Implosion of an underwater spark-generated bubble and acoustic energy evaluation using the Rayleigh model

The growth, collapse, and rebound of a vapor bubble generated by an underwater spark is studied by means of high-speed cinematography, simultaneously acquiring the emitted acoustic signature. Video recordings show that the growth and collapse phases are nearly symmetrical during the first two or three cycles, the bubble shape being approximately spherical. After 2-3 cycles the bubble behavior changes from a collapsing/rebounding regime with sound-emitting implosions to a pulsating regime with no implosions. The motion of the bubble wall during the first collapses was found to be consistent with the Rayleigh model of a cavity in an incompressible liquid, with the inclusion of a vapor pressure term at constant temperature within each bubble cycle. An estimate of the pressure inside the bubble is obtained measuring the collapse time and maximum radius, and the amount of energy converted into acoustical energy upon each implosion is deduced. The resulting value of acoustic efficiency was found to be in agreement with measurements based on the emitted acoustic pulse.     

An integrated wave-effects model for an underwater explosion bubble

A model for a moderately deep underwater explosion bubble is developed that integrates the shock wave and oscillation phases of the motion. A hyperacoustic relationship is formulated that relates bubble volume acceleration to far-field pressure profile during the shock-wave phase, thereby providing initial conditions for the subsequent oscillation phase. For the latter, equations for bubble-surface response are derived that include wave effects in both the external liquid and the internal gas. The equations are then specialized to the case of a spherical bubble, and bubble-surface displacement histories are calculated for dilational and translational motion. Agreement between these histories and experimental data is found to be substantially better than that produced by previous models.     

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.     

Diffraction losses of an asymmetric stable laser resonator using an equivalent resonator

An asymmetric stable laser resonator with one large mirror and the other mirror apertured is considered. An equivalent symmetric resonator is derived which enables the determination of the theoretical diffraction losses from Li's computations. Experiments are done to test the validity of the predictions. The experimental diffraction losses agree very well with the theoretical values and the usefulness of the concept of equivalent resonator is therefore confirmed.     

液膜气泡法测洗涤溶液的表面张力系数

利用自制装置采用液膜气泡法测定了洗涤溶液的表面张力系数．由医用注射器充气形成球形液泡，通过与之连通的微压计测出液泡内外的压强差，运用杨氏-普拉斯方程计算出洗涤溶液的表面张力系数.     

Enhanced experimental approach to measure the absolute ultrasonic wave attenuation

The absolute ultrasonic wave attenuation is an important input parameter for mathematical models, which play an increasingly important role in non-destructive testing. The measurement of the absolute ultrasonic wave attenuation however is a difficult task. When conventional measurement techniques are applied, corrections to the raw data are required to account for apparent losses. In this study, a modified experimental approach is proposed to determine the absolute ultrasonic attenuation without any further corrections of the raw data.     

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.     

A method to accurately measure and respond to an underwater sound pulse in a wide dynamic range

I.IntroductionTomeasurethesound1evelofanundcrwatersoundpulsewithunknownamplitudeisofimportanceinunderwaterapp1ication.Forinstancc,asimu1ationsystemusedtointerfcresomesonarmusthavesuchfunction,whichhastobeabIctomeasurethesoundlcvelatreceivingpointandretransmitasimulatcdsignalwithrequircdsoundlcvelafterpropcrprocessing.Itisdifficultforanorma1peakorthresho1ddetectortocompletethistask.Thereasonforthisisthattherangeofthesignalamp1itudercaches7O-8odBandthereissomedistortionofsignalwaveform,inaddit…     

Effects of coupled vibrations on the acoustical performance of underwater cylindrical shell transducers

An experimental investigation of the effects of coupled vibrations on the acoustical performance of underwater transducers made from radially polarized, thin walled, air-backed, piezoelectric cylinders as a function of their height-to-diameter aspect ratio is presented. Characteristics of the frequency response, directivity patterns, and effectiveness of the transducers are considered in comparison with analogous characteristics for the transducers comprised of mechanically separated rings, otherwise having the same geometry. Recommendations are made on the application of cylindrical piezoelectric elements with different aspect ratios for underwater transducer applications.     

Acoustic energy harvesting using an electromechanical Helmholtz resonator

This paper presents the development of an acoustic energy harvester using an electromechanical Helmholtz resonator (EMHR). The EMHR consists of an orifice, cavity, and a piezoelectric diaphragm. Acoustic energy is converted to mechanical energy when sound incident on the orifice generates an oscillatory pressure in the cavity, which in turns causes the vibration of the diaphragm. The conversion of acoustic energy to electrical energy is achieved via piezoelectric transduction in the diaphragm of the EMHR. Moreover, the diaphragm is coupled with energy reclamation circuitry to increase the efficiency of the energy conversion. Lumped element modeling of the EMHR is used to provide physical insight into the coupled energy domain dynamics governing the energy reclamation process. The feasibility of acoustic energy reclamation using an EMHR is demonstrated in a plane wave tube for two power converter topologies. The first is comprised of only a rectifier, and the second uses a rectifier connected to a flyback converter to improve load matching. Experimental results indicate that approximately 30 mW of output power is harvested for an incident sound pressure level of 160 dB with a flyback converter. Such power level is sufficient to power a variety of low power electronic devices.     

High-frequency attenuation measurements using an acoustic microscope

An acoustic microscope was used to measure excess attenuation of aqueous solutions of sugars and proteins at 1.0 GHz. Interference pattern spacing and peak amplitude reduction of V(z) curves, obtained with these solutions as the acoustic microscope coupling liquid, were related to the solution wavespeed and attenuation, respectively. Consistent with published results for lower frequencies, solutions with molecular weight greater than 10,000 had a higher specific absorption than those with a molecular weight less than 1000 and within these two molecular weight ranges specific absorption was independent of concentration.     

Piezoelectric coupling of an exciton to acoustical phonons

The effective electron-hole potential resulting from the piezoelectric coupling is derived. For CdS it is shown to be negligibly small compared to the optical phonon coupling. It is concluded that this special coupling to acoustical phonons plays no role in biexciton formation.     

Laser tattoo removal as an ablation process monitored by acoustical and optical methods

Monodisperse hollow silica nanospheres have been prepared by using a polystyrene nanosphere template-assistant approach and their potential as antireflection (AR) coatings for window applications has been discussed. The as-prepared hollow silica nanospheres have a typical inner diameter of 200 nm and a shell thickness of 15–20 nm. The AR effect over the ultraviolet-visible-near infrared spectral region has been observed for the hollow silica nanospheres, with a minimized reflection of about 5.2 % at 500 nm, compared to 8.5 % of a plain float glass substrate. By modifying the structural features of the hollow silica nanospheres, their AR properties can be further enhanced.     

Laser phase and frequency stabilization using an optical resonator

We describe a new and highly effective optical frequency discriminator and laser stabilization system based on signals reflected from a stable Fabry-Perot reference interferometer. High sensitivity for detection of resonance information is achieved by optical heterodyne detection with sidebands produced by rf phase modulation. Physical, optical, and electronic aspects of this discriminator/laser frequency stabilization system are considered in detail. We show that a high-speed domain exists in which the system responds to the phase (rather than frequency) change of the laser; thus with suitable design the servo loop bandwidth is not limited by the cavity response time. We report diagnostic experiments in which a dye laser and gas laser were independently locked to one stable cavity. Because of the precautions employed, the observed sub-100 Hz beat line width shows that the lasers were this stable. Applications of this system of laser stabilization include precision laser spectroscopy and interferometric gravity-wave detectors.     

The fast frequency stabilisation of an argon laser to an optical resonator using an extra-cavity electro-optic modulator

A frequency stabilisation system for an argon laser is described, incorporating a fast piezoelectrically driven laser mirror and an extracavity electro-optic phase shifter. We show that results obtained with this method are potentially as good as those reported from systems which use fast intra-cavity modulators. This technique avoids the loss associated with an intra-cavity device, thus allowing higher power output, and so making it an attractive alternative to present stabilisation systems.     

最大气泡压力法测液体表面张力系数的改进

对最大气泡压力法测定液体表面张力系数的实验原理、方法及仪器进行了研究,提出用螺旋活塞定量加压控制气泡的生成速度;用扩散硅气体压力传感器测量压强,测得值数字显示;用双毛细管制作实验探头,消除了毛细管插入液体一定深度产生的静压强及待测液体密度等对计算液体表面张力系数的影响,提高了液体表面张力系数的测量速度和精度.     

Reconstruction of an acoustical image using an ‘acousto-electronic’ lens device

Present acoustic imaging apparatuses give only a virtual image of the insonified object. Several studies have been conducted in order to pick up the information from such an image, such as holography and computer data processing. These techniques are very involved, and so simpler processing systems have been analysed in our laboratory in order to reconstruct a true image. Our first system used optical processing of the acoustic echo. In this paper, a second, acousto-electronic, system is described, which enables like the first the attainment of very high image rates in the B-scan mode.