This study concerns on trapped Rossby waves and local Rossby wave packets in zonal basic flows with two or more prominent extreme values (so called multiple-jet flows). For simplicity, most of the calculations are performed on the f-plane, but an extension to the ß-plane is also discussed. Under the assumption that the basic fiow is a solution to a special type of second order ordinary differential equation, we show that then the amplitudes of stationary trapped Rossby waves are solutions to an equation of the same type. We investigate the occurrence of trapped modal waves as well as the rays of wave action radiation in a particular multiple-jet flow. Further we consider the development of the local wavenumbers of Rossby wave packets in such a flow, with and without a zonally oriented reflective boundary. lt is found that wave action can propagate in the zonal direction only when the boundary is present. Otherwise the rays of wave action radiation form a closed curve. / Diese Studie beschäftigt sich mit gefangenen Rossby Wellen und lokalen gefangenen Rossbywellenpaketen in einer zonalen Strömung mit zwei oder mehreren ausgeprägten Extrema (sog. Strömungen mit mehrfachen Jets). Der Einfachheit halber werden die meisten Berechnungen auf der f-Ebene angestellt, eine Erweiterung auf die ß-Ebene wird allerdings auch diskutiert. Wenn man annimmt, dass der Grundstrom die Lösung einer bestimmten gewöhnlichen Differentialgleichung zweiter Ordnung ist, kann man zeigen, dass die Amplituden gefangener Wellen Lösungen des gleichen Gleichungstypes sind. Wir betrachten die Bedingungen für das Auftreten gefangener modaler Wellen, als auch die Pfade der Wellenenergie-Abstrahlung in einer bestimmten Mehrfach-Jet-Strömung. Ferner untersuchen wir die Entwicklung der lokalen Wellenzahlen von Wellenpaketen in einer solchen Strömung, mit und ohne eines zonal orientierten reflektierenden Randes. Wir finden, dass sich Wellenenergie auf der f-Ebene nur dann in zonaler Richtung ausbreiten kann, falls eine solcher Rand vorhanden ist. Anderenfalls ergeben die Strahlen der Wellenenergie Ausbreitung eine geschlossene Kurve.

Two separate heat transfer problems that involve jet flows impinging on a reacting target are studied through modeling and experimentation. The first system is an ablating carbon-carbon specimen exposed to high heat fluxes from an oxy-acetylene torch which has applications in atmospheric re-entry vehicles. The second system involves the penetration of hot gases into the void space in a compartment. The fire protection stands to benefit from knowledge of this system, both in building component design and informing firefighting personnel. Both problems can be modeled as a jet flow impinging on a flat surface where hot gases from the jet lead to primarily convective heat transfer. Ablation experiments are outlined and a theoretical framework is developed. A serial inversion technique is tested for predicting the recession rate observed in the experiments. A novel inversion technique that takes advantage of parallel computing is developed to circumvent the shortcomings of the serial technique. These techniques are then compared to synthetically generated and experimental data for different data streams and error signals. Compartment-scale experiments were conducted to test hot gas penetration into void spaces. Anecdotal evidence was observed outside of the intended test section prompting further investigation into the mechanics of ignition in void spaces. A theoretical framework is established to predict possibility of ignition under varied environmental factors. A leakage-scale experiment is constructed to gain insight into conditions that result in ignition of materials in void spaces. / text

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16887号 / 工博第3608号 / 新制||工||1545(附属図書館) / 29562 / 京都大学大学院工学研究科化学工学専攻 / (主査)教授 松坂 修二, 教授 宮原 稔, 教授 大嶋 正裕 / 学位規則第4条第1項該当

Une configuration en T-jonction a été étudiée pour sa simplicité en géométrie en comparant avec d’autres mélangeurs en industrie. Plus particulièrement, j’ai effectué des simulations numériques directes avec OpenFOAM des T-jonction convergent à section circulaire et rectangulaire. Les fluides Newtonien et non-Newtonien (modèle Bird-Carreau) ont été pris en compte. Dans un premier temps, j’ai comparé mes données avec le travail expérimental de Nguyen [1] sur le T-jonction circulaire en régime deflecting. J’arrive à valider la DNS avec les données expérimentales. L’organisation des structures cohérentes sont illustrées en régime laminaire et turbulent en Newtonien et en non-Newtonien. Dans un deuxième temps, j’ai simulé deux régimes (deflecting et impinging) dans un T-jonction rectangulaire en Newtonien et en non-Newtonien. J’ai montré l’existence de structures cohérentes (par example kidney vortex) qui servent de moteur au mélange du scalaire passif propre au non-Newtonien. L’efficacité de mélange est augmentée en régime impinging par rapport au régime deflecting. Le shifting du pic de turbulence est observé uniquement en régime impinging. / For the simplicity in geometry by comparing it with other mixers in the industry, flows in T-junction configuration have been studied. More specifically, Direct Numerical Simulations is carried out using OpenFOAM on a convergent T-junction configuration with circular and rectangular cross-section. Both Newtonian and non-Newtonian fluids (Bird-Carreau model) are taken into account. Firstly, DNS data is compared to Nguyen’s experimental work on the circular T-junction at regime deflecting [1]. Good agreement between simlation and experiment is achieved. The organization of coherent structures is illustrated in laminar and turbulent for both Newtonian and non-Newtonian cases. Secondly, two flow regimes (deflecting and impinging) are simulated in a rectangular T-junction for the same Newtonian and non-Newtonian fluids. The existence of non-Newtonian coherent structures (e.g. kidney vortex ) is shown. These structures are regarded as essential mixing mechanism of passive scalar mixing. The mixing efficiency is increased in regime impinging compared to regime deflecting. The shifting of the turbulence peak is only observed in regime impinging.

This PhD work was motivated by the CEA R&D program to provide solid technological basis for the use of Brayton power conversion system in Sodium-cooled Fast nuclear Reactors (SFRs). Multi-channel compact heat exchangers are necessary for the present application because of the low heat transfer capacity of the gas foreseen. In ASTRID project, a minimum size of Na channels section is required to avoid the plugging risk. However, this induces very low pressure losses in the bundle. Considering an additional inlet flow condition, a real risk of bad flow distribution remains. As a result, the thermal performance and thermal loading of the heat exchanger degrades due to it. The main goal of this work was to overcome the flow maldistribution problem by means of an innovative design of sodium distribution system (PATENT FR1657543), the development of a numerical strategy and the construction of an experimental database to validate all theoretical studies. The innovative sodium distribution system consists on an inlet header which tries to guide the evolution of the impinging jet flow while a system of bifurcating pre-distribution channels increases pressure drops in the bundle. Lateral communications between pre-distribution channels are introduced to further homogenize the flow. Two experimental facilities have been conceived to study the flow behavior in bifurcating channels and in the inlet header, respectively. At the same time, their effect on the flow distribution between channels is evaluated. The acquired PIV aerodynamic database allows to validate the numerical models and to prove the design basis for the proposed distribution system. Once having validated the CFD turbulence models and the strategy to study the flow maldistribution in the SGHE module, a decisive and trustworthy optimization of each component of the sodium distribution system has been performed. Finally, an optimal configuration has been proposed for the actual phase of ASTRID project.

This study concerns on trapped Rossby waves and local Rossby wave packets in zonal basic flows with two or more prominent extreme values (so called multiple-jet flows). For simplicity, most of the calculations are performed on the f-plane, but an extension to the ß-plane is also discussed. Under the assumption that the basic fiow is a solution to a special type of second order ordinary differential equation, we show that then the amplitudes of stationary trapped Rossby waves are solutions to an equation of the same type. We investigate the occurrence of trapped modal waves as well as the rays of wave action radiation in a particular multiple-jet flow. Further we consider the development of the local wavenumbers of Rossby wave packets in such a flow, with and without a zonally oriented reflective boundary. lt is found that wave action can propagate in the zonal direction only when the boundary is present. Otherwise the rays of wave action radiation form a closed curve. / Diese Studie beschäftigt sich mit gefangenen Rossby Wellen und lokalen gefangenen Rossbywellenpaketen in einer zonalen Strömung mit zwei oder mehreren ausgeprägten Extrema (sog. Strömungen mit mehrfachen Jets). Der Einfachheit halber werden die meisten Berechnungen auf der f-Ebene angestellt, eine Erweiterung auf die ß-Ebene wird allerdings auch diskutiert. Wenn man annimmt, dass der Grundstrom die Lösung einer bestimmten gewöhnlichen Differentialgleichung zweiter Ordnung ist, kann man zeigen, dass die Amplituden gefangener Wellen Lösungen des gleichen Gleichungstypes sind. Wir betrachten die Bedingungen für das Auftreten gefangener modaler Wellen, als auch die Pfade der Wellenenergie-Abstrahlung in einer bestimmten Mehrfach-Jet-Strömung. Ferner untersuchen wir die Entwicklung der lokalen Wellenzahlen von Wellenpaketen in einer solchen Strömung, mit und ohne eines zonal orientierten reflektierenden Randes. Wir finden, dass sich Wellenenergie auf der f-Ebene nur dann in zonaler Richtung ausbreiten kann, falls eine solcher Rand vorhanden ist. Anderenfalls ergeben die Strahlen der Wellenenergie Ausbreitung eine geschlossene Kurve.

The flow field of a twin jet impinging on ground plane with and without free-stream and at low jet-height-to-diameter ratios was investigated using the Particle Image Velocimetry (PIV) technique. Detailed, time-averaged flow field data are obtained via the high-resolution and the high-sampling rate instantaneous velocity field that is made available via the PIV technique. A model of twin jet issuing from 0.245m circular plate, with 0.019m jet exit diameter, and with jet span to diameter ratio of 3.0 is placed in a water tunnel with the jets in tandem arrangement with respect to the free-stream. The recently upgraded PIV system, in the ESM department fluid mechanics laboratory at VA-Tech, allowed us to capture instantaneous velocity field images of about 0.076m x 0.076m, at 512(H)x512(V) frame resolution. Sampling rates of 1000 and 1200 fps were employed. Understanding the flow field at lower heights is of crucial significance to the VSTOL aircraft application. Huge jet thrust is required to initiate the take-off operation due to the high lift loss encountered while the airframe is in proximity to the ground. Therefore, jet-height-to-diameter ratios of 2 and 4 were employed in this study. Jet-to-free-stream velocity ratios of 0.12, 0.18 and 0.22 were employed in addition to the no-free-stream case. In the current study, only time-averaged flow field properties were considered. These properties were extracted from the available instantaneous velocity field data. In order to provide some details in the time-averaged velocity field, the data were obtained along several planes of interrogation underneath the test model in the vicinity of the twin jet impinging flow. Images were captured in a single plane normal to the free-stream and five planes parallel to the free-stream. A vortex-like flow appears between the main jet and the fountain upwash. This flow is found to experience spiral motion. The direction of such flow spirals is found to be dependent on the jet exit height above the ground, and on the jet-to-free stream, velocity ratios. The flow spirals out towards the vortex flow periphery and upon increasing the free-stream it reverses its direction to be inward spiraling towards the core of the vortex. The flow reversal at certain height of the jet above the ground depends on the free-stream velocity. In our discussion, more emphasis is given to the case of jet-height-to-diameter ratio of two. We also found that the largest turbulent kinetic energy production rate is found to be at the fountain upwash formation zone. / Master of Science

No description available.

No description available.

Ce travail de thèse s’inscrit dans le cadre du programme R&D du CEA en support au système de conversion d’énergie à gaz du prototype industriel de Réacteur à Neutrons Rapides refroidi au Sodium (RNR-Na). Cette technologie représente une alternative aux cycles Rankine conventionnels à eau/vapeur, ayant pour avantage principal l’élimination du scenario accidentel de réaction sodium-eau. Cependant, la faible capacité de transfert de chaleur du gaz nécessite une technologie d’échangeurs compacts à plaques avec un nombre élevé de canaux à alimenter. Coté sodium, une section minimale de passage est nécessaire pour éviter le risque de bouchage par impureté. Cela induit de très faibles pertes de pression dans le faisceau qui, couplées à une condition de vitesse élevée à l’entrée, génèrent un risque réel de mauvaise distribution du débit. Les performances d’échange thermique et la tenue mécanique du composant sont alors dégradées. L’objectif principal de ce travail de thèse a été de résoudre ce problème de mauvaise distribution, en s’appuyant sur une conception innovante (BREVET FR16 57543), sur une stratégie de calcul numérique et l’établissement d’une base de données expérimentale pour la validation des travaux théoriques. Le nouveau système de distribution sodium se compose d’un collecteur d'entrée dont le design permet de guider la trajectoire du jet et d’un système de bifurcation de canaux qui augmente les pertes de pression dans le faisceau. De plus, des communications latérales entre les canaux sodium aident à homogénéiser davantage le flux. Deux installations expérimentales ont été conçues pour caractériser l'écoulement dans les canaux de bifurcation et dans le collecteur d'entrée. La conception des maquettes a permis de quantifier leur effet sur la distribution du flux entre les canaux. La base de données aérodynamiques PIV acquises a permis de valider les modèles numériques et de prouver l’efficacité du système de distribution proposé. Après avoir validé les modèles de turbulence CFD et la stratégie d'étude de la distribution dans le module SGHE, une optimisation de chaque composant du système de distribution de sodium a été réalisée. Le travail de cette thèse s’achève par la description de la conception optimale retenue pour la phase actuelle du projet ASTRID. / This PhD work was motivated by the CEA R&D program to provide solid technological basis for the use of Brayton power conversion system in Sodium-cooled Fast nuclear Reactors (SFRs). Multi-channel compact heat exchangers are necessary for the present application because of the low heat transfer capacity of the gas foreseen. In ASTRID project, a minimum size of Na channels section is required to avoid the plugging risk. However, this induces very low pressure losses in the bundle. Considering an additional inlet flow condition, a real risk of bad flow distribution remains. As a result, the thermal performance and thermal loading of the heat exchanger degrades due to it. The main goal of this work was to overcome the flow maldistribution problem by means of an innovative design of sodium distribution system (PATENT FR1657543), the development of a numerical strategy and the construction of an experimental database to validate all theoretical studies. The innovative sodium distribution system consists on an inlet header which tries to guide the evolution of the impinging jet flow while a system of bifurcating pre-distribution channels increases pressure drops in the bundle. Lateral communications between pre-distribution channels are introduced to further homogenize the flow. Two experimental facilities have been conceived to study the flow behavior in bifurcating channels and in the inlet header, respectively. At the same time, their effect on the flow distribution between channels is evaluated. The acquired PIV aerodynamic database allows to validate the numerical models and to prove the design basis for the proposed distribution system. Once having validated the CFD turbulence models and the strategy to study the flow maldistribution in the SGHE module, a decisive and trustworthy optimization of each component of the sodium distribution system has been performed. Finally, an optimal configuration has been proposed for the actual phase of ASTRID project.