L'hydrodynamisme autour des îles océaniques et dans les régions côtières est caractérisé par la présence de nombreuses structures de méso- et de subméso-échelle. L'objectif de cette thèse est d'étudier, à partir de données in situ et de modélisation numérique, d'une part la prédominance de certains forçages dans la génération de ces structures et d'autre part leurs dynamiques et leurs impacts sur la dispersion des eaux côtières. Dans un premier temps, une étude basée sur des données issues d'un modèle hydrodynamique autour de l'archipel hawaïen a permis d'évaluer les influences respectives et l'importance des forçages du vent, de la topographie et de la circulation générale sur la génération de tourbillons de méso-échelle. Des tests de sensibilité ont mis en évidence l'intérêt d'une haute-résolution spatiale du forçage atmosphérique. Dans un deuxième temps, la dynamique côtière du Golfe du Lion (GdL) a été investie. Une première étude s'est focalisée sur les caractéristiques physiques et la dynamique d'un tourbillon dans la partie ouest du golfe à l'aide de données de la campagne Latex09 et de résultats d'un modèle hydrodynamique. Leur analyse combinée a permis d'identifier un nouveau processus de génération de tourbillons de méso-échelle dans cette zone et de mettre en évidence la formation d'une structure transitoire de subméso-échelle. Basée sur les données de la campagne Latex10, une deuxième étude s'est alors orientée sur la dispersion des eaux côtières de la partie occidentale du GdL. Un suivi lagrangien des masses d'eau a permis de déterminer les coefficients horizontaux et verticaux de diffusion dans cette zone clef pour les échanges côte-large ou interrégionaux. / The hydrodynamics around oceanic islands and in coastal areas is characterized by the presence of numerous meso- and submesocale features. The aim of this PhD thesis is to study, from in situ data and numerical modeling, firstly the predominance of some forcings on the generation of these features and secondly their dynamics and their impacts on the dispersion of coastal waters. Firstly, a study based on a series of numerical simulations in the Hawaiian region, allows us to examine the relative importance of wind, topographic and inflow current forcing on the generation of mesoscale eddies. Sensitivity tests have shown the importance of high wind-forcing spatial resolution. Secondly, the coastal dynamics of the Gulf of Lions (GoL), also subject to these forcings, has been investigated. A first part focuses on the physical characteristics and the dynamics of an eddy in the western part of the gulf, using data from the Latex09 campaign and results from a realistic hydrodynamic model of the GoL. Their combined analysis has allowed to identify a new generation mechanism for the mesoscale eddies in this area and to understand the formation of a transient submesoscale structure. This work has shown the importance of these structures in modulating exchanges in this region. Based on the data of the Latex10 campaign, a second part has then focused on the dispersion of coastal waters in the western area of the GoL. The tracking of the water masses in a Lagrangian reference frame (floats, tracer) has allowed to determine the horizontal and vertical diffusion coefficients in this key area for coastal-offshore and interregional exchanges.

O sistema de correntes de contorno oeste que flui ao largo da costa leste brasileira entre 15°S e 22°S, é composto pela Corrente do Brasil (CB) fluindo para sul, a Sub-Corrente Norte do Brasil (SNB) fluindo para norte e a Corrente de Contorno Oeste Profunda (CCP) também fluindo para sul. Vigorosos meandros e vórtices são observados à jusante dos Bancos de Abrolhos (BA) e Royal Charlotte (BRC) e da Cadeia Vitória-Trindade. O objetivo central desta dissertação é o estudo da estabilidade deste sistema de correntes, aqui denomidado Sistema CB-SNB-CCP, utilizando o modelo oceânico da Universidade de Princeton (POM) em um cenário idealizado. Buscamos responder o quão instável é este sistema e quais seriam as características das ondas instáveis geradas a fim de contribuir para o conhecimento acerca da rica dinâmica de meso-escala observada nesta região. Objetivamos também, elucidar o papel dos BA e BRC nesta dinâmica. Embasados na alta baroclinicidade do sistema de correntes de contorno ao largo do sudeste brasileiro, optamos por representar o escoamento CB-SNB-CCP através de um modelo paramétrico do campo de massa, calibrado com os dados hidrográficos oriundos dos Cruzeiros Abrolhos [Silveira et al., 2006]. Mantendo o caráter idealizado do estudo, também empregamos topografia de fundo analítica, onde representamos o talude da região por uma função tangente hiporbólica. O BA e o BRC foram aproximados através de funções gaussianas devidademente ajustadas aos contornos da isóbata de 80 m extraídas do conjunto ETOPO 2. Para identificar os mecanismos de crescimento das possíveis ondas instáveis, calculou-se o balanço de energia das simulações realizadas de acordo com o método de [Xue & Bane, 1997]. Os resultados de três experimentos numéricos realizados sugerem que o sistema CB-SNB-CCP é instável. Ciclones quase-estacionários do lado costeiro da CB surgem como o principal modo de variabilidade desta corrente. De acordo com a análise do balanço energético, o crescimento dessas feições resulta primariamente de instabilidade baroclínica do escoamento. A escala horizontal típica das ondas e vórtices instáveis modelados é dada pelo raio de deformação interno, como esperado pela teoria de instabilidade baroclínica de escoamentos realisticamente estratificados. Em particular, os resultados do experimento com o BA e o BRC idealizados comprovam que estes funcionam como gatilhos para o desenvolvimento de ondas instáveis, favorecendo amplamente o crescimento das estruturas verticais. Os trens de onda instáveis quase-estacionários aqui obtidos sugerem que provavelmente o meandramento da CB observado em latitudes que se estendem até 28°S pode ser parte de um único sistema que se origina na região dos BA e BRC. / The western boundary currents system that flows off the eastern brazilian coast between 15°S e 22°S is composed by the southward-flowing Brazil Current (BC), the northward-flowing North Brazil Under Current (NBUC) and the Deep Western Boundary Current (DWBC) that flows south. Vigorous meanders and eddies are observed downstream of the Abrolhos (AB) and Royal Charlotte Banks (RCB) and the Vitória-Trindade Ridge. The main goal of this dissertation is to study the stability of this currents system, hereby named BC-NBUC-DWBC system, using the Princeton University Ocean Model (POM) in an idealized scenerio. We seek to answer how unstable is this system and what are the characteristics of the unstable waves in order to contribute to the understanding of the rich mesoescale dynamics observed in this region. We also aim to elucidate the role of the AB and of the RCB on this dynamics. Based on the high degree of baroclinicity of the western boundary currents system off the southeastern brazilian coast, we opted to represent the BC-NBUC-DWBC system through a parametric model of the mass field, calibrated with hydrographic data from the Abrolhos Cruises [Silveira et al., 2006]. Maintaining the idealized character of the study, we have also employed an analytical bottom topography, in which the region\'s continental slope is approximated by hyperbolic tangent function. The AB and RCB were approximated by gaussian functions properly adjusted to the 80 m isobath extracted from the ETOPO 2 database. To identify the growth mecanism of the unstable waves, the energy budget of the simulations was calculated according to [Xue & Bane, 1997]. The results from the three experiments conducted here suggest that the BC-NBUC-DWBC system is indeed unstable. Quasi-stationary cyclones in the coastal side of BC arise as the main mode of variability of this current. According to the energy budget analisys, the growth of these features results primarily from baroclinic instability of BC-NBUC-DWBC flow. The typical horizontal scale of the modeled unstable waves and eddies is given by the internal radius of deformation, as expected by baroclinic instability theory of realistically stratified flows. The quasi-stationary unstable wave trains modeled in the present study also suggest that the meandering of the BC observed down to 28°S are probably part of a single system that originates at the AB and RCB region.

A Corrente do Brasil é formada na bifurcação do ramo sul da Corrente Sul Equatorial e flui meridionalmente para sul ao largo das costas leste, sudeste e sul do Brasil, apresentando intensa atividade de mesoescala entre a Cadeia Vitória-Trindade (20oS) e o Cabo de Santa Marta Grande (28oS). O conhecimento de tal atividade, construído ao longo do tempo, se refere principalmente aos meandros e vórtices frontais, sendo os vórtices de borda na configuração de dipolos vorticais relatados apenas através de imagens AVHRR. Dessa forma, objetivamos caracterizar a estrutura dinâmica de um dipolo vortical, observado na Bacia de Santos, a partir de dados quase-sinóticos de velocidade e hidrográficos oriundos do Cruzeiro Dinâmica do Ecossistema da Plataforma da Região Oeste do Atlântico Sul V. O mapeamento objetivo das estruturas capturadas pela grade amostral quase-sinótica do cruzeiro foi o ponto de partida para a descrição do dipolo e avaliação de sua geostrofia. Os campos de função de corrente apresentaram um padrão clássico de dipolo vortical, com vórtices ciclônico e anticiclônico posicionados em lados opostos ao eixo da corrente, que se enfraquece após a estrutura. Em média, o diâmetro de ambos os vórtices ficou em torno de 110 km, em seus maiores eixos. Em seguida, como o número de Rossby máximo foi de 0,27, para avançar nas questões dinâmicas envolvendo a estrutura, aproximamos o oceano a um modelo quase-geostrófico de 11/2-camadas derivado das observações e dinamicamente calibrado. A análise dos campos modelados, utilizando o número de Burger, revelou que a vorticidade relativa compete com a vorticidade de estiramento pela dinâmica do sistema. Ao compararmos os campos modelados de função de corrente e vorticidade potencial, observamos que o dipolo e o escoamento da CB não se encontram em um cenário estacionário. Finalmente, evidências de retroalimentação mútua entre os vórtices do dipolo foram obtidas através da interação entre o campo de função de corrente de uma estrutura com o campo de vorticidade potencial da outra. Logo, concluímos que o dipolo capturado encontrava-se em processo de crescimento barotrópico. / The Brazil Current (BC) is formed at the bifurcation of the southern branch of the South Equatorial Current and flows south-southeastward off the east, southeast and southern Brazilian continental margin, exhibiting intense meso-scale activity between the Vitória-Trindade Ridge (20oS) and Cape Santa Marta Grande (28oS). The knowledge of such activity, built over time, refers mainly to the frontal meanders and eddies. The edge vortical structures as a dipole configuration are reported only through AVHRR images. Thus, we aimed to characterize the dynamical structure of a BC vortical dipole, observed in the Santos Basin, with a quasi-synoptic velocity (vessel-mounted ADCP) and hydrographic (CTD) dataset from DEPROAS V cruise. The objective mapping of structures captured by the cruise sampling grid was the departing point for the description of the dipole and evaluation of its degree of geostrophy. The fields of the stream function showed a classic pattern of a dipole with cyclonic and anticyclonic eddies positioned on opposite sides of the current axis. On average, the diameter of the two eddies was around 110 km in their largest axes. Then, as the Rossby number was 0.27, we advanced in investigating the issues involving the dynamic structure by we approximating the ocean to a 11/2 layer quasi-geostrofic model derived from the observations and dynamically calibrated. Burger number revealed that both relative and stretching vorticity were competing for the dynamics of the system. The comparison between the modeled stream function and potential vorticity fields showed that both the dipole and the BC flow were not in a stationary state. Finally, evidences of reinforcement between the eddies of the dipole were obtained through the interaction between the field of the stream function of vortical structure with the field of potential vorticity of the other one, and vice-versa. Therefore, we conclude that the dipole were in a barotropic growth process.

Yuen, Man Chun. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references (leaves 58-[60]). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts also in Chinese.

O sistema de correntes de contorno oeste que flui ao largo da costa leste brasileira entre 15°S e 22°S, é composto pela Corrente do Brasil (CB) fluindo para sul, a Sub-Corrente Norte do Brasil (SNB) fluindo para norte e a Corrente de Contorno Oeste Profunda (CCP) também fluindo para sul. Vigorosos meandros e vórtices são observados à jusante dos Bancos de Abrolhos (BA) e Royal Charlotte (BRC) e da Cadeia Vitória-Trindade. O objetivo central desta dissertação é o estudo da estabilidade deste sistema de correntes, aqui denomidado Sistema CB-SNB-CCP, utilizando o modelo oceânico da Universidade de Princeton (POM) em um cenário idealizado. Buscamos responder o quão instável é este sistema e quais seriam as características das ondas instáveis geradas a fim de contribuir para o conhecimento acerca da rica dinâmica de meso-escala observada nesta região. Objetivamos também, elucidar o papel dos BA e BRC nesta dinâmica. Embasados na alta baroclinicidade do sistema de correntes de contorno ao largo do sudeste brasileiro, optamos por representar o escoamento CB-SNB-CCP através de um modelo paramétrico do campo de massa, calibrado com os dados hidrográficos oriundos dos Cruzeiros Abrolhos [Silveira et al., 2006]. Mantendo o caráter idealizado do estudo, também empregamos topografia de fundo analítica, onde representamos o talude da região por uma função tangente hiporbólica. O BA e o BRC foram aproximados através de funções gaussianas devidademente ajustadas aos contornos da isóbata de 80 m extraídas do conjunto ETOPO 2. Para identificar os mecanismos de crescimento das possíveis ondas instáveis, calculou-se o balanço de energia das simulações realizadas de acordo com o método de [Xue & Bane, 1997]. Os resultados de três experimentos numéricos realizados sugerem que o sistema CB-SNB-CCP é instável. Ciclones quase-estacionários do lado costeiro da CB surgem como o principal modo de variabilidade desta corrente. De acordo com a análise do balanço energético, o crescimento dessas feições resulta primariamente de instabilidade baroclínica do escoamento. A escala horizontal típica das ondas e vórtices instáveis modelados é dada pelo raio de deformação interno, como esperado pela teoria de instabilidade baroclínica de escoamentos realisticamente estratificados. Em particular, os resultados do experimento com o BA e o BRC idealizados comprovam que estes funcionam como gatilhos para o desenvolvimento de ondas instáveis, favorecendo amplamente o crescimento das estruturas verticais. Os trens de onda instáveis quase-estacionários aqui obtidos sugerem que provavelmente o meandramento da CB observado em latitudes que se estendem até 28°S pode ser parte de um único sistema que se origina na região dos BA e BRC. / The western boundary currents system that flows off the eastern brazilian coast between 15°S e 22°S is composed by the southward-flowing Brazil Current (BC), the northward-flowing North Brazil Under Current (NBUC) and the Deep Western Boundary Current (DWBC) that flows south. Vigorous meanders and eddies are observed downstream of the Abrolhos (AB) and Royal Charlotte Banks (RCB) and the Vitória-Trindade Ridge. The main goal of this dissertation is to study the stability of this currents system, hereby named BC-NBUC-DWBC system, using the Princeton University Ocean Model (POM) in an idealized scenerio. We seek to answer how unstable is this system and what are the characteristics of the unstable waves in order to contribute to the understanding of the rich mesoescale dynamics observed in this region. We also aim to elucidate the role of the AB and of the RCB on this dynamics. Based on the high degree of baroclinicity of the western boundary currents system off the southeastern brazilian coast, we opted to represent the BC-NBUC-DWBC system through a parametric model of the mass field, calibrated with hydrographic data from the Abrolhos Cruises [Silveira et al., 2006]. Maintaining the idealized character of the study, we have also employed an analytical bottom topography, in which the region\'s continental slope is approximated by hyperbolic tangent function. The AB and RCB were approximated by gaussian functions properly adjusted to the 80 m isobath extracted from the ETOPO 2 database. To identify the growth mecanism of the unstable waves, the energy budget of the simulations was calculated according to [Xue & Bane, 1997]. The results from the three experiments conducted here suggest that the BC-NBUC-DWBC system is indeed unstable. Quasi-stationary cyclones in the coastal side of BC arise as the main mode of variability of this current. According to the energy budget analisys, the growth of these features results primarily from baroclinic instability of BC-NBUC-DWBC flow. The typical horizontal scale of the modeled unstable waves and eddies is given by the internal radius of deformation, as expected by baroclinic instability theory of realistically stratified flows. The quasi-stationary unstable wave trains modeled in the present study also suggest that the meandering of the BC observed down to 28°S are probably part of a single system that originates at the AB and RCB region.

A variety of general circulation model experiments are performed to investigate the influence of seasonality and topography on the strength of baroclinic eddies in the Martian atmosphere. Three different models are used: a full physics model, a simplified physics model, and a zonally symmetric simplified physics model. All three models are sigma coordinate, finite difference global atmospheric circulation models that have been adapted to the Martian regime. The full physics model has previously been tested extensively by researchers at the NASA Ames Research Center. The simplified physics model replaces many of the atmospheric physics routines with simple parameterizations; most importantly, the radiation code is replaced by Newtonian cooling. A Newtonian cooling code with a radiative time constant that varies in height and latitude produces superior results to one with a radiative time constant that is the same everywhere throughout the atmosphere. It is found that baroclinic eddy activity is extremely sensitive to the mean meridional temperature gradient in the simplified model. A power law fit gives an exponent of approximately six. The baroclinic eddy activity is also sensitive to the maximum growth rate in the Eady model of baroclinic activity. This is due to the close connection between the meridional temperature gradient and the maximum growth rate. Baroclinic adjustment theory, which predicts how baroclinic eddies will react to changes in the mean circulation, does not appear to be valid in the Martian regime, according to the simplified model. This finding may be related to the differences in the relative strengths of the baroclinic eddies and the mean circulation on Earth and Mars. The simplified model indicates that seasonality is more important than topography in creating stronger eddies in the northern hemisphere winter than in the southern hemisphere winter. However, the effects of topography in the simplified model may not be adequately matching the effects of topography in the full physics model, particularly in the southern hemisphere. / Graduation date: 2001

A comprehensive analysis of midlatitude intraseasonal variability in extended integrations of General Circulation Models (GCMs) developed at the National Aeronautics and Space Administration Goddard Space Flight Center is conducted in the first part of this thesis. The model statistics, three-dimensional structure, and dynamical characteristics of storm tracks and large-scale weather regimes are diagnosed and directly compared to parallel observational analyses. Results indicate that several aspects of simulated intraseasonal phenomenon are linked to errors in the zonally asymmetric circulation. During the boreal winter, anomalously large meridional temperature gradients and enhanced zonal winds in the simulations are linked to suppressions in baroclinic wave activity in the models. As a result lower-frequency modes in the GCMs are more strongly driven by baroclinic dynamics. Nonetheless, the GCMs successfully reproduce the North Pacific midwinter suppression phenomenon. In an effort to provide insight into the North Pacific midwinter suppression, the second part of this work stratifies the boreal cool season into three stages, where the early and late (middle) stages approximately correspond to the seasonal maxima (minimum) in synoptic eddy activity that occurs over the North Pacific. Analyses using this unique approach reveal that during the midwinter suppression period, cyclonic perturbations entering the North Pacific storm track core from Asia are already deficient in magnitude compared to early and late winter stages. In both observations and model simulations, the North Pacific midwinter suppression feature is discovered to have a clear organized extension upstream into Siberia. Thus, the final portion of this thesis examines the causes for the midwinter suppression of upper tropospheric Rossby wave packets propagating across Asia prior to entering the North Pacific storm track. The study unveils an increased sensitivity of synoptic-scale wave packets with a large-scale, upper-tropospheric flow pattern over Siberia during midwinter. This interaction with the large-scale pattern over Siberia results in a general dampening of synoptic eddy amplitudes over Asia, which is proposed to be the contributing factor to the North Pacific midwinter suppression phenomenon.

The jet-in-crossflow problem has been extensively studied, mainly because of its applications in film cooling and injector designs. It has been established that in low-speed flows, pulsing the jet significantly enhances mixing and jet penetration. This work investigates the effects of pulsing on mixing and jet trajectory in high speed (compressible) flow, using Large Eddy Simulation. Jets with different density ratios, velocity ratios and momentum ratios are pulsed from an injector into a crossflow. Density ratios used are 0.55 (CH4/air), 1.0 (air/air) and 1.5 (CO2/air). Results are compared with the low speed cases studied in the past and then analyzed for high speed scaling. The simulations show that the lower density jet develops faster than a higher density jet. This results in more jet spread for the lower density jet. Scaling for jet spread and the decay of centerline jet concentration for these cases are established, and variable density scaling law is developed and used to predict jet penetration in the far field. In most non-premixed combustor systems, the fuel and air being mixed are at different initial temperatures and densities. To account for these effects, heated jets at temperatures equal to 540K and 3000K have been run. It has been observed that, in addition to the lower density of heated jets, the higher kinematic viscosity effects the jet penetration. This effect has been included and validated in the scaling law for the heated jet trajectory.

In the present study syngas-air diffusion flames are simulated using LES with artificial neural network (ANN) based chemical kinetics modeling and the results are compared with previous direct numerical simulation (DNS) study, which exhibits significant extinction-reignition and forms a challenging problem for ANN. The objective is to obtain speed-up in chemistry computation while still having the accuracy of stiff ODE solver. The ANN methodology is used in two ways: 1) to compute the instantaneous source term in the linear eddy mixing (LEM) subgrid combustion model used within LES framework, i.e., laminar-ANN used within LEMLES framework (LANN-LEMLES), and 2) to compute the filtered source terms directly within the LES framework, i.e., turbulent-ANN used within LES (TANN-LES), which further dicreases the computational speed. A thermo-chemical database is generated from a standalone one-dimensional LEM simulation and used to train the LANN for species source terms on grid-size of Kolmogorov scale. To train the TANN coefficients the thermo-chemical database from the standalone LEM simulation is filtered over the LES grid-size and then used for training. To evaluate the performance of the TANN methodology, the low Re test case is simulated with direct integration for chemical kinetics modeling in LEM subgrid combustion model within the LES framework (DI-LEMLES), LANN-LEMLES andTANN-LES. The TANN is generated for a low range of Ret in order to simulate the specific test case. The conditional statistics and pdfs of key scalars and the temporal evolution of the temperature and scalar dissipation rates are compared with the data extracted from DNS. Results show that the TANN-LES methodology can capture the extinction-reignition physics with reasonable accuracy compared to the DNS. Another TANN is generated for a high range of Ret expected to simulate test cases with different Re and a range of grid resolutions. The flame structure and the scalar dissipation rate statistics are analyzed to investigate success of the same TANN in simulating a range of test cases. Results show that the TANN-LES using TANN generated fora large range of Ret is capable of capturing the extinction-reignition physics with a very little loss of accuracy compared to the TANN-LES using TANN generated for the specific test case. The speed-up obtained by TANN-LES is significant compared to DI-LEMLES and LANN-LEMLES.

A comprehensive understanding of the hydrodynamics in vegetated open-channels and flow-vegetation interaction is of high interest to researchers and practitioners alike for instance in the content of river and coastal restoration schemes. The focus of this study was to investigate the effect of the presence of vegetation on flow resistance, turbulence statistics, and the instantaneous flow in open channels by performing three-dimensional computational-fluid-dynamics (CFD) simulations. Firstly, fully developed turbulent flow in fully-vegetated channel was analyzed by employing the method of high-resolution Large-Eddy Simulation (LES). Flow through a staggered array of rigid, emergent cylinders was simulated and the LES was validated through experiments. After validation, numerical simulations were performed at an extended parameter range of two different cylinder Reynolds numbers (ReD = 500 and 1340) and three different vegetation densities (φ = 0.016, 0.063, and 0.251). Flow structures and statistics were analyzed on the instantaneous flow and the effect of the vegetation density and cylinder Reynolds number was assessed. Moreover, drag forces exerted on the cylinders were calculated explicitly, and the effect of both ReD and φ on the drag coefficient was quantified. Secondly, two new alternative simulation strategies, a RANS based strategy with a vegetative closure model and a low-resolution Large-Eddy Simulation, were devised. They were evaluated by simulating several experimental cases with diverse conditions of the cylinder arrangement (i.e., staggered vs. random distribution), vegetation densities (φ = 0.016, 0.022, 0.063, 0.087, 0.091, 0.150, and 0.251), and cylinder Reynolds number (ReD = 170 - 1700). For the RANS based strategy, the importance of a-priori knowledge was assessed, and for the low-resolution LES, the efficiency and accuracy was demonstrated. Finally, a numerical strategy based on a porosity approach was developed and applied to open-channel flow through a natural plant. The simulated velocities were compared with experimentally acquired ones and results showed reasonable agreement. The results obtained in this research contribute to the understanding of fundamental mechanism of flow-vegetation interaction in vegetated open-channels, resolving turbulent flow-vegetation interaction explicitly. In addition, the new numerical strategies developed as part of this research are expected to allow describing the behavior of turbulent flow through artificial and natural vegetation with high efficiency and accuracy.