Thesis (M.S.)--Mississippi State University. Department of Computational Engineering. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 1998. / Vita. Includes bibliographical references (p. [104]-115).

Thesis (Ph. D.)--University of Washington, 1998. / Includes bibliographical references (leaves [104]-115).

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第15356号 / 工博第3235号 / 新制||工||1487(附属図書館) / 27834 / 京都大学大学院工学研究科都市環境工学専攻 / (主査)教授 間瀬 肇, 教授 後藤 仁志, 准教授 森 信人 / 学位規則第4条第1項該当

This study reports on a new formulation of the spectral dissipation source term Sds for wind-wave modelling applications. This new form of Sds features a nonlinear dependence on the local wave spectrum, expressed in terms of the azimuthally integrated saturation parameter B(k)=k^4 F(k). The basic form of this saturation-dependent Sds is based on a new framework for the onset of deep-water wave breaking due to the nonlinear modulation of wave groups. The new form of Sds is succesfully validated through numerical experiments that include exact nonlinear computations of fetch-limited wind-wave evolution and hindcasts of two-dimensional wave fields made with an operational wind-wave model. The newly-proposed form of Sds generates integral spectral parameters that agree more closely with observations when compared to other dissipation source terms used in state-of-the-art wind-wave models. It also provides more flexibility in controlling properties of the wave spectrum within the high wavenumber range. Tests using a variety of wind speeds, three commonly-used wind input source functions and two alternative full-development evolution limits further demonstrate the robustness and flexibility of the new saturation-dependent dissipation source term. Finally, improved wave hindcasts obtained with an implementation of the new form of Sds in a version of the WAM model demonstrate its potential usefulness in operational wind-wave forecasting applications.

Wind induced turbidity within shallow lakes can greatly affect the biological functioning of a system in either a positive or negative manner. This research aims to understand and model the physical processes that cause sediment re-suspension. Lake St Lucia on the east coast of South Africa, a UNESCO World heritage site was used as a case study. Lake St. Lucia is a shallow water system which commonly experiences high levels of turbidity. Coupled with the naturally shallow depth of the lake, it is currently drought stricken, resulting in abnormally low water levels. A simple model has been developed which accounts for sediment re-suspension due to wind-driven waves and their associated bed shear stresses. The wave heights within a shallow lake such as St Lucia are controlled either by the fetch (for a large water depth), or the water depth (for a large fetch). When the wind is strong enough, the wind-driven turbulent mixing causes the water column to become fully mixed. When the wave-driven boundary layer becomes turbulent, sediment, being entrained within the water column increases significantly. The model also accounts for the effects of temporal consolidation on the re-suspension of sediments by setting a time scale for the erosion processes. It was found that the median of the monthly turbidity levels over the past ten years exceeded the average turbidity levels over the past 92 years. In all cases it was shown that mouth linkage with the uMfolozi resulted in lower turbidity levels than without any linkage due to the higher average water levels. The model was then developed to predict the spatial variation in turbidity within the Southern Lake. This was achieved through the use of existing bathymetric data for the Lake. This spatial model was then used to show how the turbidity varied for different wind and water depth conditions. Two conditions were considered, a NE and SW wind blowing at 8m/s for water levels of 0 EMSL and -0.5 EMSL. The spatial model showed that a decrease in water level increases the turbidity within the lake significantly. The wind directions appeared to yield similar results of sediment re-suspension. It was also shown that the high turbidity values were situated in the shallow depths even though the wave heights were small in comparison to those in deeper water. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2011.

In this thesis, the wind-wave records obtained at three locations along the Turkish coasts (Alanya, Dalaman and Hopa) are analyzed. Probability distributions of individual wave characteristics (wave height, wave period and wave steepness) are obtained and compared with the model distributions. Goodness of fit of the observed distributions is checked by Chi-square test and Kolmogorov-Smirnov tests. Joint probability distribution of individual wave heights and periods is also studied and compared with the theoretical distributions. The relationships among various statistical wave height parameters and statistical wave period parameters are investigated and compared with the theoretical and reported values.

Bien qu'ayant suscité de nombreuses études sur le sujet, un certain nombre de questions à propos de la formation des vagues sous l'effet du vent restent sans réponse précise. Dans ma thèse, j'aborde ce problème selon une approche peu explorée : l'étude expérimentale de la déformation sous l'effet du vent de la surface d'un liquide fortement visqueux. En effet, contrairement à la majeure partie de la littérature sur le sujet, le liquide que j'utilise n'est pas de l'eau mais un liquide sensiblement plus visqueux. Indépendamment des questions fondamentales sous-jacentes, cela a en pratique l'avantage de simplifier le problème. En effet, du fait de la forte viscosité du liquide, l'écoulement dans le liquide reste laminaire et les perturbations de l'interface qui ne sont pas amplifiées ne peuvent se propager que sur une distance limitée. Pour observer ces déformations de l'interface liquide-air, j'ai développé un nouveau montage expérimental sur lequel a été mise en œuvre la méthode de visualisation Free Surface Synthetic Schlieren. Cette technique non intrusive a permis de mesurer avec une résolution micrométrique les amplitudes de déformations de la surface et d'accéder aux premières déformations à faible vitesse de vent. Dans un premier temps, les expériences furent conduites sur un liquide trente fois plus visqueux que l'eau. Grâce aux données expérimentales obtenues par FS-SS, deux régimes de déformation de l'interface liquide-air ont été mis en évidence. A vitesse de vent faible, l'interface est recouverte de "wrinkles", des perturbations de faible amplitude désorganisées spatialement et globalement alignées dans le sens de l'écoulement. Ces wrinkles peuvent être interprétés comme l'effet sur l'interface des fluctuations de pression de l'écoulement turbulent d'air. A plus forte vitesse, au-dessus d'une vitesse critique, apparaissent des vagues transverses quasi-parallèles entre elles et perpendiculaire à la direction du vent. Les distinctions entre les deux régimes ont été détaillées et les non-linéarités émergeant au-dessus du seuil ont aussi été étudiées. Par la suite, la viscosité du liquide a été changée sur une large gamme. Il ressort des expériences que les deux régimes de déformation de l'interface sous l'effet du vent peuvent être identifiés pour l'ensemble des viscosités parcourues. Suite à ces résultats, un modèle décrivant l'évolution de l'amplitude des wrinkles en fonction du vent et de la viscosité du liquide a été développé. / Despite numerous studies on the subject, the development of waves under the action of wind still retains a certain number of open questions. In my PhD, I approach this problem through a fairly uncommon angle: the experimental study of the deformation by wind of the surface of a highly viscous liquid. Indeed, contrary to the major part of the literature on the matter, the liquid I used is not water but a significantly more viscous liquid. Regardless of the fundamental underlying questions, this has the practical advantage of simplifying the problem. Indeed, due to the high viscosity of the liquid, the flow in the liquid stays laminar and the unamplified perturbations of the interface can only propagate over a limited distance. To observe these deformations at the liquid-air interface, I have developed a new experimental set-up upon which the Free Surface Synthetic Schlieren method of visualization was implemented. This non-intrusive technique allowed to measure with a micrometric accuracy the amplitude of the surface deformation and to access the first deformations at low wind velocity. First, experiments were conducted over a liquid thirty times more viscous than water. The experimental data obtained by FS-SS show two regimes of deformation of the liquid-air interface. At low wind velocity, the interface is populated with ``wrinkles'', small-amplitude streamwise spatially disorganized perturbations. These wrinkles can be interpreted as the effect on the interface of the pressure fluctuations in the turbulent wind. At higher windspeed, above a critical velocity, transverse waves appear with quasi-parallel crests perpendicular to the wind direction. The distinctions between the two regimes have been detailed and the nonlinearities emerging above the threshold have also been studied. Then, the viscosity of the liquid has been changed over a large range. It results from the experiments that the two regimes of surface deformation by wind can be identified for all the viscosities explored. Following these results, a model was developed to account for the evolution of the wrinkles' amplitude both with wind velocity and with viscosity.

Statistical extrapolation is required to predict extreme loads, associated with a target return period, for offshore wind turbines. In statistical extrapolation, “short-term" distributions of the load random variable(s) conditional on the environment are integrated with the joint probability distribution of environmental random variables (from wind, waves, current etc.) to obtain the so-called “long-term" distribution, from which long-term loads may be obtained for any return period. The accurate prediction of long-term extreme loads for offshore wind turbines, using efficient extrapolation procedures, is our main goal. While loads data, needed for extrapolation, are obtained by simulations in a design scenario, field data can be valuable for understanding the offshore environment and the resulting turbine response. We use limited field data from a 2MW turbine at the Blyth site in the United Kingdom, and study the influence of contrasting environmental (wind) regimes and associated waves at this site on long-term loads, derived using extrapolation. This study also highlights the need for efficient extrapolation procedures and for modeling nonlinear waves at sites with shallow water depths. An important first step in extrapolation is to establish robust short-term distributions of load extremes. Using data from simulations of a 5MW onshore turbine model, we compare empirical short-term load distributions when two alternative models for extremes--global and block maxima--are used. We develop a convergence criterion, based on controlling the uncertainty in rare load fractiles, which serves to assess whether or not an adequate number of simulations has been performed. To establish long-term loads for a 5MW offshore wind turbine, we employ an inverse reliability approach, which is shown to predict reasonably accurate long-term loads, compared to a more expensive direct integration approach. We show that blade pitching control actions can be a major source of response variability, due to which a large number of simulations may be required to obtain stable tails of short-term load distributions, and to predict accurate ultimate loads. We address model uncertainty as it pertains to wave models. We investigate the effect of using irregular nonlinear (second-order) waves, compared to irregular linear waves, on loads for an offshore wind turbine. We incorporate this nonlinear irregular wave model into a procedure for integrated wind-wave-response analysis of offshore wind turbines. We show that computed loads are generally somewhat larger with nonlinear waves and, hence, that modeling nonlinear waves is important is response simulations of offshore wind turbines and prediction of long-term loads. / text