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Journal of Harbin Institute of Technology (New Series)
Received 2019-04-18.
Sponsored by the National Natural Science Foundation of China (Grant Nos. 51879159, 51490675, 11432009, and 51579145), Chang Jiang Scholars Program (Grant No. T2014099), Shanghai Excellent Academic Leaders Program (Grant No. 17XD1402300), Program for Professor of Special Appointment (Eastern Scholar)
at Shanghai Institutions of Higher Learning (Grant No. 2013022), and Innovative Special Project of Numerical Tank of Ministry of Industry and Information
Technology of China (2016-23/09).
*Corresponding author. Distinguished Professor of Yangtze River Scholar. E - mail: [email protected] .
1
DOI:10.11916/j.issn.1005-9113.19036
Review: Recent Advancement of Experimental and Numerical
Investigations for Breaking Waves
Cheng Liu, Yiding Hu, Zheng Li and Decheng Wan*
(State Key Laboratory of Ocean Engineering, School of Naval Architecture, Ocean and Civil Engineering,
Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai Jiao Tong University,
Shanghai 200240, China)
Abstract: Breaking wave is a complex physical phenomenon that takes place at the gas-fluid interface, which is
the chief reason for the generation of two-phase turbulence, wave energy dissipation, and mass transfer between air
and water. For marine hydrodynamics, the breaking bow wave of high speed vessels induces the bubble-mixed flow
travelling around the ship, eventually developing to be the turbulent wake which is easy to be detected by
photoelectric equipment. Besides, the flow-induced noise stemming from wave plunging may weaken the acoustic
stealth of water surface craft. In the oceanographic physics context, wave breaking accounts for the energy and mass
exchange of the ocean-atmosphere system, which has a great effect on the weather forecasts and global climate
predictions. Due to multi-scale properties of multiphase turbulent flows, a wide range of time and length scales
should be resolved, making it rather complicated for experimental and numerical investigations. In early reviews[1-4]
,
general mechanisms related to wave breaking problems are well-described. However, previous emphasis lies on the
phenomenological characteristics of breaking wave. Thus, this review summarizes the recent experimental and
numerical advances of the studies of air entrainment, bubble distribution, energy dissipation, capillary effect, and so
on.
Keywords: wave breaking; energy dissipation; air entrainment and void fraction; bubble and droplet size
distribution; capillary effects
CLC number: O353.2 Document code: A
1. Experimental Studies
In last few decades, a number of important
experiments have been performed and fully analyzed to
reveal the intrinsic physics of breaking waves. Different
measurement methods including the high-speed
cameras, bubble detection approach[5]
, fiber optical
probes[6-7]
, Laser Doppler Velocimer[8]
, Particle Image
Velocimetry (PIV) technologies[9]
, and acoustic
measurement[10]
have been adopted.
By applying a visualization technique, Bonmarin[11]
analyzed the steep wave breaking evolution. The wave
profile of a plunging crest, the related splash-up, and
the air-entrainment phenomenon were observed. Qiao
and Duncan[12]
performed detailed analysis for the
unsteady evolution of moderate spilling wave breakings.
Grue and Jensen[13]
measured the velocity field and
corresponding accelerations of the steep plunging
waves from experiment. They found that for the
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2
horizontal and vertical accelerations over 1.1g (g
indicates the acceleration of gravity) and 1.5g, the wave
breaking phenomenon will be induced.
The experiment of breaking waves over an
underwater hydrofoil conducted by Duncan[14-15]
was
the first attempt to study the energy dissipation caused
by wave breaking. Rapp and Melville[8]
studied the
dispersion of the deep-water wave breakings by using
Laser Doppler measurement. They measured the loss of
excess momentum and energy flux of breaking waves,
and found over 10% flux loss for single spilling wave
breakers to as much as 25% flux loss for plunging wave
breakers. Chang and Liu[16]
investigated the turbulent
flow originated from wave breaking with limited water
depth. By applying the ensemble average approach, the
mean velocity, Reynolds stress, and strain rate were
obtained. It was concluded that the generation and
evolution of turbulence flows including its dissipation
are of equal importance, whereas the effect of
turbulence diffusion occupies a small portion.
Kimmoun and Branger[17]
performed the waves
breaking test in water tank by using a wedge body.
They calculated the spatial derivatives to acquire
adequate information for describing the flow structures.
Many existing experimental studies suggest that
the air fractions in wave-turbulence interaction process
can induce low-frequency-noise in the ocean. For a
better understanding of the air entrainment caused by
wave breaking, Lamarre and Melville[18]
exploited the
bubble plumes and found that the magnitude of
containing air fractions is many orders larger than
previous expectation. It was concluded that the
contribution of bubbles to sound propagation might be
underestimated to a very large extent if these plumes of
large bubbles are out of consideration. Later on,
Lamarre and Melville[19]
gave detailed measurements
for the evolution of the void-fraction field in bubble
plumes originated from large-scale deep water breakers.
Various parameters of air fractions were calculated and
comparison was made with previous 2-D laboratory
breaking waves. In the experiment performed by
Cartmill and Su[20]
, bubble radii in the bounds of 34 to
1200 μm were measured by an acoustic sensor at
different locations and deepness in a water tank, and the
scatters of bubble size are well conformed with -3
power-law from 10 to 1200 μm . Loewen et al.[10]
applied an image processing technology to measure the
size scatters of large bubbles generated by breaking
waves in fresh and salt water. They found that the
distributions of bubble size can be described by an
exponential (or power law) equation, and concluded
that the gas transportation resulted from large bubbles
should be taken into consideration even for the
low-speed wind. By measuring the bubble dimensions
for breaking waves in open water and laboratory
experiment, Deane and Stokes[5]
gave a quantitative
illustration of the bubble generation principles. They
found that for bubbles dimensions greater than about
1 mm, the bubble density conforms a -10/3 power law
correspondent to the bubble radius, in which the bubble
size distribution is dominated by turbulence intensity.
In terms of smaller bubbles stemming from droplet and
jet impact, a -3/2 power-law can be deduced. Air
entrainment of typical gravity wave breakers on a
slope-beach was studied experimentally by Mori and
Kakuno[21]
, in which the wave surface altitude, air
fractions, and bubble transportation were analyzed in
detail. Experimental results revealed a linear
dependence between the air portions and turbulent
intensity. Besides, the bubble distributions were
observed to follow a -3.4 power law against the bubble
diameters. It was noted that the bubble transportation
has no significant relations with the breaking positions
and free surface depth. Based on the experiment of
Blenkinsopp and Chaplin[22]
, further measurements
were performed in the water tank to study the
transportation of void fractions originated from the
wave breaking of fresh water, salted water, and sea
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3
water. Experimental results suggested that the influence
of water type might be less significant compared with
the scale effect for investigating the evolution of bubble
plumes entrained due to breaking waves.
Andreas[23]
was the first to analyze the
transportation of seawater spray. In the study, previous
droplets creation functions were summarized, and new
formula for wind speeds over 32 ms−1
were proposed.
The droplet size distribution under strong wind
conditions were also studied experimentally by Veron et
al.[24]
by using high speed video camera. It showed that
the droplet dimension distribution follows -3 to -5
power laws when the droplets diameters vary between
196 and 5510 mm. The creation of droplets by two
kinds of plunging waves was studied systematically by
Towle[25]
, and results revealed that the wave breakers
induced by larger wave steepness generally lead to
larger numbers of droplets with greater radius.
By using high-image-density PIV, Lin and
Rockwell[9]
studied the evolution of quasi-steady
breaker in different stages, including the capillary wave
pattern and the fully developed breaking waves. For
high Froude number, surface tension plays an important
role to determine the breaker characteristics. In terms of
intermediate Froude numbers, the evolution of free
surface exhibits capillary wave pattern. Quasi-steady
breaking waves accompany with flow separation may
be generated by high Froude number at the breaker
zone.
Some of the representative experimental
investigations for breaking waves are summarized in
Table 1.
Table 1 Experimental and open ocean investigations for breaking waves
References Year Exp. Technique Breaking Wave Types Research Emphasize
Duncan[14-15]
1981
1983 Camera
Breaking waves due to submerged
hydrofoil
Surface profile, dimensions of
turbulent wake
Bonmarin[11]
1989 Camera Deep water wave breaking Splash phenomenon, air
entrainment
Rapp and Melville[8]
1990 LDV Deep water wave breaking Kinetic energy loss, air entrainment
and dissipation
Lamarre and
Melville[18-19]
1991
1994
Acoustic instruments,
camera Deep water breaking waves
Contribution of bubbles to sound
propagation
Cartmill and Su[20]
1993 Acoustic instruments Deep water breaking waves Bubble size distribution
Lin and Rockwell[9]
1995 PIV, camera Quasi-steady breaking wave Surface tension effect
Loewen et al.[10]
1996 Acoustic instruments,
camera
Breaking waves in freshwater and
saltwater Bubble size distribution
Chang and Liu[16]
1999 Camera Breaking waves of intermediate
depth Turbulence production, dissipation
Qiao and Duncan[12]
2001 PIV, Camera Spilling breakers Surface tension effect
Deane and Stokes[5]
2002 Bubble interface detection Breaking waves in laboratory and
open ocean Bubble density, size distributions
Grue and Jensen[13]
2006
PIV Deep water plunging breaking
Overturning jet and wave induced
acceleration
Rojas and Loewen[6]
2007 Fiber optical probes, camera Deep water breaking waves Peak, mean void fractions
Kimmoun and Branger[17]
2007 PIV Sloping beach wave breaking Full velocity field, void fraction of
the surf zone.
Blenkinsopp and
Chaplin[22]
2007 Fiber optical probes, camera
Breaking waves in fresh water,
salted water and sea water
Distribution of void fractions, effect
of scale
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4
Mori and Kakuno[21]
2008 Camera Wave breaking in sloping beach
Free surface elevation, void
fraction, bubble distribution,
turbulent intensity
Blenkinsopp and
Chaplin[7]
2010
Fiber optical probes, camera Breaking waves induced by
submerged reef
Spatial, temporal evolution of the
bubble plumes
Veron et al.[24]
2012 Camera Breaking waves under strong wind Droplet size distribution under
strong wind conditions
Towle[25]
2014 Camera Plunging breaking waves Amounts, droplets size distribution
2. Numerical Studies
Experimental research can only be done for usual
wave conditions due to technical difficulties. Thus, it is
difficult to acquire an intensive understanding of the
multi-scale wave breaking phenomenon. Due to the
existence of complex hydrodynamic characteristics
(e.g., complex breaking patterns, coalescence and
separation of bubbles/droplets, and multiscale
liquid-gas turbulence), simulating the surf zone
breaking waves is still a challenge for traditional CFD.
With recent progresses of the numerical approaches, it
becomes feasible to resolve the two-phase flow
problems numerically even with the presence of the
above difficulties. Because of the recent advances in
computer architecture, detailed numerical studies of the
intermediate scale wave breaking problems that takes
account of water droplets and air bubbles become
possible. Scardovelli and Zaleski[26]
provided a
well-documented review on different numerical
approaches.
In early numerical studies, Lemoes[27]
; Takikawa
et al. [28]
; Lin and Liu[29-30]
; Bradford [31]
adopted the
Reynolds Averaged Navier-Stokes (RANS) models for
the analysis of breaking waves. Although the
understanding of the fundamental wave breaking
phenomenon arising in the surf zone can be improved
accordingly, the turbulent intensity was over predicted
at the breaking regions. Hieu et al.[32]
and Zhao et al.[33]
developed multi-scale method that can improve original
RANS predictions. The 2-D flow structures can be fully
resolved by RANS model, while the 3-D turbulence
interactions are represented by the specified eddy
viscosity model. Although air entrainment was not
involved, the numerical results are generally consistent
with experimental data.
Large eddy simulations (LES) and direct
numerical simulations (DNS) are two major branches
for the high-fidelity simulation of the turbulent wave
breaking. In LES, the large scales eddies are modeled
directly, while sub-grid viscosity model and
bubbly-flow models are utilized for representing small
scale eddy. Thus, Navier-Stokes equations can be
solved using the computational mesh with moderate
refinement. Shi et al.[34]
, Liang et al.[35-36]
, and Derakhti
and Kirby[37]
developed turbulent bubbly flow models
for the closure of governing equations. The numerical
work of Zhao and Tanimoto[38]
was the first attempt to
simulate breaking waves of a two-dimensional
configuration by using LES method, in which good
agreement was obtained compared with experimental
measurements. Watanabe and Saeki[39]
presented the
first numerical results of 3-D LES of a plunging
breaking wave. Similar 3-D numerical experiment of
plunging breakers were conducted by Mutsuda and
Yasuda[40]
, in which air entrainment and bubble
interactions were discussed in detail. Christensen and
Deigaard[41]
studied spilling breaking wave and
plunging breaking wave with different intensity.
Phenomenology study with a visualization of the
internal velocity field were given. 3-D vortex structures
created by the wave breaker in surf zone were
investigated by Watanabe et al.[42]
with the adoption of
single-phase model.
In contrast, as a promising technology, DNS is
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5
becoming prevalent in solving the multi-phase flow
since no parametrizations are required. It is noted that
the mesh resolution should be sufficiently refined to be
taken into account of the small-scale flow structures.
The early-stage numerical models by Lin and Liu[29-30]
as well as Bradford[31]
failed in considering air
entrainment details until the studies of Lubin et al.[43]
and Biausser et al.[44]
DNS are often adopted to study
the evolution of wave breakings to provide numerical
data on wave dissipation and the splashing processes.
Periodic wave conditions with relatively small
wavelengths are usually adopted, and representative
research works can be found in Refs.[45-49]
. The physical
processes of wave breaking, velocity profiles, and
breaking intensities for plunging breakers were
investigated by Chen et al.[45]
Iafrati[50-51]
provided
quantitative statistics of the spray and air fraction in
breaking waves. Numerical investigation of plunging
breaking wave past an underwater bump were
performed by Kang et al.[52]
and Koo et al.[53]
, in which
the whole plunging wave breaking procedure including
the jet diving and secondary jet were analyzed.
Considering the computational cost for the 3-D
cases, the simulations mentioned above were mainly
conducted in 2-D domain. In addition, most of the DNS
studies aimed at low-Reynolds-number problems.
However, since 2-D simulations cannot capture the
flow structure of Langmuir revolution in wave breaking
proceedings[54]
, 3-D simulations are necessary to
fundamentally investigate the 3-D phenomenon
including bubble and spray formation. The differences
between 2-D and 3-D results were compared by Lubin
et al.[43]
, and numerical simulation revealed that the
energy dissipation after the wave breaking can be
enhanced by 3-D turbulent flow, while the enhancement
is less than 5% for half-wave period. To alleviate such
computational burdens of 3-D simulations, Sullivan et
al.[55-56]
developed an approach with intermediate
complexity. Different from the complete model of the
breaking waves, a stochastic term was added in
momentum equation with a linearizing free-surface
condition to represent the existence of wave breaking.
This model has been used to study the wave breaking
phenomenon considering Langmuir circulation.
However, forcing terms should be defined to keep
consistence with the experimental measurements. Thus
the stochastic model cannot capture the air entrainment,
and the density changes and buoyancy effect are not
involved in current stage (Sullivan et al.[56]
).
Recently, with the rapid development of numerical
schemes and computer hardware, more investigations
have been conducted to 3-D DNS focusing on the
vortex structures, energy dissipation, and air
entrainment, represented research works were given by
Christensen[57]
; Fuster et al.[58]
; Brucker et al.[59]
;
Lakehal and Liovic[60]
; Zhou et al.[61]
; Lubin and
Glockner [62]
, etc.
In the study of Brucker et al.[59]
, up to 134 million
grids were used and numerical tests showed that the
parallel scalability remain acceptable. More recent
numerical study presented by Lubin and Glockner[62]
used over 1 billion grids to investigate the vortex
filaments created at an early stage after the occurrence
of breaking. In the studies of Stern et al. [63]
and Wang
et al.[64]
, bow wave breakings adjacent to the wedge
shape obstacle and breaking wave past a underwater
bump were simulated using very large grids (up to 2.2
billion), which was the first endeavor to simulate
wave-breaking problems with the mesh resolution scale
around micrometers. Table 2 outlines the existing
simulation approaches for the fundamental analysis of
breaking waves.
Table 2 Numerical investigations for breaking waves (FD: finite difference; FV: finite volume; CIP:
constraint interpolation profile)
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References Year Model Scheme Dim. Research Emphasize
Takikawa[28]
1997 RANS FD 2 Breaking wave transformation system on a slope
Lin and Liu[29-30]
1998 RANS FD 2 Evolution of wave sequence, wave breaking in the slope beach
region
Watanabe and Saeki[39]
1999 LES FD 3 Vorticity and velocity field during the wave breaking process
Chen et al.[45]
1999 DNS FD 2 Splash-up phenomenon of plunging breakers
Bradford[31]
2000 RANS FD 2 Spilling and plunging breaking waves past a sloping region
Song, Sirviente[46]
2004 DNS FD 2 Viscous and capillary effects in breaking wave
Sullivan et al.[55]
2004 DNS FD 3 Stochastic model and its application in breaking waves
simulation
Hieu et al.[32]
2004 RANS CIP, FD 2 Wave propagation in shallow water (wave shoaling, wave
reflection and air movement)
Zhao et al.[33]
2004 RANS FD 2 Multi-scale turbulence model for breaking wave
Watanabe et al.[42]
2005 LES - 3 Wave breaking and the breaking-induced turbulent two-phase
flow
Lubin et al.[65]
2006 LES - 3 Overturning, splash-up and air entrainment of plunging
breaking processes
Christensen[57]
2006 LES FV 3 Undertow, turbulence in breaking waves
Fuster et al.[58]
2009 DNS FV 3 Atomization of a liquid sheet, capillary effect
Iafrati[49]
2009 DNS - 2 Breaking waves of different initial steepness
Brucker et al.[59]
2010 DNS FD 3 Four types of deep-water breaking waves (weak, strong, and
very strong plunging)
Shi et al.[34]
2010 LES FD 2 Air fraction evolution predicted by two-phase flow model
Iafrati[48]
2011 DNS - 2 Energy dissipation in early stage breaking
Lakehal and Liovic[60]
2011 DNS FV 3 Breaking of steep water waves on a beach of constant bed slope
Liang et al.[36]
2012 LES - 2 Multi-size multi-gas bubble model to study evolution of
bubbles
Kang et al.[52]
, Koo et al.[53]
2012 DNS - 2 Plunging breakers for wave past a submerged bump
Wang et al.[64]
2016 DNS FD 3 Small-scale two-phase flow structures in a breaking wave
Derakhti and Kirby[37]
2014 LES - 3 LES model to incorporate entrained bubble populations
Zhou et al.[61]
2014 LES FV 3 Wave breaking-induced turbulence on seabed
Lubin and Glockner[62]
2015 DNS FD 3 Visualization of vortex filaments and its evolution for plunging
breaking waves
Deike et al.[49]
2015 DNS FV 3 Void fraction, bubble distribution, energy dissipation, surface
tension effect
3. Mechanism Analysis
The breaking process of complex multiphase
turbulent flows is a challenging problem for analysis.
General conclusions of the unsteady and statistical
characteristics of the energy dissipation, air entrainment,
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7
bubble cloud, aerated vortex filaments, and the
capillary effect are summarized and presented in this
section. Corresponding statistical methodologies and
important conclusions are also introduced and
discussed.
The typical breaking procedure can be described in
following aspects. As shown in Figs.1(a)-(c)
(reproduced from Wang et al.[64]
), a jet was first formed
in the wave forward direction (Fig.1(a)), and then it
impacted the water surface due to gravity (Fig.1(b)). In
the initial breaking stage, the wave dynamics exhibited
2-D flow pattern. Afterwards, the jet impacting and
entrapment of air cavity brought numerous bubbles and
droplets, and the flow became fully three dimensional
(Fig.1(c)). As summarized by Deane and Stokes[5]
as
well as Kiger and Duncan[66]
, there are two major
reasons for the generation of bubble clouds, i.e., jet
impacting the free surface creates small scale bubbles,
while the air-bubble collapsing accounts for the
formulation of larger bubbles. The influence of initial
slope for waves (𝑆) was analyzed numerically by Deike
et al.[67]
, and they found that the wave evolution was
close to the plunging breaking with initial slope
𝑆 > 0.42, while a spilling process was observed for
0.35 < 𝑆 < 0.42, which was also conformed with the
experiment done by Rojas and Loewen[68]
.
Fig.1 (a)-(c) are reproduced from Wang et al.[64]
, and (d)-(e) are reproduced from Lubin and Glockner[62]
;
(d)-(e) show the bottom view of the free surface for breaking waves, in which the air entrainment and cavity
break-up process can be demonstrated clearly. The wave impact that induces large air cavity is accompanied
by many small-size satellite bubbles. The thin filaments of air, which is an evident phenomenon in wave
breaking, are under numerical investigation by Lubin and Glockner[62]
.
3.1 Energy Dissipation
The total energy 𝐸 = 𝐸𝑘 + 𝐸𝑔 + 𝐸𝑠 for wave
breaking can be decomposed into three components
(the kinetic energy 𝐸𝑘 , the gravitational potential
energy 𝐸𝑔 , and the capillary potential energy 𝐸𝑠 ),
according to the formulations given by Chen et al.[45]
,
Iafrati[51]
, and Deike et al.[49]
as follows:
𝐸𝑘 =1
2∫ 𝜌𝒖2 d𝑥d𝑦 (1)
𝐸𝑔 = ∫ 𝜌𝑔𝑦 d𝑥d𝑦 (2)
𝐸𝑠 =𝛾(𝔏−1)
𝜌𝑤𝑔𝜆2 (3)
where 𝔏 represents the arc length. Due to the viscosity
of the fluid, the total wave energy exhibits descending
trend in time. In the following part, the methodologies
and preliminary conclusions for breaking energy
dissipation will be introduced briefly. It is noted that in
most of the theoretical and numerical studies, the
surface tension effect 𝐸𝑠 is ignored.
Fig.2(a) shows the time evolution of wave energy,
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Journal of Harbin Institute of Technology (New Series)
8
in which▽ denotes the gravitational potential energy,
◇ represents the capillary potential energy, ⭕
indicates the kinetic energy, and □ denotes the total
energy. Fig.2(b) shows the wave breaking factors as a
function of wave steepness, in which the
semi-empirical results are represented by solid line, ▼
indicates the DNS by Deike et al.[67]
, and other symbols
are from the experimental measurements of Banner and
Peirson[69]
, Drazen et al. [70]
and Grare et al. [71]
Fig.2(c) shows the total wave energy, kinetic energy,
and gravitational potential energy as a function of time.
(a) Time evolution of wave energy regenerated
from Deike et al.[67]
(b) Wave breaking factors as a function of wave
steepness regenerated from Deike et al.[67]
(c) Total wave energy, kinetic energy, and
gravitational potential energy as a function of time
regenerated from Wang et al.[64]
Figs.2 Numerically predicted energy dissipation
as a function of time.
Followed the theoretical study of Phillips[72]
, the
energy dissipation rate caused by breaking waves can
be described by 𝐸diss = ∫ 𝜖𝑙Λ(𝒄)d𝒄, where 𝒄 is the
phase velocity, 𝜖𝑙 indicates the dissipation rate of unit
width of wave crest, and Λ(𝒄) stands for the breaking
distribution, which can be measured in experiments
(Gemmrich et al.[73]
, Kleiss and Melville[74]
, and
Sutherland and Melville[75]
). The dissipation rate of
Duncan[15]
and Phillips[72]
can be adopted by defining
𝜖𝑙 = 𝑏𝜌𝑐5/𝑔 , where 𝑏 represents the
non-dimensional breaking parameter. The
semi-empirical relationships of the wave breaking
factors derived by Romero et al.[76]
is applied as
𝐸𝑘 = 𝑏 = 0.4(𝑆 − 0.08)2.5 (4)
where 𝑆 = 0.08 is an estimation of the threshold for
breaking. The accuracy of the above inertial model was
validated by numerous experimental studies (Deike et
al.[49]
, Romero et al.[76]
, Garrett et al.[77]
, Melville and
Pizzo[78]
, and Melville and Fedorov[79]
).
Deike et al.[49]
compared the evolution of the whole
wave energy and the kinetic and gravitational energy
per unit length of breaking wave, as shown in
Figs.2(a)-(b). The total wave energy shows an abrupt
decrease during the first wave period, which means the
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Journal of Harbin Institute of Technology (New Series)
9
majority of the wave energy is dissipated. They also
provided a fitted exponential function to predict energy
decay which is written as 𝐸 = 𝐸0𝑒−𝜁𝑡 , where 𝜁
represents the decay rate and 𝐸0 stands for the
primary wave energy per unit width of the wave crest.
The breaking influencing factor 𝑏 against the wave
slope is shown in Fig.2(b), in which the experimental
data of Romero et al.[76]
, Garrett et al.[77]
, and Drazen et
al.[70]
together with the 3-D DNS solutions (Deike et
al.[49]
) were given. Deike et al.[67]
inferred that the
initial setups for breaking cause the differences in
dissipation rate between DNS and experimental result
for 𝑆 varying from 0.35 to 0.4. The kinetic, gravity,
and potential energy components derived from 3-D
NDS were also given by Wang et al.[64]
, together with
the 2-D DNS solutions by Chen et al.[45]
as shown in
Fig.2(c). Although the time evolution of energy exhibits
similar trends for 2-D and 3-D simulations in general, a
higher jump after jet plunges was observed for 3-D case,
which is consistent with the numerical predictions by
Lubin et al.[43]
and Lubin and Glockner[62]
.
Deike et al.[49]
analyzed components of the
breaking waves energy for various factors 𝜖 and 𝐵𝑜
(Bond number, 𝐵𝑜 = 𝛥𝜌𝑔/(𝛾𝑘2) ), as shown in
Figs.3(a)-(d). Considering low surface tension
( 𝐵𝑜 = 1000) circumstance, the wave energy descends
exponentially with constant decaying rate for the
steepness below the breaking threshold. In the first few
wave periods, the wave energy is shown to be dispersed
very fast. Specifically, the gravitational energy is
dissipated completely, while the kinetic fraction
remains even in the immediate wave periods. They
found that during the wave propagation, the surface
tension potential energy could be neglected. Similar to
the investigation by Iafrati[50]
, the surface tension
energy occupies less than 8% of the total energy
dissipated.
Fig.3 Wave energy components (kinetic energy: ∗; surface tension potential energy:▽; gravitational potential
energy: ⭕; and the total energy: □) as functions of time, 𝑩𝒐 = 𝟏𝟎𝟎𝟎: (a) 𝝐 = 𝟎. 𝟑 and (b) 𝝐 = 𝟎. 𝟒𝟓;
𝑩𝒐 = 𝟓𝟎: (c) 𝝐 = 𝟎. 𝟑 and (d) 𝝐 = 𝟎. 𝟒𝟓 ((a)-(d) are reproduced from Deike et al.[49]
)
When the surface tension effect becomes dominated ( 𝐵𝑜 = 50, 100), it was concluded in Ref. [49]
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10
that if the Bond number is relatively low, more energy
will be dissipated for non-breaking waves, while the
total energy dissipation shows less relevance to the
Bond number for the case of breaking wave.
The total energy dissipation affected by initial
steepness at different Bond numbers was also provided
in Ref.[49]
. For low surface tension, the energy
dissipation will be promoted for wave steepness larger
than the threshold. In terms of strong surface tension,
the existence of capillary effect will enhance the energy
dissipation rate. It was observed that a larger initial
steepness will further increase the total energy
dissipation.
3.2 Air Entrainment and Void Fraction
Air entrainment occurs when the jet re-contacts the
free surface with a large air pocket and bubbles
entrained. It also occurs when the droplets drop into the
water with high-speed, which was reviewed by Kiger
and Duncan[66]
. However, the investigation for
entrained air was not adequate in their study, so the
available corresponding studies were performed by
Deane and Stokes[5]
, Lamarre and Melville[18]
, and
Blenkinsopp and Chaplin[22]
.
The void fraction has been experimentally studied
by repeating measurements at various locations by
measuring electric conductivity[19]
and using optical
fiber probes[6,22]
. Deike et al.[49]
obtained void fraction
variation through 3-D DNS, in which the predicted
bubble flume evolution and the dynamics of void
fraction are consistent with the available experimental
measurements by Lamarre and Melville[19]
and
Blenkinsopp and Chaplin[22]
. Both plunging case
( 𝑆 = 0.55 ) and spilling breakers ( 𝑆 = 0.43 ) were
compared, and in the spilling case, bubble clouds
generated by entrained air are accumulated adjacent to
the free surface. In terms of plunging breaking waves,
larger bubbles rise above the water surface into the air,
while small bubbles remain in water phase for longer.
The entrained air evolving with time is presented
in Fig.4(a), which was reproduced from Deike et al.[49]
At 𝑡/𝑇 ≈ 1, the first peak is observed since the jet
impacts surface and a big pocket of air is entrapped.
Then the measured air volume decreases quickly due to
the collapse of the bubble and its re-contact into the
free surface. After that, bubbles are created and a
secondary air volume peak will be observed, which
corresponds to the situation where large amount of
bubbles are trapped beneath the water surface. Lamarre
and Melville[18]
described the fast decay as an
exponential function, 𝑉 = 𝑉0𝑒−𝒦𝑡/𝑇 , Deike et al.[67]
deduced that the 𝒦 factor varies from 2.5 to 4, and
approximately, it relies on the initial wave steepness.
𝒦 = 3.9 and 𝒦 = 5 were deduced respectively by
Lamarre and Melville[19]
and Blenkinsopp and
Chaplin[22]
in their experiments.
(a) Time variation of the entrained air volume
obtained from the void fraction (- - -) and the closed
surfaces (▼) reproduced from Deike et al.[67]
(b) Volumetric entrained air �̅� against 𝝐𝒍𝑳𝒄 𝝆𝑾𝒈⁄
reproduced from Deike et al.[67]
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Journal of Harbin Institute of Technology (New Series)
11
(c) Normalized air volume against the initial wave
slope 𝑺. ▼ shows the total volume of air during the
active breaking process; ◇ represents the
experimental results of Deane and Stokes[5]
; ■
represents the experimental measurement of
Lamarre and Melville[19]
; ◆ represents the
experimental results of Duncan[14]
Fig.4 Numerically predicted air volume as a function
of time.
Deike et al.
[67] further deduced the formulation for
predicting the time-averaged air entrainment volume
against the dissipation rate per unit width of wave crest
or the slope, given as, �̅� = 𝐵 𝜖𝑙𝐿𝑐 𝜌𝑊𝑔⁄ . Apply the
semi-empirical function for 𝜖𝑙 as (Romero et al.[76]
)
�̅� = 0.4𝐵 (𝑆 − 0.08)2.5𝐿𝑐𝑐5 𝑊𝑔2⁄ (5)
Fig.4(b) shows the total air volume �̅� against
𝜖𝑙𝐿𝑐 𝜌𝑊𝑔⁄ , in which a good agreement was obtained
between the theoretical and numerical scaling (Eq. (3)),
and the CFD solution is also consistent with available
experimental measurement by Deane and Stokes[5]
,
Duncan[14]
, and Lamarre and Melville[19]
. Total volume
of air against wave steepness 𝑆 is presented in Fig.4(c),
in which a reasonable match between the theoretical
model (Eq. (5)) and the DNS data is found.
3.3 Bubble and Droplet Size Distribution
The identification of the droplets and bubbles was
implemented with a neighbor searching algorithm
utilizing a non-recursive strategy. Herrmann[80]
proposed an efficient identification approach for
classifying droplets or bubbles shared by different
blocks[49,67]
.
Previous experimental investigation by Deane and
Stokes[5]
indicated that the size distribution of bubbles
is dependent with different power law scaling. A
theoretical scaling equation accounting for the turbulent
fragmentation was proposed by Garrett et al.[77]
, which
is written as
𝑁(𝑟) ∝ 𝑄𝜖−13𝑟−
103 (6)
where 𝑁(𝑟) stands for the bubble numbers in a unit
𝑚𝑚, 𝑄 and 𝜖 represent the air volume entrained per
volume and energy dissipation respectively, and 𝑟
indicates the bubble dimensions. It was found that when
the bubble dimension is smaller than Hinze scale, the
bubble shape can be held by surface tension effect. In
this situation, the influence of turbulence is expected to
be unimportant. Hinze scale was defined by Deane and
Stokes[5]
as 𝛼𝐻 = 2−8/5𝜖−2/5(𝜎𝑊𝑒𝑐/𝜌)3/5 , where
𝑊𝑒𝑐 denotes the critical Weber number and 𝜎 is the
surface tension coefficient. The scaling equation of
small bubbles was given by Deane and Stokes[5]
and is
expressed as
𝑁(𝑟) ∝ 𝑄(𝜎/𝜌)−3
2𝑣2𝑟−3
2 (7)
where 𝑣 indicates the jet falling speed. Soloviev and
Lukas[81]
derived a scaling equation for bubble size
distribution based on the analysis of buoyancy forces
dominated bubble fraction. Figs.5(a-c) illustrate that
bubbles created at the early stages of breaking waves
are relatively small while larger bubbles are formed in
the later stages of after breaking. Figs.5(a)-(b) present
the bubble size distributions for fine-grid simulations[64]
,
in which the bubble numbers (time-averaged) is close
to the experimental measurements (Deane and Stokes[5]
,
Mori and Kakuno[21]
, and Tavakolinejad[82]
). In the
simulation work of Wang et al.[64]
, the two-phase flow
is not as violent as those in the experiments, so the
slopes are slightly lower than the theoretical and
experimental results for large bubbles.
Page 12
Journal of Harbin Institute of Technology (New Series)
12
(a) Time-averaged bubble size distribution at
specified time for fine grid[64]
(b) Time-averaged results of different grid
resolution (coarse, medium, and fine grid) [64]
(c) Time-averaged bubble dimension distribution[67]
Fig.5 Numerically predicted bubble size distribution
In terms of droplet size distribution, a similar
scaling equation was given by Garrett et al.[77]
and
Deane and Stokes[5]
, which is written as
𝑁(𝑟) ∝ 𝑣−1(𝜎/𝜌)1
2𝑟−9
2 (8)
where 𝑣 represents the jet velocity of the spray. Fig.6
shows the theoretical and experimental results
(Lhuissier and Villermaux[83]
, Veron et al.[24]
, and
Towle[24]
), as well as the numerical predictions of Wang
et al.[64]
for time-averaged droplet size distribution. It is
noted that there are significant differences between the
experimental studies and simulations for analyzing the
spray formation mechanisms, (e.g., the plunging wave
breaking[26]
), the high wind speed generated spray
spume drops (Veron et al.[24]
), and bubble bursting
(Lhuissier and Villermaux[83]
), however, the distribution
of drop size also conforms a scaling power-law. Based
on the DNS results of Deike et al.[49]
, the
time-averaged bubble size distribution �̅�(𝑟) against
the wave breaking time is presented in Fig.6, in
which �̅�(𝑟) is found to obey a power law with
𝑁(𝑟) ∝ 𝑟𝑚 , and 𝑚 falls into the range of 3-3.5
according to the numerical results. The size
distribution predicted by the above formula is
consistent with many experimental measurements
(e.g., Deane and Stokes[5]
, Rojas and Loewen[6]
,
Blenkinsopp and Chaplin[7]
, and Terrill et al.[84]
).
Fig.6 Drop density distribution as a function of
radius, which is regenerated from Wang et al.[64]
3.4 Capillary Effects
Few researchers have paid attention to the
micro-mechanism inside the wave breaking processes,
considering the influence of capillary effects. The
surface tension effects were firstly studied by the
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Journal of Harbin Institute of Technology (New Series)
13
experiments of Liu and Duncan[85-87]
. They found that
the breaking kinematic energy dissipation is able to be
modified through adding surfactants. Capillary effects
on the wave breaking measured in experiment were
discussed extensively by Duncan[87]
. The numerical
investigation of capillary effects was accomplished in
the studies of Liu and Duncan[86]
and Deike et al.[49]
Deike et al.[49]
classified various small-scale wave
patterns into four groups, including the gravity induced
parasitic capillary waves, non-breaking waves, and
spilling and plunging breaking waves. By changing the
initial steepness of the wave and the Bond number, a
wave-state diagram was summarized, as shown in
Fig.7(a), where PB indicates plunging breakers, SB
represents spilling breakers, PCW represents parasitic
capillary waves, and NB stands for non-breaking
gravity waves. From Fig.7(b), the critical steepness 𝜖𝑐
for this case is around 0.32. Thus when 𝜖 < 𝜖𝑐 , no
wave breaking occurs. However, as pointed by Deike et
al.[49]
, since the critical steepness predicted by
numerical simulation only gives an ideal situation, 𝜖𝑐
in experiment or open sea is strongly corresponding to
the initial conditions, therefore the discrepancies of 𝜖𝑐
between numerical and experimental predictions are
unavoidable. Accurate evaluation of critical steepness is
challenging even in experimental studies, and the
corresponding experiments for critical steepness were
performed by Romero et al.[76]
, Drazen et al.[70]
, and
Perlin et al.[88]
As shown in Fig.7, when surface tension becomes
dominated, two types of wave breaking will be induced,
i.e., the gravity-capillary breakers and the spilling
breakers, and both are scaled as 𝜖 ∝ (1 + 𝐵𝑜−1/3),
which is consistent with the theoretical study of
Longuet-Higgins[89]
.
(a) Wave regime diagram derived from numerical
simulation for 𝑩𝒐 and 𝝐, in which ■ is the wave
breaking boundary, and ▼ is the spilling-plunging
boundary
(b) Wave state diagram derived from experiment
compares with the regimes obtained through the
simulation, in which ▼ is the plunging breakers, ● is
the spilling breakers, * is the breakers triggered by
modulation instability, and ◆ is the parasitic
capillary waves.
Fig.7 Numerically predicted wave regime diagram
3.5 Other Research Findings Related to Wave
Breaking
Recent progress in computational capabilities
offers the possibility to study multiscale phenomenon
existing in wave breaking. In this subsection, recent
development in the simulation of vortex filaments,
modulational instability, and wind turbulence over
breaking waves are introduced briefly.
The vortex filaments generated in the primary
stage of the wave breaking are often observed in
laboratories or open sea. Lubin and Glockner[62]
confirmed the existence of the unusual vorticial
Page 14
Journal of Harbin Institute of Technology (New Series)
14
structures under breaking waves and investigated the
mechanisms through 3-D DNS. From numerical results,
the streamwise vortex filaments were detected with an
upstream obliqueness (approximately 50∘ ) under
plunging breaking waves. However, Lubin and
Glockner[62]
pointed out that there is no contribution on
the dissipation process due to the presence of the vortex
filaments. The formation of large-scale vortex
structures in the air induced by the plunging wave and
its influence to the interaction between the ocean and
atmosphere were investigated numerically by Iafrati et
al.[90]
The energy dissipated by the wave breaker both in
fluid and gas was considered. It is particularly
important to note that the energy dissipation in air is
larger than that in water. Recently, the wind turbulence
over breaking waves was analyzed by Yang et al.[91]
in
detail. Effects of wave age and wave steepness were
investigated through statistics of turbulent airflow over
breaking waves.
4 Conclusion and Future Prospect
In this review, previous studies for wave breaking
phenomenon by laboratory experiments, numerical
simulations, and semi-empirical models are
summarized. The methodologies for analyzing
characteristic variables during wave breaking, including
the void fraction, the bubble size distribution, the
energy dissipation of the breaking wave, and the time
evolution of the total volume of air are illustrated.
In the last few decades, numerous experimental,
theoretical, and numerical methods have been proposed
for analyzing the breaking waves and their effects in
different scales. For experimental and open ocean
investigation, extensive measurements for the bubble
density, size distribution (Lamarre and Melville[18]
;
Cartmill and Su[20]
; Lin and Rockwell[9]
; Deane and
Stokes[5]
; Mori and Kakuno[21]
; Blenkinsopp and
Chaplin[7]
) under various initial wave steepness were
performed. Acoustic resonator, fiber optical probes, and
high speed camera were often used for those analyses.
Two experiments (Lin and Rockwell[9]
; Qiao and
Duncan[12]
) were designed aiming at analyzing the
surface tension effects, in which the PIV technology
was adopted for visualizing the velocity field of spilling
breakers. Kinetic energy dissipation and turbulence
intensity during the breaking procedure were well
studied by Rapp and Melville[8]
, Chang and Liu[16]
, and
Mori and Kakuno[21]
with PIV and high speed camera.
There were also quantitative statistics for droplets
distribution (Veron et al.[24]
; Towle[25]
) and air
entrainment (Bonmarin[11]
; Rapp and Melville[8]
) during
the violent breaking. Compared with experiment,
numerical simulations gave more details and deep
insights into the complex two-phase flow phenomenon.
Among the prevalent numerical methods mentioned in
this review, it was noticed that the RANS simulations
(e.g., Takikawa et al.[28]
; Lin and Liu[29-30]
; Bradford[31]
)
were only conducted in early years. Recently, the LES
(e.g., Watanabe et al.[42]
; Lubin et al.[62]
; Shi et al.[34]
),
especially the DNS (e.g., Lakehal and Liovic[60]
; Wang
et al.[64]
; Lubin and Glockner[62]
; Deike et al.[49]
) were
more preferred in the fundamental study of wave
breaking problems. The statistic of air entrainment
(Lubin et al.[43]
) and droplet/bubble distribution (Wang
et al.[64]
), as well as energy dissipation (Deike et al.[49]
)
were more simple and straightforward from a volume
of fluid (VOF) field than from experimental photograph.
In terms of semi-empirical method, Romero et al.[76]
proposed breaking parameter model, which can be used
for estimating dissipation rate. Deike et al.[67]
deduced
the formulation for predicting the volume of entrained
air. Garrett et al.[77]
developed bubble size scaling
equation accounting for the turbulent fragmentation.
Deane and Stokes[5]
further provided a scaling equation
of small bubbles and droplet size distribution.
Although existing theoretical, experimental, and
numerical tools are capable to provide majority
characteristics, such as the mean void fraction, and
droplet/bubble distribution, many fundamental physical
Page 15
Journal of Harbin Institute of Technology (New Series)
15
processes of breaking wave are lack of quantitative
analysis. For experimental studies, tracking and
recording evolution of single droplet/bubble are
challenging, especially for fully 3-D experiment. In
addition to the reliable approaches for accurate statistics
of air entrainment, droplets distributions need further
development, and existing PIV technologies cannot
provide adequate details for intensive investigations of
the turbulent two-phase flows, while the promising
X-ray micro-computed tomography technology[92]
is
considered to be extended for this purpose. For
numerical approaches, DNS provides an important
alternative in resolving wave breaking mechanism in
micro-scale. Due to the high requirement of the
computing resources in DNS, adaptive mesh (Liu and
Hu[93-96]
), which is capable to resolve the minimum
bubble/droplet by localized mesh refinement, shows
obvious advantage in large scale parallel computation.
Besides, the fast algorithm for two-phase flow
problems, which can greatly improve the computational
efficiency, can be considered in future work. In terms of
theoretical methods, up to now, all the semi-empirical
formulae mentioned in this review were validated by
model experiment and numerical simulations, while the
scaling effect is rarely considered. In the future, more
experimental and numerical investigations are required
to bridge the gap between model scale analysis and real
scale applications.
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