Laboratory Breaking Waves Research Articles

A Comparison of Laboratory and Field Measurements of Whitecap Foam Evolution From Breaking Waves

AbstractSufficiently energetic breaking ocean waves produce distinctive visible foam signatures on the water surface called whitecaps. The mixture of surface whitecap foam cells, and sub‐surface bubbles, results in the broad‐band scattering of light that allow whitecaps to be measured with optical cameras. In this paper the temporal evolution of whitecap foam area from laboratory and oceanic breaking waves is compared. When appropriately scaled, the foam area time series for both laboratory and oceanic breaking waves follow similar trends, despite occurring in vastly different settings. Distinct similarities of the signature of foam stabilization due to the presence of surfactants in the controlled laboratory experiments are also found in the field suggesting foam stabilization may be a means to remotely sense the presence/absence or concentration of surfactants in the ocean. In addition, probability density distributions of key whitecap variables such as foam area growth and decay timescales and maximum foam area are compared between laboratory and oceanic whitecaps. The oceanic whitecaps are much larger in scale than the laboratory breaking waves, whereas the whitecap growth and decay timescales are similar in magnitude, the latter suggesting that the depths to which bubbles are injected during active air entrainment in the field are relatively shallow. The aggregated whitecap statistics are used to estimate the energy dissipation of individual whitecaps in a novel manner.

The influence of bandwidth on the energetics of intermediate to deep water laboratory breaking waves

An experimental investigation of two-dimensional dispersively focused laboratory breaking waves is presented. We describe the bandwidth effect on breaking wave energetics, including spectral energy evolution, characteristic group velocity, energy dissipation and its rate, and breaking strength parameter, $b$ . To evaluate the role of bandwidth, three definitions of wave group steepness are adopted where $S_s$ and $S_n$ are bandwidth-dependent and $S_p$ remains constant when bandwidth is changed. Our data show two regimes of spectral energy evolution in breaking wave groups, with both regimes bandwidth-dependent: energy dissipation and gain occur at $f > 0.95f_p$ ( $f_p$ is the peak frequency) and $f < 0.95f_p$ , respectively. The characteristic group velocity, which is used in energy dissipation calculations, increases by up to 7 % after wave breaking, being larger for higher bandwidth breaking waves. An unambiguous bandwidth dependence is found between $S_p$ and both the fractional and absolute wave energy dissipation. Wave groups of larger bandwidth break at a lower value of $S_p$ and consequently lose relatively more energy. The energy dissipation rate depends on the breaking duration which itself is bandwidth dependent. Consequently, no clear bandwidth effect is observed in energy dissipation rate when compared with either $S_p$ or $S_s$ . However, there is a systematic bandwidth dependence in the variation of $b$ when parameterised in terms of $S_p$ , with their relationship becoming increasingly nonlinear as bandwidth increases. When parameterised with $S_s$ , $b$ shows a markedly reduced bandwidth dependence. Finally, the numerical breaking onset and relationship between $b$ and $S_s$ in the numerical study of Derakhti & Kirby (J. Fluid Mech., vol. 790, 2016, pp. 553–581) is validated experimentally.

High-resolution direct simulation of deep water breaking waves: transition to turbulence, bubbles and droplets production

We present high-resolution three-dimensional (3-D) direct numerical simulations of breaking waves solving for the two-phase Navier–Stokes equations. We investigate the role of the Reynolds number (Re, wave inertia relative to viscous effects) and Bond number (Bo, wave scale over the capillary length) on the energy, bubble and droplet statistics of strong plunging breakers. We explore the asymptotic regimes at highReandBo, and compare with laboratory breaking waves. Energetically, the breaking wave transitions from laminar to 3-D turbulent flow on a time scale that depends on the turbulentReup to a limiting value$Re_\lambda \sim 100$, consistent with the mixing transition in other canonical turbulent flows. We characterize the role of capillary effects on the impacting jet and ingested main cavity shape and subsequent fragmentation process, and extend the buoyant-energetic scaling from Deikeet al.(J. Fluid Mech., vol. 801, 2016, pp. 91–129) to account for the cavity shape and its scale separation from the Hinze scale,$r_H$. We confirm two regimes in the bubble size distribution,$N(r/r_H)\propto (r/r_H)^{-10/3}$for$r>r_H$, and$\propto (r/r_H)^{-3/2}$for$r<r_H$. Bubbles are resolved up to one order of magnitude below$r_H$, and we observe a good collapse of the numerical data compared to laboratory breaking waves (Deane & Stokes,Nature, vol. 418 (6900), 2002, pp. 839–844). We resolve droplet statistics at highBoin good agreement with recent experiments (Erininet al.,Geophys. Res. Lett., vol. 46 (14), 2019, pp. 8244–8251), with a distribution shape close to$N_d(r_d)\propto r_d^{-2}$. The evolution of the droplet statistics appears controlled by the details of the impact process and subsequent splash-up. We discuss velocity distributions for the droplets, finding ejection velocities up to four times the phase speed of the wave, which are produced during the most intense splashing events of the breaking process.

On the imprint of surfactant‐driven stabilization of laboratory breaking wave foam with comparison to oceanic whitecaps

AbstractSurfactants are ubiquitous in the global oceans: they help form the materially‐distinct sea surface microlayer (SML) across which global ocean‐atmosphere exchanges take place, and they reside on the surfaces of bubbles and whitecap foam cells prolonging their lifetime thus altering ocean albedo. Despite their importance, the occurrence, spatial distribution, and composition of surfactants within the upper ocean and the SML remains under‐characterized during conditions of vigorous wave breaking when in‐situ sampling methods are difficult to implement. Additionally, no quantitative framework exists to evaluate the importance of surfactant activity on ocean whitecap foam coverage estimates. Here we use individual laboratory breaking waves generated in filtered seawater and seawater with added soluble surfactant to identify the imprint of surfactant activity in whitecap foam evolution. The data show a distinct surfactant imprint in the decay phase of foam evolution. The area‐time‐integral of foam evolution is used to develop a time‐varying stabilization function, and a stabilization factor, , which can be used to identify and quantify the extent of this surfactant imprint for individual breaking waves. The approach is then applied to wind‐driven oceanic whitecaps, and the laboratory and ocean distributions overlap. It is proposed that whitecap foam evolution may be used to determine the occurrence and extent of oceanic surfactant activity to complement traditional in‐situ techniques and extend measurement capabilities to more severe sea states occurring at wind speeds in excess of about 10 m/s. The analysis procedure also provides a framework to assess surfactant‐driven variability within and between whitecap coverage data sets.

Surface tension effects on energy dissipation by small scale, experimental breaking waves

Much of the existing knowledge about breaking waves comes from physical model experiments scaled using Froude's law. A widely held assumption is that surface tension effects are not significant at typical laboratory scales and specifically for waves longer than 2 m. Since, however, smaller wavelengths are not untypical in small to medium scale laboratory facilities, a consideration of surface tension effects is indeed important. Although some emphasis has been given in the past, little is known regarding the importance of surface tension following impingement of the breaking-wave crest and especially on the overall energy dissipation by laboratory breaking waves. To answer this question a laboratory study was conducted comparing the energy dissipation rate by waves breaking in fresh-water and in a 10% isopropyl-alcohol and distilled water solution of lower surface tension. It was found that when waves shorter than 4 m and smaller than 0.11 m break under the weakened influence of surface tension they dissipate up to 65% more wave energy. Visual observations link the increased wave energy dissipation with greater breaking intensity, increased air entrainment and longer bubble lifetime. Overall, the results presented here indicate that the inability to maintain Weber similarity (ratio of fluid inertia to its surface tension) has a more significant effect on the intensity of wave energy dissipation than previously assumed.

Sound radiation by various types of laboratory breaking waves in fresh and salt water

A recent article [A. R. Kolaini and L. A. Crum, J. Acoust. Soc. Am. 96, 1755–1765 (1994)] reported the measurements of the ambient sound generated by laboratory breaking waves over the range 100–20 000 Hz in fresh water. Those observations from both spilling- and plunging-type breakers have been repeated in the same manner and wavemaker/anechoic tank with water that had 25‰ salt in its content. The observations in salt water, just like those in fresh water, reveal that the sources of sound in laboratory spilling breakers are due primarily to single bubble oscillations that can have frequencies lower than a few hundred Hertz. In the case of weak spilling breakers, the sound spectra level in fresh water was due primarily to single bubble oscillation, while the same breakers in salt water have introduced smaller size bubbles with large density. The relatively high-density populated bubble cloud generated by weak breakers shows the evidence of the onset of collective oscillation that was absent for the same breakers in the fresh water. In the case of moderate spilling and plunging breakers, it appears that both individual bubbles and bubble clouds can contribute to the acoustic emissions in fresh and salt water. The average sound spectra reveal that the peak frequencies of the spectra shift from a few kHz (weak, spilling breaker) to few hundred Hz (plunging breaker), and the high-frequency portions have slopes approximately 5–6 dB/oct, which are the slopes observed from the noise spectra of the ocean. Besides the high bubble density and smaller bubble sizes in salt water, all breakers experienced a significant increase in sound-pressure level in all observed frequency range. The ionic structure of the medium alters the sound radiation from bubbles. In this paper some of the observed acoustic signatures from breaking waves are discussed and a plausible explanation of how salt can effect the sound radiation from bubbles is given.

Scattering of sound from laboratory breaking waves

Computer-controlled gravity waves were generated to break in an anechoic tank. A sequence of 15 incident bursts was directed at the bubble cloud entrained by each breaking wave. A burst has seven cycles of a sine wave of frequency ranging from 10 to 15 kHz. The interval between adjacent bursts is set at 0.1 s in order to avoid overlapping the bursts. By analyzing the data of the 15 sound scattering events, the fluctuation was significantly reduced. The bubble clouds were observed to be roughly semicylindrical. The bubble size distribution in an entrained cloud was obtained from video images of a bubble cloud. This experimental information of the shape, size, and bubble size distribution of a bubble cloud was employed as the input into the theory adapted from Sarkar and Prosperetti [J. Acoust. Soc. Am. 93, 3128–3138 (1993)] to estimate the theoretical scattering strength. A comparison between the experimentally measured and the theoretically estimated scattering strengths will be shown. [Work supported by ONR.]

Measurements of the scattering of sound from laboratory breaking waves

The results of a laboratory experiment to characterize the backscattered sound field from the bubble cloud generated by spilling breaking waves are described. Gravity waves were generated by a computer controlled plunging-type wavemaker along the length of a 12.7-m-long channel where they were made to break in a 3.6 m×3.6 m×2.4 anechoic tank. An underwater F42A transducer with a special parabolic reflector was used to generate incident bursts of sound ranging from 15 to 40 kHz. This procedure was conducted in the presence of breaking and nonbreaking gravity waves in order to isolate the acoustic scattering strength of the bubble clouds from surface roughness. Detailed measurements of the average void fraction of the bubble cloud at the instant the sound was incident were made. These observations show that there is a significant increase in backscattering strength from the bubble cloud when compared to surface roughness. The scattering strength of the bubble cloud, with an average void fraction of 0.33%, is also shown to increase as the incident frequency approaches the resonant frequency of the individual bubbles in the cloud. A simple theory will be given to predict some of these observations.

Acoustic characterization of laboratory breaking waves in fresh and salt water

Results of a laboratory experiment to characterize the underwater acoustic noise radiated from breaking waves in fresh and salt water are described. The underwater sound field radiated by various breaking waves intensities in fresh water in the range from 0.1 to 20 kHz were reported earlier [A. R. Kolaini and L. A. Crum, J. Acoust. Soc. Am. (in press); 92, 2349(A) (1992)]. These waves were generated by a computer-controlled plunging-type wavemaker and propagated along a 12.7-m-long channel where they were made to break at the midsurface of a 3.6-×3.6-×2.4-m anechoic tank. Specific attention is given to a comparison of the results obtained in salt and fresh water. The sources of acoustic radiation from bubble clouds, the average sound-pressure level, bubble clouds shape, bubble concentration, and size distribution in these medium are discussed. These results may provide considerable insight into the likely source mechanisms for ocean ambient noise.

Observations of underwater sound from laboratory breaking waves and the implications concerning ambient noise in the ocean

The results of an experiment to characterize the underwater sound field radiated by various breaking waves intensities in fresh water in the range from 0.1 to 20 kHz are described. Waves are generated by a computer-controlled plunging-type wave maker and propagate along a 12.7-m-long channel where they are made to break at the mid-surface of a 3-×3-×2.5-m anechoic water tank. The individual bubbles and bubble clouds entrained by the breaking wave provide a mechanism for sound production. Using high-speed cinephotography, correlations were established between the hydrodynamic evolution of the cloud and the radiated acoustic emissions. The bubble size distributions inside the cloud were measured with the aid of a high-speed video camera and a fiber optic cable. These measurements indicate that single bubbles with radii as large as 7–8 mm may be entrained in this fresh-water system by moderate spilling breakers. Detailed measurements of the bubble size distribution of the bubble cloud enabled us also to obtain a measurement of the average void fraction in the cloud. These observations reveal that the sources of sound in laboratory spilling breakers is due mostly to single bubble oscillations that can have frequencies as low as 400 Hz; in the case of plunging breakers, it appears that both individual bubbles and bubble clouds can contribute to the acoustic emissions. The acoustic radiation from bubble clouds is the result of collective oscillations of the bubbles, stimulated by large scale vortices created by the plunging breaker. The sound spectra, averaged over 100 breakers, reveal the following observations. First, the peak frequencies of the average sound spectra shifts from few kHz (weak, spilling breaker) to few hundred Hz (plunging breaker). Second, the sound pressure levels increase in all frequency bands with increasing breaker severity. Lastly, the high-frequency portions of the sound spectra have slopes of about 5–6 dB/oct, which are the slopes observed for the noise spectra of the ocean. These results provide considerable insight into the likely source mechanisms for ocean ambient noise.

Void-fraction measurements and sound-speed fields in bubble plumes generated by breaking waves

Recent field and laboratory experiments have confirmed that low-frequency sound (10 to 300 Hz) is generated under breaking waves. It has been proposed that collective oscillations of the bubble plume generated by breaking may be the mechanism responsible for the generation of this sound. Confirmation of this process requires independent measurement of the void fraction, and therefore sound speed, in the bubbly mixture. Detailed measurements are presented of the evolution of the void-fraction field in bubble plumes generated by large-scale three-dimensional (3-D) laboratory breaking waves. Various moments of the void-fraction field are calculated and compared with results from two-dimensional (2-D) laboratory breaking waves [Lamarre and Melville, Nature 351, 469–472 (1991)]. The kinematics of the bubble plume reveals that the initial horizontal velocity of the plume is approximately 0.7C, where C is the wave phase speed. The centroid of the bubble plume is found to deepen at a speed of approximately 0.2H/T, where H and T are the wave height at breaking and the wave period, respectively. The radial dependence of the void-fraction and sound-speed field is characterized in terms of simple functions of time. Finally, the void-fraction measurements described here, along with independent measurements of the pressure fluctuations under breaking waves [Loewen and Melville, J. Acoust. Soc. Am. 95, 1329–1343 (1994)], support the hypothesis that low-frequency sound is generated by the collective oscillations of the bubble plume.

Acoustic characteristics of laboratory breaking waves

Results of a laboratory experiment to characterize the underwater sound field radiated from breaking waves are described. Waves are generated by a computer controlled plunging-type wave maker that propagate along a 40-ft-long channel and break at the surface of 12×12×8-ft anechoic water tank. The wave-maker parameters are controlled to produce spilling-type breakers with different breaker intensity (i.e., bubble cloud size). At smaller wave-maker amplitudes, a ‘‘gentle’’ spilling breaker can be produced to create a discrete number of bubbles. The number of bubbles and the sounds emitted by them can be measured and their type can be identified [H. Medwin and A. C. Daniel, J. Acoust. Soc. Am. 88, 408–412 (1990)]. Increasing the amplitude of the wave maker causes the breaker intensity to increase. The evolution of the bubble cloud size and its acoustic emissions are monitored by a high-speed video camera. The acoustic noise emitted at different breaker intensities by individual bubbles and by bubble clouds is presented and scaled to ocean breakers. Backscattering from bubble clouds was also observed using a parametric source. The scattered signal level was measured in the presence and absence of breaking waves as the insonifying frequency approached the collective oscillation frequencies of the bubble clouds. [Work supported by ONR.]

Void-fraction measurements in bubble plumes generated by 2-D and 3-D breaking waves.

Recent experiments have confirmed that low-frequency sound (10 to 300 Hz) is generated under breaking waves. It has been suggested that volume oscillations of the bubble plume generated by breaking is the mechanism responsible for the generation of this sound. Confirmation of this process requires independent measurement of the void fraction, and therefore sound speed, in the bubbly mixture. Detailed measurements of the evolution of the void-fraction field in bubble plumes generated by 2-D and large scale 3-D laboratory breaking waves are presented and compared. Various moments of the bubble plume void-fraction field give information on the (a) volume of air entrained, (b) cross-sectional area of the plume, (c) mean void fraction and sound speed, (d) bubble plume resonant frequencies, (e) potential energy of the bubble plume, (f) horizontal and vertical displacement and velocity of the bubble plume. These measurements along with independent measurements of the sound radiated (Loewen and Melville, this meeting) support the hypothesis that low-frequency sound is generated by the volume oscillation of the bubble plume. The radial dependence of the bubble plume’s void-fraction (and sound speed) field and its effect on low-frequency sound generation is investigated and the influence of the void-fraction instrumentation detection threshold in estimating the above moments is also examined. [Work supported by ONR (Ocean Acoustics) and NSF (Physical Oceanography).]

Instrumentation for the measurement of void-fraction in breaking waves: laboratory and field results

The authors report on the development and use of an impedance probe to measure the volume fraction of air (void-fraction) in bubble plumes generated by breaking waves. The void-fraction gauge described was found to be most useful in the initial period after breaking when large void-fractions prevail. The authors describe the instrumentation at length and report on its use in the laboratory and in the field. The instrument is found to be capable of rendering the space-time evolution of the void-fraction field from controlled laboratory breaking waves. Field results show measurements of void-fractions (up to 24%) which are several orders of magnitude greater than time averaged values previously reported. Preliminary measurements show that the fraction of breaking waves per wave is dependent on significant wave height and wind speed. The dependence on wind speed is compared with data of previous investigators. Underwater video photography from the field shows the formation and evolution of distinct bubble plumes and the presence of large bubbles (at least 6-mm radius) generated by breaking. >