Helen Czerski

A mechanism stimulating sound production from air bubbles released from a nozzle

Gas bubbles in water act as oscillators with a natural frequency inversely proportional to their radius and a quality factor determined by thermal, radiation, and viscous losses. The linear dynamics of spherical bubbles are well understood, but the excitation mechanism leading to sound production at the moment of bubble creation has been the subject of speculation. Experiments and models presented here show that sound from bubbles released from a nozzle can be excited by the rapid decrease in volume accompanying the collapse of the neck of gas which joins the bubble to its parent.

Improvements to the methods used to measure bubble attenuation using an underwater acoustical resonator

Active acoustic techniques are commonly used to measure oceanic bubble size distributions, by inverting the bulk acoustical properties of the water (usually the attenuation) to infer the bubble population. Acoustical resonators have previously been used to determine attenuation over a wide range of frequencies (10-200 kHz) in a single measurement, corresponding to the simultaneous measurement of a wide range of bubble sizes (20-300 μm radii). However, there is now also considerable interest in acquiring measurements of bubbles with radii smaller than 16 μm, since these are thought to be important for ocean optics and as tracers for near-surface flow. To extend the bubble population measurement to smaller radii, it is necessary to extend the attenuation measurements to higher frequencies. Although the principles of resonator operation do not change as the frequency increases, the assumptions previously made during the spectral analysis may no longer be valid. In order to improve the methods used to calculate attenuation from acoustical resonator outputs, a more complete analysis of the resonator operation is presented here than has been published previously. This approach allows for robust attenuation measurements over a much wider frequency range and enables accurate measurements from lower-quality spectral peaks.

A candidate mechanism for exciting sound during bubble coalescence

Coalescing bubbles are known to produce a pulse of sound at the moment of coalescence, but the mechanism driving the sound production is uncertain. A candidate mechanism for the acoustic forcing is the rapid increase in the bubble volume, as the neck of air joining the two parent bubbles expands. A simple model is presented here for the volume forcing caused by the coalescence dynamics, and its predictions are tested against the available data. The model predicts the right order of magnitude for the acoustic amplitude, and the predicted amplitudes also scale correctly with the radius of the smaller parent bubble.

Contributions to the acoustic excitation of bubbles released from a nozzle

It has recently been demonstrated that air bubbles released from a nozzle are excited into volume mode oscillations by the collapse of the neck of air formed at the moment of bubble detachment. A pulse of sound is caused by these breathing mode oscillations, and the sound of air-entraining flows is made up of many such pulses emitted as bubbles are created. This paper is an elaboration on a JASA-EL paper, which examined the acoustical excitation of bubbles released from a nozzle. Here, further details of the collapse of a neck of air formed at the moment of bubble formation and its implications for the emission of sound by newly formed bubbles are presented. The role of fluid surface tension was studied using high-speed photography and found to be consistent with a simple model for neck collapse. A re-entrant fluid jet forms inside the bubble just after detachment, and its role in acoustic excitation is assessed. It is found that for slowly-grown bubbles the jet does make a noticeable difference to the total volume decrease during neck collapse, but that it is not a dominant effect in the overall acoustic excitation.

The effect of coupling on bubble fragmentation acoustics.

Understanding the formation and evolution of bubble populations is important in a wide range of situations, including industrial processes, medical applications, and ocean science. Passive acoustical techniques can be used to track changes in the population, since each bubble formation or fragmentation event is likely to produce sound. This sound potentially contains a wealth of information about the fragmentation process and the products, but to fully exploit these data it is necessary to understand the physical processes that determine its characteristics. The focus of this paper is binary fragmentation, when turbulence causes one bubble to split into two. Specifically, the effect that bubble-bubble coupling has on the sound produced is examined. A numerical simulation of the acoustical excitation of fragmenting bubbles is used to generate model acoustic signals, which are compared with experimental data. A frequency range with a suppressed acoustic output which is observed in the experimental data can be explained when coupling is taken into account. In addition, although the driving mechanism of neck collapse is always consistent with the data for the larger bubble of the newly formed pair, a different mechanism must be driving the smaller bubble in some situations.

Extending bubble measuremenets below 20 μm: in memory of Ralph Goodman’s contributions.

No matter which aspect of underwater acoustics we worked on, conversations with Ralph Goodman invariably added insight and often motivated new lines of investigation. This was certainly true of our interest in measuring bubble size distributions. We discuss the extension of acoustical measurement of bubbles to radii below 20 μm, a topic of particular relevance to the study of optical scatter in the ocean. Extending the frequency range of a resonator to 1 Mhz would in principle allow measurement down to ∼3 μm, but significant challenges arise due to the confounding effects of geometric scatter from larger bubbles. This problem becomes greater for measurements of smaller bubbles, requiring a careful analysis of the accuracy limits imposed on the inversion procedure. Preliminary steps along this path are discussed with examples drawn from recent field measurements.

The influence of temperature on bubble formation under breaking waves

The bubble plumes generated by breaking waves have a complex structure that is highly dependent on the local environment. These temporary bubble populations have a strong influence on many processes at the air-sea interface, for example, air-sea gas transfer, aerosol production, sound transmission, and optical absorption. To quantify the importance of the bubble plume, two things are required: the state of the initial bubble plume and an understanding of the longer-term plume evolution. This paper focuses on the formation of the initial bubble plume. I’ll present evidence showing the effect of temperature on the fragmentation of single bubbles in the laboratory. These results imply that changing the water temperature has consequences for ocean bubble plume structure, bubble size distribution, and whole bubble plume acoustics, and these will be discussed.

Acoustical resonators: A versatile tool for bubble detection

It can be tricky to quantify bubble size distributions accurately, especially for broad distributions of sub-millimeter bubbles. One solution to the problem is an acoustical resonator, a device which has been developed over the past two or three decades for use in the ocean. A single broadband measurement can provide a detailed bubble size distribution for bubbles from 5 microns to 500 microns in radius. Resonators are physically robust, provide data which needs little post-processing, and they have the potential for use in many other situations. In the ocean, we have used it to understand bubble coatings, follow bubble dissolution, and monitor void fraction in real time. I will describe the current state of resonator technology, show ocean data, and discuss its potential advantages and disadvantages for use in other environments.

The acoustic excitation mechanism of bubbles released from a nozzle

At the moment of their formation, bubbles emit a short pulse of sound. Bubble noise is associated with sound from a variety of natural processes, including whitecaps, waterfalls, breaking surf and rain. A number of acoustic excitation mechanisms for bubble noise have been proposed, including the increase in internal pressure of the bubble associated with the Laplace pressure, hydrostatic pressure effects, shape mode to volume mode coupling, and a fluid jet associated with the collapse of the neck of air formed during bubble creation. Using bubbles released from a nozzle as a model system, we have determined that sound production is excited by a sudden decrease in bubble volume driven by the collapse of the neck of gas joining the bubble to its parent. A simple analytical model of neck collapse driven by surface tension energy is in agreement with high speed photographic measurements, and sufficient to explain the details of acoustic excitation. [Work supported by ONR and NSF]

The origin of storm wave noise and its application to passive acoustic remote sensing of bubbles

Breaking waves in storms create large numbers of bubbles at the ocean surface. Direct measurement of the bubbles is difficult, and there are few data on the numbers, sizes, and creation rates of the large, transient bubbles formed in whitecaps. These bubbles could be measured using passive acoustic remote sensing if the excitation mechanism of sound production from newly created bubbles was understood. Using bubbles released from a nozzle as a model system, we have determined that sound production is driven by the collapse of an air neck formed immediately after bubble pinch‐off. Laboratory experiments and model calculations of the mechanism will be presented. [Work supported by ONR and NSF.]