Study Of Wind Waves Research Articles

The Dynamics of the Oceanic Air-Sea Boundary Layer

This paper presents a model for the similarity structure of the velocity profiles in air and water in the wave boundary layer, which provides predictions in terms of two parameters, F and R, of all its important properties, including the Charnock parameter, the surface drift velocity and the condition for the cancellation of the surface Stokes velocity by the surface current. The parameter, F, arises from the fetch variability of the wave field, and the parameter, R, arises from the duration variability of the wave field. In the analysis two regimes emerge, namely the Ekman regime and the Hasselmann regime. In the Ekman regime, which occurs for R > ½(1 + F), there is a net loss of energy from the wave field to the deep ocean, and in the Hasselmann regime which occurs for R < ½(1 + F), there is a net gain of energy from the atmosphere to the wave field. The predictions are compared with observations from classical wave formulae, wind-wave studies, and also ROMS and SWAN modelling in the South Australian Basin. A general conclusion is that the condition, F = 1, which was used in the original inertial coupling model of Bye [1], is a good approximation for large scale theoretical ocean studies, and hence the wind-wave interaction is determined principally by R.

Stereo wave imaging from moving vessels: Practical use and applications

Stereo wave imaging of the sea surface elevation has become an effective instrumentation to gather small- and medium-range 3-D wind wave data. Indeed, fruitful applications of stereo techniques have provided new insights into directional wave spectra, space–time distributions of wave maxima, and small-scale wave statistics. So far, however, stereo systems have been deployed mainly on fixed structures (e.g. oceanographic platforms or lighthouses) in order to simplify the installation and maintenance procedures. Nonetheless, advances in stereo calibration and processing suggest that stereo deployments are also feasible onboard moving vessels, thus broadening the impact of these observations on the study of wind waves. In this context, this study aims at discussing how the stereo processing designed to gather reliable wave data from fixed structures should be managed to operate on a moving structure. In particular, estimate of stereo cameras orientation and position with respect to the mean sea plane is of utmost importance. We discuss this aspect by using a synthetic sea state and stereo data collected during an oceanographic campaign onboard a research vessel. Results suggest that, without complementary data sources for ship motion compensation, the sea surface elevation field should include at least about sixteen spatial (2-D) waves to gather a robust estimate of the mean sea plane and consequently realistic wave parameters (e.g. the significant wave height). In this respect, our results provide also insight into the uncertainty of estimates in case of a limited number of 2-D waves is collected by the stereo system. Finally, applications of stereo wave imaging on a moving structure are discussed, with particular emphasis on the collection of space–time wave fields for assessment of numerical models and operational wave observation onboard vessels.

Reminiscences on the study of wind waves.

The wind blowing over sea surface generates tiny wind waves. They develop with time and space absorbing wind energy, and become huge wind waves usually referred to ocean surface waves. The wind waves cause not only serious sea disasters but also take important roles in the local and global climate changes by affecting the fluxes of momentum, heat and gases (e.g. CO2) through the air-sea boundary. The present paper reviews the selected studies on wind waves conducted by our group in the Research Institute for Applied Mechanics (RIAM), Kyushu University. The themes discussed are interactions between water waves and winds, the energy spectrum of wind waves, nonlinear properties of wind waves, and the effects of surfactant on some air-sea interaction phenomena.

Aplicação da Transformada de Hilbert-Huang para Caracterização da Dinâmica Costeira - Parte 1: Grandezas Escalares

This work presents a comparison among three data analysis techniques and their application to the study of wind waves: Fourier spectrum, wavelet and Hilbert-Huang Transform (HHT). Several examples are developed using synthetic free surface elevation data, emphasizing nonstationary and non linear properties, and their respective spectral analysis. In order to test the methodology, free surface elevation data measured during the second campaign of the BRISA project (“BReaking waves and Induced SAnd transport”) were analyzed

Theoretical and experimental investigation on nonlinear interaction among wave groups

Wave group is important in ocean wave theory and applications. In the past, nonlinear interaction among wave groups has been studied on the basis of the nonlinear Schrodinger equation. Using this theoretical approach, we found that the nonlinear interaction among wave groups causes asymmetry in the shape of the wave envelope (steeper in the front of the curve of the envelope). An important consequence of this asymmetry is that the highest wave in a wave group appears one individual wave length ahead of the center of the wave group. Further results show that the degree of envelope asymmetry increases with increasing spectral width and the wave steepness. This theoretical analysis has been supplemented by a systematic experimental study of wind waves. Laboratory and some open sea wave data were analyzed. The results show that the shape of the wind wave envelope of wind waves has the same asymmetry predicted by the theoretical approach. The observed degree of deformation of the envelope also increases with increasing spectral width and the wave steepness as predicted by theory. These conclusions have important ramifications for practical applications of ocean wave theory.

Is the wind wave frequency spectrum outdated

This paper presents a detailed examination of the practice of using the frequency spectrum to characterize wind waves. In particular, the issue of stationarity and Gaussian random process in connection with wind wave studies is addressed. We describe a test for nonstationarity based on the wavelet spectrum. When this test is applied to wind wave time series, the results significantly diverge from those expected for a Gaussian random process, thus casting critical doubts on the conventional concept of the wind wave frequency spectrum.

Discussion: Study of Wind Waves in the Gulf Intracoastal Waterway at Aransas National Wildlife Refuge

Discussion: Study of Wind Waves in the Gulf Intracoastal Waterway at Aransas National Wildlife Refuge

Study of Wind Waves in the Gulf Intracoastal Waterway at Aransas National Wildlife Refuge

This study describes a field experiment on wind waves in the Gulf Intracoastal Waterway. Wind-wave height and period were measured at the Aransas National Wildlife Refuge in Texas and compared to hindcast results generated by the wind-wave model for restricted fetches in the Automated Coastal Engineering System. The comparisons showed that the present technology did not predict the measurements well. The restricted fetch method showed a tendency to overpredict waves when the wind was blowing along the channel and underpredict waves when the wind was blowing across the channel. The poor performance of the restricted fetch model in long, narrow channels was attributed to the measured data being far outside the bounds of the data used to develop the model (fetch width-to-length ratio, as well as wave height and period). It was shown that wave damping could reduce wave heights for along-channel winds. Analysis of an elliptical fetch suggested that fetch width-to-length ratio is an important variable. Because this parameter is not included in the wave-prediction equations, the formulas should only be applied to water bodies similar in width-to-length ratio to those used to develop the model.

On the Importance of the Significant Slope in Empirical Wind-Wave Studies

Abstract The significant slope of a random wave field is found to be an important parameter in empirical wind-wave studies. This significant slope Ss is defined as Ss = (ζ2)½/λ0, with ζ2 as the mean-square surface elevation and λ0 as the wavelength corresponding to the waves at the peak of the spectrum. With this parameter, the relationship between Ẽ and n is reduced to an identity expressing a pure geometric measure of the sea state, because Ẽn4 = (2πSs)2. By applying the significant slope as a parameter explicitly, we proposed that the traditional empirical formulas relating the nondimensional energy Ẽ, fetch x and frequency n be combined into a single unified relationship as Ẽn/x = (9/40)Ss9/4. This unified empirical formula governs the wind-wave data equally as well in the field as in the laboratory.

Forced convection accompanying wind waves

Wind-wave tunnel experiments reveal, by use of techniques of the flow visualization, that wind waves are accompanied by the wind drift surface current with large velocity shear and with horizontal variation of velocity relative to the wave profile. The surface current converges from the crest to a little leeward face of the crest, making a downward flow there, even though the wave is not breaking. Namely, wind waves are accompanied by forced convections relative to the crests of the waves. Since the location of the convergence and the downward flow travels on the water surface as the crest of the wave propagates, the motion as a whole is characterized by turbulent structure as well as by the nature of water-surface waves. In this meaning, the term of real wind waves is proposed in contrast with ordinary water waves. The study of real wind waves will be essential in future development of the study of wind waves.

BASIC SYSTEMS OF WIND WAVE FIELD

The study of wind waves is usually carried out in the following manner. At the first moment a homogenous wind field with the constant speed directed from the shore to the basin is occurred over the water surface restricted by a straight shore line. It is required to calculate statistic wave characteristics as functions of time and distance from the shore. When solving the problem in such a way the explorers [.1-6 ~] usually came to a conclusion of the system development of gravitational waves with a main energy maximum the amplitude and period of which rise in process developing from small magnitudes to limiting values. Some explorers noted that the two -or three-wave systems under the conditions of constant wind are available. The first results of this theory were obtained by L.Ph. Tytov. In studies of stereophotographs of sea waves he noted and described quantitatively two types of waves: "prevailing" and "large", f7l I* is possible to show that the first type of waves has a phase speed that is less than wind speed, and the second one is equal to wind speed. At a later time G.Neumann [6J generalizing results of ocean observations has come to the conclusion that under the action of constant wind three "specific" wave systems which have phase speeds less, equal and 1.2 more than the speed of wind are developed. However, Tytov's and Neumann's results didn't receive a progress, and later on they were substituted by the conception of continuous wave spectrum with one energy maximum £3-6J - Nevertheless, the opinions of availability of two-or three-wave systems as a typical feature of wind rough sea [8,9] were published in the press.

Icosahedral-gnomonic projection and grid of the world ocean for wave studies

The world ocean has been mapped in a special projection, icosahedral-gnomonic, which is optimized for wind-wave studies. The map allows for relatively simple propagation of the waves. The graticule of the map has been computed in detail. The inverse function describing geographical coordinates in terms of map coordinates has been derived and used to tabulate grid point locations for a world network; the function describing great circle track adjustments on any icosahedral-gnomonic projection has been derived and tabulated. This projection features segmented straightline representation of great circles and small simple distortions of direction and scale. Large parts of the globe can be represented without interruption, and a regular, nearly equidistant grid can be fitted over the globe. This projection should be useful in other problems, especially problems involving great circles.

Laboratory Study of Wind Waves in Shallow Water

Very little quantitative information exists on growth of wind waves in waters where relatively shallow depth could influence wave growth; results of laboratory investigation of growth of waves in shallow water is presented, showing experimental relationships between wind velocity, fetch, and wave parameters for two different water depths.