Void fraction measurements in breaking waves

Blenkinsopp, Christopher; Chaplin, J.R.

doi:10.1098/rspa.2007.1901

Cited by 102 publications

(154 citation statements)

References 49 publications

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“…As described by Lamarre & Melville (1991), the fast decay is exponential, with V = V 0 exp (−κt/T ), with κ a numerical factor varying from approximately 2.5 to 4 in our DNS depending on the initial wave slope. These values appear in reasonable agreement with the experimental values, considering the difficulty of the experiments of Lamarre & Melville (1991) and Blenkinsopp & Chaplin (2007), who found κ = 3.9 and κ ≈ 5 respectively. This rapid decay corresponds to the fast degassing of the plume caused by large bubbles rising back to the surface.…”

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

confidence: 79%

“…The bubble cloud dynamics, as well as the void fraction observed during the breaking are consistent with previous experimental observations (Lamarre & Melville 1991, 1994Blenkinsopp & Chaplin 2007): the air cavity is first entrained with α = 100%, collapses and gives birth to a bubble cloud with α up to 30% during the active breaking stages.…”

Section: Air Entrainment and Void Fractionsupporting

confidence: 76%

“…This measure of the void fraction can then be integrated in space in order to obtain another estimate the amount of air entrained by the breaking wave. The main interface during the breaking process can then be defined by the α(x, z, t) = 50% value, as proposed experimentally by several authors (Lamarre & Melville 1991;Blenkinsopp & Chaplin 2007). Figure 6 shows the evolution of the void fraction (volume of air per unit volume) α(x, z, t) during the breaking process, for a plunging breaker (S = 0.55) and a spilling breaker (S = 0.43).…”

Section: Air Entrainment and Void Fractionmentioning

confidence: 99%

“…(1.1)), while undoubtelly correct for constant Q, does not describe the temporal evolution in which the bubble size distribution experiences very rapid change (Terrill et al 2001;Deane & Stokes 2002). Moreover, measurements of the volume of entrained air have shown that Q is not a constant parameter in this problem (Lamarre & Melville 1991;Blenkinsopp & Chaplin 2007). Finally the dependence onε and Q in Eq.…”

Section: Bubble Size Distribution Models and Observationsmentioning

confidence: 99%

“…Experimentally, the local void fraction α(x, z, t) (volume of air per unit volume of water) is measured using conductivity (Lamarre & Melville 1991) or optical probes (Blenkinsopp & Chaplin 2007;Rojas & Loewen 2007). A spatial map of the void fraction is then obtained by repeating this measure at various locations.…”

Section: Air Entrainment and Void Fractionmentioning

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: Time Evolution Of the Bubble Size Distribution And Integral supporting

confidence: 79%

Section: Air Entrainment and Void Fractionsupporting

confidence: 76%

Section: Air Entrainment and Void Fractionmentioning

confidence: 99%

Section: Bubble Size Distribution Models and Observationsmentioning

confidence: 99%

Section: Air Entrainment and Void Fractionmentioning

confidence: 99%

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Air entrainment and bubble statistics in breaking waves

2016

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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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The effect of water temperature on air entrainment, bubble plumes, and surface foam in a laboratory breaking-wave analog

Callaghan

Stokes

Deane

2014

J. Geophys. Res. Oceans

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Air-entraining breaking waves form oceanic whitecaps and play a key role in climate regulation through air-sea bubble-mediated gas transfer, and sea spray aerosol production. The effect of varying sea surface temperature on air entrainment, subsurface bubble plume dynamics, and surface foam evolution intrinsic to oceanic whitecaps has not been well studied. By using a breaking wave analog in the laboratory over a range of water temperatures (T w 5 5 C to T w 5 30 C) and different source waters, we have examined changes in air entrainment, subsurface bubble plumes, and surface foam evolution over the course of a breaking event. For filtered seawater, air entrainment was estimated to increase by 6% between T w 5 6 C and T w 5 30 C, driven by increases of about 43% in the measured surface roughness of the plunging water sheet. After active air entrainment, the rate of loss of air through bubble degassing was more rapid at colder water temperatures within the first 0.5 s of plume evolution. Thereafter, the trend reversed and bubbles degassed more quickly in warmer water. The largest observed temperature-dependent differences in subsurface bubble distributions occurred at radii greater than about 700 lm. Temperature-dependent trends observed in the subsurface bubble plume were mirrored in the temporal evolution of the surface whitecap foam area demonstrating the intrinsic link between surface whitecap foam and the subsurface bubble plume. Differences in foam and plume characteristics due to different water sources were greater than the temperature dependencies for the filtered seawater examined.

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Void fraction measurements in breaking waves

Cited by 102 publications

References 49 publications

Air entrainment and bubble statistics in breaking waves

Air entrainment and bubble statistics in breaking waves

Experimental study on plunging breaking waves in deep water

The effect of water temperature on air entrainment, bubble plumes, and surface foam in a laboratory breaking-wave analog

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