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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Stationary Waves in the Stratosphere-troposphere Circulation

Wang, Lei 23 February 2011 (has links)
Stationary wave theory elucidates the dynamics of the time mean zonally asymmetric component of the atmospheric circulation and separates it from the dynamics of the zonal mean climatological flow. This thesis focuses on the dynamics of stationary wave nonlinearity and its applications in stationary wave modelling and the stationary wave response to climate change. Stationary wave nonlinearity describes the self-interaction of stationary waves and is important in maintaining the observed zonally asymmetric atmospheric general circulation. Stationary wave nonlinearity is examined in quasi-geostrophic barotropic dynamics in both the presence and absence of transient waves. Stationary wave nonlinearity is shown to account for most of the difference between the linear and full nonlinear stationary waves, particularly if the zonal-mean flow adjustment to the stationary waves is taken into account. Wave activity analysis shows that stationary wave nonlinearity in this setting is associated with Rossby wave critical layer reflection. A time-integration type nonlinear stationary wave modelling technique is tested in this simple barotropic setting and is shown to be able to predict stationary wave nonlinearity and capture the basic features of the full nonlinear stationary wave. A baroclinic nonlinear stationary wave model is then developed using this technique and is applied to the problem of the stationary wave response to climate change. Previous stationary wave modelling has largely focused on the tropospheric circulation, but the stationary wave field extends into the stratosphere and plays an important dynamical role there. This stationary wave model is able to represent the stratospheric stationary wave field and is used to analyze the Northern Hemisphere stationary wave response to climate change simulated by the Canadian Middle Atmosphere Model (CMAM). In the CMAM simulation changes to the zonal mean basic state alone can explain much of the stationary wave response, which is largely controlled by changes of the zonal mean circulation in the Northern Hemisphere subtropical upper troposphere. However, details of the stratospheric wave driving response are also sensitive to other aspects of the zonal-mean response and to the heating response. Many climate change related effects appear to contribute robustly to an increased wave activity flux into the stratosphere.
12

Stationary Waves in the Stratosphere-troposphere Circulation

Wang, Lei 23 February 2011 (has links)
Stationary wave theory elucidates the dynamics of the time mean zonally asymmetric component of the atmospheric circulation and separates it from the dynamics of the zonal mean climatological flow. This thesis focuses on the dynamics of stationary wave nonlinearity and its applications in stationary wave modelling and the stationary wave response to climate change. Stationary wave nonlinearity describes the self-interaction of stationary waves and is important in maintaining the observed zonally asymmetric atmospheric general circulation. Stationary wave nonlinearity is examined in quasi-geostrophic barotropic dynamics in both the presence and absence of transient waves. Stationary wave nonlinearity is shown to account for most of the difference between the linear and full nonlinear stationary waves, particularly if the zonal-mean flow adjustment to the stationary waves is taken into account. Wave activity analysis shows that stationary wave nonlinearity in this setting is associated with Rossby wave critical layer reflection. A time-integration type nonlinear stationary wave modelling technique is tested in this simple barotropic setting and is shown to be able to predict stationary wave nonlinearity and capture the basic features of the full nonlinear stationary wave. A baroclinic nonlinear stationary wave model is then developed using this technique and is applied to the problem of the stationary wave response to climate change. Previous stationary wave modelling has largely focused on the tropospheric circulation, but the stationary wave field extends into the stratosphere and plays an important dynamical role there. This stationary wave model is able to represent the stratospheric stationary wave field and is used to analyze the Northern Hemisphere stationary wave response to climate change simulated by the Canadian Middle Atmosphere Model (CMAM). In the CMAM simulation changes to the zonal mean basic state alone can explain much of the stationary wave response, which is largely controlled by changes of the zonal mean circulation in the Northern Hemisphere subtropical upper troposphere. However, details of the stratospheric wave driving response are also sensitive to other aspects of the zonal-mean response and to the heating response. Many climate change related effects appear to contribute robustly to an increased wave activity flux into the stratosphere.
13

Wave resource assessment using numerical wave modelling

Vijay Kumar, Akash January 2020 (has links)
Wave resource assessment is the collection of site-specific meteorological data to estimate the wave potential in a region and is carried out to mitigate the risks involved in the setup of wave energy converters. Wave resource assessment has been typically done using wave buoys. But wave buoys are expensive and require constant maintenance for smooth operation. An alternative to wave buoys is numerical wave modelling. Numerical wave modelling uses mathematical models to generate computer simulations that describe wave parameters in a region. This project is carried out in the region of Hvide Sande, a western port city of Denmark situated in the North Sea. Wave resource assessment is conducted for this region using the SWAN numerical wave model. The data from a wave buoy in the region is used to perform comparative analysis, in order to study the SWAN wave model and to compare the accuracy of the wave data between the measured buoy and the calculated model. The accuracy of the numerical predictions is further quantified by using a correlation coefficient and the relative square root error. This is followed by a scatter diagram for the region to find the most frequent sea states. The corresponding energy flux of the most frequent sea states was found to be in the range of 1-5 kW/m and the highest energy flux for the area is about 40 kW/m for the region.
14

Study of Nano-structures with Applications on Single-mode Lasers

Deng, Lanxin 04 1900 (has links)
<p>Semiconductor laser diode has been a popular research topic for longer than half a century and plays a crucial role in optical communication systems. The work in this thesis focuses on the development of the semiconductor laser diode with rapid-evolving nanotechnologies: by incorporating specific semiconductor or metal structures in the nanometer scale into the laser cavity, several key advantages are achieved.</p> <p>One category of the nano-materials is semiconductor quantum dots (QD). QD laser is a promising product by providing three-dimensional confinement to the injected electrons and holes. However, in order to realize the single-longitudinal-mode operation, which is critical to optical communications in purpose of reducing the dispersion and partition noise, the Fabry-Perot (FP) QD laser still needs further development to suppress the gain-broadening effects; otherwise the mode-selective structure must be adopted, such as the distributed feedback (DFB) cavity. In this thesis, the QD FP laser and QD DFB laser are both researched by advanced modelling techniques and the work is summarized as follows.</p> <p>1) For the QD FP laser, a comprehensive rate-equation model has been applied for simulation, with the emphasis on describing the interplay of inhomogeneous and homogeneous gain-broadening effects. According to the laser-behaviour simulations, it is found that for each given inhomogeneous broadening, the optimum homogeneous broadening can be obtained for the single longitudinal-mode selectivity. Based on the optimal gain-broadening parameters, the single-mode QD FP laser is designed and analysed. The quantitative conditions for the performance feasibility are examined with respect to the gain-broadening parameters.</p> <p>2) A one-dimensional (1D) standing wave model is developed for the QD DFB laser. This model can provide more information for the laser operation and better describe the dynamic behaviour compared with the rate-equation model. Based on it, the statistic operation and output spectrum of a typical QD DFB laser are simulated; and then the dynamic properties of the laser are analysed.</p> <p>The other category is the metal nano-structure, including the metal nano-particle and the metal nano-strip Bragg grating. The related work is summarized as follows.</p> <p>1) The optical properties of a single metal nano-particle with different size, composition and shape are researched by Mie theory, with respect to the localized surface plasmon polariton (LSPP) effect. It shows that both the resonance wavelength and Q-factor can be tuned in a large scale by proper methods.</p> <p>2) A novel metal nano-strip distributed Bragg grating (DBR) laser is proposed and investigated theoretically. Firstly the metal nano-strip Bragg grating is simulated by the couple-mode theory and the mode-matching method. It shows that the coupling constant and reflection spectrum can be tuned to meet different requirements when varying the grating parameters. Then for the designed metal-grating DBR laser, the rate-equation simulation results show that it works under the single-mode operation for a broad range of the design parameters.<br /> <strong></strong></p> / Doctor of Philosophy (PhD)
15

Numerical Modelling and Statistical Analysis of Ocean Wave Energy Converters and Wave Climates

Li, Wei January 2016 (has links)
Ocean wave energy is considered to be one of the important potential renewable energy resources for sustainable development. Various wave energy converter technologies have been proposed to harvest the energy from ocean waves. This thesis is based on the linear generator wave energy converter developed at Uppsala University. The research in this thesis focuses on the foundation optimization and the power absorption optimization of the wave energy converters and on the wave climate modelling at the Lysekil wave converter test site. The foundation optimization study of the gravity-based foundation of the linear wave energy converter is based on statistical analysis of wave climate data measured at the Lysekil test site. The 25 years return extreme significant wave height and its associated mean zero-crossing period are chosen as the maximum wave for the maximum heave and surge forces evaluation. The power absorption optimization study on the linear generator wave energy converter is based on the wave climate at the Lysekil test site. A frequency-domain simplified numerical model is used with the power take-off damping coefficient chosen as the control parameter for optimizing the power absorption. The results show a large improvement with an optimized power take-off damping coefficient adjusted to the characteristics of the wave climate at the test site. The wave climate modelling studies are based on the wave climate data measured at the Lysekil test site. A new mixed distribution method is proposed for modelling the significant wave height. This method gives impressive goodness of fit with the measured wave data. A copula method is applied to the bivariate joint distribution of the significant wave height and the wave period. The results show an excellent goodness of fit for the Gumbel model. The general applicability of the proposed mixed-distribution method and the copula method are illustrated with wave climate data from four other sites. The results confirm the good performance of the mixed-distribution and the Gumbel copula model for the modelling of significant wave height and bivariate wave climate.
16

Two-dimensional Depth-averaged Beach Evolution Modelling

Baykal, Cuneyt 01 February 2012 (has links) (PDF)
In this study, a two-dimensional depth-averaged beach evolution numerical model is developed to study the medium and long term nearshore sea bottom evolution due to non-cohesive sediment transport under the action of wind waves only over the arbitrary land and sea topographies around existing coastal structures and formations. The developed beach evolution numerical model is composed of four sub-models: a nearshore spectral wave transformation model based on energy balance equation including random wave breaking and diffraction terms to compute the nearshore wave characteristics, a nearshore wave-induced circulation model based on the non-linear shallow water equations to compute the nearshore depth averaged wave-induced current velocities and mean water level changes, a sediment transport model to compute the local total sediment transport rates occurring under the action of wind waves and a bottom evolution model to compute the bed level changes in time due to gradients of sediment transport rates in cross-shore and longshore directions. The governing partial differential equations are solved utilizing finite difference schemes. The developed models are applied successfully to several theoretical and conceptual benchmark cases and an extensive data set of laboratory and field measurements. As an alternative approach to be used in beach evolution problems, a distributed total sediment load formula is proposed based on the assumption that the local total sediment transport rates across the surf zone are proportional to the product of the rate of dissipation of wave energies due to wave breaking and wave-induced current velocities. The proposed distribute load approach is validated with the available laboratory and field measurements.
17

The impact of climate change effects on the planform of a headland-bay beach on the southern coast of South Africa

Hugo, Pierre-Malan 03 1900 (has links)
Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The various consequences of climate change pose a significant threat to developments near the coast. These threats include saltwater intrusion, coastal erosion and flooding. In the coastal context, the climate change effect often raising the most concern is that of sea-level rise. Much work has therefore been done on the linear setback caused by a rise in sea-level. In order to get the full picture of possible changes caused by sea-level rise, the secondary effects of a rising sea-level also need to be considered. Sea-level rise could cause changes to the nearshore wave climate and could have impacts such as coastal erosion and changes to the coastline shape. The primary objective of this study was therefore to investigate the effects of sea-level rise on the nearshore wave climate and, consequently, the coastline stability. Other consequences of climate change considered in this study include increasing average wave heights and a rotation of offshore wave directions. The many headland-bay beaches on the South African coastline are generally in a state of dynamic equilibrium and find their planforms based on the local wave climate. Changes to the wave climate may therefore disrupt the equilibrium shapes of these bays. This study was therefore also aimed at investigating the effects of the changes to the wave climate on the stability of headland-bay beaches. The three consequences of climate change expected to affect the nearshore wave climate were identified as (1) sea-level rise; (2) an increased wave height; and (3) changing offshore wave angles. Although changes to storm frequency and intensity are also possible, the impacts of these changes were not studied. In order to assess the impacts of the three considered changes on a typical headland-bay beach, two numerical models were set up for Mossel Bay – a headland-bay beach on the southern coast of South Africa. The modelling approach included a wave transformation model to calculate nearshore wave climates from offshore data and a coastline model to assess the stability of the bay under the changed nearshore wave climates. The model results indicated that the rising sea-level alone would cause changes in the nearshore wave direction. These changes were shown to alter the longshore sediment transport regime such that rotations are expected in the south-western corner and eastern end of Mossel Bay. These rotations do not include the cross-shore effects of inundation and erosion, as suggested by models such as the Bruun Model. The results for an increased offshore wave height were inconclusive. The southerly rotation in offshore wave angles was shown to affect the nearshore wave angles. These changes affected the longshore transport regime such that the outward sediment transports were reduced. A minor accretion resulted in the centre of the bay for a 1° southerly rotation in offshore wave angles. For a 2° rotation, the extent of accretion increased and shifted towards the eastern end of the bay, primarily due to the dominance of south-westerly waves in the local wave climate. A valuable observation was made regarding the current stability of Mossel Bay. Inter-tidal reefs are present along three sections of the bay. These reefs protect the coastline such that the current bay shape contains sharp bends between the reefs. Under a rising sea-level, however, the effect of the reefs will become less pronounced. If a water level should be reached where these reefs become less significant, the planform of the bay is expected to smooth out through a significant redistribution of sediment. This smoothing effect was shown to cause erosion of the coastline in the order of 80m near the town of Klein Brak River. / AFRIKAANSE OPSOMMING: Die verskeie gevolge van klimaatsverandering bied ‘n merkwaardige bedreiging vir ontwikkelings naby die kus. Hierdie bedreigings sluit die versouting van varswaterbronne, kuserosie en oorstromings in. Vir kusgebiede is seevlakstyging gereeld die effek van klimaatsverandering wat die meeste kommer wek. Dus is heelwat navorsing rakende die direkte erosie as gevolg van seevlakstyging reeds gedoen. Om die volle beeld van die gevolge van ‘n stygende seevlak te verkry, is dit egter nodig om ook die sekondêre effekte hiervan in ag te neem. Seevlakstyging kan veranderinge in die golfklimaat naby die kus veroorsaak, en kan impakte soos kuserosie en veranderende baaivorms tot gevolg hê. Die primêre doel van hierdie studie is dus om die effek van seevlakstyging op die golfklimaat by die kus en gevolglik die stabiliteit van die kuslyn, te ondersoek. Benewens die styging van die seevlak word die effekte van groter gemiddelde golfhoogtes en die rotasie van diepsee golfrigtings ook in hierdie studie ondersoek. Die vele landpunt-baaie (headlandbay beaches) op die Suid-Afrikaanse kus is meestal in ‘n dinamiese ekwilibriumtoestand, waarvan die vorm deur die lokale golfklimaat bepaal word. Veranderinge aan dié golfklimaat mag dus die ekwilibrium vorms van sulke baaie versteur. Hierdie studie het dus ook die stabilititeit van landpuntbaaie onder ‘n veranderende golfklimaat ondersoek. Die drie gevolge van klimaatsverandering wat verwag word om die golfklimaat naby die kus te beïnvloed is geïdentifiseer as (1) seevlakstyging; (2) vergrote golfhoogtes; en (3) veranderende diepsee golfhoeke. Veranderinge aan die frekwensie en intensiteit van storms is ook moontlike gevolge van klimaatsverandering, maar die impakte hiervan is nie in die studie ondersoek nie. Twee numeriese modelle is toegepas om die impak van die drie bogenoemde gevolge op Mosselbaai – ‘n tipiese landpunt-baai aan die suidkus van Suid-Afrika – te ondersoek. ‘n Golfmodel is ingespan om die golfklimaat naby die kus te bepaal waarna ‘n kuslynmodel gebruik is om die stabiliteit van die baai onder die veranderde golfklimaat te ondersoek. Die resultate van die studie dui daarop dat die golfhoeke naby die kus beïnvloed word deur seevlakstyging. Daar is aangetoon dat dié veranderinge die langsstroomvervoer sodanig sal verander dat kuslynrotasies in die suid-westelike hoek asook die oostelike rand van Mosselbaai verwag word. Hierdie rotasies sluit nie die lineêre landwaartse verplasing van die kuslyn as gevolg van erosie en oorstroming in nie. Die effek van vergrote golfhoogtes kon nie met akkuraatheid ondersoek word nie. Daar is wel gevind dat die suidwaartse rotasie van diepsee golfhoeke rotasies in die golfklimaat naby die kus veroorsaak. Hierdie rotasies verander die langsstroom sedimentvervoer sodanig dat die uitwaartse sedimentvervoer verminder word en ‘n klein opbou van sediment in die middel van die baai vir ‘n 1° diepsee rotasie verwag word. Vir ‘n 2° suidwaartse rotasie is daar ‘n groter opbou van sediment wat verder ooswaarts veplaas is. Die ooswaartse veplasing is primêr ‘n gevolg van die oorheersing van suid-westelike golftoestande in die golfklimaat. ‘n Waardevolle gevolgtrekking rakende die huidige stabiliteit van Mosselbaai is ook gemaak. Langs drie gedeeltes van die Mosselbaaise kus word riwwe in die gebied tussen hoog- en laagwater aangetref. Hierdie riwwe beskerm die kus sodanig dat skerp kinkels in die vorm van die baai tussen die riwwe gesien kan word. Wanneer die seevlak styg, word die beskermende effek van die riwwe egter minder doeltreffend. Indien ‘n watervlak bereik word waar dié effek genoegsaam verminder is, word daar verwag dat die baai deur ‘n merkwaardige verplasing van sediment die kinkels sal uitstryk. Deur hierdie proses word erosie in die orde van 80m naby die dorp van Klein Brakrivier verwag.
18

Climatologie des états de mer en Atlantique nord-est : analyse du climat actuelet des évolutions futures sous scénarios de changement climatique par descente d'échelle dynamique et statistique / Sea state climatology in the North-East Atlantic Ocean : analysis of the present climate and future evolutions under climate change scenarios by means of dynamical and statistical downscaling methods

Laugel, Amélie 11 December 2013 (has links)
L'analyse de la climatologie des aléas océano-météorologiques tels que les états de mer est fondamentale pour comprendre l'évolution et la dynamique des zones côtières, estimer les risques naturels survenant lors d'événements de tempête majeurs, définir les moyens optimaux de protection des ports et infrastructures onshore et offshore, caractériser la ressource houlomotrice pour des projets de récupération d'énergie des vagues, comprendre les processus d'érosion et accrétion des plages, etc. Pour répondre à ces problématiques dans un contexte de questionnement croissant sur les conséquences potentielles associées au changement climatique, le travail de thèse s'inscrit dans une démarche double : (i) approfondissement de la connaissance du climat de vagues actuel le long des côtes Atlantique, Manche et Mer du Nord en France d'une part, et (ii) estimation des évolutions futures potentielles de cette climatologie des vagues pour différents scénarios d'évolution climatique. L'estimation de l'impact du changement climatique sur le climat de vague se compose de trois éléments principaux : (i) une connaissance détaillée de la variabilité climatique actuelle des états de mer, (ii) l'utilisation de scénarios de changement climatique à l'horizon 2100 et (iii) la définition d'une méthodologie de descente d'échelle adaptée. Pour appréhender ces sujets, l'Atlas Numérique d'Etats de Mer Océanique et Côtier ANEMOC-2 a été construit à l'aide du modèle spectral de 3ème génération TOMAWAC (Benoit et al., 1996) sur la période 1979-2009 et le climat de vagues futur a été simulé à l'horizon 2100 par des méthodes de descente d'échelle dynamique et statistique en considérant les scénarios de changement climatique du quatrième rapport du GIEC (IPCC, 2007).En particulier, un travail original de comparaison de projections d'états de mer par approche dynamique et par approche statistique des types de temps a été réalisé sur la période 2061-2100 pour les scénarios B1, A1B et A2 simulés par le modèle ARPEGE-CLIMAT de Météo-France (Salas-Mélia, et al. 2005). Les résultats des deux approches (à savoir hauteur significative, période moyenne, direction moyenne et flux d'énergie des vagues) ont été comparés en termes de valeurs moyennes, écarts-types, distributions jointes et variabilités saisonnière et interannuelle. Ce travail a abouti à une estimation de l'impact du changement climatique sur la climatologie des états de mer le long des côtes Atlantique, Manche et Mer du Nord françaises sur la période 2061-2100 en tenant compte des incertitudes intrinsèques aux méthodes de descente d'échelle et aux scénarios de changement climatique. En hiver par exemple, nous observons une augmentation des valeurs moyennes et de la variabilité des paramètres de hauteur significative, période moyenne et flux d'énergie des vagues, notamment en Mer du Nord (pour les scénarios B1, A1B et A2) et dans le Golfe de Gascogne pour le scénario B1. En complément, ces paramètres d'états de mer ont tendance à diminuer dans le Golfe de Gascogne pour les saisons printemps, été et automne. Enfin, les paramètres d'états de mer associés aux hauteurs de vagues du quantile 95 tendent à augmenter sur une large emprise de l'Atlantique nord-est / Wave climate analysis is of utmost importance to understand the evolution and dynamics of coastal zones, to estimate the occurrence of extreme events, to design protections for ports, onshore and offshore infrastructure, to characterize wave resources for wave energy conversion, to quantify sediment erosion and accretion processes, et cetera. Thus, this thesis project aims to improve knowledge of wave climatology in the growing context of climate change prediction with a two-step approach: (i) enhancement of the understanding of the present wave climate along the French coastline facing the Atlantic Ocean, English Channel and North Sea and (ii) estimation of possible future wave climate evolution. For this purpose, the estimation of climate change impacts on the wave climate requires three key parameters: (i) detailed knowledge of current wave climate variability, (ii) the application of climate change scenarios from Global Climate Models and (iii) the definition of an appropriate downscaling method. To answer these questions, ANEMOC-2, a hindcast sea-state data base has been built based on the third-generation spectral wave model TOMAWAC (Benoit et al., 1996) over the period 1979-2009, and the future wave climate has been simulated over the period 2061-2100 by means of dynamical and statistical downscaling methods. In particular, an original approach comparing sea-state projections obtained from dynamical and statistical downscaling methods has been applied over the period 2061-2100 for B1, A1B and A2 scenarios (Forth Assessments Reports, IPCC, 2007), based on the ARPEGE-CLIMAT (Salas-Mélia et al., 2005) model simulations. The wave spectral parameters resulting from the projections (i.e. significant wave height, mean period, mean direction and wave energy flux) have been compared in term of mean, joint distribution and seasonal and interannual variability.The possible climate change impacts on the wave climate along the Atlantic, English Channel and North Sea French coastline have also been evaluated. The analysis provides estimations of the inherent uncertainties of climate change scenarios and downscaling methods. Wave climate evolution trends are presented in terms of the mean, joint distribution, and seasonal and interannual variability of significant wave height, mean period, mean direction and wave energy flux
19

Morphodynamic modelling of a wave-dominated tidal inlet : the Albufeira lagoon / Modélisation morphodynamique d'une embouchure tidale dominée par la houle : la lagune d'Albufeira

Dodet, Guillaume 19 December 2013 (has links)
Les embouchures tidales dominées par la houle sont des systèmes côtiers particulièrement dynamiques dont la morphologie est continuellement remodelée par l’action des vagues et de la marée. Les rapides évolutions morphologiques auxquelles elles sont sujettes impactent directement leurs environnements écologiques et socio-économiques. Afin de mettre en œuvre des réglementations adaptées à la gestion durable des embouchures tidales, des études environnementales systématiques sont nécessaires. L’objectif principal de cette thèse est de mieux comprendre les processus physiques qui contrôlent les évolutions morphologiques d’une embouchure tidale éphémère au Portugal - l’embouchure de la lagune d’Albufeira – à partir de l’analyse de mesures hydrodynamiques et topographiques et de résultats d’un système de modélisation morphodynamique récemment développé. Les processus qui influent sur la dynamique de l’embouchure tidale à court terme, notamment ceux liés aux interactions vague-courant, ont été étudiés à travers l’application du système de modélisation à l’embouchure. Les modulations saisonnières du climat de vagues et du niveau moyen de la mer affectent fortement la dynamique sédimentaire de l’embouchure et contribuent au comblement naturel de l’embouchure pendant l’hiver. Les processus à long terme ont également été étudiés à partir de simulations rétrospectives de paramètres moyens de vagues pour des échelles régionales et locales sur une période 65 ans. Les fortes variabilités interannuelles du climat de vagues et de la dérive littorale qui lui est associée pourraient expliquer les différences d’évolutions morphologiques du système embouchure-lagune sur des échelles de temps pluri–annuels. / Wave-dominated tidal inlets are very dynamic coastal systems, whose morphology is continuously shaped by the combined action of the waves and the tides. The rapid morphological changes they experience impact directly their ecological and socio-economic environments. In order to implement adequate regulations for the sustainable management of tidal inlets, systematic environmental studies are necessary. The main objective of this PhD research work is to gain a better understanding of the physical processes that control the morphological evolutions of an ephemeral tidal inlet in Portugal - the Albufeira Lagoon inlet - based on the analysis of hydrodynamic and topographic data and on the results of a newly developed morphodynamic modelling system. The processes that impact the dynamics of the inlet at short time-scales, particularly those related to wave-current interactions, are investigated through the application of the modelling system to the inlet. The seasonal modulations of the wave climate and mean sea level strongly affect the sediment dynamics of the inlet and contribute to the natural closure of the inlet during the winter period. Long-term processes are also investigated based on a 65-year hindcast of mean wave parameters at regional and local scales. The large inter-annual variability of the wave climate and the associated longshore sediment transport – both correlated to the North-Atlantic Oscillation – are proposed to explain the differences in the morphological behaviour of the inlet-lagoon system at pluri-annual time-scales.
20

Examining Optimal Form of Two Scale Approximation (TSA) for Calculating Snl Source Term

Ardag, Dorukhan 01 January 2014 (has links)
Nonlinear four-wave interactions (Snl) are critical for acquiring realistic spectra needed by operational wave models. High computational demand to calculate these interactions led to an approximation method named the Discrete Interaction Approximation (DIA) to be used broadly in the major operational wave models for a long time. However, the accuracy of the DIA has been controversial since it was first introduced and more precise approximations such as the Two Scale Approximation (TSA in short) are now available. The only issue with the initial TSA`s efficiency is performing an order of a magnitude slower than the DIA in speed. This study questions the exactness of the DIA while trying to increase the competence of the TSA by making improvements on its execution time. Particularly, in this thesis, the main effort is on the local scale term of the TSA since it is the part that consumes the most time while running the code. The findings of this work imply that the TSA can improve its operation speed significantly while maintaining its accuracy with making alterations in the code. By decreasing the number of bands in the local scale it is possible to run the TSA up to 7.5 faster than its initial version.

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