On the microwave reflectivity of small-scale breaking water waves

1985 ◽  
Vol 399 (1816) ◽  
pp. 93-109 ◽  
Keyword(s):  
Water Waves ◽  
Microwave Remote Sensing ◽  
Breaking Waves ◽  
Small Scale ◽  
Breaking Wave ◽  
Bragg Scattering ◽  
Wave Flume ◽  
X Band ◽  
Scale Breaking ◽  
Active Microwave Remote Sensing

The aim of this paper is to elucidate the microwave reflectivity properties of small-scale breaking water waves, which are a widespread feature of the wind-driven air-sea interface. By using a laboratory wave flume in which a small-scale breaking wave was held stationary against an opposing current, a detailed investigation of the microwave reflectivity at X-band revealed significantly enhanced levels of local backscattered power from the crest regions of small-scale breaking waves. A surprising level of organization is discovered in the hydrodynamic disturbances generated in such breaking zones. Their wavenumber-frequency spectral properties are reported in detail, from which it is concluded that the microwave reflectivity is consistent with Bragg scattering from these disturbances. The application of these findings to active microwave remote sensing of the oceans is discussed.

scholarly journals Turbulence structure and interaction with steep breaking waves

2011 ◽  
Vol 674 ◽  
pp. 522-577 ◽  
Author(s):  
DJAMEL LAKEHAL ◽  
PETAR LIOVIC
Keyword(s):  
Fluid Flow ◽  
Water Waves ◽  
Three Dimensional ◽  
Breaking Waves ◽  
Small Scale ◽  
Breaking Wave ◽  
Turbulence Structure ◽  
Mean Flow ◽  
Scale Breaking ◽  
The Mean

Large-eddy and interface simulation using an interface tracking-based multi-fluid flow solver is conducted to investigate the breaking of steep water waves on a beach of constant bed slope. The present investigation focuses mainly on the ‘weak plunger’ breaking wave type and provides a detailed analysis of the two-way interaction between the mean fluid flow and the sub-modal motions, encompassing wave dynamics and turbulence. The flow is analysed from two points of views: mean to sub-modal exchange, and wave to turbulence interaction within the sub-modal range. Wave growth and propagation are due to energy transfer from the mean flow to the waves, and transport of mean momentum by these waves. The vigorous downwelling–upwelling patterns developing at the head and tail of each breaker are shown to generate both negative- and positive-signed energy exchange contributions in the thin sublayer underneath the water surface. The details of these exchange mechanisms are thoroughly discussed in this paper, together with the interplay between three-dimensional small-scale breaking associated with turbulence and the dominant two-dimensional wave motion. A conditional zonal analysis is proposed for the first time to understand the transient mechanisms of turbulent kinetic energy production, decay, diffusion and transport and their dependence and/or impact on surface wrinkling over the entire breaking process. The simulations provide a thorough picture of air–liquid coherent structures that develop over the breaking process, and link them to the transient mechanisms responsible for their local incidence.

scholarly journals HYDRODYNAMICS UNDER LARGE-SCALE REGULAR AND BICHROMATIC BREAKING WAVES

2018 ◽  
pp. 90
Author(s):  
Dominic Van der A ◽  
Joep Van der Zanden ◽  
Ming Li ◽  
James Cooper ◽  
Simon Clark ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Surf Zone ◽  
Experimental Studies ◽  
Breaking Waves ◽  
Irregular Waves ◽  
Small Scale ◽  
Breaking Wave ◽  
Wave Flume ◽  
Cfd Models

Multiphase CFD models recently have proved promising in modelling cross‐shore sediment transport and morphodynamics (Jacobsen et al 2014). However, modelling breaking wave turbulence remains a major challenge for these models, because it occurs at very different spatial and temporal length scales and involves the interaction between surface generated turbulence and turbulence generated in the bottom boundary layer. To an extent these challenges arise from a lack of appropriate experimental data, since most previous experimental studies involved breaking waves at small-scale, and have not permitted investigation of the turbulent boundary layer processes. Moreover, most existing studies have concentrated on regular waves, thereby excluding the flow and turbulence dynamics occurring at wave group time-scales under irregular waves within the surf zone. These limitations motivated a new experiment in the large-scale CIEM wave flume in Barcelona involving regular and irregular waves. The experiment was conducted in May-July 2017 within the HYDRALAB+ Transnational Access project HYBRID.

scholarly journals The Role of Micro Breaking of Small-Scale Wind Waves in Radar Backscattering from Sea Surface

2020 ◽  
Vol 12 (24) ◽  
pp. 4159
Author(s):  
Irina A. Sergievskaya ◽  
Stanislav A. Ermakov ◽  
Aleksey V. Ermoshkin ◽  
Ivan A. Kapustin ◽  
Olga V. Shomina ◽  
...  
Keyword(s):  
Wind Waves ◽  
Breaking Waves ◽  
Sea Surface ◽  
Small Scale ◽  
Breaking Wave ◽  
Generation Process ◽  
Bragg Scattering ◽  
Radar Backscattering ◽  
Radar Sensing ◽  
The Impact

The study of the microwave scattering mechanisms of the sea surface is extremely important for the development of radar sensing methods. Some time ago, Bragg (resonance) scattering of electromagnetic waves from the sea surface was proposed as the main mechanism of radar backscattering at moderate incidence angles of microwaves. However, it has been recently confirmed that Bragg scattering is often unable to correctly explain observational data and that some other physical mechanisms should be taken into consideration. The newly introduced additional scattering mechanism was characterized as non-polarized, or non-Bragg scattering, from quasi-specular facets appearing due to breaking wave crests, the latter usually occurring in moderate and strong winds. In this paper, it was determined experimentally that such non-polarized radar backscattering appeared not only for rough sea conditions in which wave crests strongly break and “white caps” occur, but also at very low wind velocities close to their threshold values for the wave generation process. The experiments were performed using two polarized Doppler radars. The experiments demonstrated that a polarization ratio, which characterizes relative contributions of non-polarized and Bragg components to the total backscatter, changed slightly with wind velocity and wind direction. Detailed analysis of radar Doppler shifts revealed two types of scatterers responsible for the non-polarized component. One type of scatterer, moving with the velocities of decimeter-scale wind waves, determined radar backscattering at low winds. We identified these scatterers as “microbreakers” and related them to nonlinear features in the profile of decimeter-scale waves, like bulges, toes and parasitic capillary ripples. The scatterers of the second type were associated with strong breaking, moved with the phase velocities of meter-scale breaking waves and appeared at moderate winds additionally to the “microbreakers”. Along with strong breakers, the impact of microbreaking in non-polarized backscattering at moderate winds remained significant; specifically the microbreakers were found to be responsible for about half of the non-polarized component of the radar return. The presence of surfactant films on the sea surface led to a significant suppression of the small-scale non-Bragg scattering and practically did not change the non-Bragg scatterer speed. This effect was explained by the fact that the nonlinear structures associated with dm-scale waves were strongly reduced in the presence of a film due to the cascade mechanism, even if the reduction of the amplitude of dm waves was weak. At the same time, the velocities of non-Bragg scatterers remained practically the same as in non-slick areas since the phase velocity of dm waves was not affected by the film.

scholarly journals A Mild-Slope System for Bragg Scattering of Water Waves by Sinusoidal Bathymetry in the Presence of Vertically Sheared Currents

2019 ◽  
Vol 7 (1) ◽  
pp. 9 ◽  
Author(s):  
Kostas Belibassakis ◽  
Julien Touboul ◽  
Elodie Laffitte ◽  
Vincent Rey
Keyword(s):  
Resonant Frequency ◽  
Water Waves ◽  
Bottom Topography ◽  
Wave System ◽  
Bragg Scattering ◽  
Wave Flume ◽  
Seabed Topography ◽  
Incident Waves ◽  
Slope System ◽  
A Current

Extended mild-slope models (MMSs) are examined for predicting the characteristics of normally incident waves propagating over sinusoidal bottom topography in the presence of opposing shearing currents. It is shown that MMSs are able to provide quite good predictions in the case of Bragg scattering of waves over rippled bathymetry without a current, but fail to provide good predictions concerning the resonant frequency in the additional presence of a current. In order to resolve the above mismatch, a two-equation mild-slope system (CMS2) is derived from a variational principle based on the representation of the wave potential expressed as a superposition of the forward and backward components. The latter system is compared against experimentally measured data collected in a wave flume and is shown to provide more accurate predictions concerning both the resonant frequency and the amplitude of the reflection coefficient. Future work will be devoted to the examination of the derived model for a more general wave system over realistic seabed topography.

The surface velocity field in steep and breaking waves

1988 ◽  
Vol 189 ◽  
pp. 1-22 ◽  
Author(s):  
W. K. Melville ◽  
Ronald J. Rapp
Keyword(s):  
Water Waves ◽  
Surface Displacement ◽  
Breaking Waves ◽  
Surface Velocity ◽  
Breaking Wave ◽  
Laser Anemometry ◽  
Air Water Interface ◽  
Velocity Spectra ◽  
Displacement Measurements ◽  
Surface Drift

Coincident simultaneous measurements of the surface displacement and the horizontal velocity at the surface of steep and breaking waves are presented. The measurements involve a novel use of laser anemometry at the fluctuating air-water interface and clearly show the limitations of surface displacement measurements in characterizing steep and breaking wave fields. The measurements are used to examine the evolution of the surface drift velocity, spectra, wave envelopes, and forced long waves in unstable deep-water waves. Preliminary results of this work were reported by Melville & Rapp (1983).

scholarly journals A Model of the Air–Sea Momentum Flux and Breaking-Wave Distribution for Strongly Forced Wind Waves

2007 ◽  
Vol 37 (7) ◽  
pp. 1811-1828 ◽  
Author(s):  
Tobias Kukulka ◽  
Tetsu Hara ◽  
Stephen E. Belcher
Keyword(s):  
Momentum Flux ◽  
Breaking Waves ◽  
Wind Forcing ◽  
Small Scale ◽  
Breaking Wave ◽  
Unit Surface Area ◽  
Wave Age ◽  
Coupled Equations ◽  
Wave Distribution ◽  
Equilibrium Range

Abstract Under high-wind conditions, breaking surface waves likely play an important role in the air–sea momentum flux. A coupled wind–wave model is developed based on the assumption that in the equilibrium range of surface wave spectra the wind stress is dominated by the form drag of breaking waves. By conserving both momentum and energy in the air and also imposing the wave energy balance, coupled equations are derived governing the turbulent stress, wind speed, and the breaking-wave distribution (total breaking crest length per unit surface area as a function of wavenumber). It is assumed that smaller-scale breaking waves are sheltered from wind forcing if they are in airflow separation regions of longer breaking waves (spatial sheltering effect). Without this spatial sheltering, exact analytic solutions are obtained; with spatial sheltering asymptotic solutions for small- and large-scale breakers are derived. In both cases, the breaking-wave distribution approaches a constant value for large wavenumbers (small-scale breakers). For low wavenumbers, the breaking-wave distribution strongly depends on wind forcing. If the equilibrium range model is extended to the spectral peak, the model yields the normalized roughness length (Charnock coefficient) of growing seas, which increases with wave age and is roughly consistent with earlier laboratory observations. However, the model does not yield physical solutions beyond a critical wave age, implying that the wind input to the wave field cannot be dominated by breaking waves at all wavenumbers for developed seas (including field conditions).

Observation of the Occurrence of Air Flow Separation Over Water Waves

2010 ◽  
Author(s):  
Zhigang Tian ◽  
Marc Perlin ◽  
Wooyoung Choi
Keyword(s):  
Wind Speed ◽  
Flow Separation ◽  
Water Waves ◽  
High Speed ◽  
Air Flow ◽  
Breaking Waves ◽  
Wave Crest ◽  
Breaking Wave ◽  
Wind Condition ◽  
Separation Criterion

A preliminary study on the occurrence of air flow separation over mechanically generated water waves under following wind conditions is presented. Separated air flows over both non-breaking and breaking waves are observed in the flow visualization. A first attempt to identify an air flow separation criterion based on both wind speed and wave steepness is made. It was believed that, in the case of water waves propagating in the following wind condition, air flow separation will occur only in the presence of breaking waves. However, some laboratory experiments and field measurements suggested the occurrence of air flow separation over nonbreaking waves. Therefore, we conducted lab experiments to observe the air flow over mechanically generated waves. In the experiments, the air is seeded with water droplets generated with a high-pressure spray gun and is illuminated with a thin laser light sheet. A high-speed imaging system is used to record and observe the air flow over the mechanically generated wave waves. Our observations show that the separation of air flow occurs above both breaking and non-breaking wave crests, implying that wave breaking is sufficient, but not necessary for air flow separation. In addition, as compared to the separation over breaking waves, a higher wind speed is necessary for the separation over non-breaking ones, indicating that a robust air flow separation criterion likely depends on both the wave crest geometry and the wind speed above the crest. Our preliminary results support, to a certain degree, such a criterion. To the best of our knowledge, this criterion has not been reported previously in laboratory studies.

scholarly journals Gas transfer under breaking waves: experiments and an improved vorticity-based model

2008 ◽  
Vol 26 (8) ◽  
pp. 2131-2142 ◽  
Author(s):  
V. K. Tsoukala ◽  
C. I. Moutzouris
Keyword(s):  
Experimental Data ◽  
Oxygen Transfer ◽  
Large Scale ◽  
Breaking Waves ◽  
Small Scale ◽  
Gas Transfer ◽  
Transfer Coefficients ◽  
Wave Flume ◽  
Proposed Model ◽  
Sloping Beach

Abstract. In the present paper a modified vorticity-based model for gas transfer under breaking waves in the absence of significant wind forcing is presented. A theoretically valid and practically applicable mathematical expression is suggested for the assessment of the oxygen transfer coefficient in the area of wave-breaking. The proposed model is based on the theory of surface renewal that expresses the oxygen transfer coefficient as a function of both the wave vorticity and the Reynolds wave number for breaking waves. Experimental data were collected in wave flumes of various scales: a) small-scale experiments were carried out using both a sloping beach and a rubble-mound breakwater in the wave flume of the Laboratory of Harbor Works, NTUA, Greece; b) large-scale experiments were carried out with a sloping beach in the wind-wave flume of Delft Hydraulics, the Netherlands, and with a three-layer rubble mound breakwater in the Schneideberg Wave Flume of the Franzius Institute, University of Hannover, Germany. The experimental data acquired from both the small- and large-scale experiments were in good agreement with the proposed model. Although the apparent transfer coefficients from the large-scale experiments were lower than those determined from the small-scale experiments, the actual oxygen transfer coefficients, as calculated using a discretized form of the transport equation, are in the same order of magnitude for both the small- and large-scale experiments. The validity of the proposed model is compared to experimental results from other researchers. Although the results are encouraging, additional research is needed, to incorporate the influence of bubble mediated gas exchange, before these results are used for an environmental friendly design of harbor works, or for projects involving waste disposal at sea.

scholarly journals Spectrally Resolved Energy Dissipation Rate and Momentum Flux of Breaking Waves

2008 ◽  
Vol 38 (6) ◽  
pp. 1296-1312 ◽  
Author(s):  
Johannes R. Gemmrich ◽  
Michael L. Banner ◽  
Chris Garrett
Keyword(s):  
Energy Dissipation ◽  
Wave Field ◽  
Wave Energy ◽  
Dissipation Rate ◽  
Momentum Flux ◽  
Phase Speed ◽  
Breaking Waves ◽  
Energy Dissipation Rate ◽  
Small Scale ◽  
Breaking Wave

Abstract Video observations of the ocean surface taken from aboard the Research Platform FLIP reveal the distribution of the along-crest length and propagation velocity of breaking wave crests that generate visible whitecaps. The key quantity assessed is Λ(c)dc, the average length of breaking crests per unit area propagating with speeds in the range (c, c + dc). Independent of the wave field development, Λ(c) is found to peak at intermediate wave scales and to drop off sharply at larger and smaller scales. In developing seas breakers occur at a wide range of scales corresponding to phase speeds from about 0.1 cp to cp, where cp is the phase speed of the waves at the spectral peak. However, in developed seas, breaking is hardly observed at scales corresponding to phase speeds greater than 0.5 cp. The phase speed of the most frequent breakers shifts from 0.4 cp to 0.2 cp as the wave field develops. The occurrence of breakers at a particular scale as well as the rate of surface turnover are well correlated with the wave saturation. The fourth and fifth moments of Λ(c) are used to estimate breaking-wave-supported momentum fluxes, energy dissipation rate, and the fraction of momentum flux supported by air-entraining breaking waves. No indication of a Kolmogorov-type wave energy cascade was found; that is, there is no evidence that the wave energy dissipation is dominated by small-scale waves. The proportionality factor b linking breaking crest distributions to the energy dissipation rate is found to be (7 ± 3) × 10−5, much smaller than previous estimates.

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