Underwater Nozzle Research Articles

Sound amplitude of discrete bubbles entrained by an impacting water stream.

Experiments were undertaken to develop a relationship between bubble size and acoustic-emission amplitude for a vertical stream of water impacting a water pool. A particular focus is the formation of the discrete bubbles. Although the relationship between bubble diameter and the natural frequency of sound emissions has been established through Minnaert's work, a comprehensive investigation into the amplitude of sound emissions is missing. Air bubbles were generated from the impact of falling-water streams of varying diameters on an underlying water pool and their acoustic emissions were recorded using a nearby hydrophone. Sound amplitude was found to increase monotonically with bubble size. A second-order polynomial relationship between logarithmic acoustic sound pressure level (L) and bubble diameter (Db) was found, L=-0.0401Db2+1.5781Db+110.7225 within the ±3 dB margin of error. The relationship between linear sound pressure level (P) and bubble diameter (Db) is expressed by the equation P=0.0059Db2+0.0505Db+0.3591, within the ±3 dB margin of error. Results demonstrate that larger bubbles (D > 4 mm) exhibit noise emissions similar to bubbles produced by other mechanisms, such as the underwater nozzle, while smaller diameters tend to produce higher noise levels compared to the same mechanism.

Passive acoustic measurement of bubble size and number from a bubble chain

Experiments and matching simulations are presented on the passive-acoustic emissions of a chain of bubbles, with the aim of deducing both the bubble size and number of bubbles from the emitted sound. It is well known that bubble-acoustic interactions cause shifts in frequency from those predicted for single bubbles, complicating the goal of measuring bubble size by measuring frequency, and also affecting the collective resonances of bubbles driven by ultrasound. A horizontal line of identical bubbles was held fixed in space, eliminating uncertainties in earlier studies due to the variable shape and location of freely rising bubbles. At one end of the line, a first, identical bubble naturally emitted a pulse of sound on formation from an underwater nozzle, exciting the rest of the bubbles into a coupled acoustic emission. It was found that a theoretically predicted relation between frequency and the number of bubbles, which can be derived from the coupled equations of motion, permitted an accurate fit to the data. Measurements of two independent spectral peaks unambiguously determined the two unknowns of bubble size and bubble number. However, experiments diverge from theory and simulations for greater than six bubbles, which are speculated may be explicable by network theory.

Numerical simulations of bubble formation and acoustic characteristics from a submerged orifice: The effects of nozzle wall configurations

The detachment of bubbles from an underwater nozzle inevitably generates distinct acoustic signals. This work proposes numerical simulation models to investigate the effect of different nozzle wall configurations on bubble formation and acoustic characteristics. A combination of large eddy simulation (LES) model and Ffowcs Williams–Hawkings (FW-H) equation is successfully applied to predict bubble acoustic pressures. The adaptive filtering techniques of signal processing are utilized in bubble acoustic analyses. In particular, transient spectrum based on time–frequency analyses can not only precisely count the bubbling period but also effectively measure the bubble sizes. The numerical results for bubble size are in good agreement with the theoretical data (less than 10% deviation). The main results of this research show that the structure of the nozzle wall plays a guiding role in vortex motion around bubbles. The bubble size also increases with the increase in exit-lip thickness, whereas the bubbling rate decreases.

Bubble-Forming Regime Identification Based on Image Textural Features and the MCWA Feature Selection Method

Knowledge of the bubble-forming regime is important for studying the mechanism of gas–liquid two-phase flow. The process of bubble formation through an underwater nozzle at the bottom of a rectangular tank was observed using a high-speed camera. Five bubble-forming regimes were classified by considering both the bubble interaction and the periodic behavior. To automatically identify the bubble-forming regimes, 22 image textural features were extracted based on the grey-level co-occurrence matrix and grey-level gradient co-occurrence matrix. A feature selection method, the mathematical correlation weight algorithm (MCWA), was proposed to select the optimal textural feature subset for bubble-forming regime recognition. Experimental results demonstrated that the identification rates based on the feature subset selected by MCWA were 99.81% and 99.24% for support vector machine and artificial neural network, respectively, which were higher than those by the between-class distance method and the Fisher algorithm. The MCWA proposed in this paper is an effective feature selection method and can be widely used in pattern recognition.

Perturbed breakup of gas bubbles in water: Memory, gas flow, and coalescence

The pinch-off of an air bubble from an underwater nozzle ends in a singularity with a remarkable sensitivity to a variety of perturbations. I report on experiments that break both the axial (i.e., vertical) and azimuthal symmetry of the singularity formation. The density of the inner gas influences the axial asymmetry of the neck near pinch-off. For denser gases, flow through the neck late in collapse changes the pinch-off dynamics. Gas density is also implicated in the formation of satellite bubbles. The azimuthal shape oscillations described by Schmidt et al. can be initiated by anisotropic boundary conditions in the liquid as well as with an asymmetric nozzle shape. I measure the n=3 oscillatory mode and observe the nonlinear, highly three-dimensional outcomes of pinch-off with large azimuthal perturbations. These are consistent with prior theory.

Generation and breakup of Worthington jets after cavity collapse. Part 1. Jet formation

At the beginning of the last century Worthington and Cole discovered that the high-speed jets ejected after the impact of an axisymmetric solid on a liquid surface are intimately related to the formation and collapse of an air cavity created in the wake of the impactor. In this paper, we combine detailed boundary-integral simulations with analytical modelling to describe the formation of such Worthington jets after the impact of a circular disk on water. We extend our earlier model in Gekle et al. (Phys. Rev. Lett., vol. 102, 2009a, 034502), valid for describing only the jet base dynamics, to describe the whole jet. We find that the flow structure inside the jet may be divided into three different regions: the axial acceleration region, where the radial momentum of the incoming liquid is converted to axial momentum; the ballistic region, where fluid particles experience no further acceleration and move constantly with the velocity obtained at the end of the acceleration region; and the jet tip region, where the jet eventually breaks into droplets. From our modelling of the ballistic region we conclude that, contrary to the case of other physical situations where high-speed jets are also ejected, the types of Worthington jets studied here cannot be described using the theory of hyperbolic jets of Longuet-Higgins (J. Fluid Mech., vol. 127, 1983, p. 103). Most importantly, we find that the velocity and the shape of the ejected jets can be well predicted at any instant in time with the only knowledge of quantities obtained before pinch-off occurs. This fact allows us to provide closed expressions for the jet velocity and the sizes of the ejected droplets as a function of the velocity and the size of the impactor. We show that our results are also applicable to Worthington jets emerging after the collapse of a bubble growing from an underwater nozzle, although this system creates thicker jets than the disk impact.

Experimental comparison between acoustic and pressure signals from a bubbling flow

Experiments were performed on both the acoustic and pressure signals produced by bubbles formed from an underwater nozzle. The data were analyzed by a comprehensive set of spectral and nonlinear-dynamical techniques. As air flow rates increase, it is well known that such flows became chaotic. However, the present study of both acoustic and pressure signals showed that chaos could appear in different regimes and was manifested in different ways in the acoustic and pressure signals. The use of different time delays for the chaotic analysis of acoustic and pressure signals were found necessary. The acoustic signals offer data primarily on the bubble size while the pressure signals offer data primarily on the bubble production rate. The present results suggest that chaos can appear in the bubble size and bubble production rate independently.

Breakup of Air Bubbles in Water: Memory and Breakdown of Cylindrical Symmetry

Using high-speed video, we have studied air bubbles detaching from an underwater nozzle. As a bubble distorts, it forms a thin neck which develops a singular shape as it pinches off. As in other singularities, the minimum neck radius scales with the time until the breakup. However, because the air-water interfacial tension does not drive the breakup, even small initial cylindrical asymmetries are preserved throughout the collapse. This novel, nonuniversal singularity retains a memory of the nozzle shape, size, and tilt angle. In the last stages, the air appears to tear instead of pinch.

수중 노즐에서 발생하는 기포의 형상 및 음향 특성 연구

It is well known that the acoustic characteristics of the sea are significantly affected by bubbles which have their own inherent characteristics at the undersea. In this study, the shape and acoustic characteristics of air bubbles generated by an underwater nozzle are calculated numerically, and are measured with a high speed camera and a hydrophone at various air flow rates in the experimental apparatus. As a result of analysis, the shape calculated numerically well matched with measured values at low flow rates, but in case of relatively higher flow rates. the use of correction coefficient is needed for more accurate estimation of the bubble shape. And also the rising velocity of a single bubble is constant regardless of both the bubble size and the flow rate. and the acoustic signal generated when the bubble is produced by an underwater nozzle has the same characteristic of natural frequency of the bubble pulsation, and is agreed with Minnaert's equation if the correction coefficient is considered in accordance with the flow rate.

Sound radiation by a bubble encountering a vortex ring

The dynamics of a bubble and subsequent acoustic radiation interacting with a vortex ring created by an underwater nozzle are studied both theoretically and experimentally. The velocity field of the flow containing the bubble may be represented as a ‘‘basic flow’’ related to the vortex ring and a ‘‘secondary flow’’ related to the bubble motion [G. Chahine, 20th Symposium on Naval Hydrodynamics (1994)]. The ‘‘bubble flow’’ is assumed to be irrotational satisfying the Laplace equation. The vortex related velocities are given by the stream function and are taken into account by imposing ‘‘correct’’ kinematics and dynamic boundary conditions at the bubble surface. The interaction of the bubble with a vortex ring has been shown to cause severe deformation. Under special circumstances the deformed bubble may be at resonance with these vortex filaments and force it to radiate sound to the far field. The boundary integral technique (BIT) is employed to solve governing equations. The BIT is utilized to simulate the ‘‘bubble flow,’’ its surface distortions and subsequent acoustic radiation. The critical parameters responsible for the acoustic re-excitation of the bubble encountering the vortex filaments are discussed in some details. Some conclusions are arrived at on the acoustical characteristics of the bubble encountering a fully developed turbulent flow field. [Work supported by ONR.]

The release of air bubbles from an underwater nozzle

Air bubbles released from an underwater nozzle emit an acoustical pulse which is of interest both for the study of bubble detachment and for elucidating the mechanism of sound generation by a newly formed bubble. In this paper we calculate theoretically the sequence of bubble shapes from a given nozzle and show that there is for each nozzle a bubble of maximum volume vmax Assuming that the bubble becomes detached at its ‘neck’, and that the volume of the detached bubble equals the volume V* of the undetached bubble above its ’neck’, we determine for each nozzle diameter D an acoustic frequency f* corresponding to 'slow’ bubble release.Experiments show that the acoustic frequency, hence the bubble size, depends on the rate of air.flow to the bubble, but for slow rates of flow the frequency f is very close to the theoretical frequency f*.High-speed photographs suggest that when the bubble pinches off. the limiting form of the surface is almost a cone. This is accounted for by assuming a line sink along the axis of symmetry. Immediately following pinch-off there is evidence of the formation of an axial jet going upwards into the bubble. This may play a part in stimulating the emission of sound.

The release of air bubbles from an underwater nozzle

Air bubbles released from an underwater nozzle emit an acoustical pulse that is of interest both for the study of bubble detachment and for elucidating the mechanism of sound generation by a newly formed bubble. In this paper, the sequence of bubble shapes is calculated theoretically from a given nozzle, and it is shown that there is for each nozzle a bubble of maximum volume Vmax. Assuming that the bubble becomes detached at its “neck,” and that the volume of the detached bubble equals the volume V* of the undetached bubble above its “neck,” it is determined for each nozzle diameter D an acoustic frequency f* corresponding to “slow” bubble release. Experiments show that the acoustic frequency, hence the bubble size, depends on the rate of air flow to the bubble, but for slow rates of flow the frequency f is very close to the theoretical frequency f*. High-speed photographs suggest that when the bubble pinches off, the limiting form of the surface is almost a cone. This is accounted for by assuming a line sink along the axis of symmetry. Immediately following pinch off, there is evidence of the formation of an axial jet going upward into the bubble. This may play a part in stimulating the emission of sound.

Noise generation by newly created bubbles

Air bubbles trapped at a free surface by breaking waves, or emerging from an underwater nozzle, or splitting, or coalescing in various ways, all undergo strong “shape oscillations” before settling into a spherical shape. In a fully nonlinear theory (not the usual linearized approximation), the shape oscillations can be shown to emit a strong monopole radiation of sound. Second‐order calculations suggest that this is a significant, perhaps dominant, source of bubble noise in the ocean. Resonance between the second harmonic of each shape oscillation and the radial “breathing” mode of a given bubble leads to enhanced emission of sound at or near the breathing‐mode frequency. There can also be coupling between a bubble and the free surface from which it is detached. Oceanic data and laboratory experiments relating to these predictions will be discussed, particularly the well‐known experiments by Fitzpatrick and Strasberg (1957); also, broadband spectra of oceanic noise from a variety of locations will be discussed.