Natural stream systems contain a variety of flow geometries which contain flow separation, turbulent shear layers, and recirculation zones. This work focuses on streams with dead zones. Characterized by slower flow and recirculation, dead zones are naturally occurring cutouts in stream banks. These dead zones play an important role in stream nutrient retention and solute transport. Previous experimental work has focused on idealized dead zone geometries studied in laboratory flumes. This work explores the capabilities of computational fluid dynamics (CFD) to investigate the scaling relationships between flow parameters of idealized geometries and the time scales of transport. The stream geometry can be split into two main regions, the main stream flow and the dead zone. Geometric parameters of the dead zone as well as the bulk stream velocity were varied to determine a scaling relationship for the transport time scales. These flow geometries are simulated using the RANS turbulence model with the standard k-ω closure. The standard first order dead zone model is expanded to a two region model to accommodate the multiple time scales observed in the simulation results. While this model currently has limited predictive capability, it provides physical insight into the functional dependence of the dead zone time scales. LES is used to evaluate the performance of the Reynolds Averaged Navier-Stokes (RANS) turbulence model and to describe the anisotropic turbulence characteristics. The differences between the time averaged flow field for Large Eddy Simulation (LES) and RANS was determined to have a significant impact on passive scalar transport. / Graduation date: 2012

The barotropic tide dissipates a well established estimate of 2.5 TW of energy at the M2 frequency. Bottom topography is responsible for part of this dissipation, and the generation of the internal tide is also partly responsible. The fate of this energy is largely described by a cascade from large scales to small scales by non-linear wave-wave interactions where it gets dissipated. This thesis aims to investigate how the presence of mesoscale eddies (vortices) in the ocean affect the internal tide. Previous work has looked at the interaction of the barotropic tide with eddies. Krauss (1999) found that the interaction can produce a modulated internal tide, however a scaling analysis suggests that the effect may not be as strong as reported. The MITgcm is used to simulate internal wave generation by barotropic flow over topography and comparisons are made with Dr. Lamb's IGW model. Baroclinic eddies are analytically prescribed and then geostrophically adjusted also using the MITgcm. Finally, the two are combined, and the internal tide field is analysed with and without the presence of eddies of various magnitude and length scales. The results of this investigation do not find a strong transfer of energy between modes; the modal distribution of energy in the internal tide remains the same when an eddy is added. However, focusing and shadow beams of internal waves are produced in the wake of an eddy as the internal waves pass through it. The beams show very strong variations in intensity, vertically integrated energy flux can reduce almost to zero in the shadow regions and increase more than double in the focusing regions. Modal decomposition of the horizontal flow field reveals that mode 2 and 3 waves are most strongly affected by the eddies and contribute strongly to the formation of the beams. Mode 1 appears to be less affected by the eddy. The larger wavelength and faster group velocity of mode 1 supports the notion that the eddy interacts with it less.

The barotropic tide dissipates a well established estimate of 2.5 TW of energy at the M2 frequency. Bottom topography is responsible for part of this dissipation, and the generation of the internal tide is also partly responsible. The fate of this energy is largely described by a cascade from large scales to small scales by non-linear wave-wave interactions where it gets dissipated. This thesis aims to investigate how the presence of mesoscale eddies (vortices) in the ocean affect the internal tide. Previous work has looked at the interaction of the barotropic tide with eddies. Krauss (1999) found that the interaction can produce a modulated internal tide, however a scaling analysis suggests that the effect may not be as strong as reported. The MITgcm is used to simulate internal wave generation by barotropic flow over topography and comparisons are made with Dr. Lamb's IGW model. Baroclinic eddies are analytically prescribed and then geostrophically adjusted also using the MITgcm. Finally, the two are combined, and the internal tide field is analysed with and without the presence of eddies of various magnitude and length scales. The results of this investigation do not find a strong transfer of energy between modes; the modal distribution of energy in the internal tide remains the same when an eddy is added. However, focusing and shadow beams of internal waves are produced in the wake of an eddy as the internal waves pass through it. The beams show very strong variations in intensity, vertically integrated energy flux can reduce almost to zero in the shadow regions and increase more than double in the focusing regions. Modal decomposition of the horizontal flow field reveals that mode 2 and 3 waves are most strongly affected by the eddies and contribute strongly to the formation of the beams. Mode 1 appears to be less affected by the eddy. The larger wavelength and faster group velocity of mode 1 supports the notion that the eddy interacts with it less.

Two new models to calculate the species instantaneous and filtered reaction rates for multi-step, multi-species chemical kinetics mechanisms are developed based on the artificial neural networks (ANN) approach. The proposed methodologies depend on training the ANNs off-line on a thermo-chemical database representative of the actual composition and turbulence level of interest. The thermo-chemical database is constructed by stand-alone linear eddy mixing (LEM) model simulations under both premixed and non-premixed conditions, where the unsteady interaction of turbulence with chemical kinetics is included as a part of the training database. In this approach, the information regarding the actual geometry of interest is not needed within the LEM computations. The developed models are validated extensively on the large eddy simulations (LES) of (i) premixed laminar-flame-vortex-turbulence interaction, (ii) temporally mixing non-premixed flame with extinction-reignition characteristics, and (iii) stagnation point reverse flow combustor, which utilizes exhaust gas re-circulation technique. Results in general are satisfactory, and it is shown that the ANN provides considerable amount of memory saving and speed-up with reasonable and reliable accuracy. The speed-up is strongly affected by the stiffness of the reduced mechanism used for the computations, whereas the memory saving is considerable regardless.

The present study describes the flow characteristics of swirling flows induced by twisted tape inserts in circular pipes. The study is focused on the secondary flow which is investigated experimentally and with numerical models. The results are expected to improve the paper manufacturing process by identifying and removing the detrimental secondary flow. Experimental tests show for the first time the existence of two co-rotating helical vortices superimposed over the main swirling flow, downstream of twisted tapes. The close proximity of the two co-rotating vortices creates a local counter-rotating flow at the pipe centerline. The flow is analyzed using LDV measurements and high speed camera visualization with fine air bubbles seeding which confirm that the helical vortices are stable. After extracting the characteristic tangential velocity profiles of the main vortex and of the two secondary vortices, it was observed that the maximum tangential velocity of all three vortices is the same, approximately half of the bulk velocity. The winding of the helical vortices is in the swirl direction and the pitch of the helical vortices is found to be independent of the inlet velocity. The experimental findings are confirmed by numerical simulations. The numerical results show that the helical vortices originate inside the swirler and evolve from single co-rotating vortices on each side of the tape. The flow characteristics are analyzed in detail. Swirlers with multiple twists and multiple chambers are shown to have less stable secondary motion and could be employed in applications were the secondary motion is detrimental.

A new combustor, referred to as Stagnation Point Reverse Flow (SPRF) combustor has been developed at Georgia Tech to meet increasingly stringent emission regulations. The combustor incorporates a novel design to meet the conflicting requirements of low pollution and high efficiency in both premixed and non-premixed modes. The objective of this thesis is to perform Large Eddy Simulations (LES) on this lab-scale combustor and explain the underlying physics. To achieve this, numerical simulations are performed in both the premixed and non-premixed combustion modes. The velocity field, species field, entrainment characteristics, flame structure, emissions and mixing characteristics are then analyzed. Simulations have been carried out first for a non-reactive case and the flow features in the combustor are analyzed. Next, the simulations have been extended for the premixed reactive case by employing different sub-grid scale combustion chemistry closures - Eddy Break Up (EBU), Artificially Thickened Flame (TF) and Linear Eddy Mixing (LEM) models. Only LEMLES which is an advanced scalar approach is able to accurately predict both the velocity and species field in the combustor. The results from LEM with LES (LEMLES) using a reduced chemical mechanism have been analyzed in the premixed mode. The results showed that mass entrainment occurs along the shear layer in the combustor. The entrained mass carried products into the reactant stream and provided preheating. The product entrainment enhances the reaction rates and stabilizes the flame even at very lean conditions. These products are shown to enter into the flame through local extinction zones present on the flame surface. The flame structure is further analyzed and the combustion mode is found to be primarily in thin reaction zones. The emissions in the combustor are studied using simple global mechanisms for NOx. Computations show extremely low NOx values comparable to the measured emissions. These low emissions are shown to be primarily due to the low temperatures in the combustor. LEMLES computations are also performed with detailed chemistry to capture more accurately the flame structure. The flame in the detailed chemistry case is more sensitive to strain effects and show more extinction zones very near to the injector. LEMLES approach is also used to resolve the combustion mode in the non-premixed case. The studies indicate that mixing of fuel and air close to the injector controls the combustion process. The predictions in the near field are shown to be very sensitive to the inflow conditions. Analysis shows that fuel and air mixing occurs to lean proportions in the combustor before any burning takes place. The flame structure in the non-premixed mode is very similar to the premixed mode. Along with fuel-air mixing, the products also mix with the reactants and provide the preheating effects to stabilize the flame in the downstream region of the combustor.

Dans la partie ouest du golfe du Lion, Millot (1979, 1982) avait postulé l'existence de tourbillons anticycloniques de mésoéchelle. Comme déjà bien étudié dans l'océan ouvert, ce type de structure pourrait aussi jouer, en zone côtière, un rôle important sur la circulation et sur la biogéochimie, ainsi que dans les échanges de matière et d'énergie entre les zones côtières et hauturières. Les objectifs de cette thèse sont de caractériser les tourbillons anticycloniques à l'ouest du Golfe du Lion et de comprendre leur processus de génération, en utilisant à la fois la modélisation numérique et les données in situ. Le modèle numérique SYMPHONIE est utilisé pour reproduire de manière réaliste la circulation du golfe du Lion afin de nous aider à comprendre la dynamique de ces structures. Une étude de sensibilité sur la résolution spatiale du modèle et sur la diffusion horizontale a été effectuée pour obtenir la configuration optimisée du modèle. L'application de la technique d'analyse en ondelettes sur les résultats du modèle a permis d'identifier les tourbillons et d'estimer leurs caractéristiques (durée de vie, taille,...). Les résultats de la modélisation numérique ont été utilisés pour établir la stratégie des campagnes en mer LATEX et pour nous aider à mieux comprendre les données des mesures in situ. L'analyse des données collectées pendant la campagne Latex08 (1-6 septembre 2008) a confirmé l'existence de tourbillons sur la partie ouest du plateau continental du golfe du Lion et a montré que les caractéristiques des tourbillons observés in situ sont bien comparables avec celles issues des résultats du modèle. Combiné avec les résultats numériques, les mesures in situ ont permis d'améliorer notre connaissance des tourbillons. Grâce à une simulation pluri-annuelle (2001-2008) avec la configuration optimisée choisie, nous avons pu étudier la reproductibilité et la variabilité des tourbillons, et appréhender leur processus de génération. Les résultats de simulation ont montré que, pour chaque année de 2001 à 2008, des tourbillons anticycloniques se produisent régulièrement, de mai à octobre, à l'ouest du Golfe du Lion avec une durée de vie allant de quelques jours jusqu'à plus que deux mois en maximum. Il a été montré que pour qu'un tourbillon de « longue durée de vie » (plus de 15 jours) se développe, deux conditions sont nécessaires: un vent du nord-ouest fort et persistant et une forte stratification de la masse d'eau. L'analyse des effets combinés de ces deux facteurs en fonction de leurs différents degrés d'intensité ont permis d'expliquer la variabilité annuelle et interannuelle des tourbillons générés au cours des 8 ans de simulation numérique. / In the western part of the Gulf of Lion, Millot (1979, 1982) had postulated the presence of mesoscale anticyclonic eddies. As already well studied in the open ocean, such structures could also play, in the coastal zone, an important role on the circulation and biogeochemistry, as well as on the coastal-offshore exchanges of energy and mass. The objectives of this PhD thesis is to characterize the anticyclonic eddies present in the western part of the Gulf of Lion and to understand their generation processes, using in a combined way numerical modelling and in situ measurements. The numerical model SYMPHONIE is used to simulate realistically the circulation of the Gulf of Lion, in order to help us understand the dynamics of such structures. A sensitivity analysis has been carried out, varying the model spatial resolution and the horizontal viscosity, to choose the best model configuration. A wavelet technique is applied on model outputs to identify eddies and estimate their characteristics (duration, size...). The numerical results have been used to set up the sampling strategy of the LATEX cruises and help us interpret in situ data. The analysis of the measurements collected during Latex08 (September 1-6 , 2008) confirmed the presence of an anticyclonic eddy in the western part of the Gulf of Lion and showed that its features agree with the ones of the simulated eddy. These in situ data, combined with the modeling results, allow us to better understand the eddies' dynamics. Thanks to a long period simulation (2001-2008) with the appropriate model configuration, we studied the repetitiveness and variability of the eddies, as well as their generation processes. Modeling results showed that during the 8 years of simulation, coastal anticyclonic eddies are present regularly, from May to October, in the western part of the Gulf of Lion with a life duration varying from several days to more than two months. The eddies with a life duration longer than 15 days need two conditions to be generated: a persistent and strong northwest wind and a strong stratification. Annual and interannual differences between the eddies present in the 8 years of simulation can be explained by varying the intensity of each of the two conditions.

Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2006. / Includes bibliographical references (p. 139-143). / The continuous supply of heat and fresh water from the boundaries to the interior of the Labrador Sea plays an important role for the dynamics of the region and in particular, for the Labrador Sea Water formation. Thus, it is necessary to understand the factors governing the exchange of properties between the boundary and interior. A significant fraction of heat and fresh water, needed to balance the annual heat loss and to contribute to the seasonal freshening of the Labrador Sea, is thought to be provided by coherent long-lived anticyclonic eddies shed by the Irminger Current. The population, some properties, rates and direction of propagation of these anomalies are known but the evolution and the mechanism of their decay are still far from obvious. In this work I investigated their water mass properties and evolution under the strong wintertime forcing using hydrographic data from 1990-2004 and a 1-dimensional mixed layer model. There were 50 eddies found in the hydrographic data record, 48 of which were identified as anticyclones. Vertical structure of the eddies was investigated, leading to the categorization of all the anticyclones into three classes: 12 - with a fresh surface layer and no mixed layer, 18 - without a fresh layer and at least one mixed layer, and 18 with ambiguous vertical structure. Four eddies of the second group appeared to have cores extending to as deep as 1500 m vertically and an isopycnal displacement of 400-600 m. A number of eddies without a fresh water cap contained Labrador Sea Water from the previous year at mid-depths. / (cont.) Vertical structure of the eddies was investigated, leading to the categorization of all the anticyclones into three classes: 12 - with a fresh surface layer and no mixed layer, 18 - without a fresh layer and at least one mixed layer, and 18 with ambiguous vertical structure. Four eddies of the second group appeared to have cores extending to as deep as 1500 m vertically and an isopycnal displacement of 400-600 m. A number of eddies without a fresh water cap contained Labrador Sea Water from the previous year at mid-depths. / by Tatiana Rykova. / S.M.

L'océan Austral joue un rôle crucial dans la régulation du système climatique en absorbant de grandes quantités de CO2 atmosphérique. Toutefois de nombreuses incertitudes demeurent quant à l'évolution récente du puits de carbone austral notamment en raison du manque d'observations et des lacunes des modèles océaniques dans la représentation de processus dynamiques comme les tourbillons. Depuis quelques décennies notamment, l'efficacité du puits de carbone austral diminuerait en raison d'une intensification des vents liée à une tendance positive du Mode Annulaire Austral (SAM). L'objectif de ces travaux de thèse est de décrire et comprendre la variabilité spatiale et temporelle récente des flux air-mer de CO2 dans l'océan Austral. Pour cela, des simulations de sensibilité aux phases positives du SAM sont réalisées dans une configuration régionale de l'océan Austral (sud de 30°S), basée sur un modèle couplé dynamique-biogéochimie forcé par l'atmosphère et résolvant partiellement la méso-échelle océanique. Dans l'océan Austral, la réponse des flux de CO2 au SAM correspond à un dégazage intense de CO2 dans la zone antarctique dû à une augmentation des concentrations de surface de carbone inorganique dissous (DIC). Cette augmentation est pilotée par la dynamique de la couche de mélange et alimentée par un transport méridien de DIC qui résulte essentiellement de la compétition entre circulation induite par les vents et par les méandres stationnaires. Ces travaux montrent l'apport d'une augmentation de la résolution numérique des modèles pour la simulation des flux de CO2. / By taking up large amounts of atmospheric CO2, the Southern Ocean helps to regulate the climate system. Southern Ocean carbon sink is poorly constrained, in part because data coverage is sparse and also because ocean models that have been used in such assessments fail to explicitly resolve key physical features such as mesoscale eddies. In recent decades, the growth of the Southern Ocean carbon sink may have been partly counteracted due to a loss of natural CO2 from the ocean driven by an intensification of westerlies, related to a positive trend in the Southern Annular Mode (SAM). This thesis focuses on documenting and understanding recent spatial and temporal variability of air-sea CO2 fluxes in the Southern Ocean. Sensitivity to positive phases of the SAM are tested by making simulations with a regional model of the Southern Ocean (south of 30°S) that couples biogeochemistry to the dynamics, is forced by atmosphere reanalysis data, and partially resolves the mesoscale. The resulting response of Southern Ocean CO2 fluxes to the SAM is dominated by a strong CO2 efflux to the atmosphere from the Antarctic Zone due to an increase in surface dissolved inorganic carbon (DIC). This increase is driven by the mixed-layer dynamics and is supplied by a meridional transport of DIC, a competition between the wind-driven circulation and the standing eddy-induced circulation. This work discusses the effect of increasing model resolution on simulated air-sea CO2 fluxes.

By collocating 10 years (1999-2009) of remotely sensed surface turbulent heat fluxes with satellite altimetry data, we investigate the impact of ocean mesoscale eddies on the latent and sensible heat fluxes in the South Atlantic ocean. Eddies were identified using the method proposed by Chaigneau et al. (2009), which is based on closed contours of sea level anomaly. Most of the identified eddies had a radius of ~70 km and amplitude of ~5 cm. On average, in the South Atlantic, eddies play a minor role on the ocean-atmosphere heat exchange. However, in strongly energetic regions such as the Brazil-Malvinas confluence or Agulhas Current retroflection regions, eddies can account up to 20-30% of the total variance of the surface turbulent heat fluxes with averaged anomalies of ±10-20 W/m2 for both heat flux components. Cyclonic (anticyclonic) eddies, associated with negative (positive) heat fluxes anomalies tend to cool (warm) the overlying atmosphere. A composite analysis of the turbulent heat fluxes anomalies within the eddies reveals a direct relationship between the eddy amplitude and the intensity of the latent and sensible fluxes anomalies, such that large-amplitude eddies have a stronger signature in the turbulent surface heat fluxes. Heat fluxes anomalies are also much stronger near the eddy centers and decay radially to reach minimum values outside the eddies. / Uma combinação de 10 anos (1999-2009) de fluxos turbulentos de calor pela superfície, medidos a partir de satélites, e dados altimétricos de anomalia da altura da superfície do mar, foram o utilizados com objetivo de investigar o impacto de vórtices de meso-escala nos fluxos de calor sensível e latente na bacia do Atlântico Sul. Para a detecção dos vórtices foi aplicado o método proposto por Chaigneau et al. (2009), que baseia-se em contornos fechados de anomalia da altura da superfície do mar. A maior parte dos vórtices identificados possui raio de ~70 km e amplitude de ~5 cm. Em média, no Atlântico Sul, o impacto dos vórtices para as trocas de calor entre oceano e atmosfera é relativamente fraco. Entretanto, em regiões de alta variabilidade energética como na Confluência Brasil- Malvinas e na retroflecção da Corrente das Agulhas, vórtices de meso-escala podem contribuir com anomalias médias de até ±10-20 W/m2 nos fluxos turbulentos. Vórtices ciclônicos (anti-ciclônicos), associados com anomalias negativas (positivas) de fluxos de calor, tendem a esfriar (esquentar) a atmosfera adjacente. Mapas composite foram analisados para milhares de vórtices, mostrando um relação direta entre a magnitude das anomalias dos fluxos e a amplitude dos vórtices, de tal modo que vórtices de maior amplitude contribuem com maiores anomalias de calor latente e sensível. Além disso, os padrões espaciais dos composites médios revelam que as anomalias são significativamente maiores próximo ao centro dos vórtices e decaem radialmente até atingirem valores absolutos mínimos fora dos contornos dos vórtices.