Showing papers on "Breaking wave published in 2022"

Mass Transfer at the Ocean–Atmosphere Interface: The Role of Wave Breaking, Droplets, and Bubbles

Abstract: Breaking waves modulate the transfer of energy, momentum, and mass between the ocean and atmosphere, controlling processes critical to the climate system, from gas exchange of carbon dioxide and oxygen to the generation of sea spray aerosols that can be transported in the atmosphere and serve as cloud condensation nuclei. The smallest components, i.e., drops and bubbles generated by breaking waves, play an outsize role. This fascinating problem is characterized by a wide range of length scales, from wind forcing the wave field at scales of [Formula: see text](1 km–0.1 m) to the dynamics of wave breaking at [Formula: see text](10–0.1 m); air bubble entrainment, dynamics, and dissolution in the water column at [Formula: see text](1 m–10 μm); and bubbles bursting at [Formula: see text](10 mm–1 μm), generating sea spray droplets at [Formula: see text](0.5 mm–0.5 μm) that are ejected into atmospheric turbulent boundary layers. I discuss recent progress to bridge these length scales, identifying the controlling processes and proposing a path toward mechanistic parameterizations of air–sea mass exchange that naturally accounts for sea state effects.

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

Abstract: 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 high Re and Bo , 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 turbulent Re up 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 Deike et 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 high Bo in good agreement with recent experiments (Erinin et 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.

Periodic Wave Trains in Nonlinear Media: Talbot Revivals, Akhmediev Breathers, and Asymmetry Breaking.

Abstract: We study theoretically and observe experimentally the evolution of periodic wave trains by utilizing surface gravity water wave packets. Our experimental system enables us to observe both the amplitude and the phase of these wave packets. For low steepness waves, the propagation dynamics is in the linear regime, and these waves unfold a Talbot carpet. By increasing the steepness of the waves and the corresponding nonlinear response, the waves follow the Akhmediev breather solution, where the higher frequency periodic patterns at the fractional Talbot distance disappear. Further increase in the wave steepness leads to deviations from the Akhmediev breather solution and to asymmetric breaking of the wave function. Unlike the periodic revival that occurs in the linear regime, here the wave crests exhibit self acceleration, followed by self deceleration at half the Talbot distance, thus completing a smooth transition of the periodic pulse train by half a period. Such phenomena can be theoretically modeled by using the Dysthe equation.

Cross‐shore distribution of the wave‐induced circulation over a dissipative beach under storm wave conditions

Abstract: This study explores the spatial distribution and the driving mechanisms of the wave-induced cross-shore flow within the shoreface and surf zone of a dissipative beach. Unpublished results from a field campaign carried out in early 2021 under storm wave conditions are presented and compared with the predictions from a state-of-the-art phase-averaged three-dimensional circulation modeling system based on the vortex force formalism. Under storm wave conditions, the cross-shore flow is dominated by a strong seaward-directed current in the lower part of the water column. The largest current velocities of this return current are located in the surf zone, where the dissipation by depth-induced breaking is most intense, but offshore-directed velocities up to 0.25 m/s are observed as far as 4 km from the shoreline (≃12 m-depth). Numerical experiments further highlight the key control exerted by non-conservative wave forces and wave-enhanced mixing on the cross-shore flow across a transition zone, where depth-induced breaking, whitecapping, and bottom friction all significantly contribute to the wave energy dissipation. Under storm conditions, this transition zone extended almost 6 km offshore and the cross-shore Lagrangian circulation shows a strong seaward-directed jet in the lower part of the water column, whose intensity progressively decreases offshore. In contrast, the surf zone edge appears clearly delimited under fair weather conditions and the seaward-directed current is weakened by a near bottom shoreward-directed current associated with wave bottom streaming in the shoaling region, such that the clockwise Lagrangian overturning circulation is constrained by an additional anti-clockwise overturning cell at the surf zone edge.

Wind Turbulence over Misaligned Surface Waves and Air–Sea Momentum Flux. Part I: Waves Following and Opposing Wind

Abstract: Abstract Air–sea momentum and scalar fluxes are strongly influenced by the coupling dynamics between turbulent winds and a spectrum of waves. Because direct field observations are difficult, particularly in high winds, many modeling and laboratory studies have aimed to elucidate the impacts of the sea state and other surface wave features on momentum and energy fluxes between wind and waves as well as on the mean wind profile and drag coefficient. Opposing wind is common under transient winds, for example, under tropical cyclones, but few studies have examined its impacts on air–sea fluxes. In this study, we employ a large-eddy simulation for wind blowing over steep sinusoidal waves of varying phase speeds, both following and opposing wind, to investigate impacts on the mean wind profile, drag coefficient, and wave growth/decay rates. The airflow dynamics and impacts rapidly change as the wave age increases for waves following wind. However, there is a rather smooth transition from the slowest waves following wind to the fastest waves opposing wind, with gradual enhancement of a flow perturbation identified by a strong vorticity layer detached from the crest despite the absence of apparent airflow separation. The vorticity layer appears to increase the effective surface roughness and wave form drag (wave attenuation rate) substantially for faster waves opposing wind. Significance Statement Surface waves increase friction at the sea surface and modify how wind forces upper-ocean currents and turbulence. Therefore, it is important to include effects of different wave conditions in weather and climate forecasts. We aim to inform more accurate forecasts by investigating wind blowing over waves propagating in the opposite direction using large-eddy simulation. We find that when waves oppose wind, they decay as expected, but also increase the surface friction much more drastically than when waves follow wind. This finding has important implications for how waves opposing wind are represented as a source of surface friction in forecast models.

The Rise and Fall of Banana Puree: Non-Newtonian Annular Wave Formation by Transonic Self-Pulsating Flow

Abstract: We reveal mechanisms driving pre-filming wave formation of non-Newtonian banana puree inside a twin-fluid atomizer at a steam-puree mass ratio of 2.7%. Waves with a high blockage ratio form periodically at a frequency of 1000 Hz, where the collapse of one wave corresponds to the formation of another (i.e., no wave train). Wave formation and collapse occur at very regular intervals, while instabilities result in distinctly unique waves each cycle. The average wave angle and wavelength are 50{degree sign} and 0.7 nozzle diameters, respectively. Kelvin-Helmholtz instability (KHI) dominates during wave formation, while pressure effects dominate during wave collapse. Annular injection of the puree into the steam channel provides a wave pool, allowing KHI to deform the surface; then, steam shear and acceleration from decreased flow area lift the newly formed wave. The onset of flow separation appears to occur as the waves' rounded geometry transitions to a more pointed shape. Steam compression caused by wave sheltering increases pressure and temperature on the windward side of the wave, forcing both pressure and temperature to cycle with wave frequency. Wave growth peaks at the nozzle exit, at which point the pressure build-up overcomes inertia and surface tension to collapse and disintegrate the wave. Truncation of wave life by pressure build-up and shear-induced puree viscosity reduction is a prominent feature of the system, and steam turbulence does not contribute significantly to wave formation. The wave birth-death process creates bulk system pulsation, which in turn affects wave formation.

Eulerian and Lagrangian transport by shallow-water breaking waves

Abstract: This study examines the mass and Lagrangian transport, kinematic and dynamic characteristics of shallow-water breaking waves, focusing on the wave breaking, and jet impingement processes. A multiphase Navier–Stokes flow model has been developed to track the origin and trajectory for the jet and the splash-up using both a geometric piece-wise linear interface calculation volume-of-fluid (PLIC-VOF) and the Lagrangian particle tracking approaches. The model is first validated both quantitatively and qualitatively against the experimental data for the plunging jet and the splash-up during wave breaking, in which a good agreement is obtained. The mass transport and the origin of the jet and splash-up are studied using the new multi-component PLIC-VOF approach, and the different regions in the interior of the wave are tracked in an Eulerian way. Both horizontal and vertical drifts for the interior and surface particles are shown using the Lagrangian particles. The location and origin of the plunging jet can be clearly seen from the simulations. Various wave steepness and beach slopes have been investigated for different types of breakers. Furthermore, the detailed jet impingement, velocity, pressure, vorticity, and turbulence fields during wave breaking are discussed and presented, providing more detailed flow fields to gain further insight into the plunging jet and splash-up in shallow-water breaking waves.

Experimental investigation of hydrodynamic loading induced by regular, steep non-breaking and breaking focused waves on a fixed and moving cylinder

Abstract: Monopiles are commonly adopted in marine structures and subject to combined loading from waves and currents. The nature of superposition of wave and current loads needs to be known during the design stage. In the present paper, combined hydrodynamic loading induced by nonlinear waves and uniform currents on a cylinder is experimentally investigated. The current is represented by towing the cylinder along the flume. By this, nonlinear wave–current interactions are excluded physically, but the loading of a proportional current following or opposing a wave group is captured and analyzed. It is argued that towing makes provision for analyzing the nature of superposition of wave and current loads using Morison theory (which is not applicable to true combined wave–current fields) and also facilitates experimentation of a wide range of nonlinear wave and uniform current loading combinations onto the structure. Accordingly, regular, steep non-breaking and breaking focused waves interacting with the cylinder towed along and in opposition to the wave-field at different speeds have been investigated. The non-breaking wave–structure interactions have been analyzed within the framework of Morison theory using Fully Nonlinear Potential Theory (FNPT) based kinematics. Breaking wave–cylinder interactions have been analyzed through a spectral approach. The experiments demonstrate that wave and locally-acting current loads on the structure can be linearly superimposed, irrespective of the nature of waves and towing speed. Hence, provided wave–current interactions are excluded, steep breaking wave and uniform current loads can be linearly superimposed, despite focused wave generation itself being inherently nonlinear.

Reynolds stress turbulence modelling of surf zone breaking waves

Abstract: Abstract Computational fluid dynamics is increasingly used to investigate the inherently complicated phenomenon of wave breaking. To date, however, no single model has proved capable of accurately simulating the breaking process across the entirety of the surf zone for both spilling and plunging breakers. The present study newly considers the Reynolds stress–$\omega$ turbulence closure model for this purpose, where $\omega$ is the specific dissipation rate. Novel stability analysis proves that, unlike two-equation closures (at least in their standard forms), the stress–$\omega$ model is neutrally stable in the idealized potential flow region beneath surface waves. It thus naturally avoids unphysical exponential growth of turbulence prior to breaking, which has plagued numerous prior studies. The analysis is confirmed through simulation of a progressive surface wave train. The stress–$\omega$ model is then applied to simulate a turbulent wave boundary layer, demonstrating superior accuracy relative to a two-equation model, especially during flow deceleration. Finally, the stress–$\omega$ model is employed to simulate spilling and plunging breaking waves, with seemingly unprecedented accuracy. Specifically, the present work marks the first time that a single turbulence closure model collectively: (1) avoids turbulence over-production prior to breaking, (2) accurately predicts the breaking point, (3) provides reasonable evolution of turbulent normal stresses, while also (4) yielding accurate evolution of undertow velocity structure and magnitude across the surf zone, for both spilling and plunging cases. Differences in the predicted Reynolds shear stresses (hence flow resistance) are identified as key to the improved inner surf zone performance, relative to a state-of-the-art two-equation model.

Experimental Investigation of the Hydrodynamic Performance of Land-Fixed Nearshore and Onshore Oscillating Water Column Systems with a Thick Front Wall

Abstract: Most experimental research on land-fixed Oscillating Water Column (OWC) systems assume that the OWC-water wave interaction happens with waves that propagate normally towards the device. However, the angle of incidence of the waves can determine the performance of the OWC, in particular the wave period at which the device resonates. In this study, an experimental investigation to examine the interaction of regular, oblique, water waves with a land-fixed, thick-front wall OWC device in terms of its hydrodynamic performance is reported. A 1:20 Froude scale was used to replicate a single chamber of the Mutriku Wave Energy Plant (MWEP), and a series of tests were carried out in a spectral wave basin. The goal of this study is to look at how incident wave direction and device location affect the hydrodynamic performance of land-fixed OWC systems in regular wave conditions with varying wave heights. The hydraulic performance includes the assessment of the wave amplification factor, hydrodynamic efficiency, the non-dimensional air pressure inside the chamber and non-dimensional water pressures on the chamber walls. The findings show that, for the nearshore OWC device, the period at which resonance occurs decreases when the incident wave angle increases. For the corresponding wave angles, similar results were found for the onshore and nearshore OWC devices, with a slight frequency shift in the bandwidth of the hydrodynamic efficiency. Furthermore, it was found that when wave height increases, the hydrodynamic efficiency improves for both short and long wave periods, with the exception of the resonance period, where the trend is reversed. Finally, regardless of the location, an OWC device with a thick front wall performs well when interacting with intermediate and long-period waves.

A review of breaking wave force on the bridge pier: Experiment, simulation, calculation, and structural response

Abstract: In the course of the propagation of waves from the offshore to the nearshore zone, the wave may break due to the shoaling effect. Strong impact forces are observed when the breaking wave acts on the pier of the bridge. This impact force might not only change the dynamic load pattern on the pier but also cause strong structural vibration, which may threaten the driving and structural safety of the bridge. Many studies have been carried out to study the issues in the aspect of wave flume experiment, numerical simulation, calculation of breaking wave force, and random vibration response of the structure. Considering the studies of breaking wave load on bridge piers are lack of systematic summaries, this paper presents a comprehensive and up-to-date literature review of breaking wave research and practice related to bridges. Firstly, a brief introduction is given, which includes recent cases of bridge failures caused by breaking waves. Then, both scientific and technical studies are reviewed, categorized into four aspects: experimental study, numerical simulation, analytical calculation of breaking wave force, and the structural response under breaking wave. Finally, Discussion is provided on four emerging ideas to investigate breaking wave forces on the pier from both science and engineering perspectives. • Breaking waves threaten the bridge located in the nearshore water. • A comprehensive and up-to-date literature review of breaking wave research and practice related to bridges is presented. • Prospects on breaking wave force in bridge engineering are put forward from both science and engineering perspectives.

Nonlinear wave evolution with data-driven breaking

Abstract: Wave breaking is the main mechanism that dissipates energy input into ocean waves by wind and transferred across the spectrum by nonlinearity. It determines the properties of a sea state and plays a crucial role in ocean-atmosphere interaction, ocean pollution, and rogue waves. Owing to its turbulent nature, wave breaking remains too computationally demanding to solve using direct numerical simulations except in simple, short-duration circumstances. To overcome this challenge, we present a blended machine learning framework in which a physics-based nonlinear evolution model for deep-water, non-breaking waves and a recurrent neural network are combined to predict the evolution of breaking waves. We use wave tank measurements rather than simulations to provide training data and use a long short-term memory neural network to apply a finite-domain correction to the evolution model. Our blended machine learning framework gives excellent predictions of breaking and its effects on wave evolution, including for external data.

Revisiting wind wave growth with fully coupled direct numerical simulations

Abstract: Abstract We investigate wind wave growth by direct numerical simulations solving for the two-phase Navier–Stokes equations. We consider the ratio of the wave speed $c$ to the wind friction velocity $u_*$ from $c/u_*= 2$ to 8, i.e. in the slow to intermediate wave regime; and initial wave steepness $ak$ from 0.1 to 0.3; the two being varied independently. The turbulent wind and the travelling, nearly monochromatic waves are fully coupled without any subgrid-scale models. The wall friction Reynolds number is 720. The novel fully coupled approach captures the simultaneous evolution of the wave amplitude and shape, together with the underwater boundary layer (drift current), up to wave breaking. The wave energy growth computed from the time-dependent surface elevation is in quantitative agreement with that computed from the surface pressure distribution, which confirms the leading role of the pressure forcing for finite amplitude gravity waves. The phase shift and the amplitude of the principal mode of surface pressure distribution are systematically reported, to provide direct evidence for possible wind wave growth theories. Intermittent and localised airflow separation is observed for steep waves with small wave age, but its effect on setting the phase-averaged pressure distribution is not drastically different from that of non-separated sheltering. We find that the wave form drag force is not a strong function of wave age but closely related to wave steepness. In addition, the history of wind wave coupling can affect the wave form drag, due to the wave crest shape and other complex coupling effects. The normalised wave growth rate we obtain agrees with previous studies. We make an effort to clarify various commonly adopted underlying assumptions, and to reconcile the scattering of the data between different previous theoretical, numerical and experimental results, as we revisit this longstanding problem with new numerical evidence.

Wave‐driven hydrodynamic processes over fringing reefs with varying slopes, depths, and roughness: Implications for coastal protection

Abstract: Wave breaking on the steep fore-reef slopes of shallow fringing reefs can be effective at dissipating incident sea-swell waves prior to reaching reef shorelines. However, wave setup and free infragravity waves generated during the sea-swell breaking process are often the largest contributors to wave-driven water levels (wave runup) at the shoreline. Laboratory flume experiments and a two-dimensional vertical phase-resolving nonhydrostatic wave-flow model, which includes a canopy model to predict drag forces generated by roughness elements, were used to investigate wave-driven water levels for along-shore uniform fringing reefs. In contrast to many previous studies, both the laboratory experiment and the numerical model account for the effects of large bottom roughness. The numerical model reproduced the observations of the wave transformation and runup over both smooth and rough reef profiles. The numerical model was then extended to quantify the influence of reef geometry and compared to simulations of plane beaches lacking a reef. For a fixed offshore forcing condition, the fore-reef slope controlled wave runup on reef-fronted beaches, whereas the beach slope controlled wave runup on plane beaches. As a result, the coastal protection utility of reefs is dependent on these slopes. For our examples, with a fore-reef slope of 1/5 and a 500 m prototype reef flat length, a beach slope of ∼1/30 marked the transition between the reef providing runup reduction for steeper beach slopes and enhancing wave runup for milder slopes. Roughness coverage, spacing, dimensions, and drag coefficient were investigated, with results indicating the greatest runup reductions were due to tall roughness elements on the reef flat.

Computational Fluid Dynamics Simulation of Deep-Water Wave Instabilities Involving Wave Breaking

Abstract: Instabilities of deep-water wave trains subject to initially small perturbations (which then grow exponentially) can lead to extreme waves in offshore regions. The present study focuses on the two-dimensional Benjamin–Feir (or modulational) instability and the three-dimensional crescent (or horseshoe) waves, also known as Class I and Class II instabilities, respectively. Numerical studies on Class I and Class II wave instabilities to date have been limited to models founded on potential flow theory, thus they could only properly investigate the process from initial growth of the perturbations to the initial breaking point. The present study conducts numerical simulations to investigate the generation and development of wave instabilities involving the wave breaking process. A CFD model solving Reynolds-averaged Navier-Stokes (RANS) equations coupled with turbulence closure model in terms of the Reynolds stress model is applied. Wave form evolutions, Fourier amplitudes, and the turbulence beneath the broken waves are investigated.

Wave breaking and jet formation on axisymmetric surface gravity waves

Abstract: Abstract Axisymmetric standing waves occur across a wide range of free surface flows. When these waves reach a critical height (steepness), wave breaking and jet formation occur. For travelling surface gravity waves, wave breaking is generally considered to limit wave height and reversible wave motion. In the ocean, the behaviour of directionally spread waves lies between the limits of purely travelling (two dimensions) and axisymmetric (three dimensions). Hence, understanding wave breaking and jet formation on axisymmetric surface gravity waves is an important step in understanding extreme and breaking waves in the ocean. We examine an example of axisymmetric wave breaking and jet formation colloquially known as the ‘spike wave’, created in the FloWave circular wave tank at the University of Edinburgh, UK. We generate this spike wave with maximum crest amplitudes of 0.15–6.0 m (0.024–0.98 when made non-dimensional by characteristic radius), with wave breaking occurring for crest amplitudes greater than 1.0 m (0.16 non-dimensionalised). Unlike two-dimensional travelling waves, wave breaking does not limit maximum crest amplitude, and our measurements approximately follow the jet height scaling proposed by Ghabache et al. (J. Fluid Mech., vol. 761, 2014, pp. 206–219) for cavity collapse. The spike wave is predominantly created by linear dispersive focusing. A trough forms, then collapses producing a jet, which is sensitive to the trough's shape. The evolution of the jets that form in our experiments is predicted well by the hyperbolic jet model proposed by Longuet–Higgins (J. Fluid Mech., vol. 127, 1983, pp. 103–121), previously applied to jets forming on bubbles.

Beach morphodynamic classification using high-resolution nearshore bathymetry and process-based wave modelling

Abstract: Classification of beach morphodynamic state relies on accurate representation of breaking wave conditions, Hb (plus grain size and spring tidal range). Measured breaking wave data, however, are absent from all but a handful of sites worldwide. Here, we apply process-based wave modelling for propagating offshore waves to the breaking zone using high-resolution nearshore bathymetry, obtaining representative and accurate Hb values for multiple beaches at regional scale, and thereby derive meaningful morphodynamic classifications that accord with observed beach state. Ninety-five beaches on the north coast of Ireland were investigated, with observed beach types and states compared to predictions based on morphodynamic parameters determined using wave, tide and sediment data, obtained from field surveys and detailed numerical wave modelling. The coast is exposed to micro-through meso-tides (0.43–3.90 m) and low sea through high swell waves (Hb = 0.13–1.18 m) and is composed of fine to medium sand resulting in a full range of beach types (wave-dominated, tide-modified and tide-dominated) and most beach states, thereby providing a comprehensive field laboratory to undertake such a comparison. We found that modal beach types reside within their predicted Relative Tide Range (RTR) and modal beach states close to the predicted dimensionless fall velocity (Ω) range. The use of high-resolution nearshore wave modelling to determine Hb was deemed the most appropriate approach for deriving predicted beach classification. The work follows the investigation of the same coast by Jackson et al. (2005) who found shortcomings in relating beach types to breaker wave conditions. However, advances in inshore wave modelling and access to high-resolution nearshore bathymetry since then have enabled improved estimates of breaker height, producing more accurate results and enhancing previous work. The results highlight the need to obtain accurate estimates of Hb and Tp if they are to be used effectively in predicting beach parameters. This work therefore sets a precedence for other coastal sites worldwide where detailed nearshore bathymetry is available and Hb can be derived from process-based wave modelling, improving the classification and prediction of morphodynamic beach type and state.