Dissertations / Theses on the topic 'Wave condition'

Surfzone wave transformation under storm conditions is investigated through field and laboratory measurements in this study. The observations have been used to examine currently available models of wave energy dissipation. Detailed field data has been collected by means of a novel method which was first introduced by Nielsen (1988). This method has been utilised through a common program between Griffith University and The University of Queensland at The Spit on the Gold Coast in Southeast Queensland. The facility primarily consists of a manometer tube array with 12 different manometer tube lengths varying from 60 m to 500 m offshore and a concrete manhole excavated into the dune system to house the monitoring station. Accordingly, this system has enabled the monitoring of a detailed wave height profile across the surfzone under any conditions from the safety of the “bunker” on land. The findings of new laboratory experiments on the frequency response of the semi-rigid manometer tubes are also presented which extend and improve upon the previous work of Nielsen et al. (1993). Testing was conducted over a range of frequencies (0.0067 Hz< f <2 Hz) and tube lengths (10 m< L <900 m). New frequency response factors are determined by fitting the semiempirical gain function of Nielsen et al. (1993) to the observed gain data. As a result, new predictive formulas for the empirical coefficients as a function of tube parameters are provided in this study. Wave induced pore pressure in the surfzone seabed is investigated based on the recorded field data. Two well-known models, i.e. Hsu and Jeng (1994) and Sleath (1970), are assessed against the field measurements. The findings validate the accuracy of the models and indicate that the extent of energy dissipation due to the overlying sand is less than 5% and depends on the incident wave length.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Engineering
Science, Environment, Engineering and Technology
Full Text

The study of the behaviour of the extreme values of a variable such as wave height is very important in engineering applications such as flood risk assessment and coastal design. Storm wave modelling usually adopts a univariate extreme value theory approach, essentially identifying the extreme observations of one variable and fitting a standard extreme value distribution to these values. Often it is of interest to understand how extremes of a variable such as wave height depend on a covariate such as wave direction. An important associated concept is that of return level, a value that is expected to be exceeded once in a certain time period. The main areas of research discussed in this thesis involve making improvements to the way that extreme observations are identified and to the use of quantile regression as an alternative methodology for understanding the dependence of extreme values on a covariate. Both areas of research provide developments to existing return level methodology so enhancing the accuracy of predicted future storm wave events. We illustrate the methodology that we have developed using both coastal and offshore wave data sets. In particular, we present an automated and computationally inexpensive method to select the threshold used to identify observations for extreme value modelling. Our method is based on the distribution of model parameter estimates across a range of thresholds. We also assess the effect of the uncertainty associated with threshold selection on return level estimation by using a bootstrap procedure. Furthermore, we extend our approach so that the selection of the threshold can also depends on the value of a covariate such as wave direction. As a biproduct of our methodological development we have improved existing techniques for estimating and making inference about the parameters of a standard extreme value distribution. We also present a new technique that extends existing Bayesian quantile regression methodology by modelling the dependence of a quantile of one variable on the values of another using a natural cubic spline. Inference is based on the posterior density of the spline and an associated smoothing parameter and is performed by means of a specially tuned Markov chain Monte Carlo algorithm. We show that our nonparametric methodology provides more flexible modelling than the current polynomial based approach for a range of examples.

This thesis focuses on the micro/nanoparticle size measurement by using Diffusing-wave Spectroscopy, investigating the laser power and concentration effect on the measurement and researching the measuring method for particles in shear flow. A DWS-CCD backscattering experiment set-up was used in this project. By using this set-up, in all about 2000 experiments were performed during the project, including system testing, laser power influence study, concentration effect study and shear flow study. In the beginning, a detailed analysis of the particle sizing for particles in static condition was carried out by summarising the principles and procedures. The results revealed that the experimental set-up in this work was reliable and repeatable. A calibration process was still required for the CCD’s frame rate and resolution, the light absorption and the CCD’s position in the set-up. After determining some important parameters, the research was extended to laser power and concentration influence study. The autocorrelation functions were produced under different laser power and for different concentration of particle solution. Analysis confirmed the influences, and the results were expressed in formulas to describe specific effects for laser power and solution concentration. Based on the formulas, new equations for particle sizing were derived for different concentration ranges. After that, particle’s motion and light scattering in shear flow were investigated. It was concluded that three regions could be used to describe the particle’s movement under shear force; in different regions, the autocorrelation functions were different due to the variation of the characteristic time scales. The Brownian motion and shear strain dominated particle’s movement under specific flow velocities. Therefore, for particles subjected to high flow velocities, a new particle sizing formula was produced to distinguish the general formula which was only valid for particles under Brownian motion. Contributions made by this research are applying DWS application to micro/nanoparticle sizing in different conditions. In static condition, the laser power and concentration influence were described in formulas; new equations were produced for particle sizing for different concentration range. In shear flow, the thresholds of Brownian motion domination and shear flow domination were found; a new particle sizing equation was derived for particles only controlled by shear force.

In this paper, the problem on formation and construction of a shock wave for three dimensional compressible Euler equations with the small perturbed spherical initial data is studied. If the given smooth initial data satisfies certain nondegenerate condition, then from the results in [20], we know that there exists a unique blowup point at the blowup time such that the first order derivates of smooth solution blow up meanwhile the solution itself is still continuous at the blowup point. From the blowup point, we construct a weak entropy solution which is not uniformly Lipschitz continuous on two sides of shock curve, moreover the strength of the constructed shock is zero at the blowup point and then gradually increases. Additionally, some detailed and precise estimates on the solution are obtained in the neighbourhood of the blowup point.

Thesis (M.S. in Electrical and Computer Engineering)--Naval Postgraduate School, December 1990.
Thesis Advisor(s): Janaswamy, R. Second Reader: Adler, Richard W. "December 1990." Description based on title screen as viewed on March 30, 2010. DTIC Descriptor(s): Linear systems, thickness, scattering, polarization, distribution, formulations, integrals, shape, structures, theses, electromagnetic wave propagation, reflection, surfaces, surface properties, wave propagation, cylindrical bodies, width, integral equations, frequency domain, plane waves, circular, truncation, concave bodies, convex bodies, radiation DTIC Identifier(s): Radar reflections, hollow cylinders, slit cylinders, OSRC (on surface radiation condition). Author(s) subject terms: Surface radiation conditions; transverse electric; circular cylinders; hollow cylinder slit cylinders Includes bibliographical references (p. 68-69 ). Also available in print.

My thesis is a theoretical and numerical investigation of the feasibility of enhancing the four wave mixing bandwidth by tuning the phase matching condition via stimulated Brillouin scattering. This technique was experimentally demonstrated in 2013 by introducing the amplitude and phase optical gain/loss frequency responses of stimulated Brillouin scattering (SBS) pump and Stokes waves onto the FWM pump or the FWM signal. Both SBS amplitude and phase responses originate from modifying the refractive index of the optical fibre while the FWM interaction takes place. As a result the FWM phase-matching condition and hence the signal gain can be flexibly controlled. The theoretical investigation was carried out with both processes considered separately. In this thesis, we show that with modest optical power, exponential gain is achievable for normal dispersion, which leads to a gain bandwidth which is comparable in size to that for anomalous dispersion without SBS. This is equivalent to a sign change of the nonlinear contribution to the phase matching condition. However the method is limited by SBS-induced power variations and by the need to limit the SBS power levels. The thesis is illustrated while using highly nonlinear silica fibre, a well-known platform for efficient gain in telecommunications as an example. Our results can be equally applied to other materials and to other geometries, for example that of chalcogenide photonic chips, which is an interesting and exciting platform for all-optical signal processing devices. This platform is considered in Chapter 3.

Photonic crystals have allowed unprecedented control of light and have allowed bringing new functionalities on chip. Photonic crystal waveguides (PCWs), which are linear defects in a photonic crystal, have unique features that distinguish these waveguides from other waveguides. The unique features include very large dispersion, existence of slow light, and the possibility of tailoring the dispersion properties for guiding light. In my research, I have overcome some of the challenges in using slow light in PCWs. In this work, I have demonstrated (i) high efficiency coupling of light into slow group velocity modes of a PCW, (ii) large bandwidth high transmission and low dispersion bends in PCWs, (iii) accurate modeling of pulse propagation in PCWs, (iv) high efficiency absorbing boundary conditions for dispersive slow group velocity modes of PCWs. To demonstrate the utility of slow light in designing high efficiency devices, I have demonstrated refractive index sensors using slow light in PCWs. In the end, a few high efficiency devices based on slow light in PCWs are mentioned. The remaining issues in the widespread use of PCW are also discussed in the last chapter.

Combustion instabilities (thermo-acoustic pressure oscillations) have been recognised for some time as a problem limiting the development of low emissions (e.g., lean burn) gas turbine combustion systems, particularly for aviation propulsion applications. Recently, significant research efforts have been focused on acoustic damping for suppression of combustion instability. Most of this work has either been experimental or based on linear acoustic theory. The last 3-5 years has seen application of density based CFD methods to this problem, but no attempts to use pressure-based CFD methods which are much more commonly used in combustion predictions. The goal of the present work is therefore to develop a pressure-based CFD algorithm in order to predict accurately acoustic propagation and acoustic damping processes, as relevant to gas turbine combustors. The developed computational algorithm described in this thesis is based on the classical pressure-correction approach, which was modified to allow fluid density variation as a function of pressure in order to simulate acoustic phenomena, which are fundamentally compressible in nature. The fact that the overall flow Mach number of relevance was likely to be low ( mildly compressible flow) also influenced the chosen methodology. For accurate capture of acoustic wave propagation at minimum grid resolution and avoiding excessive numerical smearing/dispersion, a fifth order accurate Weighted Essentially Non-Oscillatory scheme (WENO) was introduced. Characteristic-based boundary conditions were incorporated to enable accurate representation of acoustic excitation (e.g. via a loudspeaker or siren) as well as enable precise evaluation of acoustic reflection and transmission coefficients. The new methodology was first validated against simple (1D and 2D) but well proven test cases for wave propagation and demonstrated low numerical diffusion/dispersion. The proper incorporation of Characteristic-based boundary conditions was validated by comparison against classical linear acoustic analysis of acoustic and entropy waves in quasi-1D variable area duct flows. The developed method was then applied to the prediction of experimental measurements of the acoustic absorption coefficient for a single round orifice flow. Excellent agreement with experimental data was obtained in both linear and non-linear regimes. Analysis of predicted flow fields both with and without bias flow showed that non-linear acoustic behavior occurred when flow reversal begins inside the orifice. Finally, the method was applied to study acoustic excitation of combustor external aerodynamics using a pre-diffuser/dump diffuser geometry previously studied experimentally at Loughborough University and showed the significance of boundary conditions and shear layer instability to produce a sustained pressure fluctuation in the external aerodynamics.

Thesis (Ph. D.)--Case Western Reserve University, 2010.
Title from PDF (viewed 2009-12-30). Department of History. Includes abstract. Includes bibliographical references and appendices. Available online via the OhioLINK ETD Center.

The polarization properties of embedded centro-symmetric and periodic multilayer stacks under conditions of frustrated total internal reflection (FTIR) are considered. The centro-symmetric multilayer stack consists of a high-index center layer sandwiched between two identical low-index films and high-index – low-index bilayers repeated on both sides of the central trilayer maintaining the symmetry of the entire stack. The periodic multilayer consists of periodically repeated low-index – high-index bilayers. Each multilayer stack is embedded in a high-index prism. Embedded centro-symmetric multilayer stacks are designed to function as efficient polarizers or polarizing beam splitters (PBSs) under conditions of FTIR over an extended range of incidence angles. For a given set of refractive indices, all possible solutions for the thicknesses of the layers that suppress the reflection of p-polarized light at a specified angle, and the associated reflectance of the system for the orthogonal s polarization, are determined. The angular and spectral sensitivities of polarizing multilayer stacks employing 3, 7, 11, 15 and 19 layers of BaF2 and PbTe thin films embedded in a ZnS prism, operating at ë = 10.6 ìm, are presented. Embedded centro-symmetric multilayer stacks are also designed to function as complete-transmission quarter-wave or half-wave retardation (QWR or HWR) devices under conditions of FTIR. QWR and HWR designs at ë =1.55 mì are presented that employ 11 and 7 layers of Si and SiO2 thin films embedded in GaP and Si cube prisms, respectively. The angular and spectral sensitivities of these devices are also considered. Embedded centro-symmetric multilayer stacks under FTIR conditions are also designed to produce various 50%-50% beam splitters. Embedded periodic multilayer stacks are designed to function as polarizers and PBSs at discrete multiple angles of incidence and wavelengths under condition of FTIR. For a given set of refractive indices, all possible solutions for the thicknesses of the layers that suppress the reflection of p-polarized light at a specified angle, and the associated reflectance of the system for the orthogonal s polarization, are determined. The angular and spectral sensitivities of polarizing multilayer stacks employing 4, 6, 8, 10, 12, 14, 16 and 18 layers of BaF2 and PbTe thin films embedded in a ZnS prism, operating at ë= 10.6 ìm, are presented.

L’objectif de cette thèse est d’améliorer les procédures de génération et de qualification des vagues pour les études de génie océanique. Le cadre est limité aux états de mer unidirectionnels générés dans des bassins d’essais expérimentaux et numériques. Les campagnes expérimentales ont été réalisées dans les bassins de l’ECN et les études numériques à l’aide du solveur HOS-NWT développé par l’ECN. Dans la première partie de la thèse, la propagation des états de mer dans les bassins d’essais est abordée d’un point de vue théorique, expérimental et numérique. Une attention particulière est accordée à l’évolution spatiale du spectre et des statistiques. En outre, les incertitudes expérimentales sont étudiées endétail. La deuxième partie de la thèse se concentre sur le contrôle des champs de vague à n’importe quelle position cible dans le domaine. Tout d’abord, une procédure axée sur la qualité du spectre de houle est étudiée. Ensuite, face à l’influence de la position cible sur les statistiques, une nouvelle procédure est introduite. Elle permet de mieux contrôler les distributions statistiques, indépendamment de la position cible
The objective of this thesis is to improve the wave generation and qualification procedures in the context of ocean engineering studies. The framework is limited to unidirectional irregular sea states generated in experimental and numerical wave tanks. Experiments were carried out using the ECN facilities and numerical studies were performed using the nonlinear potential wave solver HOS-NWT developed by ECN. In the first part of the thesis, the problem of irregular wave propagation in wave tank environments is addressed from theoretical, experimental, and numerical points of view. Paticular attention is paid to the evolution in space of the wave spectrum and statistics. In addition, experimental uncertainties arestudied in detail. The second part of the thesis focuses on developing methods to better control the wave fields at any target position in the domain. First, a procedure focusing on the quality of the wave spectrum is studied. Then, facing the dependence of the wave statistics on the target location, a new procedure is developed and tested to better control the statistical distributions independently of the target location

In this thesis we analyse the high-order in time discontinuous Galerkin nite element method (DGFEM) for second-order in time linear abstract wave equations. Our abstract approximation analysis is a generalisation of the approach introduced by Claes Johnson (in Comput. Methods Appl. Mech. Engrg., 107:117-129, 1993), writing the second order problem as a system of fi rst order problems. We consider abstract spatial (time independent) operators, highorder in time basis functions when discretising in time; we also prove approximation results in case of linear constraints, e.g. non-homogeneous boundary data. We take the two steps approximation approach i.e. using high-order in time DGFEM; the discretisation approach in time introduced by D Schötzau (PhD thesis, Swiss Federal institute of technology, Zürich, 1999) to fi rst obtain the semidiscrete scheme and then conformal spatial discretisation to obtain the fully-discrete formulation. We have shown solvability, unconditional stability and conditional a priori error estimates within our abstract framework for the fully discretized problem. The skew-symmetric spatial forms arising in our abstract framework for the semi- and fully-discrete schemes do not full ll the underlying assumptions in D. Schötzau's work. But the semi-discrete and fully discrete forms satisfy an Inf-sup condition, essential for our proofs; in this sense our approach is also a generalisation of D. Schötzau's work. All estimates are given in a norm in space and time which is weaker than the Hilbert norm belonging to our abstract function spaces, a typical complication in evolution problems. To the best of the author's knowledge, with the approximation approach we used, these stability and a priori error estimates with their abstract structure have not been shown before for the abstract variational formulation used in this thesis. Finally we apply our abstract framework to the acoustic and an elasto-dynamic linear equations with non-homogeneous Dirichlet boundary data.

La condition superieure absorbante et la condition superieure radiative ont ete examinees dans la simulation numerique des ondes orographiques. Nous constatons que la premiere condition, bien que permettant de simuler les ecoulements perturbes, n'est pas tres satisfaisante du fait de la sensibilite de la couche absorbant aux facteurs qui la definissent et egalement du fait de la resonance du modele. Quant a la seconde, elle permet d'exclure les insuffisances constatees avec la premiere, et fournit des resultats satisfaisants. L'influence de la complexite du terrain sur les ecoulements perturbes est liee etroitement a la valeur et a la variation verticale du parametre de scorer. Les conditions dynamiques de formation des precipitations orographiques liees aux fronts froids a ete egalement etudiee. Ce sont les zones statiquement instables formees par advection differentielle qui declenchent les precipitations en amont du relief, et non pas en raison de l'ascendance foree par la montagne. En l'absence de precipitations, l'humidite a pour effet de reduire l'interaction constatee entre le mouvement moyen et les perturbations forcees par relief

Thesis (M. S.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Jacobs, Laurence; Committee Co-Chair: Qu, Jianmin; Committee Member: DesRoches, Reginald. Part of the SMARTech Electronic Thesis and Dissertation Collection.

À Paris environ 44% des bâtiments ont été construits avant 1914 avec des murs épais non isolés et des matériaux poreux, caractérisés par une forte inertie thermique et des propriétés hygroscopiques. Les propriétés hygrothermiques des matériaux utilisés dans les constructions anciennes ont des effets qui : (i) confèrent (aux bâtiments) de bonnes qualités thermiques en période estivale et (ii) contribuent à réguler la température et l’humidité relative intérieure. En France les politiques d’économie d’énergie et la régulation thermique ont abouti à la mise en place de l’isolation thermique afin de réduire les consommations d'énergie pendant l'hiver. L'installation de l'isolation thermique dans la rénovation des bâtiments existants pose cependant des difficultés. D’autre part, il est prévu que des conditions extrêmes de chaleur deviennent plus fréquentes dans des scénarios du climat futur. Ainsi, les évolutions possibles du climat futur doivent être intégrées dans l'évaluation des stratégies de rénovation dans le bâtiment ancien. Ce travail de thèse porte sur l'évaluation du comportement hygrothermique de constructions anciennes rénovées à Paris, dans des conditions météorologiques actuelles et de vague de chaleur. A l’échelle des logements un modèle a été construit sur un outil de simulation thermique dynamique, calé et validé à travers des données enregistrées dans une campagne de mesure lancée en 2014 dans les logements étudiés. À l’échelle de la paroi un modèle macroscopique en 2D est proposé afin d'étudier les transferts de chaleur et de masse dans un mur poreux constitué de plusieurs couches avec de l’isolation thermique par intérieur et par l’extérieur
In Paris 44% of the dwellings were built before 1914 with uninsulated thick walls made of porous materials, characterized by high thermal inertia and hygroscopic properties. The hygrothermal properties of existing buildings materials have effects that: (i) give (to these buildings) good thermal qualities in summer and (ii) help to regulate indoor temperature and relative humidity. In France the energy saving policies and thermal regulation have resulted in the implementation of thermal regulation to reduce energy consumption during winter. Though the installation of thermal insulation in existing buildings poses a number of difficulties. Moreover, it is expected that extreme heat conditions become more frequent in future climate scenarios. Thus, the possible evolutions of future climate must be integrated into the evaluation of renovation strategies in old buildings. This thesis focuses on the assessment of the hygrothermal behavior of old renovated Parisian buildings, in current and heat wave weather. At the building scale (housing), a model was built in dynamic thermal simulation tool. The model was calibrated and validated through recorded data from a measurement campaign launched in 2014 on the studied housings. At the wall scale, a macroscopic model in 2D is proposed to study the heat and mass transfer through a multilayered porous wall, renovated with internal thermal insulation and external thermal insulation

This thesis represents a small step toward improved generation of realistic sea states in laboratory wave basins. Experiments were conducted in an offshore directional wave basin equipped with a segmented wave generator. Regular waves were generated for several periods and propagation directions, and the resulting wave elevations were measured throughout the basin. Motions of the 60 wavemaker segments were based on the snake principle of directional wave generation. Results are summarized and compared with predictions of a boundary element numerical method. Findings encourage further development of this linear diffraction numerical technique. It should be used to correct generator segment motions as prescribed by the snake principle, so as to account for diffraction and reflection effects which affect the wave field. The experiments were conducted at the Hydraulics Laboratory of the National Research Council of Canada, in Ottawa. A six metre square array of wave probes was used to measure wave elevations at discrete points spaced on a 2m grid around the wave basin. Three wave periods were investigated, with waves directed normal to the generator face, as well as in three oblique propagation directions. For all but the most severe propagation direction, tests were run a second time with a large surface piercing cylindrical structure positioned in the test area. Maximum waveheight results were plotted, and compared with numerical model predictions at the same locations around the basin. A linear diffraction computer program based on the boundary element method was used to predict the wave field at the same points around the basin. By this method, generator segment faces and reflecting walls are represented by a distribution of discrete sources around the basin boundaries. Measured elevation time series were analysed using a multiple regression screening program which extracts a prescribed number of sinusoidal components from the signal of interest. The program was modified to accommodate harmonic analysis. Fundamental and second harmonic components were synthesized from each time series. The second harmonic component was generally an order of magnitude smaller than the fundamental component. Discrepancies between these results and the linear numerical model predictions are attributed to nonlinear effects, and to basin resonance. The linear diffraction computer model is seen to predict the wave field to a high degree of accuracy, even though imprecise boundary definition was necessary at the current level of program development.
Applied Science, Faculty of
Civil Engineering, Department of
Graduate

In this thesis, first a higher order dynamic theory for anisotropic thermoelastic plates is developed. Then, based on this plate theory, two dynamic models, discrete and continuum models (DM and CM), are proposed for layered composites consisting of anisotropic thermoelastic layers. Of the two models, CM is more important, which is established in the study of periodic layered composites using smoothing operations. CM has the properties: it contains inherently the interface and Floquet conditions and facilitates the analysis of the composite, in particular, when the number of laminae in the composite is large
it contains all kinds of deformation modes of the layered composite
its validity range for frequencies and wave numbers may be enlarged by increasing, respectively, the orders of the theory and interface conditions. CM is assessed by comparing its prediction with the exact for the spectra of harmonic waves propagating in various directions of a two-phase periodic layered composite, as well as, for transient dynamic response of a composite slab induced by waves propagating perpendicular to layering. A good comparison is observed in the results and it is found that the model predicts very well the periodic structure of spectra with passing and stopping bands for harmonic waves propagating perpendicular to layering. In view of the results, the physical significance of Floquet wave number is also discussed in the study.

Esta tese é dedicada ao estudo de uma classe de equações de ondas com condições de fronteira da acústica. Investigamos a dinâmica assintótica de tais equações no caso em que o sistema está sujeito à ação de uma força externa não autônoma. Nessa situação, adicionando uma dissipação fraca, provamos que o problema gera um processo de evolução dissipativo. O nosso objetivo é então o estudo da existência de atratores não autônomos. Num primeiro momento estabelecemos a existência de um atrator do tipo \\pullback\", minimal, dentro de um universo de conjuntos temperados. Também estudamos a semicontinuidade superior dos atratores quando a perturbação não autônoma tende para zero. Nosso resultado permite considerar forcas externas não limitadas e perturbações não lineares com crescimento crítico (de Sobolev). Num segundo momento, fazemos um estudo sobre a existência de atratores uniformes. Em vista de resultados recentes de Zelik (2015), consideramos forcas externas mais gerais do que a dita classe das forcas compactas por translação (translation-compact). Parte desta tese foi aceita para publicação na revista \\Differential and Integral Equations\" sob o ttulo \\Pullback dynamics of non-autonomous wave equations with acoustic boundary condition\".
This thesis is concerned with the study of a class of wave equations with acoustic boundary conditions. We investigate the long-time dynamics of such equations in the case where the system is subject to a non-autonomous external force. In this situation, by adding a weak dissipation, we prove that the problem generates a dissipative evolution process. Our goal is then the existence of non-autonomous attractors. In this direction, we first establishes the existence of a minimal pullback attractor within a universe of tempered sets. We also studied the upper semi-continuity of attractors when the non-autonomous perturbation tends to zero. Our result allows to consider unbounded external forces and nonlinear perturbation with critical (Sobolev) growth. Secondly, we establish the existence of uniform attractors, as well. In view of recent results Zelik (2015) we consider more general external forces than the so called class of translation-compact forces. Part of this thesis was accepted for publication in the journal \\Differential and Integral Equations\" under the title \\Pullback dynamics of non-autonomous wave equations with acoustic boundary condition\".

The amount of energy enclosed in ocean waves has been classified as one of the most promising renewable energy sources. Nowadays, different wave energy conversion (WEC) systems are being investigated, but only a few concepts have been operated in a sea environment. One of the largest challenges is to guarantee the offshore survivability of the devices in extreme wave conditions. However, there are large uncertainties related to the prediction of extreme wave loads on WECs.  Highfidelity computational fluid dynamics (CFD) simulations can resolve nonlinear hydrodynamic effects associated with wave-structure interaction (WSI). This thesis explores the point-absorbing WEC developed by Uppsala University in extreme wave conditions. The dynamic response and the forces on key components (mooring line, buoy, generator's end-stop spring) of the device are studied and compared. The high nonlinear phenomena accompany the steep and high waves, i.e., breaking behavior, slamming loads can be well-captured by the highfidelity CFD simulations. A commonly used methodology for extreme waves selection, recommended by technical specifications and guidelines, is the environmental contour approach. The 100-year contour in Hamboldt Bay site in California and the 50-year contour in the Dowsing site, outside the UK, are utilized to extract the extreme waves examined in the present thesis. Popular methodologies and data from different sources (observational and hindcast data) are examined for the environmental contour generation providing useful insights. Moreover, two popular approaches for the numerical representation of the extreme sea states, either as focused wave or as equivalent regular wave, were examined and compared. A midfidelity model of the WEC is successfully verified, as the utilization of lower fidelity tools in the design stage would reduce the computational cost. Last but not least, in CFD simulations the computational grid is sensitive in large motions, something often occurs during extreme-WSI. The solution of this issue for the open source CFD software OpenFOAM is provided here.

Mes travaux de thèse portent sur le développement de schémas numériques d'ordre élevé en temps et en espace pour la simulation de la propagation des ondes. Nous avons proposé de discrétiser dans un premier temps l'équation des ondes par rapport au temps, en utilisant une technique de type équation modifiée. Puis nous avons utilisé une méthode d'éléments finis de type Galerkine discontinue pour la discrétisation en espace. En modifiant l'ordre de la discrétisation, nous avons construit des schémas tout aussi précis que ceux déjà existants pour un coût de mise en oeuvre très intéressant. Après avoir validé numériquement la nouvelle méthode, nous nous sommes intéressés à sa stabilité ainsi qu'à son adaptivité en temps et en espace. Pour arriver à cela, nous avons dû faire une étude précise de la stabilité de la méthode de Galerkine discontinue et nous avons proposé des améliorations à cette technique entraînant des gains de temps significatifs.

We consider two aspects of non-reflecting boundaryconditions for wave propagation problems. First we evaluate aproposed Perfectly Matched Layer (PML) method for thesimulation of advective acoustics. It is shown that theproposed PML becomes unstable for a certain combination ofparameters. A stabilizing procedure is proposed andimplemented. By numerical experiments the performance of thePML for a problem with nonuniform flow is investigated. Furtherthe performance for different types of waves, vorticity andsound waves, are investigated.

The second aspect concerns spurious waves, which areintroduced by any discretization procedure. We constructdiscrete boundary conditions, that are nonreflecting for bothphysical and spurious waves, when combined with a fourth orderaccurate explicit discretization of one-way wave equations. Theboundary condition is shown to be GKS-stable. The boundaryconditions are extended to hyperbolic systems in two spacedimensions, by combining exact continuous non-reflectingboundary conditions and the one dimensional discretelynon-reflecting boundary condition. The resulting boundarycondition is localized by the standard Pad´eapproximation.

Numerical experiments reveal that the resulting methodsuffers from boundary instabilities. Analysis of a relatedcontinuous problem suggests that the discrete boundarycondition can be stabilized by adding tangential viscosity atthe boundary. For the lowest order Pad´e approximation weare able to stabilize the discrete boundary condition.

The purpose of this research is to provide a better understanding of the physical and social aspects of rip currents in ocean areas that will lead to better forecasts, better governmental policies in beach areas, and ultimately to save lives. A rip current is a nearshore circulation in which breaking waves run up onto the beach then retreat rapidly in deeper channels back toward the sea. Rip currents pose a significant threat to beachgoers and can pull even the strongest swimmers out to sea. The primary factors associated with rip current formation on unarmored beaches are variations in the local beach bathymetry, wind-generated longshore waves of varying height, and lower tidal stages. The rationale for this study is highlighted when rip current deaths are put in context with deaths from other weather related deaths. The average number of rip current deaths per year in the United States is 46 and in the year 2010 rip currents were responsible for 64 deaths which was higher than the deaths associated with lightning, tornadoes, hurricanes and the cold winter during the year. The methodology followed for this study includes a review of demographics from over 500 rip current drowning reports along the Atlantic Ocean, Pacific Ocean and Gulf of Mexico coasts of the United States from 1994-2012. This research indicates that tourists are often victims, and rescuers can become the victims. For each state or sub-state area where rip current drownings are prevalent, an analysis of social aspects, beach areas, and associated ocean and weather patterns was conducted using averaged wind and pressure fields over wave generation areas, buoy data, and tide data. It is important to understand the evolution of these drowning events and seek solutions to mitigate the problem.

Dans cette thèse nous considérons l’équation des ondes amorties vectorielle sur une variété riemannienne compacte, lisse et sans bord. L’amortisseur est ici une fonction lisse allant de la variété dans l’espace des matrices hermitiennes de taille n. Les solutions de cette équation sont donc à valeurs vectorielles. Nous commençons dans un premier temps par calculer le meilleur taux de décroissance exponentiel de l’énergie en fonction du terme d’amortissement. Ceci nous permet d’obtenir une condition nécessaire et suffisante la stabilisation forte de l’équation des ondes amorties vectorielle. Nous mettons aussi en évidence l’apparition d’un phénomène de sur-amortissement haute fréquence qui n’existait pas dans le cas scalaire. Dans un second temps nous nous intéressons à la répartition asymptotique des fréquences propres de l’équation des ondes amorties vectorielle. Nous démontrons que, à un sous ensemble de densité nulle près, l’ensemble des fréquences propres est contenu dans une bande parallèle à l’axe imaginaire. La largeur de cette bande est déterminée par les exposants de Lyapunov d’un système dynamique défini à partir du coefficient d’amortissement
In this thesis we are considering the vectorial damped wave equation on a compact and smooth Riemannian manifold without boundary. The damping term is a smooth function from the manifold to the space of Hermitian matrices of size n. The solutions of this équation are thus vectorial. We start by computing the best exponential energy decay rate of the solutions in terms of the damping term. This allows us to deduce a sufficient and necessary condition for strong stabilization of the vectorial damped wave equation. We also show the appearance of a new phenomenon of high-frequency overdamping that did not exists in the scalar case. In the second half of the thesis we look at the asymptotic distribution of eigenfrequencies of the vectorial damped wave equation. Were show that, up to a null density subset, all the eigenfrequencies are in a strip parallel to the imaginary axis. The width of this strip is determined by the Lyapunov exponents of a dynamical system defined from the damping term

Accurate knowledge of the centimeter- and millimeter-wavelength absorptivity of ammonia is necessary for the interpretation of the emission spectra of the jovian planets. The objective of this research has been to advance the understanding of the centimeter- and millimeter-wavelength opacity spectra of ammonia under jovian conditions using a combination of laboratory measurements and theoretical formulations. As part of this research, over 1000 laboratory measurements of the 2-4 mm-wavelength properties of ammonia under simulated upper and middle tropospheric conditions of the jovian planets, and approximately 1200 laboratory measurements of the 5-20 cm-wavelength properties of ammonia under simulated deep tropospheric conditions of the jovian planets have been performed. Using these and pre-existing measurements, a consistent mathematical formalism has been developed to reconcile the centimeter- and millimeter-wavelength opacity spectra of ammonia. This formalism can be used to estimate the opacity of ammonia in a hydrogen/helium atmosphere in the centimeter-wavelength range at pressures up to 100 bar and temperatures in the 200 to 500 K range and in the millimeter-wavelength range at pressures up to 3 bar and temperatures in the 200 to 300 K range. In addition, a preliminary investigation of the influence of water vapor on the centimeter-wavelength ammonia absorptivity spectra has been conducted. This work addresses the areas of high-sensitivity centimeter- and millimeter-wavelength laboratory measurements, and planetary science, and contributes to the body of knowledge that provides clues into the origin of our solar system. The laboratory measurements and the model developed as part of this doctoral research work can be used for interpreting the emission spectra of jovian atmospheres obtained from ground-based and spacecraft-based observations. The results of the high-pressure ammonia opacity measurements will also be used to support the interpretation of the microwave radiometer (MWR) measurements on board the NASA Juno spacecraft at Jupiter.

There is no guidance available for either mean or wave-by-wave overtopping discharges over vertical seawalls subject to oblique and impulsive wave attack.  Impulsive wave attack (i.e. waves break onto the structure) may lead to substantially higher overtopping volumes than pulsating wave attach and can arise at relatively low water levels. Mean and wave-by-wave overtopping discharges represent important quantities in the design of coastal structures. The not only affect the structural safety and determine the capacity of the drainage system behind the structure, but also pose a hazard to communications, buildings, and members of the public. Previous work has shown that impulsive overtopping can substantially increase overtopping volumes and velocities, thus underlining the importance of robust prediction tools. This PhD thesis describes experiments and analysis to provide design guidance in wave overtopping of vertical seawalls under oblique and impulsive wave attach. The guidance extends existing design tools for wave overtopping which cover both wave conditions under oblique wave attack. Special attention is given to the transition from impulsive to reflecting wave conditions, which goes along with a significant reduction in overtopping towards higher obliquities. For moderate obliquities (15° and 30°) a new intermediate wave condition is defined as the “impact-like” condition, which – in terms of overtopping – has still to be treated as impulsive. Contrary to a few previous investigations on sloping walls, no increase in overtopping could be found at small obliquities (15°). Spatial variability, however, could be measured along the seawall and has been considered in the design guidance offered in this thesis.

The design of rubble mound breakwaters usually focuses on the main armor layer. A review of the existing literature reveals that different equations are used to design rock armors in non-breaking wave conditions. However, most rubble mound breakwaters are constructed in the depth-induced breaking zone where they are attacked by waves breaking in the foreshore; in these conditions, existing design equations are not valid. Therefore, in this PhD thesis, the hydraulic stability of double-layer rock armors is analyzed through a series of small-scale tests conducted with a bottom slope m=1/50. Based on test results, a new potential relationship is given to design rock armors in depth-limited breaking wave conditions with armor slope cot¿=1.5, stability numbers within the range 0.98¿Hm0/(¿Dn50)¿2.5, and relative water depth at the toe 3.75¿hs/(¿Dn50)¿7.50. When concrete units are used for the armor layer, mound breakwaters are usually protected by a toe berm. This toe berm is placed on the seafloor or underlayer, providing support for the concrete armor units which are placed later on the structure slope. Toe berm design is commonly related to the armor design; in non-breaking wave conditions, the mass of toe berm rocks is one order of magnitude lower than the units of the layer. In breaking wave conditions, however, the highest waves start breaking on the bottom and impact directly on the toe berm. This is the common case of rocky sea bottoms with m=1/10 or higher slopes and thus, a correct design of the toe berm is crucial to guarantee the armor stability. The present PhD thesis examines the hydraulic stability of rock toe berms placed on a m=1/10 bottom slope and in very shallow waters (0.53Dn50): (1) the nominal toe berm or the most shoreward toe berm area which effectively supports the armor layer, and (2) the sacrificial toe berm or the most seaward toe berm area which serves to protect the nominal toe berm. Considering the nominal toe berm damage, a new method is developed to reduce the rock toe berm size (Dn50) by increasing the toe berm width (Bt) if the required rock size is not available at the quarries. Finally, cube armor damage is examined, and the influence of the placement technique on armor stability is also characterized from physical tests conducted with cubes randomly- and uniformly- placed on the armor in two layers.
El manto principal de los diques en talud suele estar formado por escollera natural o elementos prefabricados de hormigón; su función es resistir la acción del oleaje. Una revisión del estado del arte pone de manifiesto que son numerosas las fórmulas existentes para el diseño de mantos derivadas de ensayos físicos a escala reducida con oleaje sin rotura por fondo. Sin embargo, la mayoría de diques en talud se construyen en la zona de rompientes con oleaje limitado por fondo, donde las ecuaciones de diseño habituales no son del todo válidas. En esta tesis doctoral se analiza la estabilidad hidráulica de mantos bicapa de escollera, a partir de ensayos a escala reducida con pendiente de fondo m=1/50. En base a los resultados obtenidos de los ensayos físicos, se propone una nueva relación potencial para el diseño de mantos de escollera en condiciones de oleaje limitado por fondo, válida para taludes con cot¿=1.5, números de estabilidad 0.98¿Hm0/(¿Dn50)¿2.5, y profundidades relativas a pie de dique de 3.75¿hs/(¿Dn50)¿7.50. Cuando el manto principal está formado por elementos de hormigón, es habitual construir una berma de pie que proporciona apoyo a los elementos del manto y, en su caso, colabora en la protección de la zona inferior del dique contra la socavación. Dicha berma suele construirse con escollera natural y su peso está condicionado al de los elementos del manto en el caso de no haber rotura por fondo. El peso de los elementos de la berma de pie suele ser un orden de magnitud inferior al peso de las unidades del manto; sin embargo, si la pendiente de fondo es fuerte (p.e. m=1/10) y las aguas someras esta regla no se cumple ya que algunas olas rompen sobre el fondo impactando directamente sobre la berma de pie. En estos casos, el peso de la escollera de la berma puede sobrepasar el de las unidades del manto y su correcto diseño es crucial para garantizar la estabilidad del dique. Además de estudiar la estabilidad del manto principal de diques de escollera, la presente tesis doctoral analiza también la estabilidad hidráulica de bermas de pie de escollera ubicadas en fondos con pendiente m=1/10 y aguas someras (0.53Dn50): (1) berma nominal o zona de la berma de pie sobre la que realmente apoya el manto principal, y (2) berma de sacrificio o zona de la berma de pie que protege a la berma nominal. A partir del daño de la berma de pie nominal, se propone un nuevo método para reducir el tamaño de piedra (Dn50) incrementando el ancho de la berma (Bt) cuando no se disponga del tamaño requerido en cantera. Finalmente, se examina el daño del manto de cubos y se analiza la influencia del método de colocación sobre el mismo, a partir de ensayos realizados con mantos bicapa de cubos con colocación aleatoria y uniforme.
El mantell principal dels dics en talús sol estar format per roca o elements prefabricats de formigó, la seva funció és resistir l'acció de l'onatge. Una revisió de l'estat de l'art manifesta que són nombroses les equacions de disseny existents per a condicions d'onatge no trencat. No obstant això, la majoria de dics en talús es construeixen a la zona de rompents amb onatge limitat per fons, on les equacions de disseny existents no són del tot vàlides. En aquesta tesi doctoral s'analitza l'estabilitat hidràulica de mantells bicapa de roca, a partir d'assajos a escala reduïda realitzats amb pendent de fons m = 1/50. En base als resultats obtinguts dels assajos, es proposa una relació potencial per al disseny de mantells de roca en condicions d'onatge limitat per fons vàlida per a talussos amb cot¿ = 1.5, nombres d'estabilitat 0.98¿Hm0/(¿Dn50) ¿2.5, i profunditats relatives a peu de dic de 3.75¿hs/(¿Dn50)¿7.50. Quan mantell principal està format per elements de formigó , és habitual construir una berma de peu que proporciona suport als elements del mantell i, si escau, col¿labora en la protecció de la zona inferior del dic contra la soscavació. Aquesta berma sol construir amb roca i el seu pes està condicionat al dels elements del mantell en el cas de no haver trencament per fons. El pes dels elements de la berma de peu sol ser un ordre de magnitud inferior al pes de les unitats del mantell; però, si el pendent de fons és fort ( p.e. m = 1 /10) i les aigües someres aquesta regla no es compleix ja que algunes onades trenquen sobre el fons impactant directament sobre la berma de peu. En aquests casos, el pes de la roca de la berma pot sobrepassar el de les unitats del mantell, i el seu correcte disseny és crucial per garantir l'estabilitat del dic. A més d'estudiar l'estabilitat del mantell principal de dics de roca, la present tesi doctoral analitza també l'estabilitat hidràulica de bermes de roca ubicades en fons amb pendents m = 1/10 i aigües someres (0.5 3 Dn50): (1) berma nominal o zona de la berma de peu sobre la qual recolza el mantell principal, i (2) berma de sacrifici o zona de la berma de peu que protegeix la berma nominal. A partir del dany de la berma de peu nominal, es proposa un nou mètode per reduir el tamany de roca (Dn50) incrementant l'amplada de la berma (Bt) quan no es disposi de la mida requerit en pedrera. Finalment, s'examina el dany del mantell de cubs i s'analitza la influència del mètode de col¿locació sobre el mateix , a partir d'assajos realitzats amb mantells bicapa de cubs amb col¿locació aleatòria i uniforme.
Herrera Gamboa, MP. (2017). Mound Breakwater Design in Depth-Limited Breaking Wave Conditions [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/82553
TESIS

Simulating electromagnetic waves is of increasing importance, for example, due to the rapidly growing demand of wireless communication in the fields of antenna design, photonics and electromagnetic compatibility (EMC). Many numerical and asymptotic techniques have been developed and one of the most common is the Finite-Difference Time-Domain (FDTD) method, also known as the Yee scheme. This centered difference scheme was introduced by Yee in 1966. The success of the Yee scheme is based on its relatively high accuracy, energy conservation and superior memory efficiency from the staggered form of defining unknowns. The scheme uses a structured Cartesian grid, which is excellent for implementations on modern computer architectures. However, the structured grid results in loss of accuracy due to general geometry of boundaries and material interfaces. A natural challenge is thus to keep the overall structure of Yee scheme while modifying the coefficients in the algorithm near boundaries and interfaces in order to improve the overall accuracy. Initial results in this direction have been presented by Engquist, Gustafsson, Tornberg and Wahlund in a series of papers. Our contributions are new formulations and extensions to higher dimensions. These new formulations give improved stability properties, suitable for longer simulation times. The development of the algorithmsis supported by rigorous stability analysis. We also tackle the problem of controlling the divergence free property of the solution—which is of extra importance in three dimensions—and present results of a number of numerical tests.
QC 20101101

Wave tank tests facilitate the understanding of how complex sea conditions influence the dynamics of man-made structures. If a potential deployment location is known, site data can be used to improve the relevance and realism of the test conditions, thus helping de-risk device development. Generally this data is difficult to obtain and even if available is used simplistically due to established practices and limitations of test facilities. In this work four years of buoy data from the European Marine Energy Centre is characterised and simulated at the FloWave Ocean Energy Research Facility; a circular combined wave-current test tank. Particular emphasis is placed on the characterisation and validation processes, aiming to preserve spectral and directional complexity of the site, whilst proving that the defined representative conditions can be effectively created. When creating representative site-specific sea states, particular focus is given to the application of clustering algorithms, which enable the entire spectral (frequency or directional) form to be considered in the characterisation process. This enables the true complex nature of the site to be considered in the data reduction process. Prior to generating and measuring the resulting sea states, issues with scaling are explored, the facility itself is characterised, and emphasis is placed on developing measurement strategies for the validation of directional spectra. Wave gauge arrays are designed and used to characterise various elements of the FloWave tank, including reflections, spatio-temporal variability and wave shape. A new method for directional spectrum reconstruction (SPAIR) is also developed, enabling more effective measurement and validation of the resulting directional sea states. Through comparison with other characterisation methods, inherent method-induced trade-offs are understood, and it is found that there is no absolute favourable approach, necessitating an application specific procedure. Despite this, a useful set of 'generic' sea states are created for the simulation of both production and extreme conditions. For sea state measurement, the SPAIR method is proven to be significantly more effective than current approaches, reducing errors and introducing additional capability. This method is used in combination with a directional wave gauge array to effectively measure, correct, and validate the resulting directional wave conditions. It is also demonstrated that site-specific wave-current scenarios can be effectively re-created, thus demonstrating that truly complex ocean conditions can be simulated at FloWave. This ability, along with the considered characterisation approach used, means that representative site-specific sea states can be simulated with confidence, increasing the realism of the test environment and helping de-risk device development.

The demands of railway transport have been changing over the 150 years of existence of this type of transport in South Africa, specifically the performance requirements of the formation to cater for new traffic requirements. As such, it is important to assess the condition of this vital part of a railway track. This dissertation covers a research project conducted on two railway lines in which measurements of ground vibration were conducted in order to perform geophysical analysis and characterise the formation based on the results obtained. Measurements were taken on a 26 ton axle load track (Coal line, at Bloubank) and on a 20 ton axle load track (at Amandelbult) in South Africa. Planning and implementation of several test procedures to characterise track formation require considerable effort to minimize the impact on railway operations. Coupled with track occupation and the destructive nature of some of the test procedures, it is relevant to investigate alternative testing techniques to address the issues stated above. The use of surface waves for geotechnical characterization of sites is increasing worldwide. Applications to railway engineering have so far been limited to light load, high speed lines to minimize the use of poor geomaterials with reduced Rayleigh wave velocity. Four sites were identified where trains are operated at heavy loads, with the formation condition varying from poor to good. Seismic testing (geophysical) and conventional testing (deflection measurements) were performed at the identified sites. Seismic measurements were recorded using geophones as receivers, coupled to an amplifier and a computer. The source of the seismic events was the trains operating on the track and a hammer for impact testing. For the deflection measurements, the Remote Video Monitoring (RVM) technique was adopted. Dispersion analysis of the ground vibration experimental data was conducted using the multiple receiver method. The main conclusions reached with the analysis indicated that: __ Dispersion analysis had a good correlation with the formation deflection analysis; __ Phase velocity can be used as an indicator of the quality of a certain site; __ There are limitations when using trains as the energy source in terms of the generation of excitation frequency, which greatly reduces the phase velocity information in individual layers in the formation (i.e. wavelengths are not short enough).
Dissertation (MSc)--University of Pretoria, 2016.
Civil Engineering
MSc
Unrestricted

The purpose of this project is to evaluate the differences in the wake of a NACA 4412 airfoil in free-flight condition compared to wind tunnel and also investigate wall-interference. This was done using two sets of angles of attack, 5 degrees and 8 degrees. We collected data using Computational Fluid Dynamics to simulate free-flight condition and experimental data gathered from a wind tunnel, there were some key differences between these two cases. The key differences were the size of the area of high velocity deficit as well as the wake thickness and angle.

Wave radiation is an important process in many geophysical flows. In particular, it is by wave radiation that flows may adjust to a state for which the dynamics is slow. Such a state is described as “balanced”, meaning there is an approximate balance between the Coriolis force and horizontal pressure gradients, and between buoyancy and vertical pressure gradients. In this thesis, wave radiation processes relevant to these enormously complex flows are studied through the use of some highly simplified models, and a parallel aim is to develop accurate numerical techniques for doing so. This thesis is divided into three main parts. 1. We consider accurate numerical boundary conditions for various equations which support wave radiation to infinity. Particular attention is given to discretely non-reflecting boundary conditions, which are derived directly from a discretised scheme. Such a boundary condition is studied in the case of the 1-d Klein-Gordon equation. The limitations concerning the practical implementation of this scheme are explored and some possible improvements are suggested. A stability analysis is developed which yields a simple stability criterion that is useful when tuning the boundary condition. The practical use of higher-order boundary conditions for the 2-d shallow water equations is also explored; the accuracy of such a method is assessed when combined with a particular interior scheme, and an analysis based on matrix pseudospectra reveals something of the stability of such a method. 2. Large-scale atmospheric and oceanic flows are examples of systems with a wide timescale separation, determined by a small parameter. In addition they both undergo constant random forcing. The five component Lorenz-Krishnamurthy system is a system with a timescale separation controlled by a small parameter, and we employ it as a model of the forced ocean by further adding a random forcing of the slow variables, and introduce wave radiation to infinity by the addition of a dispersive PDE. The dynamics are reduced by deriving balance relations, and numerical experiments are used to assess the effects of energy radiation by fast waves. 3. We study quasimodes, which demonstrate the existence of associated Landau poles of a system. In this thesis, we consider a simple model of wave radiation that exhibits quasimodes, that allows us to derive some explicit analytical results, as opposed to physically realistic geophysical fluid systems for which such results are often unavailable, necessitating recourse to numerical techniques. The growth rates obtained for this system, which is an extension of one considered by Lamb, are confirmed using numerical experiments.

The presence of wave motion is the defining feature in many fields of application,such as electro-magnetics, seismics, acoustics, aerodynamics,oceanography and optics. In these fields, accurate numerical simulation of wave phenomena is important for the enhanced understanding of basic phenomenon, but also in design and development of various engineering applications. In general, numerical simulations must be confined to truncated domains, much smaller than the physical space were the wave phenomena takes place. To truncate the physical space, artificial boundaries, and corresponding boundary conditions, are introduced. There are four main classes of methods that can be used to truncate problems on unbounded or large domains: boundary integral methods, infinite element methods, non-reflecting boundary condition methods and absorbing layer methods. In this thesis, we consider different aspects of non-reflecting boundary conditions and absorbing layers. In paper I, we construct discretely non-reflecting boundary conditions for a high order centered finite difference scheme. This is done by separating the numerical solution into spurious and physical waves, using the discrete dispersion relation. In paper II-IV, we focus on the perfectly matched layer method, which is a particular absorbing layer method. An open issue is whether stable perfectly matched layers can be constructed for a general hyperbolic system. In paper II, we present a stable perfectly matched layer formulation for 2 x 2 symmetric hyperbolic systems in (2 + 1) dimensions. We also show how to choose the layer parameters as functions of the coefficient matrices to guarantee stability. In paper III, we construct a new perfectly matched layer for the simulation of elastic waves in an anisotropic media. We present theoretical and numerical results, showing that the stability properties of the present layer are better than previously suggested layers. In paper IV, we develop general tools for constructing PMLs for first order hyperbolic systems. We present a model with many parameters which is applicable to all hyperbolic systems, and which we prove is well-posed and perfectly matched. We also use an automatic method, derived in paper V, for analyzing the stability of the model and establishing energy inequalities. We illustrate our techniques with applications to Maxwell s equations, the linearized Euler equations, as well as arbitrary 2 x 2 systems in (2 + 1) dimensions. In paper V, we use the method of Sturm sequences for bounding the real parts of roots of polynomials, to construct an automatic method for checking Petrowsky well-posedness of a general Cauchy problem. We prove that this method can be adapted to automatically symmetrize any well-posed problem, producing an energy estimate involving only local quantities.
QC 20100830

In this thesis, the effects on regular waves when propagating from deep to shallow water have been investigated, assuming linear wave theory. The effects of shoaling, refraction, reflection and diffraction have been studied. The process and types of breaking waves has also been reviewed.Linear wave theory, second and higher order Stokes theory, Stream function theory, Solitary wave theory and Cnoidal theory have been reviewed. The relative validity of the different wave theories was also assessed.The effects on the wave spectrum as a sea state travels from deep to shallow water, described by the energy balance equation, has also been addressed. The effects addressed were wind, nonlinear wave-wave interactions, white-capping, bottom friction and surf-breaking. Two computer models for numerically solving the energy balance equation was mentioned, being SWAN and STWAVE where SWAN was chosen for use in the analyses.The NORA10 hindcast was believed not to properly take the change in water depth into account at the Dogger Bank Zone. At a point north of the location of interest, the water depth was of such a magnitude (81m) that the NORA10 hindcast was believed to yield credible data for the significant wave height and spectral peak period. A long term estimate of the 50 year significant wave height was performed from the NORA10 data at the point north of the Dogger Bank Zone, by the environmental contour method. This resulted in an estimated 50 year significant wave height of 11.74m and spectral peak period of 15.68s.The SWAN model was used on a test case from Svangstu (2011) to get familiar with the program, and investigate how the different physical effects influence the solution. With the knowledge acquired from the test case, the Dogger Bank case was analyzed in SWAN, to obtain the 50 year sea state parameters at the location of interest. The 50 year sea state north of the Dogger Bank Zone, the Dogger Bank bathymetry, as well as a constant wind of 23.3 m/s was used as input. The 50 year sea state at the location of interest was found to be characterized by a significant wave height of 7.34m and a spectral peak period of 15.56s. SWAN was found to result in a significant wave height of some 15-27% lower than what was found in NORA10.By performing a short term analysis on the 50-year sea state at the location of interest, the 50-year design wave height was found to be 12.5m, assuming the individual wave heights to be modeled by the Gluhovski distribution. The 90% confidence interval of the design wave period was found to be 9.6s <T< 16.3s. This was estimated from studying the ratio between the period of the three largest waves in a time series, and the spectral peak period of the sea state in 95 time series from Svangstu (2011) By evaluating only the limiting values of the period range, the longest and shortest design wave was found to be 80% and 97.4% of the breaking wave height respectively, assuming linear theory.The wind turbine structure geometry was simplified to be modeled by a cylinder with a diameter of 6m. By computing the kinematics from the Stream Function theory, the maximum base shear and overturning moment using Morison’s equation was found to be 3.67MN and 65.93MN respectively. The effects of the rate of change of added mass momentum was also assessed.From a simplified and conservative approach, the impact loads from a breaking wave was estimated. This resulted in a base shear and overturning moment of such a magnitude that en extensive analysis is recommended on this topic in the future.

[ES] El cambio climático y la conciencia social sobre el impacto de las infraestructuras en el medio está llevando a la necesidad de diseñar diques en talud con cotas de coronación reducidas frente a eventos de rebase más extremos. Además, la mayoría de estos diques se construyen en zonas de profundidades reducidas, donde el oleaje rompe a causa de la limitación por fondo. Estudios recientes apuntan a la necesidad de considerar no sólo la caudal medio de rebase (q) sino también el máximo volumen individual de rebase (Vmax), el espesor de lámina de agua (OLT) y la velocidad del flujo de rebase (OFV) en el diseño de la cota de coronación de un dique en talud según criterios de rebase. No obstante, existen pocos estudios en la literatura científica centrados en Vmax en estructuras costeras sometidas a oleaje limitado por fondo. Además, estos estudios proporcionan resultados contradictorios en relación a la influencia de la limitación por fondo del oleaje sobre Vmax. En cuanto a OLT y OFV, no se han encontrado estudios en la literatura científica que permitan su predicción en diques en talud. En esta tesis doctoral, se han realizado ensayos físicos 2D en diques en talud rebasables (0.3≤Rc/Hm0≤2.5) sin espaldón y con tres mantos principales (Cubípodo®-1L, cubo-2L y escollera-2L) sobre dos pendientes de fondo suaves (m=2% and 4%) en condiciones de oleaje limitado por fondo (0.2≤Hm0/h≤0.9). Vmax junto con q son las variables más recomendadas en la literatura científica para diseñar la cota de coronación de diques en talud según criterios de rebase. En el presente estudio, los mejores resultados en la estimación de Vmax*=Vmax/(gHm0T012) se han obtenido empleando la función de distribución Weibull de dos parámetros con un coeficiente de determinación R2=0.833. Durante la fase de diseño de un dique en talud, es necesario predecir q para calcular Vmax cuando se emplean los métodos dados en la literatura científica. Por tanto, se debe estimar q con fines de diseño si no se dispone de observaciones directas. En caso de emplear la red neuronal CLASH NN para estimar q (R2=0.636), la bondad de ajuste de la función de distribución Weibull de dos parámetros propuesta en esta tesis para predecir Vmax* es R2=0.617. Así, el ratio entre Vmax* medido y estimado cae dentro del rango de 1/2 a 2 (banda de confianza del 90%) cuando se emplea q estimado con CLASH NN. Los nuevos estimadores desarrollados en la presente disertación proporcionan resultados satisfactorios en la predicción de Vmax* con un método más simple que aquellos propuestos en la literatura científica. No se ha encontrado una influencia significativa de la pendiente de fondo ni de la limitación por fondo del oleaje sobre Vmax* en este estudio. OLT y OFV están directamente relacionados con la estabilidad hidráulica de la coronación del dique y la seguridad peatonal frente a rebase. Por tanto, se requiere estimar OLT y OFV en la coronación del dique para diseñar apropiadamente su cota de coronación empleando criterios de rebase. En este estudio, se han empleado redes neuronales para desarrollar nuevos estimadores explícitos que permiten predecir OLT y OFV superados por el 2% del oleaje incidente con un alto coeficiente de determinación (0.866≤R2≤0.867). El número de cifras significativas apropiado para los coeficientes experimentales de dichos estimadores se ha determinado en base a su variabilidad. El punto óptimo en el que las características del oleaje deben ser estimadas para predecir OLT y OFV se ha identificado a una distancia de 3h desde el pie de la estructura (siendo h la profundidad a pie de dique). La pendiente de fondo tiene influencia sobre OLT y OFV. Los valores más extremos de OLT y OFV se han descrito empleando las distribuciones Exponencial de un parámetro y Rayleigh, respectivamente, con resultados satisfactorios (0.803≤R2≤0.812).
[CA] El canvi climàtic i la consciència social sobre l'impacte de les infraestructures al medi està portant a la necessitat de dissenyar dics en talús amb cotes de coronació reduïdes front a esdeveniments d'ultrapassament més extrems. A més, la majoria dels dics es construeixen en zones amb profunditats reduïdes on l'onatge es trenca a causa de la limitació per fons. Estudis recents apunten a la necessitat de considerar no solament el cabal mitjà de sobrepasse (q) sinó també el màxim volum individual de sobrepasse (Vmax), l'espessor de la làmina d'aigua (OLT) i la velocitat del flux de sobrepasse (OFV) pel disseny de la cota de coronació d'un dic en talús segons criteris de sobrepasse. No obstant, existeixen pocs estudis a la literatura científica centrats en Vmax en estructures costeres sotmeses a onatge limitat per fons. Addicionalment, aquests estudis proporcionen resultats contradictoris en relació a la influència de la limitació per fons de l'onatge sobre Vmax. Quant a OLT i OFV, no s'han trobat estudis a la literatura científica que permeten la seua predicció a dics en talús. En aquesta tesi doctoral, s'han realitzat assajos físics 2D amb dics en talús amb sobrepassos rellevants (0.3≤Rc/Hm0≤2.5) sense espatlló i amb tres elements al mantell principal (Cubípode-1L, cubs-2L i esculleres-2L) ubicats sobre pendents de fons suaus (m=2% i 4%) en condicions d'onatge limitat pel fons (0.2≤Hm0/h≤0.9). Vmax conjuntament amb q són les variables més recomanades a la literatura científica per dissenyar la cota de coronació en dics en talús segons criteris d'ultrapassament. Al present estudi, els millors resultats en l'estimació de Vmax*=Vmax/(gHm0T012) s'han obtingut utilitzant la funció de distribució Weibull de dos paràmetres amb un elevat coeficient de determinació R2=0.833. Durant la fase de disseny d'un dic en talús, és necessari predir q per calcular Vmax quan s'utilitzen els mètodes donats a la literatura científica. Per tant, es deu estimar q amb fins de disseny si no es disposa d'observacions directes. Si s'aplica la xarxa neuronal de CLASH NN per estimar q (R2=0.636), la bondat d'ajust de la funció de distribució Weibull de dos paràmetres proposada a aquesta tesi per predir Vmax* és R2=0.617. Així doncs, el ràtio entre el Vmax* mesurat i estimat es troba dins del rang de 1/2 a 2 (banda de confiança del 90%) quan s'usa q predit amb CLASH NN. Els nous estimadors desenvolupats a aquesta dissertació proporcionen resultats satisfactoris en la predicció de Vmax* amb un mètode més senzill que aquells proposats a la literatura científica. No s'ha trobat una influència significativa de la pendent de fons ni de la limitació de l'onatge per fons sobre Vmax* a aquest estudi. OLT i OFV estan directament relacionats amb l'estabilitat hidràulica de la coronació de dics i la seguretat de vianants front a ultrapassaments. Per tant, es requereix estimar OLT i OFV en la coronació de dics per dissenyar apropiadament la seua cota de coronació utilitzant criteris de sobrepasse. En aquest estudi, s'han usat xarxes neuronals per desenvolupar nous estimadors explícits que permeten predir OLT i OFV superats pel 2% de l'onatge incident amb un elevat coeficient de determinació (0.866≤R2≤0.867). El nombre de xifres significatives apropiat per als coeficients experimentals dels mencionats estimadors s'ha determinat basant-se en la seua variabilitat. El punt òptim on determinar les característiques de l'onatge deuen ser estimades per predir OLT i OFV s'ha identificat a una distància de 3h des del peu de l'estructura (on h és la profunditat a peu de dic). La pendent de fons té influència sobre OLT i OFV. Els valors més extrems de OLT i OFV s'han descrit amb les distribucions Exponencial d'un paràmetre i Rayleigh, respectivament, amb resultats satisfactoris (0.803≤R2≤0.812).
[EN] Climate change and the social concern about the impact of infrastructures is leading to mound breakwaters with reduced crest freeboards facing higher extreme overtopping events. In addition, most mound breakwaters are built in the surf zone where depth-limited wave breaking takes place. Recent studies point out the need of considering not only the mean wave overtopping discharge (q) but also the maximum individual wave overtopping volume (Vmax), the overtopping layer thickness (OLT) and the overtopping flow velocity (OFV) when designing mound breakwater crest elevation using overtopping criteria. However, few studies in the literature are focused on Vmax on coastal structures under depth-limited breaking wave conditions. In addition, those few studies report contradictory conclusions regarding the significance of depth-limited breaking waves on Vmax. With respect to OLT and OFV, no studies are found in the literature for their prediction on mound breakwaters. In this PhD thesis, 2D physical model tests were conducted on overtopped mound breakwaters (0.3≤Rc/Hm0≤2.5) without a crown wall armored with three armor layers (Cubipod®-1L, cube-2L and rock-2L) on two gentle bottom slopes (m=2% and 4%) in depth-limited breaking wave conditions (0.2≤Hm0/h≤0.9). Vmax together with q are the most recommended variables in the literature to design mound breakwater crest elevation based on overtopping criteria. In the present study, the 2-parameter Weibull distribution provides the best results when estimating Vmax*=Vmax/(gHm0T012) with coefficient of determination R2=0.833. During the design phase of a mound breakwater, q is needed to predict Vmax using methods given in the literature. Thus, q must be estimated for design purposes when direct observations are not available. If CLASH NN is used to estimate q (R2=0.636), the goodness-of-fit of the 2-parameter Weibull distribution proposed in this thesis to predict Vmax* is R2=0.617. Hence, the ratio between the estimated and measured Vmax* falls within the range 1/2 to 2 (90% error band) when q is predicted using CLASH NN. The new estimators derived in this study provide satisfactory estimations of Vmax* with a method simpler than those found in the literature. Neither the bottom slope nor the depth-induced wave breaking seem to significantly influence the dimensionless Vmax* in this study. OLT and OFV are directly related to the hydraulic stability of the armored crest and the pedestrian safety. Thus, OLT and OFV are required to properly design crest elevation using overtopping criteria. Neural Networks (NNs) are used in this study to develop new explicit unbiased estimators for the OLT and OFV exceeded by 2% of the incoming waves with a high coefficient of determination (0.866≤R2≤0.867). The appropriate number of significant figures of the empirical coefficients of such estimators is selected according to their variability. The optimum point where wave characteristics are determined to predict OLT and OFV was identified at a distance of 3h from the toe of the structure (where h is the water depth at the toe of the structure). The bottom slope does influence both OLT and OFV. The most extreme values of OLT and OFV are described with the 1-parameter Exponential and Rayleigh distribution functions, respectively, with satisfactory results (0.803≤R2≤0.812).
Al Ministerio de Educación, Cultura y Deporte, por la financiación brindada con el programa de Formación de Profesorado Universitario (FPU16/05081). Al Ministerio de Economía y Competitividad, por la financiación de los proyectos ESBECO (EStabilidad hidráulica del manto, BErmas y COronación de diques en talud con rebase y rotura por fondo, BIA2015-70436-R) y HOLOBREAK (Estabilidad Hidráulica y Transmisión de Diques Rompeolas Homogéneos de Baja Cota Diseñados a Rotura por Fondo, RTI2018-101073-B-I00-AR).
Mares Nasarre, P. (2021). Overtopping flow on mound breakwaters under depth-limited breaking wave conditions [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/163154
TESIS

Beaches composed of pebble to boulder-sized material are a common feature of rocky coastlines globally and provide effective protection against wave attack. However difficulties with the proximal deployment of wave gauges and measuring the transport of coarse clasts in the field has limited research in this area. The overarching aim of this thesis is to use contemporary ‘off-the-shelf’ technology in a series of interlinked, field-based experiments to improve quantitative understanding of relationship between coarse clastic beach dynamics and the nearshore wave conditions. In the first of the empirical chapters (Chapter 3), indigenous clasts implanted with motion loggers provided a unique insight into the threshold-entrainment within the littoral zone on: (1) an exposed macro-tidal field site in Abbotsham, UK and; (2) a fetch-limited system in Flathead Lake, USA. The threshold-entrainment was positively related to the power of waves that prevailed over the hour-long experiment and accurately predicted with the equations of Lorang (2000). The relationship between clast mass and power of individual incident waves linked to each entrainment event was more complex. There was the propensity of large clasts to occasionally become entrained by exceedingly weak waves. The processes behind this result remain unclear. Comprehensive experiments measuring variability in the concurrent wave conditions along the inner shore platform and offshore are presented in Chapter 4, along with daily longshore clast displacement and depth of activation measurements across the adjacent coarse clastic beach at Abbotsham. The longshore transport rate varies in concert with the longshore component of wave power, which is dependent on interactions between incident waves and shore platform morphology. These findings are used in Chapter 5 to model the (positive) longshore flux of clastic material towards the economically-valuable natural barrier, which has diminished in recent years. It was found that a disproportionately large fraction of positive transport was generated by storm events, the effectiveness of which was moderated by water depth. Net annual positive transport was predicted to increase with sea level rise and storminess. However, the increased influx was small in comparison to the volume of the barriers itself, and therefore unlikely to halt, or ameliorate future degradation. The experiments presented in this thesis provide new insight into nearshore wave transformations and the relationship between wave properties and clast transport at a time when sea level rise and the potential increase in storminess is threatening coastlines globally.

Coarse-grained beaches, such as pure gravel (PG), mixed sand-gravel (MSG) and composite (CSG) beaches, can be considered as one of the most resilient non-cohesive morpho-sedimentary coastal environments to energetic wave forcing (e.g., storms). The hydraulically-rough and permeable nature of gravel (D50 > 2 mm), together with the steep (reflective) beach face, provide efficient mechanisms of wave energy dissipation in the swash zone and provide a natural means of coastal defence. Despite their potential for shore protection very little is known about the response of these environments during high energetic wave conditions. Field measurements of sediment transport and hydrodynamics on coarse-grained beaches are difficult, because there are few instruments capable of taking direct measurements in an energetic swash zone in which large clasts are moving, and significant morphological changes occur within a short period of time. Remote sensing methods emerge in this context as the most appropriate solution for these types of field measurement. A new remote sensing method, based around a mid-range (~ 50 m) 2D laser-scanner was developed, which allows the collection of swash zone hydrodynamics (e.g., vertical and horizontal runup position, swash depth and velocity) and bed changes on wave-by-wave time scale. This instrument allowed the complete coverage of the swash zone on several coarse-grained beaches with a vertical accuracy of approximately 0.015 m and an average horizontal resolution of 0.07 m. The measurements performed with this new methodology are within the accuracy of traditional field techniques (e.g. video cameras, ultrasonic bed-level sensors or dGPS). Seven field experiments were performed between March 2012 and January 2014 on six different coarse-grained beaches (Loe Bar, Chesil, Slapton, Hayling Island, Westward Ho! and Seascale), with each deployment comprising the 2D laser-scanner together with complementary in-situ instrumentation (e.g., pressure transducer, ADV current meter). These datasets were used to explore the hydrodynamics and morphological response of the swash zone of these different environments under different energetic hydrodynamic regimes, ranging from positive, to zero, to negative freeboard regimes. With reference to the swash zone dynamics under storms with positive freeboard regimes (when runup was confined to the foreshore) it was found that extreme runup has an inverse relationship with the surf scaling parameter (=2Hs /gTptan2). The highest vertical runup excursions were found on the steepest beaches (PG beaches) and under long-period swell, while lower vertical runup excursions where linked to short-period waves and beaches with intermediate and dissipative surf zones, thus demonstrating that the contrasting degree of wave dissipation observed in the different types of surf zones is a key factor that control the extreme runup on coarse-grained beaches. Contrasting morphological responses were observed on the different coarse-grained beaches as a result of the distinct swash\surf zone hydrodynamics. PG beaches with narrow surf zone presented an asymmetric morphological response during the tide cycle (accretion during the rising and erosion during the falling tide) as a result of beach step adjustments to the prevailing hydrodynamics. On dissipative MSG and CSG beaches the morphological response was limited due to the very dissipative surf zone, while on an intermediate CSG beach significant erosion of the beach face and berm was observed during the entire tide cycle as a result of the absence of moderate surf zone wave dissipation and beach step dynamics. Fundamental processes related to the link between the beach step dynamics and the asymmetrical morphological response during the tidal cycle were for the first time measured under energetic wave conditions. During the rising tide the onshore shift of the breaking point triggers the onshore translation of the step and favors accretion (step deposit development), while during the falling tide the offshore translation of the wave breaking point triggers retreat of the step and favours backwash sediment transport (erosion of the step deposit). Under zero and negative freeboard storm regimes (when runup exceeds the crest of the barrier or foredune), field measurements complimented by numerical modelling (Xbeach-G) provide clear evidence that the presence of a bimodal wave spectrum enhances the vertical runup and can increase the likelihood of the occurrence of overtopping and overwash events over a gravel barrier. Most runup equations (e.g., Stockdon et al., 2006) used to predict the thresholds for storm impact regime (e.g., swash, overtopping and overwash) on barriers lack adequate characterisation of the full wave spectra; therefore, they may miss important aspects of the incident wave field, such as wave bimodality. XBeach-G allows a full characterization of the incident wave field and is capable of predicting the effect of wave spectra bimodality on the runup, thus demonstrating that is a more appropriate tool for predicting the storm impact regimes on gravel barriers. Regarding the definition of storm impact regimes on gravel barriers, it was found that wave period and wave spectra bimodality are key parameters that can affect significantly the definition of the thresholds for these different regimes. While short-period waves dissipate most of their energy before reaching the swash zone (due to breaking) and produce short runup excursions, long-period waves arrive at the swash zone with enhanced heights (due to shoaling) and break at the edge of the swash, thus promoting large runup excursions. When offshore wave spectrum presents a bimodal shape, the wave transformation on shallow waters favours the long period peak (even if the short-period peak is the most energetic offshore) and large runup excursions occur. XBeach-G simulations show that the morphological response of fine gravel barriers is distinct from coarse gravel barriers under similar overtopping conditions. While on coarser barriers overtopping regimes are expected to increase the crest elevation and narrow the barrier, on fine barriers sedimentation occurs on the back of the barrier and in the lower beach face. Such different sedimentation patterns are attributed to the different hydraulic conductivity of the different sediment sizes which control the amount of flow dissipation (due to infiltration) and, therefore, the capacity of the flow to transport sediment across and over the barrier crest. The present findings have significantly improved our conceptual understanding of the response of coarse-grained beaches during storms. A new field technique to measure swash dynamics in the field was developed during this thesis and has great potential to become widely used in a variety of coastal applications.

A Boiling Liquid Expanding Vapour Explosion (BLEVE) is a physical explosion caused by the sudden bursting of a vessel containing a superheated liquid. The scientific community describes the BLEVE as a physical explosion and is trying to develop models to predict the strength of the shock waves generated. Taking into account the data provided by the experimental campaign on the BLEVE water, this paper focuses on the causes that may lead to the formation of the second external pressure peak. Many authors assume that this peak is influenced by the liquid/vapour phase transition that occurs in the tank after the sudden pressure drop. Using Scilab, a numerical model is created that can solve Euler's equations for the shock tube problem, simulating only the behavior of the vapour phase. The quality of the model is tested taking into account data obtained experimentally in laboratory-scale tests. Then, based on the EVUT (equal-velocity-unequal-temperature) model proposed in the literature, the boiling phenomenon caused by the sudden pressure drop is analysed. The "relaxation time model" is discussed for modelling the source terms. Using the developed model, the effects of boiling on the density, velocity and internal pressure profiles are investigated. Finally, the model is discussed by comparing it with the experimental data from the E27 test of the water BLEVE campaign

Les fluctuations du vent sont un des mecanismes pouvant generer la variabilite oceanique d'echelle moyenne, observee notamment dans l'atlantiuqe nord-est. La reponse de l'ocean est calculee par un modele numerique spectral, qui resout les equations quasi-geostrophiques dans le cas non lineaire et stratifie. Dans une premiere partie, nous decrivons la turbulence d'echelle moyenne generee par les fluctuations du vent dans un ocean a fond plat. L'energie de la reponse se partage entre un regime lineaire constitue d'ondes de rossby barotropes a grande echelle, et un regime non lineaire. Nous etudions la sensibilite de la reponse a tous les parametres physiques (echelle et energie du vent, frottement, rotation de la terre,stratification). Nos resulats montrent que le parametre cle est le rapport de la plus grande echelle forcee au rayon de rossby. Dans une seconde partie, nous etudions lm'influence d'une topographie aleatoire de faible hauteur sur la dynamique des tourbillons forces par le vent. La generation de courants moyens correles a la topographie est mise en evidence, ainsi qu'une diminution importante des echelles spatiales au fond. Enfin, nous montrons que la presence de topographie induit une intensification au fond dans le cas de la turbulence libre, et par contre une plus grande intensification en surface dans le cas de la turbulence forcee. La reponse de notre modele, surtout en presence de topographie, presente des caracteristiques proches de la variabilite observee dans l'atlantique nord-est, ce qui confirme l'importance des fluctuations du vent pour la generation de cette variabilite

The motivation for this thesis is to investigate how storm sea states in deep water transforms as the waves propagate towards shallow water. This is connection with the design of bottom fixed wind turbines in finite water depths. In order to investigate how the sea state is transformed, there have been performed a model test where the generated waves are measured as they propagate over a sloping beach. Theory behind different shallow water effects and how these will transform the sea state, is presented. The results obtained from the present model test have also been compared to similar model tests, and the comparison generally shows the same behavior. The results show that the surface process of the waves transforms into a nonlinear process, and the deviations from a Gaussian process shows this clearly in terms of values for skewness and kurtosis. It is seen that wave breaking will be an dissipation important in the wave spectra, significant wave height and the distributions of wave and crest heights in the sea state. Where wave breaking is seen to reduce the energy content in the wave spectrum, and contributes to make the proposed conventional distribution functions for both wave and crest height distributions conservative. The significant wave height is also seen to be transformed by effects from shoaling. For the largest individual waves it is seen that the waves in the measured time series are asymmetrical with respect to the front and back of the wave. This effect along with the calculated Ursell number for these waves indicates that there is a need for sophisticated wave model in order to model the surface elevation of the waves with corresponding wave kinematics.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2008.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references (p. 343-370).
The objective of this study is to develop an integrated nondestructive testing (NDT) capability, termed FAR NDT (Far-field Airborne Radar NDT), for the detection of defects, damages, and rebars in the near-surface region of glass fiber reinforced polymer (GFRP)-retrofitted concrete cylinders through the use of far-field radar measurements (electromagnetic or EM waves). In this development, two far-field monostatic ISAR (inverse synthetic aperture radar) measurement schemes are identified for collecting radar measurements, and the backprojection algorithm is applied for processing radar measurements into spatial images for visualization and condition assessment. Reconstructed images are further analyzed by mathematical morphology to extract a numerical index representing the feature of the image as a basis for quantitative evaluation. The components of the development include dielectric modeling of materials, laboratory radar measurements, numerical simulation, and image reconstruction. It is found that using the developed technique the presence of near-surface defects can be detected by the oblique incidence measurements. Radar signals in the frequency range of 8 GHz to 18 GHz are found effective for damage detection in the near-surface region of the specimens. Numerical simulation using the finite-difference time-domain (FDTD) method is conducted to understand the propagation and scattering of EM waves from the defects and inclusions in two-dimensional and three-dimensional GFRP-concrete models. The FDTD simulation is capable of predicting the far-field response of GFRP-concrete cylinders and beneficial to better understanding the pattern of field measurements in the application of the FAR NDT technique.
(cont.) Dielectric properties of materials are investigated for their use in numerical simulation and for improving the precision of reconstructed images. Reconstructed images of GFRP-concrete cylinders with and without artificial features (rebar and defect) clearly indicate the presence of these features. Normal incidence scheme is found to be effective for rebar detection, and the oblique incidence scheme can discover near-surface defects such as GFRP debonding and delamination. The proposed FAR NDT technique is found to be capable of detecting near-surface defects in GFRP-concrete cylinders and potentially applicable for the field condition assessment of GFRP-retrofitted reinforced concrete and other reinforced concrete civil infrastructure systems.
by Tzu-Yang Yu.
Ph.D.