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1

Arratia, Cristobal. "Mécanismes d'instabilité non-modaux dans les écoulements cisaillés avec et sans stratification en densité". Phd thesis, Palaiseau, Ecole polytechnique, 2011. http://www.theses.fr/2011EPXX0084.

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Cette thèse est dediée à l'étude des mécanismes d'instabilité non-modaux dans les écoulements cisaillés, principalement des couches de mélange. On se concentre sur les perturbations linéarisées qui ont la plus grande croissance d'énergie à un temps donné, les 'perturbations optimales', différentes du mode propre le plus instable pour les systémes non-normaux. Une description originale de la non-normalité et ses conséquences est donnée dans le chapitre 2. Le chapitre 3 traite du mécanisme de 'lift-up' dans le cas des perturbations longitudinales non-visqueuses sur un écoulement de base parallèle et arbitraire. On trouve une nouvelle équation 1D qui détermine l'ensemble des perturbations orthogonales, dont l'optimale, et permet de trouver des nouveaux résultats exacts et asymptotiques. Dans le chapitre 4 on s'intéresse aux instabilités secondaires d'une couche de mélange en utilisant comme état de base l'écoulement 2D instationnaire et non-linéaire pour calculer les perturbations optimales 3D. Selon le nombre d'onde et les temps d'optimisation (initial et final), on retrouve comme réponses optimales les perturbations de types 'elliptique' et 'hyperbolique'. Dans les chapitres 5 et 6 on considère une stratification en densité dans la direction orthogonale au plan de l'état de base, et on utilise une décomposition de 'Craya-Herring' pour analyser les perturbations en termes de leur contenu en ondes internes et vorticité verticale. Les équations d'évolution des perturbations linéarisées autour d'un état de base 2D général sont obtenues et analysées au niveau de l'énergie onde/tourbillon. Ces résultats sont appliqués dans le chapitre 6 pour analyser la génération et l'émission d'ondes dans les perturbations optimales sur une couche de cisaillement horizontale.
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2

Arratia, Cristobal. "Mécanismes d'instabilité non-modaux dans les écoulements cisaillés avec et sans stratification en densité". Phd thesis, Ecole Polytechnique X, 2011. http://pastel.archives-ouvertes.fr/pastel-00672072.

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Cette thèse est dediée à l'étude des mécanismes d'instabilité non-modaux dans les écoulements cisaillés, principalement des couches de mélange. On se concentre sur les perturbations linéarisées qui ont la plus grande croissance d'énergie à un temps donné, les 'perturbations optimales', différentes du mode propre le plus instable pour les systémes non-normaux. Une description originale de la non-normalité et ses conséquences est donnée dans le chapitre 2. Le chapitre 3 traite du mécanisme de 'lift-up' dans le cas des perturbations longitudinales non-visqueuses sur un écoulement de base parallèle et arbitraire. On trouve une nouvelle équation 1D qui détermine l'ensemble des perturbations orthogonales, dont l'optimale, et permet de trouver des nouveaux résultats exacts et asymptotiques. Dans le chapitre 4 on s'intéresse aux instabilités secondaires d'une couche de mélange en utilisant comme état de base l'écoulement 2D instationnaire et non-linéaire pour calculer les perturbations optimales 3D. Selon le nombre d'onde et les temps d'optimisation (initial et final), on retrouve comme réponses optimales les perturbations de types 'elliptique' et 'hyperbolique'. Dans les chapitres 5 et 6 on considère une stratification en densité dans la direction orthogonale au plan de l'état de base, et on utilise une décomposition de 'Craya-Herring' pour analyser les perturbations en termes de leur contenu en ondes internes et vorticité verticale. Les équations d'évolution des perturbations linéarisées autour d'un état de base 2D général sont obtenues et analysées au niveau de l'énergie onde/tourbillon. Ces résultats sont appliqués dans le chapitre 6 pour analyser la génération et l'émission d'ondes dans les perturbations optimales sur une couche de cisaillement horizontale.
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3

Fallon, Benoit. "Simulation des grandes échelles d'écoulements turbulents stratifiés en densité". Grenoble INPG, 1994. http://www.theses.fr/1994INPG0162.

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Par la technique de simulation des grandes echelles, on etudie l'influence de la stratification stable sur la dynamique des structures coherentes se developpant en aval d'une marche. En l'absence de forces de flottaison, on montre que l'ecoulement developpe des tourbillons de kelvin-helmholtz fortement distortus suivant l'envergure, conduisant a des interactions tourbillonnaires d'appariements helicoidaux. Nous l'expliquons par le forcage induit par la recirculation. Les predictions statistiques sont en bon accord avec les mesures experimentales. Pour les configurations stratifiees, l'ecoulement tend a se bidimensionnaliser par stabilisation de la poche de recirculation. On visualise pour des intensites de stratification croissante le blocage de la croissance de la couche cisaillee par inhibition des processus d'appariements puis des instabilites primaires de kelvin-helmholtz, le developpement d'ondes de gravite et d'interfaces stables de densite. La topologie de l'ecoulement est comparee aux visualisations sur la couche de melange stratifiee. Des simulations complementaires ont permis de mettre en evidence les mecanismes instables du couple barocline. On montre, en accord avec le critere de stabilite lineaire, la formation d'une cascade de tourbillons de kelvin-helmholtz baroclines sur l'interface de densite entre deux structures primaires, favorisant les processus de melange turbulent. Enfin, les potentialites de la simulation des grandes echelles sont demontres sur un ecoulement industriel dans une cavite stratifiee. Les fluctuations de temperature sont comparees aux mesures experimentales. Les visualisations graphiques sont completees par des animations tridimensionnelles instationnaires
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4

Lemembre, Alain. "Evolution par convection naturelle d'une stratification formée de deux couches liquides dans un réservoir cylindrique". Châtenay-Malabry, Ecole centrale de Paris, 1999. http://www.theses.fr/1999ECAP0618.

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Au moyen de simulations numériques, ce travail identifie les mécanismes qui pilotent le mélange au sein d'une stratification thermique et massique. Le mélange est provoqué par la convection naturelle induite au sein d'un liquide contenu dans un réservoir cylindrique vertical et chauffe latéralement. La stratification se présente initialement sous la forme de deux couches séparées par une interface diffusive d'épaisseur finie, la couche inferieure étant plus chaude et plus concentrée en soluté. La principale motivation de cette étude est la compréhension du phénomène de rollover, un mélange au sein des réservoirs de gaz naturel liquéfié (GNL) induisant une soudaine augmentation de l'évaporation à la surface libre. Le premier chapitre fait le point des connaissances sur le phénomène de rollover. Le deuxième chapitre propose une description du système en termes de groupements sans dimensions. L'épineux problème du respect des conditions de similitude est abordé. Le troisième chapitre expose les méthodes numériques employées dans les simulations. L'accent est mis sur les problèmes numériques qui constituent le premier obstacle à la réalisation de la similitude. Le délicat problème de viscosité artificielle, récurrent en simulation numérique, est caractérisé quantitativement dans la configuration qui nous intéresse. Ces considérations numériques sont suivies d'aspects physiques par l'étude d'une part d'un mélange dans l'eau salée, d'autre part d'un mélange dans le GNL. Le quatrième et dernier chapitre est axé sur l'évolution du GNL avec évaporation en surface libre. Des structures convectives originales sont mises en évidence. Ce chapitre propose une vision du rollover dont on espère qu'elle puisse inspirer les modélisateurs du rollover.
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5

Laroche, Hugues. "Simulations numériques directes des écoulements d'un fluide tournant, ou stratifié en densité de façon stable, autour d'un obstacle". Grenoble 1, 1991. http://www.theses.fr/1991GRE10044.

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On considere l'evolution temporelle de perturbations spatialement periodiques, appliquees a un ecoulement zonal cisaille dont le profil de vitesse est en tangente hyperbolique. Un schema aux differences finies utilise a haute resolution permet d'etudier l'evolution de l'ecoulement perturbe a haut nombre de reynolds. On fait varier la rotation differentielle depuis zero jusqu'a des valeurs ou elle inhibe totalement l'instabilite lineaire. On montre que la rotation differentielle supprime le phenomene d'appariement de tourbillons, qui suit l'instabilite dans le cas non-tournant. Ensuite, on etudie les ecoulements tri-dimensionnels d'un fluide stratifie en densite de facon stable, autour d'un obstacle. L'etude est basee sur la simulation numerique directe des equations de boussinesq, realisee en volumes finis au moyen du code trio du cea. Le sillage de l'obstacle est tridimensionnel dans le cas neutre ou peu stratifie (grand nombre de froude). On montre que pour des stratifications assez fortes (nombre de froude de l'ordre de l'unite), les perturbations dues a l'obstacle se propagent sous la forme d'ondes de gravite internes, dont l'amplitude peut etre importante. Dans les cas tres stratifies (nombre de froude tres inferieur a l'unite), le mouvement du fluide se trouve confine dans des plans horizontaux, avec formation eventuelle d'une allee tourbillonnaire. On met ainsi en evidence des regimes distincts suivant le nombre de froude, en bon accord avec les experiences de laboratoire. Les resultats ont principalement ete obtenus par analyse fine du champ de vorticite et par visualisation graphique tri-dimensionnelle de l'energie ondulatoire propagee
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6

Patrier, Laure. "Stratification du risque cardio-vasculaire en insuffisance rénale chronique : place des biomarqueurs émergents". Thesis, Montpellier 2, 2014. http://www.theses.fr/2014MON20227.

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L'insuffisance rénale chronique (IRC) demeure un problème de santé publique du fait de l'augmentation de sa prévalence. Malgré l'amélioration de la prise en charge, le taux de mortalité reste plus élevé comparé à la population générale. Parmi les causes de décès, les maladies cardiovasculaires, d'origine multifactorielle (élargissement et hypertrophie des artères, athérosclérose, calcifications vasculaires et valvulaires) sont au premier plan. A côté des facteurs de risque classiques, des facteurs non traditionnels, liés aux perturbations métaboliques de l'IRC, ont été mis en évidence, comme l'inflammation, la malnutrition, le stress oxydant, les anomalies du métabolisme minéralo-osseux. La meilleure connaissance de la physiopathologie de la vasculopathie de l'IRC permet d'émerger de nouveaux biomarqueurs pour stratifier le risque cardiovasculaire chez l'IRC.OBJECTIFS-METHODOLOGIE GENERALE : Nous avons réalisé une approche biochimique pour explorer trois composantes du risque cardiovasculaire chez l'IRC : stress oxydant, perturbations qualitatives des HDL (high-density lipoprotein) et métabolisme minéralo-osseux.RESULTATS : Dans une première publication la production d'anion superoxyde a été évaluée, via une méthode de chemoluminescence, en fonction du stade de l'IRC. Alors que la surproduction de formes réactives de l'oxygène est bien connue au stade 5d et peut être liée à la procédure dialytique, il existe peu de données aux stades précoces. Notre étude a porté sur 136 patients IRC non dialysés des stades 1à 5. Les résultats montrent que la production de FRO est assurée aux stades 4 et 5. Un bas débit de filtration glomérulaire (MDRD<30ml/min/1.73m2), l'inflammation (fibrinogène >3.7g/l) et des taux anormaux d' HDL (<1.42mM et >1.75mM) apparaissent comme les principaux déterminants du stress oxydant chez l'IRC non dialysé.Alors que dans la population générale, un taux bas de HDL est reconnu comme un facteur de risque important, nous avons montré (publication 1) que des taux anormaux de HDL, bas comme hauts, étaient indépendamment associés au stress oxydant chez les sujets IRC. Dans une deuxième publication, nous avons précisé la composition des HDL en se basant sur d'éventuelles modifications qualitatives des protéines associées à la structure des lipoprotéines. Une étude protéomique a été réalisée chez 7 patients hémodialysés versus 7 sujets sains. Nous avons retrouvé 40 protéines exprimées différemment sur les 122 identifiées, dont l'apoCII, l'apoCIII qui sont significativement augmentées et la transferrine abaissée. Ces protéines interviennent dans de nombreuses fonctions comme la réponse inflammatoire, l'activation du complément, la régulation de l'oxydation des lipoprotéines, l'homéostasie des cations.Dans une troisième publication, l'épuration du FGF23, phosphatonine impliquée dans les anomalies du métabolisme minéralo-osseux, été étudiée chez l'hémodialysé chronique en fonction de la techniques de dialyse (hémodialyse (HD) high flux versus hémodiafiltration on line (OL-HDF)). Notre étude a porté sur 53 patients dans le groupe HD et 32 patients dans le groupe OL-HDF. Dans les deux groupes le taux de FGF23 en post-dialyse est significativement plus bas qu'en pré-dialyse. Cependant, le taux de réduction, la clairance et le KT/V du FGF23 sont significativement plus bas dans le groupe OL-HDF.CONCLUSION-PERSPECTIVES : Chez l'IRC, avec l'appariation de facteurs de risque non traditionnels, de nouveaux biomarqueurs ont émergés dans la stratification du risque cardio-vasculaire. Ces biomarqueurs peuvent devenir des bioacteurs et représenter de nouvelles cibles d'action et de prévention de l'atteinte cardio-vasculaire chez l'IRC. La complexité des mécanismes physiopathologiques impliqués, nous incite à proposer des approches multimarqueurs. Actuellement des études biocliniques se poursuivent en mettant en place des cohortes régionales de patients aux stades 1 à 5 et de patients incidents en dialyse
BACKGROUND: Chronic kidney disease (CKD) is a public health problem because of its increasing prevalence. Despite care improvements, the mortality rate remains higher compared to general population. Among causes of death, cardiovascular diseases with multifactorial origins (enlargement and hypertrophy of arteries, atherosclerosis, vascular and valvular calcifications) are in the foreground. Besides the traditional risk factors, non-traditional factors associated with metabolic disorders in CKD were bring out, such as inflammation, malnutrition, oxidative stress, mineral and bone disorder. A better knowledge of vasculopathy physiopathology in CKD allows the emergence of new biomarkers to stratify cardiovascular risk in CKD.AIMS-METHODOLOGY: We performed a biochemical approach to explore three components of cardiovascular risk in CKD: oxidative stress, qualitative alterations of HDL (high-density lipoprotein) and mineral and bone disorder.RESULTS: In a first publication, the superoxide anion production, according to the stage of CKD, was assessed using a chemiluminescence method. While the overproduction of reactive oxygen species is well known at the 5d stage of CKD and may be related to the dialysis procedure, there are few data in the early stages. Our study included 136 non-dialysis patients at stages 1 to 5 of CKD. Results showed an enhanced superoxide production at the pre-dialysis phase, stages 4 and 5 of CKD. Reduced glomerular filtration rate (MDRD <30 ml / min / 1.73m2), inflammation (fibrinogène≥3.7g / l) and abnormal levels of HDL (<1.42mM and ≥1.75mM) appears as main determinants of oxidative stress in non-dialysis CKD patients.While in general population, a low HDL rate is recognized as an important risk factor, we showed (publication 1) that abnormal levels of HDL, low as high, were independently associated with oxidative stress in CKD subjects. In a second publication, we have defined the HDL composition based on qualitative changes in the structure of proteins associated with lipoproteins. A proteomic study was performed in 7 patients on hemodialysis versus 7 healthy subjects. We found 40 proteins differently expressed on the 122 identified, including apoCII, apoCIII which are significantly increased and transferrin lowered. These proteins are involved in many functions such as inflammatory response, complement activation, regulation of lipoprotein oxidation and homeostasis cations. In a third publication, the removal of FGF23, phosphatonin involved in mineral and bone metabolism, was studied in chronic hemodialysis according to the dialysis techniques (high flux hemodialysis (HD) versus on line hemodiafiltration (OL- HDF)). Our study included 53 patients in the HD group and 32 patients in the OL-HDF group. In both groups the rate of FGF23 in post-dialysis was significantly lower than in pre-dialysis. However, rate of reduction, clearance and KT / V of FGF23 were significantly lower in the OL-HDF group.CONCLUSION-PROSPECTS: In the IRC, with the appearance of non traditional risk-factors, new biomarkers have emerged in the stratification of cardiovascular risk. These biomarkers can become bioactors and represent novel targets of action and prevention in the cardiovascular disease in CKD. The complexity of the involved physiopatholological mechanisms, leads us to propose multimarkers approaches. Currently bioclinical studies continue with the constitution of regional cohorts of patients at stages 1 to 5 of CKD and incident dialysis
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7

Adim, Mahieddine. "Modèles continûment stratifiés et systèmes multi-couches pour les écoulements géophysiques". Electronic Thesis or Diss., Université de Rennes (2023-....), 2024. http://www.theses.fr/2024URENS026.

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Dans cette thèse, nous établissons rigoureusement des ponts entre les écoulements continument stratifiés et les écoulements multi-couches. Dans une première partie, nous considérons le système de Saint-Venant multi-couches avec un terme supplémentaire diffusif qui a un effet régularisant, dont la motivation provient des travaux des océanographes Gent & McWilliams sur le mélange isopycnal et la diffusivité des tourbillons, et qui pourrait être interprété comme un terme de turbulence. En exploitant la structure de ce système, nous obtenons un dictionnaire qui nous permet d'interpréter ce système multi-couches comme une discrétisation de la formulation en coordonnées isopycnales du système hydrostatique continument stratifié avec le terme diffusif de Gent & McWilliams ajouté de manière similaire. Nous montrons la convergence de la solution discrète vers la solution continue à mesure que le nombre de couches tend vers l'infini, et nous fournissons un taux de convergence explicite. Dans une deuxième partie, dans cette thèse, nous abordons la limite "inverse", nous montrons rigoureusement que, sous certaines conditions d'hyperbolicité et dans un cadre topologique bien choisi, la solution du système continument stratifié converge vers le système de Saint-Venant bi-couches dans la limite de stratification nette
In this thesis, we rigorously establish bridges between continuously stratified flows and multi-layer flows. In the first part, we consider the multi-layer shallow water system with an additional diffusive term that has a regularizing effect, motivated by the work of oceanographers Gent & McWilliams on isopycnal mixing and eddy diffusivity, which can be interpreted as a turbulence term. By exploiting the structure of this system, we derive a dictionary that allows us to interpret this multi-layer system as a discretization of the formulation in isopycnal coordinates of the continuously stratified hydrostatic system with the Gent & McWilliams diffusive term added in a similar manner. We demonstrate the convergence of the discrete solution to the continuous solution as the number of layers tends to infinity, and we provide an explicit convergence rate. In the second part of this thesis, we address the "inverse" limit. We rigorously show that, under certain hyperbolicity conditions and within a well-chosen topological framework, the solution of the continuously stratified system converges to the bi-layer shallow water system in the limit of sharp stratification
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8

Alberti, Lionel. "Propriétés Quantitatives des Singularités des Variétés Algébriques Réelles". Phd thesis, Nice, 2008. http://www.theses.fr/2008NICE4064.

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La section 2 explique une procédure de subdivision triangulant une courbe algébrique réelle plane. Les outils mathématiques sont le degré topologique, alias l'application de Gauss, ainsi que la représentation des polynômes dans la base de Bernstein, le tout dans une méthode de subdivision très rapide et certifiée. La section 3 présente une théorie de la mesure de la transversalité à une application semi-algébrique non nécessairement lisse. Il en découle: une version quantitative du théorème de trivialité topologique de Thom-Mather, une version ``métriquement stable'' du théorème de structure conique local et de l'existence d'un ``tube de Milnor'' autour des strates. Un algorithme de triangulation utilisant des partitions de Voronoi (sa mise en place n'est pas complète car l'estimation effective de la transversalité n'est pas complètement traité)La section 4 présente une borne sur le nombre générique de composantes connexes dans une section d'un germe analytique réel par un espace affine en fonction de la multiplicité et de la dimension de l'espace. Ces deux paramètres ne suffisent pas toujours à borner le nombre de composantes connexes. Le résultat est donc prouvé sous certaines conditions, dont on prouve la minimalité
Section 2 explains a subdivision procedure triangulating an algebraic plane curve. The mathematical tools are the topological degree, alias Gauss's application, the representation of polynomials in the Bernstein basis, all of it wrapped up in a subdivision very fast and certified subdivision method. Section 3 presents a quantitative theory for measuring transversality to a semi-algebraic map (not necessarily smooth). Stem from it: A quantitative version of Thom-Mather's topological triviality theorem, A ``metrically stable'' version of the local conic structure theorem and of the existence of a ``Milnor tube'' around strata. An triangulation algorithm based on Voronoi partitions (not completely implementable because the effective estimation of transversality is not completely detailed)Section 4 presents a bound on the generic number of connected components in an affine section of a real analytic germ in terms of the multiplicity and of the dimension of the ambient space. These two parameters are not always enough to bound the number of connected components. The result is thus proved under some conditions which are shown to be minimal
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9

Nagata, Koji. "TURBULENCE STRUCTURE AND TRANSPORT PHENOMENA IN DENSITY STRATIFICATION". Kyoto University, 2000. http://hdl.handle.net/2433/151546.

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10

Westlake, P. C. "Interfacial and internal waves generated by a submerged prolate spheroid". Thesis, University of Southampton, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.242629.

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11

Klaiber, Andreas [Verfasser]. "On the Spectral Stability of Internal Solitary Waves in Fluids with Density Stratification / Andreas Klaiber". Konstanz : Bibliothek der Universität Konstanz, 2013. http://d-nb.info/1045840556/34.

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12

Lee, Allison Marie. "An Experimental and Theoretical Investigation of Internal Wave Kinetic Energy Density in Variable Stratifications". BYU ScholarsArchive, 2019. https://scholarsarchive.byu.edu/etd/7737.

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Internal waves are generated in a fluid if the density increases continuously with depth. The variation in density with depth, or stratification, defines the natural frequency of the fluid N. Two common examples of stratified fluids are the ocean and atmosphere; internal waves are generated continuously in both mediums. Although there are many internal wave generation mechanisms, one common and frequently studied method is tidal flow over oceanic bathymetry. If the local natural frequency of the water near the topography is greater than the tidal frequencyω, internal waves will be generated by the tidal flow over the topography. If N=ω, only evanescent waves will be formed. Unlike internal waves, evanescent waves decay rapidly as they move vertically away from their generation site. As evanescent waves pass from an evanescent region (N=ω),through a turning depth (N=ω) and into a propagating region (N=ω), they become propagating internal waves. Because internal waves can propagate energy across large distances, they play an important role in oceanic mixing and the overall energy budget of the ocean. Knowing where these waves are formed from evanescent waves and their corresponding energy improves understanding of the impact on their surrounding area.Kinetic energy density of evanescent and internal waves formed from oscillatory flow over topography in evanescent regions is first estimated using synthetic schlieren experiments and a novel linear theory model. Experiments were performed with two Gaussian topographies in an exponential density profile. The linear theory model, which uses a set of equations that links the evanescent and propagating regions with the Airy function to overcome the discontinuity inherent with a turning depth, was compared to the experiments. Both methods showed that increasing Fr1,the strength of the evanescent region relative to the excitation frequency, causes the propagating kinetic energy to decrease. In addition, kinetic energy decreased with increasing distance between the topography and the turning depth. Because the model does not account for non-linearities such as turbulence generation, it regularly overestimates propagating kinetic energy relative to the experiments. After comparing the model with synthetic schlieren experiments, it was used to estimate that 25% of the evanescent wave energy generated by an oceanic topography located at 15◦N, 130◦E can become propagating wave energy.The influence of topography shape and fluid density profile on kinetic energy density was also explored through a combination of experiments, a linear theory model, and numerical simulations. From numerical simulations, kinetic energy can be directly calculated with the velocity pro-file and indirectly with the density perturbation field, in the same manner as the synthetic schlieren experiments. Average propagating internal wave kinetic energy (KE∗ 2) as a function of Fr1D/H,which combines Fr1 with the relative distance between the topography and the turning depth D/H,was compared for all methods. KE∗ 2 decreases with increasing Fr1D/H for all methods. Also, far from the turning depth, the direct and indirect simulations indicate similar kinetic energy when in the propagating region, where a distance from the turning depth can be quantified based on N and ω. This work was expanded to include a medium Gaussian, steep Gaussian, sinusoidal, and complex topography with two layer linear, parabolic, cubic, and exponential density profiles to investigate the validity of assuming an average natural frequency in the evanescent region and the impact of the topographic slope on KE∗ 2. A comparison of the density profiles indicated that using a two layer linear density profile has similar results compared to the other density profiles for estimating KE∗ 2 as a function of Fr1D/H. Also, KE∗ 2 is non-negligible for Fr1D/H<4. Increasing the maximum slope of a topography shape decreases the kinetic energy of the generated internal waves, though it was found that the energy is dependent upon the actual shape of the topography as well.Particle image velocimetry (PIV) experiments were performed and compared to synthetic schlieren (SS). While SS experiments generally resulted in an overestimate of kinetic energy relative to the PIV results, the trends from each experimental method matched well. It is recommended that SS be used in regions away from turning depths, but that either are valid in the evanescent and propagating regions. PIV methods should be used when results near the turning depth or the topography are desired.
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Saevarsdottir, Thorbjoerg. "The structural, serviceability and durability performance of variable density concrete panels". Thesis, University of Canterbury. Civil Engineering, 2008. http://hdl.handle.net/10092/1237.

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Conventional concrete is a poor insulating material but has good thermal mass, while lightweight concrete provides good insulation at the price of thermal mass. Precast concrete wall systems have not been widely used in residential homes due to poor thermal and acoustic performance, despite being high quality products that are easy to construct. The variable density concrete panel was designed to combine good thermal storage, insulation and high quality precast concrete. It is produced from a single concrete mix which is vibrated to get a lightweight top layer and a normal/heavyweight bottom layer. The lightweight layer is the wall exterior, having low thermal conductivity providing good thermal insulation while the normal/heavyweight layer is the dense wall interior, having high specific heat to provide good thermal mass and sufficient strength for construction handling and to withstand service loads. The intention of this research was to estimate the hardened performance; that is the structural, serviceability and durability performance of the variable density concrete panel. Further developments to the mix design were made where the fresh properties were measured and thermal performance estimated on hardened specimens. Most of the major technical concerns were proved not being as severe as first thought, making the production of variable density concrete panels promising. To ensure that the variable density concrete would stratify, the concrete mix had to have defined fresh properties. Defined rheological ranges gave a good indication of the stratification potential, but the degree of stratification was also found to be dependent on the intensity and time of vibration. Slump flow had to be within a certain range to achieve good stratification but this alone did not guarantee stratification. Variable density concrete was found to have adequate strength capacity both in axial compression and in tension for likely service loads but the strength required to withstand handling loads at early ages was not assessed. The strength of the variable density concrete was found to be affected by several factors such as; degree of stratification, relative strength and thickness of the layers, curing environment and amount of defects. As the stratification of the concrete increased the thermal insulation improved whereas the strength decreased. Warping was found not to significantly affect the serviceability of panels despite differential shrinkage within the element. The amount of warping was mainly related to the degree of stratification. Warping decreased with better stratification as more stress and strain was relieved in the lightweight layer. The lightweight concrete was significantly weaker as well as being less stiff than the structural concrete and therefore creeps to follow the structural concrete. The thermal properties aimed for were generally not reached, but these mixes were not designed to optimise the thermal performance and were tested before the concrete was fully dried. This increased thermal conductivity and therefore reduced the measured R-values. Stratified concrete had good absorption resistance, poor permeability properties and was highly porous. If the concrete was over-vibrated it tended to have a rough surface finish that would require a coating. Delamination of the panels was not assessed in this research but is a likely mode of failure.
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14

Smith, Alyson Rae. "Designing density". Thesis, Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/28147.

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15

Pinaud, Johan. "Etude expérimentale d'anneaux tourbillonnaires impactant une stratification : dynamique tourbillonnaire et génération d'ondes internes". Thesis, Toulouse 3, 2020. http://www.theses.fr/2020TOU30271.

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Cette thèse porte sur l'interaction entre des structures tourbillonnaires modèles et observées dans de nombreux types d'écoulement, les anneaux tourbillonnaires, et un milieu linéairement stratifié en densité. Un anneau tourbillonnaire constitué d'un fluide homogène léger pénètre sous l'effet de son inertie dans une stratification. Les paramètres de contrôle sont les dimensions de l'anneau, sa vitesse de propagation, son orientation par rapport à l'axe vertical, le gradient de densité de la stratification linéaire et la densité du fluide constituant l'anneau tourbillonnaire. Aux temps courts, lors de la phase de pénétration, la distorsion des isopycnes par l'anneau tourbillonnaire est responsable de la génération de vorticité barocline. Aux temps longs, la stratification se relaxe et des ondes internes sont générées. La dynamique de l'anneau est considérablement affectée par son interaction avec le milieu stratifié et la vorticité est amenée à se réorganiser sur des échelles de temps et d'espace qui dépendent des paramètres de contrôle. Différents verrous scientifiques sont abordés au fil des chapitres. Quelle est la profondeur maximale de pénétration de l'anneau et quels sont les mécanismes précurseurs de l'arrêt de la structure tourbillonnaire avant retour en arrière ? Quelles sont les échelles de temps associées à la réorganisation tourbillonnaire et à la génération d'ondes internes ? Quelles sont les caractéristiques des ondes internes forcées par l'impact ponctuel d'une structure tourbillonnaire localisée et comment les décrire ? Quelle est l'influence de l'angle de propagation de l'anneau tourbillonnaire sur la réorganisation de l'écoulement ? Comment est-ce que l'écoulement est modifié lorsque deux anneaux tourbillonnaires sont générés consécutivement ? Existe-t-il une influence du délai de génération entre les deux anneaux tourbillonnaires sur la profondeur de pénétration et sur les ondes internes générées ? Des campagnes expérimentales où des mesures qualitatives (visualisations) et quantitatives (vélocimétrie par image de particule 2D et vélocimétrie par suivi de particules 4D) ont été réalisées pour répondre à ces questions
In this work, the interaction between model vortex structures observed in many flows, vortex rings, and a linearly stratified fluid is studied. A light homogeneous vortex ring is generated and penetrates by inertia into a density stratification. The resulting interaction depends on several control parameters which are: the dimensions of the vortex ring, its propagating speed, its orientation relative to the vertical, its initial density and the density gradient of the stratification. For short times, baroclinic vorticity is generated as the vortex ring pushes isopycnal during the penetration phase. The vortex ring dynamics is highly affected by its interaction with the stratified zone leading to reorganisation of the vorticity distribution depending on the control parameters. For long times, internal gravity waves are generated as the stratification relaxes. Several key points are adressed throughout the chapters. What is the maximum penetration depth a vortex ring can reach? What is the mechanism for the vortex ring recoil? What are the time scales of the vorticity reorganisation and generation of internal waves? What are the characteristics of the internal waves generated by the impact of such a localized fluid structure and how to quantify them? What is the influence of the angle of propagation of the vortex ring on the reorganisation of the flow? How is the flow modified when two vortex rings are launched consecutively? What is the role of the time delay between the generation of the vortex rings on the penetration depth reached and the internal waves generated? Qualitative (visualizations) and quantitative (2D-Particles Image Velocimetry and 4D-Particle Tracking Velocimetry) have been deployed to answer these questions
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16

Yadav, Rakesh Kumar [Verfasser], Ulrich [Akademischer Betreuer] Christensen, Ansgar [Akademischer Betreuer] Reiners e Julien [Akademischer Betreuer] Aubert. "Effect of density stratification on dynamos in gas planets and low-mass stars / Rakesh Kumar Yadav. Gutachter: Ansgar Reiners ; Ulrich Christensen ; Julien Aubert. Betreuer: Ulrich Christensen". Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2015. http://d-nb.info/1071991612/34.

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17

AQUINO, Eveline Pinheiro de. "Fitoplâncton como indicativo das condições oceanográficas de um arquipélago no Atlântico Equatorial (Fernando de Noronha, Brasil)". Universidade Federal de Pernambuco, 2016. https://repositorio.ufpe.br/handle/123456789/17881.

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CAPES
A dinâmica espaço-temporal do fitoplâncton está relacionada a um conjunto de variáveis ambientais. As camadas de estratificação vertical e a presença de ilhas oceânicas podem influenciar na distribuição desses organismos. Esse estudo objetivou investigar a distribuição e composição da comunidade fitoplanctônica nas águas oceânicas do arquipélago de Fernando de Noronha (Atlântico Sul Equatorial, Brasil), como um fator biológico indicativo das condições oceanográficas, evidenciando sua relação com as variáveis abióticas e sob influências da proximidade do arquipélago e da estratificação vertical, bem como registrar as populações de Ornithocercus sp. que apresentam diferentes horários e estágios de reprodução, sendo esta última uma abordagem complementar. Para tanto, as amostras da água foram coletadas ao longo de seis pontos, em escalas espacial (horizontal e vertical) e temporal, a saber: em diferentes distâncias do arquipélago, em duas posições geográficas (nordeste e sudoeste), em seis profundidades (entre 1 e 130 m), em dois períodos do dia (diurno, noturno e nictemeral) e durante dois anos (2010 e 2012). Foram coletadas as variáveis abióticas, como sendo os nutrientes inorgânicos dissolvidos, temperatura e salinidade. A clorofila a fitoplanctônica foi determinada, bem como a densidade celular, riqueza de espécies, abundância relativa, frequência de ocorrência, diversidade e uniformidade. Como resultados, horizontal e verticalmente, foram identificadas 115 espécies fitoplanctônicas, representadas essencialmente por dinoflagelados (>60% do total), seguidos por diatomáceas, cianobactérias e dictiofíceas. Foi registrado um decréscimo gradual da densidade de 1 m a 130 m e, portanto, mais elevada na camada de mistura. Por outro lado, não houve efeito significativo da estratificação para a clorofila a. Os dinoflagelados Oxytoxum gracile Schiller e O. laticeps Schiller foram dominantes, para todos os tratamentos em estudo. A alta dominância de dinoflagelados indica a estabilidade da água, devido à estratificação. Nitrato e fosfato estiveram relacionados com O. laticeps, favorecendo a sua dominância. A diversidade e composição de espécies, horizontal e verticalmente, foram homogêneas, sugerindo não ocorrer efeito de massa insular durante o período em estudo e para os tratamentos amostrais aplicados.
The spatio-temporal dynamics of phytoplankton is connected to any environmental variables. The layers of vertical stratification and the occurrence of oceanic islands can induce the distribution of these organisms. This study aimed to investigate the distribution and composition of phytoplankton in the ocean waters of Fernando de Noronha archipelago (South Atlantic, Brazil), as an indicative biological of oceanographic conditions and their relationship with abiotic variables and under archipelago's proximity influences and vertical stratification, as well as record the populations of Ornithocercus sp. under different timings and reproduction stages and this is a complementary approach. Samples of water sampled over six points in spatial (horizontal and vertical) and temporal scales: at different distances, in two geographic positions (northeast and southwest), six depths (between 1 and 130 m), different periods of the day (daytime, night and nyctemeral) and two years (2010 and 2012). The abiotic variables were sampled (dissolved inorganic nutrients, temperature and salinity). Chlorophyll a was determined, and the phytoplanktonic density, species richness, abundance, frequency of occurrence, diversity and uniformity. In horizontal and vertical scale, were identified 115 phytoplankton species, represented by dinoflagellates (> 60% of the total), followed by diatoms, cyanobacteria and dyctiophyceans. Were observed a gradual decrease in density from 1 m to 130 m and higher density in the mixing layer. Were dominant Oxytoxum gracile Schiller and O. laticeps Schiller in all samples. The high dominance of dinoflagellates indicates water stability due to stratification. On the other hand, there was no significant effect of stratification for chlorophyll a. Nitrate and phosphate were in significant relationship with O. laticeps, benefiting its dominance. The diversity and species composition, horizontal and vertical, were homogeneous, suggesting that no occur insular mass effect during the study period and for the applied sample treatments.
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18

Umbel, Matthew R. "Prediction of Turbulent Mixing at the Interface of Density Stratified, Shear Flows Using CFD". Ft. Belvoir : Defense Technical Information Center, 1998. http://handle.dtic.mil/100.2/ADA477048.

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19

Obaton, Dominique. "Etude expérimentale de la stabilité d'un courant côtier de gravité : application au courant algérien". Grenoble 1, 1994. http://www.theses.fr/1994GRE10045.

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On etudie en laboratoire la stabilite d'un courant cotier de densite, s'ecoulant a la surface d'une eau plus dense immobile. Le systeme est en rotation. Le courant a un debit constant et une vitesse uniforme a sa source. L'etude parametrique en fonction des nombres de burger bu, ekman vertical ek et du rapport d'aspect du courant a la source permettent de mettre en evidence un type d'instabilite. Lorsque le nombre d'ekman est en dessous d'une valeur critique, et quelle que soit la valeur du nombre de burger, le courant se destabilise. (cependant, les ecoulements avec les plus petits nombres d'ekman correspondent a ceux avec le plus grand rapport de l'epaisseur du courant sur la hauteur totale d'eau, et on n'a pas distingue l'effet de chacun de ces parametres. ) on observe alors un elargissement local du courant, et on mesure une legere augmentation de la couche d'eau douce pres de la paroi. Au niveau du front, un cyclone se forme a cause du cisaillement horizontal. Dans le cur du courant apparait un anticyclone. Celui-ci grandit au detriment du cyclone qui disparait. Sous l'anticyclone, l'interface s'abaisse. Anticyclone et cyclone forment un meandre. Le declenchement de l'instabilite n'est vraisemblablement pas du a une onde venant de la source ni a une onde reflechie par le deversoir. Lorsque ce phenomene survient, le nombre de burger local (calcule sur une section du courant) chute brutalement. Cependant, il ne doit pas atteindre un minimum critique pour que l'instabilite puisse exister. De plus, ce declenchement a lieu d'autant plus loin de la source et plus tard que bu est grand. Une fois que le premier meandre existe, d'autres se forment en aval et pratiquement tous en meme temps. L'ecoulement stable ou regulier est geostrophique, ce n'est plus le cas du meandre. L'instabilite ne semble pas etre quasi-geostrophique. L'analyse des bilans d'energie nous permet de conclure que le mecanisme de l'instabilite est essentiellement barotrope. Les mesures dans le courant algerien montrent qu'un evenement, ou meandre, est lie a une baisse de l'interface. Vitesse de phase, taux de croissance et distance entre les meandres obtenus dans nos experiences sont en tres bon accord avec ceux de cet ecoulement naturel. Nos resultats nous permettent donc de proposer une explication au mecanisme d'instabilite du courant algerien
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20

Sayssouk, Salim. "Contrôle du phasage de la combustion dans un moteur HCCI par ajout d’ozone : Modélisation et Contrôle". Thesis, Orléans, 2017. http://www.theses.fr/2017ORLE2065/document.

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Pour franchir les prochaines étapes réglementaires, une des solutions adoptées par les constructeurs automobiles est la dépollution à la source par des nouveaux concepts de combustion. Une piste d’étude est le moteur à charge homogène allumé par compression, le moteur HCCI. Le défi majeur est de contrôler le phasage de la combustion lors des transitions. Or, l’ozone est un additif prometteur de la combustion. La première partie de ce travail est consacrée au développement d’un modèle 0D physique de la combustion dans le moteur HCCI à l’aide d’une approche statistique basée sur une fonction de densité de probabilité (PDF) de la température. Pour cela, un modèle de variance d’enthalpie est développé. Après la validation expérimentale du modèle, il est utilisé pour développer des cartographies du moteur HCCI avec et sans ajout de l’ozone afin d’évaluer le gain apporté par cet actuateur chimique en terme de charge et régime. La deuxième partie porte sur le contrôle du phasage de combustion par ajout d’ozone. Une étude de simulation est effectuée où des lois de commandes sont appliquées sur un modèle orienté contrôle. Les résultats montrent que l’ajout d’ozone permet de contrôler cycle-à-cycle le phasage de la combustion. En parallèle, une étude expérimentale sur un banc moteur est facilitée grâce à un système d’acquisition des paramètres de combustion (Pmax, CA50) en temps réel, développé au cours de cette étude. En intégrant les lois de commande par ajout d’ozone dans le calculateur du moteur (ECU), les résultats expérimentaux montrent la possibilité de contrôler non seulement cycle-à-cycle le phasage de la combustion par ajout d’ozone lors des transitions mais aussi de stabiliser le phasage de la combustion d’un point instable
To pass the next legislator steps, one of the alternative solutions proposed for the depollution at the source by new concepts of combustion. One of proposed solution is the Homogeneous Charge Compression Ignition (HCCI) engine. The major challenge is to control combustion phasing during transitions. Ozone is promising additive to combustion. During this work, a 0D physical model is developed based on temperature fluctuations inside the combustion chamber by using Probability Density Function (PDF) approach. For this, an enthalpy variance model is developed to be used in Probability Density Function (PDF) of temperature. This model presents a good agreement with the experiments. It is used to develop HCCI engine map with and without ozone addition in order to evaluate the benefit of using ozone in extending the map in term of charge-speed. The second part deals with control the combustion phasing by ozone addition. A Control Oriented Model (COM) coupled with control laws demonstrates the possibility to control combustion phasing cycle-to-cycle. Thereafter, an experimental test bench is developed to prove this possibility. A real time data acquisition system is developed to capture combustion parameters (Pmax, CA50). By integrating control laws into Engine Control Unit (ECU), results demonstrate not only the controllability of combustion phasing cycle-to-cycle during transitions but also to stabilize it for an instable operating point
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21

Chiu, Ji-Ru, e 邱繼儒. "Large-eddy simulation of suspension-induced density stratification in turbulent flow". Thesis, 2016. http://ndltd.ncl.edu.tw/handle/26025616521645632545.

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碩士
國立臺灣大學
應用力學研究所
105
In this study, we use large-eddy simulation (LES) with a dynamic mixed model (DMM) to investigate the stratified channel flow due to sediment resuspension. The flow is driven by a constant pressure gradient. We apply a pick-up function at the bottom to simulate the erosional phenomenon. The empirical erosion rate and settling velocity are chosen as parameters to control the strength of density stratification. Based on our results, we found that if the erosion rate is greater, it enhances density stratification, suppressing the vertical transport of mass and momentum. In this case, turbulent kinetic energy decays to a certain degree. In the case of the greatest erosion rate, the turbulent kinetic energy is totally damped, and the flow eventually becomes the laminar flow. For the larger settling velocity, the time sediment is retained in water column is shorter, and the influence of density stratification is relatively weak. The analysis of Richardson number is presented. Moreover, we analyze the relation between settling velocity and sediment concentration profiles using the self-similarity analysis.
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22

Yadav, Rakesh Kumar. "Effect of density stratification on dynamos in gas planets and low-mass stars". Doctoral thesis, 2015. http://hdl.handle.net/11858/00-1735-0000-0022-5FFF-D.

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23

Wong, Haw L. "Slender ship procedures that include the effects of yaw, vortex shedding and density stratification". Thesis, 1994. http://hdl.handle.net/2429/7135.

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The accurate determination of hydrodynamic loads on moving ships is important for hull form design and optimization and structural design purposes. This is especially true at the preliminary design stage during which time quick predictions of the forces and moments acting on a ship advancing steadily with, and without, yaw would be extremely useful. In view of this, simple numerical cross-flow algorithms has been developed. The numerical procedures are based on slender body theory, which is used to convert the three dimensional problem into a series of two dimensional wavemaker problems in the plane of transverse sections, marching in small steps from the bow section towards the stem. Fluid density stratification, vortex shedding, finite water depth and nonlinear free surface effects can be allowed for in the algorithms. A procedure for handling density stratified flow has been developed and successfully used for the calculation of surface and interfacial waves created by a prolate spheroid. Vortex shedding is modelled using the discrete vortex method. A hybridization of the discrete vortex and boundary element methods is achieved and illustrated in a test case of predicting the forces acting on an oscillating flat plate. The wavemaker, with the fully nonlinear free surface conditions, is used for calculating the generated wave pattern and wavemaking resistance of a Wigley hull. The effects of finite water depth on wavemaking resistance are calculated. The hybrid boundary element-discrete vortex method is used for determining the hydrodynamic forces and moments acting on a yawed Wigley hull.
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24

Advaith, S. "Density Stratified Thermal Energy Storage System and Associated Fluid Dynamic Perturbations". Thesis, 2020. https://etd.iisc.ac.in/handle/2005/4411.

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The hunger of humankind for energy has reached unprecedented levels with the ever-rising industrialization and global population surge. We are witnessing a global economic evolution towards a clean, affordable, sustainable and reliable sources of energy which could transform our lives and the planet itself. The mismatch between the supply of solar energy which is one among the most promising renewable energy and the demand for its utilization, compel us to incorporate a solar energy storage. Such storage systems are essential in various fields including power generation sectors like power plants based on solar thermal, thermal (non-renewable) and nuclear. The study is primarily an experimental investigation of single tank sensible stratified thermal energy storage which sheds light on the experimental procedure to establish a stable and sustainable thermocline thermal energy storage. Moreover, in order to have an understanding of the causes of capacity loss in such stratified storage tanks, various studies are conducted, both numerically as well as experimentally. The performance of the TES depends on the integrity of the temperature gradient region (thermocline thickness). Mixing and spatio-temporal perturbations at the thermocline region is the foremost cause of capacity loss and the most important mechanism which destroys the stratification is vortex/plume entrainment in a thermocline-based storage tank. In a stably stratified TES, vortex entrainment occurs when a relatively cold mass of fluid is injected into the tank. Vortex-thermocline interaction creates vortices by baroclinicity, leading to entrainment and mixing. Hence any vortex entrainment in the thermocline region is critical and determines the efficiency of such thermal storage. Density stratification formed in terms of salinity as an analogy to that due to temperature as well as the effect of disturbances are studied. As a corollary, various distributors are compared numerically and tested the advantage of a novel distributor design. The work provides quality experimental data in order to meet with its inadequacy in the related literature as well as deeper understanding into the establishment of a stable and sustainable thermocline thermal energy storage
This thesis is based upon work supported in part by the India– US partnership to Advance Clean Energy-Research (PACE-R) for the Solar Energy Research Institute for India and the United States (SERIIUS), funded jointly by the U.S. Department of Energy (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, under Subcontract DE-AC36-08GO28308 to the National Renewable Energy Laboratory, Golden, Colorado) and the Government of India, through the Department of Science and Technology under Subcontract IUSSTF/JCERDC-SERIIUS/2012 dated 22nd November 2012 and DST-SERI-Project No: DST/TMC/SERI/FR/136.
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25

Martyn, Michael T., e Philip D. Coates. "Pressure Variation during Interfacial Instability in the Coextrusion of Low Density Polyethylene Melts". 2013. http://hdl.handle.net/10454/9688.

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No
Pressure variation during the coextrusion of two low density polyethylene melts was investigated. Melt streams were delivered to a die from two separate extruders to converge in a 30 degrees degrees geometry to form a two layer extrudate. Melt flow in the confluent region and die land to the die exit was observed through side windows of a visualisation cell. Stream velocity ratio was varied by control of extruder screw speeds. Layer thickness ratios producing wave type interfacial instability were quantified for each melt coextruded on itself and for the combined melts. Stream pressures and screw speeds were monitored and analysed. Wave type interfacial instability was present during the processing of the melts at specific, repeatable, stream layer ratios. Increased melt elasticity appeared to promote this type of instability. Analysis of process data indicates little correlation between perturbations in extruder screw speeds and stream pressures. The analysis did however show covariance between the individual stream pressure perturbations. Interestingly there was significant correlation even when interfacial instability was not present. We conclude that naturally occurring variation in extruder screw speeds do not perturb stream pressures and, more importantly, natural perturbations in stream pressures do not promote interfacial instability.
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26

Chen, Yuning. "Evaluation of marker density for population stratification adjustment and of a family-informed phenotype imputation method for single variant and variant-set tests of association". Thesis, 2018. https://hdl.handle.net/2144/33081.

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Whole exome sequencing (WES) data cover only 1% of the genome and is designed to capture variants in coding regions of genes. When associating genetic variations with an outcome, there are multiple issues that could affect the association test results. This dissertation will explore two of these issues: population stratification and missing data. Population stratification may cause spurious association in analysis using WES data, an issue also encountered in genome-wide association studies (GWAS) using genotyping array data. Population stratification adjustments have been well studied with array-based genotypes but need to be evaluated in the context of WES genotypes where a smaller portion of the genome is covered. Secondly, sample size is a major component of statistical power, which can be reduced by missingness in phenotypic data. While some phenotypes are hard to collect due to cost and loss to follow-up, correlated phenotypes that are easily collected and are complete can be leveraged in tests of association. First, we compare the performance of GWAS and WES markers for population stratification adjustments in tests of association. We evaluate two established approaches: principal components (PCs) and mixed effects models. Our results illustrate that WES markers are sufficient to correct for population stratification. Next, we develop a family-informed phenotype imputation method that incorporates information contained in family structure and correlated phenotypes. Our method has higher imputation accuracy than methods that do not use family members and can help improve power while achieving the correct type-I error rate. Finally, we extend the family-informed phenotype imputation method to variant-set tests. Single variant tests do not have enough power to identify rare variants with small effect sizes. Variant-set association tests have been proven to be a powerful alternative approach to detect associations with rare variants. We derive a theoretical statistical power approximation for both burden tests and Sequence Kernel Association Test (SKAT) and investigate situations where our imputation approach can improve power in association tests.
2020-11-07T00:00:00Z
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