Model calculations of the underwater noise of breaking waves and comparison with experiment

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.

Statistical analysis and hybrid modeling of high-frequency underwater acoustic channels affected by wind-driven surface waves

High frequency is a solution to high data-rate underwater acoustic communications. Extensive studies have been conducted on high-frequency (>40 kHz) acoustic channels, which are strongly susceptible to surface waves. The corresponding channel statistics related to acoustic communications, however, still deserve systematic investigation. Here, an efficient channel modeling method based on statistical analysis is proposed. Three wind-associated environmental models are integrated into this hybrid model. The Texel-Marsen-Arsole spectral model is adopted to generate a three-dimensional shallow-water surface, which affects the Doppler shifts of large-scale paths. Small-scale micropaths are statistically analyzed and modeled according to the measured channels. The Hall-Novarini model is adopted to simulate the refraction and attenuation caused by wind-generated bubbles. An existing wind-generated noise model is applied to calculate the noise spectrum. The proposed model has been validated by the at-sea measurements collected in the Gulf of Mexico in 2016 and 2017. This model can be used to further analyze the channels at different carrier frequencies, bandwidths, and wind speeds for certain transmission conditions.

Analysis of sound pressure levels generated by nozzle-emitted large bubbles

The sound radiated by newly formed bubbles can be used to determine their properties. However, details of the fluid dynamics driving the acoustic emission remain unclear. A neck-collapsing model has been proposed to explain the sound generation at bubble pinch-off. The model uses a forcing function which drives the Rayleigh-Plesset equation and is linked to the bubble acoustic pressure. Here, the model is tested on bubbles of diameter up to 7 mm generated in distilled water, tap water, and alcohol-water solution. The model works well for bubbles less than 2.2 mm radius but the error increases up to 71% for larger diameters.

An Experimental Study on Measuring Breaking-Wave Bubbles with LiDAR Remote Sensing

Laboratory experiments were conducted to evaluate the feasibility of profiling and characterizing subsurface bubble plumes following a breaking wave event from an above-water Light Detection and Ranging (LiDAR) system. Measurements of LiDAR backscatter profiles of bubble plumes under mechanically generated breaking waves in a wave tank were collected and analyzed. After onset of wave breaking, the LiDAR backscatter increases rapidly by injected bubble plumes of active wave breaking. This intensification reaches a depth of one wave height within one wave period. After active wave breaking, the LiDAR backscatter from dissipated bubble plumes in the upper layer of water column decreases very slowly. The temporal variations of LiDAR backscatter are comparable to the collocated in-water measurements of optical backscatter at 850 nm wavelength and acoustic backscatter at 2000 kHz frequency. The decay rate of LiDAR backscatter of dissipated bubble plumes follows a power-law function consistent with decay rate of void fraction measurements in previous studies. This study demonstrates the viability and potential of using above-water LiDAR remote sensing to characterize subsurface bubble plumes.

A numerical simulation framework for bubbly flow and sound generation in laboratory-scale breaking waves

A simulation framework for bubbly flow and the sound radiated by breaking waves is presented. It consists of a two-phase flow solver, an algorithm to track bubbles and bubble creation rates, and a module to compute the sound generated by newly-formed bubbles. The sounds from breaking, third-order Stokes waves of 0.25 m wavelength and two slopes are calculated. The results show encouraging agreement with existing laboratory observations and identify the importance of air cylinder breakup in bubble creation. Remaining problems include modeling boundary effects that inhibit bubble coalescence in seawater and the generation of sound by the breakup of air cylinders.

Observations of wind-generated noise by the tropical cyclone

An analysis of the measured ocean noise during the tropical cyclone period is presented. While the observed noise is highly correlated with wind, this study reveals the dispersion of noise spectra. A wind-driven noise model within the framework of the bubble oscillation is developed. The noise spectrum for frequencies from hundreds of hertz to kilohertz due to the effective bubble oscillation within the bubble cloud is assumed instead of the collective bubble oscillation. The proposed model addresses the arbitrarily-shaped bubble clouds in a stratified ocean and the relation between the wind speed and the noise level, and these aspects develop from the existing models in the literature. The wind-driven ocean noise spectrum is estimated as a function of frequency and wind speed based on available information of the bubble creation rate. The comparison shows that the proposed model with optimized oceanographic parameters could fit the noise spectra of data for the frequencies from about 0.5 to 4 kHz, which indicates that the modified model based on the bubble oscillations within the bubble clouds could account for the mechanism of wind-generated noise data.

The role of jet and film drops in controlling the mixing state of submicron sea spray aerosol particles

The oceans represent a significant global source of atmospheric aerosols. Sea spray aerosol (SSA) particles comprise sea salts and organic species in varying proportions. In addition to size, the overall composition of SSA particles determines how effectively they can form cloud droplets and ice crystals. Thus, understanding the factors controlling SSA composition is critical to predicting aerosol impacts on clouds and climate. It is often assumed that submicrometer SSAs are mainly formed by film drops produced from bursting bubble-cap films, which become enriched with hydrophobic organic species contained within the sea surface microlayer. In contrast, jet drops formed from the base of bursting bubbles are postulated to mainly produce larger supermicrometer particles from bulk seawater, which comprises largely salts and water-soluble organic species. However, here we demonstrate that jet drops produce up to 43% of total submicrometer SSA number concentrations, and that the fraction of SSA produced by jet drops can be modulated by marine biological activity. We show that the chemical composition, organic volume fraction, and ice nucleating ability of submicrometer particles from jet drops differ from those formed from film drops. Thus, the chemical composition of a substantial fraction of submicrometer particles will not be controlled by the composition of the sea surface microlayer, a major assumption in previous studies. This finding has significant ramifications for understanding the factors controlling the mixing state of submicrometer SSA particles and must be taken into consideration when predicting SSA impacts on clouds and climate.

Temporal and Spatial Comparisons of Underwater Sound Signatures of Different Reef Habitats in Moorea Island, French Polynesia

As environmental sounds are used by larval fish and crustaceans to locate and orientate towards habitat during settlement, variations in the acoustic signature produced by habitats could provide valuable information about habitat quality, helping larvae to differentiate between potential settlement sites. However, very little is known about how acoustic signatures differ between proximate habitats. This study described within- and between-site differences in the sound spectra of five contiguous habitats at Moorea Island, French Polynesia: the inner reef crest, the barrier reef, the fringing reef, a pass and a coastal mangrove forest. Habitats with coral (inner, barrier and fringing reefs) were characterized by a similar sound spectrum with average intensities ranging from 70 to 78 dB re 1 μPa.Hz(-1). The mangrove forest had a lower sound intensity of 70 dB re 1 μPa.Hz(-1) while the pass was characterized by a higher sound level with an average intensity of 91 dB re 1 μPa.Hz(-1). Habitats showed significantly different intensities for most frequencies, and a decreasing intensity gradient was observed from the reef to the shore. While habitats close to the shore showed no significant diel variation in sound intensities, sound levels increased at the pass during the night and barrier reef during the day. These two habitats also appeared to be louder in the North than in the West. These findings suggest that daily variations in sound intensity and across-reef sound gradients could be a valuable source of information for settling larvae. They also provide further evidence that closely related habitats, separated by less than 1 km, can differ significantly in their spectral composition and that these signatures might be typical and conserved along the coast of Moorea.

Surface tension effects in breaking wave noise

The role of surface active materials in the sea surface microlayer on the production of underwater noise by breaking waves is considered. Wave noise is assumed to be generated by bubbles formed within actively breaking whitecaps, driven into breathing mode oscillation at the moment of their formation by non-equilibrium, surface tension forces. Two significant effects associated with surface tension are identified-a reduction in low frequency noise (<1000 Hz) due to the re-fragmentation of actively radiating bubbles by fluid turbulence within the whitecap and a reduction in overall noise level due to a decrease in the excitation amplitude of bubbles associated with reduced surface tension. The impact of the latter effect on the accuracy of Weather Observations Through Ambient Noise estimates of wind speed is assessed and generally found to be less than ±1 m s(-1) for wind speeds less than 10 m s(-1) and typical values of surfactant film pressure within sea slicks.

Review of scattering and extinction cross-sections, damping factors, and resonance frequencies of a spherical gas bubble

Perhaps the most familiar concepts when discussing acoustic scattering by bubbles are the resonance frequency for bubble pulsation, the bubbles' damping, and their scattering and extinction cross-sections, all of which are used routinely in oceanography, sonochemistry, and biomedicine. The apparent simplicity of these concepts is illusory: there exist multiple, sometimes contradictory definitions for their components. This paper reviews expressions and definitions in the literature for acoustical cross-sections, resonance frequencies, and damping factors of a spherically pulsating gas bubble in an infinite liquid medium, deriving two expressions for "resonance frequency" that are compared and reconciled with two others from the reviewed literature. In order to prevent errors, care is needed by researchers when combining results from different publications that might have used internally correct but mutually inconsistent definitions. Expressions are presented for acoustical cross-sections associated with forced pulsations damped by liquid shear and (oft-neglected) bulk or dilatational viscosities, gas thermal diffusivity, and acoustic re-radiation. The concept of a dimensionless "damping coefficient" is unsuitable for radiation damping because different cross-sections would require different functional forms for this parameter. Instead, terms based on the ratio of bubble radius to acoustic wavelength are included explicitly in the cross-sections where needed.

The effect of wind-generated bubbles on sea-surface backscattering at 940 Hz

Reliable predictions of sea-surface backscattering strength are required for sonar performance modeling. These are, however, difficult to obtain as measurements of sea-surface backscattering are not available at small grazing angles relevant to low-frequency active sonar (1-3 kHz). Accurate theoretical predictions of scattering strength require a good understanding of physical mechanisms giving rise to the scattering and the relative importance of these. In this paper, scattering from individual resonant bubbles is introduced as a potential mechanism and a scattering model is derived that incorporates the contribution from these together with that of rough surface scattering. The model results are fitted to Critical Sea Test (CST) measurements at a frequency of 940 Hz, treating the number of large bubbles, parameterized through the spectral slope of the size spectrum for bubbles whose radii exceed 1 mm, as a free parameter. This procedure illustrates that the CST data can be explained by scattering from a small number of large resonant bubbles, indicating that these provide an alternative mechanism to that of scattering from bubble clouds.

Quantification of undersea gas leaks from carbon capture and storage facilities, from pipelines and from methane seeps, by their acoustic emissions

In recent years, because of the importance of leak detection from carbon capture and storage facilities and the need to monitor methane seeps and undersea gas pipelines, there has been an increased requirement for methods of detecting bubbles released from the seabed into the water column. If undetected and uncorrected, such leaks can generate huge financial and environmental losses. This paper describes a theory by which the passive acoustic signals detected by a hydrophone array can be used to quantify gas leakage, providing a practical (as opposed to research), passive and remote detection system which can monitor over a period of years using simple instrumentation. The sensitivity in detecting and quantifying the flux of gas is shown to exceed by more than two orders of magnitude the sensitivity of the current model-based techniques used commercially for gas leaks from large, long pipelines.

Analysis of high-frequency wind-driven ambient noise in shallow brackish water

Ambient noise spectra in a shallow brackish water environment were found to be steeper than expected at frequencies above 10 kHz. The high-frequency behavior of the spectra was resolved by modeling dispersion and noise in bubbly water. Bubble size distributions fitted to the brackish water spectra exhibit a distinctive maximum in the radius range 0.1-0.3 mm, and a substantial drop in bubble density below a radius of 0.1 mm. The brackish water distributions were tied to an oceanic spectrum with a spectral slope of 5.7 dB/octave obtained with a -3 / 2 power law dependence of bubble size density on radius.