Structure and modeling of surf zone turbulence due to wave breaking (original) (raw)

Structure of the turbulent flow field under breaking waves in the surf zone

Journal of Fluid Mechanics, 1989

The structure of turbulence and its role in the breaking wave dynamics within the surf zone have been investigated through laboratory experiments using several flow visualization techniques and a fibre-optic LDV system. The results indicate that there exists a characteristic structure of large-scale eddies referred to here as 'horizontal eddies ' and 'obliquely descending eddies ', which has a significant role in the generation of Reynolds stress and thus affects the deformation of the mean flow field. The experiments also reveal that these eddies caused by the wave breaking bring a large amount of vorticity (with non-zero average) into otherwise almost irrotational velocity fields, resulting in the generation of vorticity-related mean flow fields as well as turbulence (vorticity-containing velocity fluctuation). This means that the breaking waves in the surf zone can be regarded as pseudowaves which consist of irrotational velocity components as 'wave motion ' and appreciable amounts of rotational mean velocity components as 'eddying motion ' (with non-zero mean vorticity) together with turbulence. It is found that the generation of the mean rotational velocity component due to wave breaking causes considerable increase in mass and momentum transport, as compared with ordinary non-breaking waves, and thus a decrease in wave height.

A modeling investigation of the breaking wave roller with application to cross-shore currents

Journal of Geophysical Research, 1995

A mathematical model is developed for the creation and evolution of the aerated region, or "roller," that appears as a wave breaks and passes through the surf zone. The model, which calculates the roller's cross-sectional area, is based on a shortwave averaged energy balance. The vertically integrated energy flux is split between the turbulent motion in the roller and the underlying organized wave motion, and the dissipation of energy is assumed to take place in the shear layer that exists at the interface between the two flow regimes. Calibration of the roller model is done by numerically solving equations for the cross-shore balances of mass and momentum, with roller contributions included, and then optimizing predictions of depth-averaged cross-shore currents. The laboratory data of Hansen and Svendsen [1984] for setup and cross-shore currents, driven by regular waves breaking on a planar beach, are used to set the roller model's fitting coefficient. The model is then validated utilizing five additional laboratory data sets found in the literature. Results indicate that employing stream function theory in calculating integral properties for the organized wave motion (wave celerity, and mass, momentum, and energy fluxes) significantly improves agreement as compared to results generated using linear wave theory. Using the roller model and stream function theory, root-mean-square error for the mean current is typically 19%. The bed stress is found to play a negligible role in the cross-shore mean momentum balance, relative to the radiation stress, setup, roller momentum flux, and convective acceleration of the current.

Evaluation of turbulence closure models under spilling and plunging breakers in the surf zone

Turbulence closure models are evaluated for application to spilling and plunging breakers in the surf zone using open source computational fluid dynamics software. A new library of turbulence models for application to mul-tiphase flows has been developed and is assessed for numerical efficiency and accuracy by comparing against existing laboratory data for surface elevation, velocity and turbulent kinetic energy profiles. Out of the models considered, it was found that, overall, the best model is the nonlinear k-ϵ model. The model is also shown to exhibit different turbulent characteristics between the different breaker types, consistent with experimental data.

Toward a Simple Model of the Wave Breaking Transition Region in Surf Zones

Coastal Engineering 1986, 1987

Breaking waves undergo a transition from oscillatory, irrotational motion, to highly rotational (turbulent) motion with some particle translation. On plane or monotonically decreasing beach profiles, this physically takes place in such a way that the mean water level remains essentially constant within the transition region. Further shoreward a rapid set-up takes place within the inner, bore-like region. The new surf zone model of begins at this transition point and the new wave there contains a trapped volume of water within the surface roller moving with the wave speed. This paper describes a simple model over the transition zone designed to match the Svendsen (1984) model at the end of the transition region. It uses a simple, linear growth model for the surface roller area development and semi-empirical model for the variation of the wave shape factor. Breaking wave type can vary from spilling through plunging as given by a surf similarity parameter.

The Bottleneck Problem for Turbulence in Relation to Suspended Sediment in the Surf Zone

Coastal Engineering 1986, 1987

In the present paper the vertical distribution of turbulent kinetic energy k under broken waves is calculated by application of a one-equation turbulence model. The contributions to the energy level originate partly from the production in the wave boundary layer, partly from the production in the roller. Further on, the findings for k are used to calculate the vertical distribution of suspended sediment in broken waves.

Turbulence in the swash and surf zones: a review

This paper reviews mainly conceptual models and experimental work, in the field and in the laboratory, dedicated during the last decades to studying turbulence of breaking waves and bores moving in very shallow water and in the swash zone. The phenomena associated with vorticity and turbulence structures measured are summarised, including the measurement techniques and the laboratory generation of breaking waves or of flow fields sharing several characteristics with breaking waves. The effect of air entrapment during breaking is discussed. The limits of the present knowledge, especially in modelling a two- or three-phase system, with air and sediment entrapped at high turbulence level, and perspectives of future research are discussed.

Vertical Variations of Fluid Velocities and Shear Stress in Surf Zones

Coastal Engineering 1994, 1995

Detailed laboratory measurements are made of the velocity fluctuations to investigate the processes of the turbulence generation, advection, diffusion and dissipation in the surf zone. An order of magnitude analysis of the transport equation of the turbulent kinetic energy using the normalization adopted by Kobayashi and Wurjanto (1992) indicates an approximate local equilibrium of turbulence for shallow water waves in the surf zone. Estimates are found for common surf zone turbulence parameters. The calibrated values are used to show that the eddy viscosity varies gradually over depth and is nearly time-invariant and that the local equilibrium of turbulence is a reasonable approximation for spilling waves in the inner surf zone.

Vertical variation of the flow across the surf zone

Coastal Engineering, 2002

This paper reviews recent advances that have been made in the numerical modelling and measurement techniques of the surf zone. The review is restricted by the assumption of a long and uniform coastline case. Therefore, the frame of reference is the 2DV case, but including tree-dimensional processes important for this topic. During the last two decades, new measurement techniques have become available (e.g. Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV)), which have successfully been applied in numerous laboratory experiments. These methods have enabled detailed measurements of, for instance, the production, transport and dissipation of turbulence and have made a valuable contribution to our understanding of the processes in the surf zone. The first models that were developed were primarily based on assumptions directly derived from such observations. Since the development of the first numerical models in the mid-eighties, much research effort has been put into trying to improve these wave-averaged models because they can be applied at relatively low computational cost. The improved understanding of the surf-zone processes has also led to the development of more advanced intrawave models such as the Boussinesq-based models as well as the use of Navier -Stokes solvers. These new modelling techniques give a detailed description of the processes in the surf zone. D

BREAKING WAVE TURBULENCE IN THE SURF ZONE

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Numerical simulation of surf–swash zone motions and turbulent flow

2009

A two-dimensional numerical model was presented for the simulation of wave breaking, runup and turbulence in the surf and swash zones. The main components of the model are the Reynolds-Averaged Navier-Stokes equations describing the average motion of a turbulent flow, a k-e turbulence closure model describing the transformation and dissipation processes of turbulence and a volume of fluid technique for tracking the free surface motion. Nearshore wave evolution on a sloping bed, the velocity field and other wave characteristics were investigated. First, the results of the model were compared with experimental results for different surf zone hydrodynamic conditions. Spilling and plunging breakers were simulated and the numerical model investigated for different wave parameters. The turbulence field was also considered and the spatial and time-dependent variations of turbulence parameters were discussed. In the next stage of the study, numerical results were compared with two sets of experimental data in the swash zone. Generally, there is good agreement except for turbulence predictions near the breaking point where the model does not represent well the physical processes. On the other hand, turbulence predictions were found to be excellent for the swash zone. The model provides a precise and efficient tool for the simulation of the flow field and wave transformations in the nearshore, especially in the swash zone. The numerical model can simulate the surface elevation of the vertical shoreline excursion on sloping beaches, while swash-swash interactions within the swash zone are accounted for.