆炸冲击波传播和衰减特性及气泡脉动特性.研究了金属导爆索水下爆炸的声压级、声持续时间、混响效应和功率谱特性,结果表明:(1)金属导爆索水下爆炸声压级较高,具有很强的声功率;(2)金属导爆索水下爆炸后产生的气泡脉动和随后产生的大量小气...     

Dynamics and radiation of a single bubble under conditions of abnormal compressibility of a bubbly liquid

An equation is proposed for the pulsation of a single cavity in an abnormally compressible bubbly liquid which is in pressure equilibrium and whose state is described by the Lyakhov equation. In the equilibrium case, this equation is significantly simplified. Numerical analysis is performed of the bubble dynamics and acoustic losses (the profile and amplitude of the radiation wave generated on the bubble wall from the side of the liquid). It is shown that as the volumetric gas concentration k₀ in the equilibrium bubbly medium increases, the degree of compression of the cavity by stationary shock wave decreases and its pulsations decrease considerably and disappear already at k₀ = 3%. In the compression process, the cavity asymptotically reaches an equilibrium state that does not depend on the value of k₀ and is determined only by the shock-wave amplitude. The radiation wave takes the shape of a soliton whose amplitude is much smaller and whose width is considerably greater than the corresponding parameters in a single-phase liquid. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 3, pp. 51–57, May–June, 2007.     

Motion of a gas bubble in fluid under vibration

This paper is concerned with the analysis of motion of a gas bubble in a uniformly oscillating incompressible fluid. A theoretical model explaining the effect of sinking of gas bubbles in the absence of a standing pressure wave is validated experimentally. The conditions under which this effect occurs are determined, and a simple formula is derived for the average velocity of a gas bubble in the fluid.     

BUBBLE CHARACTERISTICS IN A TWO-DIMENSIONAL VERTICALLY VIBRO-FLUIDIZED BED

Measurement of bubble size and local average bubble rise velocity was carried out in a vertically sinusoidal vibro-fluidized bed. Glass beads of Geldart group B particles were fluidized at different gas velocities, while the bed was vibrated at different frequencies and amplitudes to study their effects on the bubble behavior. This is compared with the case of no vibration in a two-dimensional bed and it is concluded that with vibration the local average bubble size, dbav, decreases significantly, especially at minimum bubbling velocity. The average bubble size increases slightly with increasing vibration frequency and amplitude. The local average bubble rise velocity is higher than that with no vibration, though with increasing vibration frequency and amplitude, it does not change significantly.     

Oscillation and breakup of a bubble under forced vibration

Coupled shape oscillations and translational motion of an incompressible gas bubble in a vibrating liquid container is studied numerically. The bubble oscillation characteristics are mapped based on the bubble Bond number (Bo) and the ratio of the vibration amplitude of the container to the bubble diameter (A/D). At small Bo and A/D, the bubble oscillation is found to be linear with small amplitudes, and at large Bo and A/D, it is nonlinear and chaotic. This chaotic bubble oscillation is similar to those observed in two coupled nonlinear systems, here being the gas inside the bubble and its surrounding liquid. Further increases in the forcing, results in the bubble breakup due to large liquid inertia.     

Behavior of a small single bubble rising in a rotating flow field

A small single bubble was generated with a single-hole nozzle facing upward in a water bath contained in a rotating cylindrical vessel. The bubble size falls in the surface tension force dominant regime. The vertical, radial, and tangential migration velocities of the bubble were measured with two CCD cameras and a high-speed video camera. The tangential velocity component of water flow was measured with particle image velocimetry. A helical motion of the bubble was observed under every experimental condition. The direction of the helical motion was the same as that of the tangential velocity component. This helical motion is associated with the large initial shape deformation of the bubble near the nozzle exit and the subsequent regular shedding of vortices behind it. The period and amplitude of the helical motion were obtained by analyzing the trajectory of the bubble. These quantities were non-dimensionalized by the volume equivalent bubble diameter and the terminal bubble velocity in the vertical direction and correlated as functions of the Eotvos number. Empirical equations were proposed for the period and amplitude. Originally published in the Journal of JSEM, Vol. 4, No. 2, pp. 38–45 (2004).     

ANALYSIS OF SOUND RADIATION FROM THE RING－STIFFENED CYLINDRICAL SHELL COATED WITH VISCOELASTIC LAYER IN FLUID MEDIUM

The Donnell theory of shell is applied to describe shell motion and layer motion is described by means of three-dimensional Navier equations. Using deformation harmonious conditions of the interface, the effects of stiffeners and layer are treated as reverse forces and moments acting on the cylindrical shell. In studying the acoustic field produced by vibration of the submerged ring-stiffened cylindrical coated shell, the structure dynamic equation, Helmholtz equation in the fluid field and the continuous conditions of the fluid-structure interface compose the cou-pling vibration equation of the sound-fluid-structure. The extract of sound pressure comes down to the extract of coupling vibration equation. By use of the solution of the equation, the influences of hydrostatic pressure, physical characters and geometric parameters of the layer on sound radiation are discussed.     

气泡弹性对同心筒水下发射影响研究

潜射导弹自同心简装置发射过程中在筒口处会形成燃气泡,由于气泡在水下的弹性效应,水气耦合作用导致气泡周期性地膨胀压缩,会对导弹弹道参数、受载特性产生一定影响.以CFD为技术手段,建立了导弹水下发射动态仿真模型,通过数值模拟获得了发射过程中多相流场、导弹弹道参数及受力的时间历程曲线,分析了气体弹性效应对参数变化的影响;并针对发射深度及其发射艇速对气泡弹性效应的影响进行了仿真分析,给出了其影响规律.仿真方法和结果对工程研究有一定参考价值.     

接地式三要素型动力吸振器性能分析

利用固定点理论优化接地类型的动力吸振器得到的结果可能不是全局最优参数,在选择其他参数时主系统可以获得更小的振幅, 接地类型动力吸振器的优化问题值得进一步研究. 因此,以一种接地式三要素型动力吸振器为对象,通过研究系统参数变化对固 定点位置与主系统最大振幅的影响,得到了此吸振器的局部最优参数并分析了它的性能. 首先建立了此系统模型的运动微分方程, 得到了主系统振幅放大因子,发现系统存在3个与阻尼无关的固定点. 固定点中幅值较大点随系统参数变化的趋势可以代表最大振 幅随系统参数变化的趋势,因此利用盛金公式得到了固定点幅值的表达式. 为了更加精确,进一步使用数值算法得到了最大振幅与 系统参数的关系图,发现系统中存在局部最优参数. 通过对比接地式吸振器与接地三要素型吸振器的最大振幅随系统参数变化的趋 势,得到了接地式三要素型吸振器的局部最优参数,并发现当固有频率比小于局部最优频率比时,接地式三要素型吸振器模型主系 统的最大振幅要远小于接地式动力吸振器模型.      

Dissipative effects of an isolated bubble in water on the sound wave

ＤＩＳＳＩＰＡＴＩＶＥＥＦＦＥＣＴＳＯＦＡＮＩＳＯＬＡＴＥＤＢＵＢＢＬＥＩＮＷＡＴＥＲＯＮＴＨＥＳＯＵＮＤＷＡＶＥＨｕｎａｇＪｉｎｇ－ｑｕａｎ（黄景泉）ＬｉＦｕ－ｘｉｎ（李福新）（ＮｏｒｔｈｗｅｓｔｅｒｎＰｏｌｙｔｅｃｈｎｉｃＵｎｉｖｅｒｓｉｔｙ,Ｘ...     

Radiation and scattering of sound from a vortex ring

The mechanisms of generation and scattering of sound by a vortex ring are investigated on the basis of fluid dynamics. The vortex ring can serve as a simple dynamic model of the large-scale structures observed in shear flows. Moreover, it is probably the most easily studied vortex element that can be created experimentally. The sound scattering investigation also served to determine the extent to which the vortex is affected by sound, its selectivity with respect to such parameters as the acoustic frequency, the angle of incidence of the wave, etc. The perturbed motion is considered against the background of the steady-state motion of the ring. The perturbed motion in the vortex core is determined on the basis of linear incompressible fluid dynamics. Two terms of the expansion in the M number of the far acoustic field generated by the perturbations in the core are found in accordance with Lighthill's theory. The acoustic power and directivity of the radiation and the acoustic instability growth rate are calculated. It is shown that the scattering of sound by the vortex ring is a resonance effect, and the scattering amplitude near resonance is determined. The acoustic action on the hydrodynamic structure of the flow in the core of the ring is especially intense near the resonances and extends over a period short as compared with the characteristic time of the acoustic instability.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 83–95, May–June, 1987.     

On the mechanism of unsteady shock oscillation in shock wave/turbulent boundary layer interactions

An experimental investigation into the mechanism of shock wave oscillation in compression ramp-generated shock wave/turbulent boundary layer interactions is presented. Particular emphasis is focused upon documenting the respective roles played by both burst-sweep events in the turbulent boundary layer immediately upstream of the interaction and the downstream separated shear layer upon unsteady shock front motion. Unlike the majority of compression ramp experiments which involve bulk separation and large-scale shock motion, consideration is given here to comparatively “weak” interactions in which the streamwise spatial excursion of the shock front is always less than one boundary layer thickness. In this manner any shock motion due to upstream burst-sweep events should be more apparent in relation to that oscillation associated with the separated region. A discrete Hilbert transform-based conditional sampling technique is used to obtain wall pressure measurements conditioned to burst-sweep events. The conditional sampling technique forms the basis by which the instantaneous shock motion is conditioned to the occurrence of upstream bursting. The relationship between the separation bubble and shock motion is also explored in detail. The results of the experiments indicate that the separation bubble represents a first-order effect on shock oscillation. Although it is demonstrated theoretically that the burst-sweep cycle can also give rise to unsteady shock motion of much lower amplitude, the experiments clearly demonstrate that there is no discernible statistical relationship between burst events and spanwise coherent shock front motion.