Air-Entrainment Mechanisms in Plunging Jets and Breaking Waves

Kiger, Ken; Duncan, James H.

doi:10.1146/annurev-fluid-122109-160724

Cited by 193 publications

(153 citation statements)

References 89 publications

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“…wave focusing, modulation instability), and the Stokes waves used here have a breaking threshold higher than the one observed in laboratory experiments using focusing wave packet. Second, the low values of the entrained air in the DNS for small slopes can also be related to the relatively short wavelength of our breaking wave, λ = 0.24, and the influence of surface tension in the shape of the breaking wave, which reduces the amount of entrained air, as discussed in Song & Sirviente (2004); Liu & Duncan (2003; Kiger & Duncan (2012).…”

Section: Volume Scaling Of the Dns And Available Experimental Datamentioning

confidence: 95%

“…Bubbles between 2 mm and the mesh size are first created at the impact (t/T ≈ 1), then a rapid growth of the bubble size distribution is observed (1 t/T 1.6), with bubbles as large as 10 mm, corresponding to the collapse of the air cavity. Therefore, bubbles of various sizes are created during the initial impact and entrapment of the air cavity (visible in Figure 4), due to entrainment by the jet, and at the edges of the air cavity, as discussed by Deane & Stokes (2002) and Kiger & Duncan (2012). The maximum number of bubbles in the system is reached at the end of this growing stage.…”

Section: Time Evolution Of the Bubble Size Distribution And Integral mentioning

confidence: 96%

“…Before the jet reconnects, the wave dynamics remain mostly two-dimensional. Air entrainment then occurs through different mechanisms, recently summarized in the review by Kiger & Duncan (2012): the entrapment of an air pocket when the jet reconnects the water, entraining a large cavity and large bubbles; entrainment around the jet impact site entraining smaller bubbles; entrapment by the subsequent splashes events; entrainment by the turbulent breakdown of the forward face of the wave. Indeed, numerous bubbles and droplets are visible (c,d,e) while the flow has become fully three-dimensional.…”

Section: Summary Of the Runsmentioning

confidence: 99%

“…Entrainment of air also occurs when the jet impacts the surface (c), during the subsequent splashing (d,e), and when high velocity droplets fall back into the water (f,g). As discussed by Deane & Stokes (2002); Kiger & Duncan (2012), there are two main steps in bubble formation, first when the jet impact the water surface, creating relatively small bubbles and second when the air cavity collapses, creating larger bubbles that are then fragmented by turbulent fluctuations. Up to a thousand bubbles are counted in this simulation (f,g).…”

Section: Summary Of the Runsmentioning

confidence: 99%

“…Figure 14 shows the DNS data for the total volume entrained during the breaking event, where all the variables are measured, together with the available laboratory data. As discussed in the review by Kiger & Duncan (2012), only few measurements of the entrained air exist: Lamarre & Melville (1991) and Blenkinsopp & Chaplin (2007). The measurements from Blenkinsopp & Chaplin (2007) can not be used here, since wave breaking is obtained when the wave propagates over a shoal.…”

Section: Volume Scaling Of the Dns And Available Experimental Datamentioning

confidence: 99%

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Air entrainment and bubble statistics in breaking waves

2016

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We investigate air entrainment and bubble statistics in three-dimensional breaking waves through novel direct numerical simulations of the two-phase air-water flow, resolving the length scales relevant for the bubble formation problem, the capillary length and the Hinze scale. The dissipation due to breaking is found to be in good agreement with previous experimental observations and inertial-scaling arguments. The air-entrainment properties and bubble-size statistics are investigated for various initial characteristic wave slopes. For radii larger than the Hinze scale, the bubble size distribution, can be described by N (r, t) = B(V 0 /2π)(ε(t − ∆τ )/W g)r −10/3 r −2/3 m during the active breaking stages, where ε(t − ∆τ ) is the time dependent turbulent dissipation rate, with ∆τ the collapse time of the initial air pocket entrained by the breaking wave, W a weighted vertical velocity of the bubble plume, r m the maximum bubble radius, g gravity, V 0 the initial volume of air entrained, r the bubble radius and B a dimensionless constant. The active breaking time-averaged bubble size distribution is described bȳ N (r) = B(1/2π)( l L c /W gρ)r −10/3 r −2/3 m , where l is the wave dissipation rate per unit length of breaking crest, ρ the water density and L c the length of breaking crest. Finally, the averaged total volume of entrained air,V , per breaking event can be simply related to l byV = B( l L c /W gρ), which leads to a relationship to a characteristic slope, S, of V ∝ S 5/2 . We propose a phenomenological turbulent bubble break-up model, based on earlier models and the balance between mechanical dissipation and work done against buoyancy forces. The model is consistent with the numerical results and existing experimental results.

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Section: Volume Scaling Of the Dns And Available Experimental Datamentioning

confidence: 95%

Section: Time Evolution Of the Bubble Size Distribution And Integral mentioning

confidence: 96%

Section: Summary Of the Runsmentioning

confidence: 99%

Section: Summary Of the Runsmentioning

confidence: 99%

Section: Volume Scaling Of the Dns And Available Experimental Datamentioning

confidence: 99%

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Air entrainment and bubble statistics in breaking waves

2016

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On the seawater temperature dependence of the sea spray aerosol generated by a continuous plunging jet

et al. 2014

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Key Points:• Particle concentrations decreased as seawater temperature increased to ∼9°C • Bubbles with r film < 2 mm decreased as seawater temperature increased to ∼9°C• Particle concentration was correlated with bubble density at the water surface Abstract Breaking waves on the ocean surface produce bubbles which, upon bursting, deliver seawater constituents into the atmosphere as sea spray aerosol particles. One way of investigating this process in the laboratory is to generate a bubble plume by a continuous plunging jet. We performed a series of laboratory experiments to elucidate the role of seawater temperature on aerosol production from artificial seawater free from organic contamination using a plunging jet. The seawater temperature was varied from −1.3• C to 30.1 • C, while the volume of air entrained by the jet, surface bubble size distributions, and size distribution of the aerosol particles produced was monitored. We observed that the volume of air entrained decreased as the seawater temperature was increased. The number of surface bubbles with film radius smaller than 2 mm decreased nonlinearly with seawater temperature. This decrease was coincident with a substantial reduction in particle production. The number concentrations of particles with dry diameter less than ∼1 m decreased substantially as the seawater temperature was increased from −1.3With further increase in seawater temperature (up to 30• C), a small increase in the number concentration of larger particles (dry diameter >∼0.3 m) was observed. Based on these observations, we infer that as seawater temperature increases, the process of bubble fragmentation changes, resulting in decreased air entrainment by the plunging jet, as well as the number of bubbles with film radius smaller than 2 mm. This again results in decreased particle production with increasing seawater temperature.

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Experimental study on plunging breaking waves in deep water

et al. 2015

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This study presents a unique data set that combines measurements of velocities and void fraction under an unsteady deep water plunging breaker in a laboratory. Flow properties in the aerated crest region of the breaking wave were measured using modified particle image velocimetry (PIV) and bubble image velocimetry (BIV). Results show that the maximum velocity in the plunging breaker reached 1.68C at the first impingement of the overturning water jet with C being the phase speed of the primary breaking wave, while the maximum velocity reached 2.14C at the beginning of the first splash-up. A similarity profile of void fraction was found in the successive impinging and splash-up rollers. In the highly foamy splashing roller, the increase of turbulent level and vorticity level were strongly correlated with the increase of void fraction when the range of void fraction was between 0 and 0.4 (from the trough level to approximately the center of the roller). The levels became constant when void fraction was greater than 0.5. The mass flux, momentum flux, kinetic energy, potential energy, and total energy were computed and compared with and without the void fraction being accounted for. The results show that all the mean and turbulence properties related to the air-water mixture are considerably overestimated unless void fraction is considered. When including the density variation due to the air bubbles, the wave energy dissipated exponentially a short distance after breaking; about 54% and 85% of the total energy dissipated within one and two wavelengths beyond the breaking wave impingement point, respectively.

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Air-Entrainment Mechanisms in Plunging Jets and Breaking Waves

Cited by 193 publications

References 89 publications

Air entrainment and bubble statistics in breaking waves

Air entrainment and bubble statistics in breaking waves

On the seawater temperature dependence of the sea spray aerosol generated by a continuous plunging jet

Experimental study on plunging breaking waves in deep water

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