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Thesis. 1977. M.S.--Massachusetts Institute of Technology. Dept. of Meteorology. / Microfiche copy available in Archives and Science. / Bibliography : leaves 62-65. / M.S.

In this work, a three-dimensional turbulent boundary layer experiment was set up with alternating stream-wise and span-wise pressure gradients. The pressure gradients are generated as a result of the test section wavy side wall shape. Each side had six sine waves with a trough to peak magnitude to wavelength ratio of 0.25. Boundary layer control was used so that the flow over the side walls remains attached. The mean flow velocity components, static and total pressures were measured at six plane along the stream-wise direction. The alternating mean span-wise and stream-wise pressure gradients created alternating stream-wise and span-wise vorticity fluxes, respectively, along the test section. As the flow developed downstream the vorticity created at the tunnel floor and ceiling diffused away from the wall. The vorticity components in the stream-wise and span-wise directions are strengthened due to stretching and tilting terms in the vorticity transport equations. The positive-z half of the test section contains large areas that generate positive vorticity flux in the trough region and smaller areas generating negative vorticity around the wave peak. The opposite is true for the negative-z half of the test-section. This results in a large positive stream-wise vorticity in the positive-z half and negative stream-wise vorticity in the negative-z half of the test-section. The smaller regions of opposite sign vorticity in each half tend to mix the flow such that as they diffuse away from the wall, the turbulent stresses are more uniform. Turbulent fluctuating velocity components were measured using Laser Doppler Velocimetery. Mean velocities as well as Reynolds stresses and triple velocity component correlations were measured at thirty stations along the last wave in the test section. Profiles at the center of the test section showed three dimensionality, but exhibited high turbulence intensities in the outer layer. Profiles off the test section centerline are highly three dimensional with multiple peaks in the normal stress profiles. The flow also reaches a state where all the normal stresses have equal magnitudes while the shear stresses are non-zero. Flow angles, flow gradient angles and shear stress angles show very large differences between wall values and outer layer vlaues. The shear stress angle lagged the flow gradient angle indicating non-equilibrium. A turbulent kinetic energy transport budget is performed for all profiles and the turbulence kinetic energy dissipation rate is estimated. Spectral measurements were also made and an independent estimate of the kinetic energy dissipation rate is made. These estimates agree very well with those estimates made by balancing the turbulence kinetic energy transport equation. Multiple turbulent diffusion models are compared to measured quantities. The models varied in agreement with experimental data. However, fair agreement with turbulence kinetic energy turbulent diffusion is observed. A model for the dissipation rate tensor anisotropy is used to extract estimates of the pressure-strain tensor from the Reynolds stress transport equations. The pressure-strain estimates are compared with some of the models in the literature. The comparison showed poor agreement with estimated pressure-strain values extracted from experimental data. A tentative model for the turbulent Reynolds shear stress angle is developed that captures the shear stress angle near wall behavior to a very good extent. The model contains one constant that is related to mean flow variables. However, the developed expression needs modification so that the prediction is improved along the entire boundary layer thickness. / Ph. D.

This thesis presents a multiscale model of an industrial nylon-6 leacher. We develop several models at various spatial scales and implement them together in a simplistic, efficient way to develop an overall leacher model. We solve dynamic transport differential equations using the finite-volume method and method of lines in an in-house-developed FORTRAN program. We use the ODEPACK package of ordinary differential equation (ODE) solvers to solve our system of coupled ODEs. Our multiscale model performs transport, thermodynamic, physical property, and mass-transfer calculations at a finite-volume scale. We introduce two additional scales: a mesoscale, in which we perform computational fluid dynamic (CFD) simulations, and a molecular scale. Our CFD simulations solve for turbulent properties of fluid flowing over a packed bed. We incorporate the turbulent diffusivity of the fluid into our finite-volume leacher model. We perform molecular simulations and use the conductor-like screening model-segment activity coefficient (COSMO-SAC) model to generate solubility predictions of small, cyclic oligomers in water and ε-caprolactam. Additionally, we develop an extension of COSMO-SAC to model polymer species, which we refer to as Polymer-COSMO-SAC, and apply it to solve liquid-liquid equilibrium equations. We present a unique methodology to apply COSMO-based models to polymer species, which shows reasonable results for nylon-6. Because of the computational intensity of our Polymer-COSMO-SAC liquid-liquid equilibrium algorithm, we generate pre-computed tables of equilibrium predictions that we may import into our leacher model. Our integration of multiscale models maximizes efficiency and feasibility with accuracy. We are able to use our multiscale models to estimate necessary parameters, but we need to fit two mass-transfer related parameters to industrial data. We validate our model against the plant data and find average-absolute errors in the final mass percent of ε-caprolactam and cyclic dimer in polymer chips of 25.0% and 54.7%, respectively. Several plant data sets are suspected outliers and we believe an unforeseen equilibrium limitation may cause this discrepancy. If we remove these outlying data sets, we then find average-absolute errors of 7.5% and 19.3% for ε-caprolactam and cyclic dimer, respectively. We then use our validated model to perform application and sensitivity studies to gain critical insight into the leacher's operating conditions. / Master of Science

Le rayonnement joue un rôle fondamental dans les problèmes d'incendie puisque c'est le mode dominant de transfert de chaleur entre la flamme et le milieu environnant. Il contrôle la pyrolyse, et donc la puissance de flamme, et la vitesse de croissance de l'incendie. Étudier les flammes de diffusion contrôlées par les forces de flottabilité est une première étape pour comprendre et de prédire les incendies. Le principal objectif de ce travail est de modéliser le transfert radiatif et les processus de production/destruction de la suie dans ce type de flammes. Premièrement, différents modèles de propriétés radiatives des gaz ont été comparés dans des configurations tests. Il est apparu que le modèle FSCK couplé avec le schéma de mélange de Modest et Riazzi est le meilleur compromis entre précision et temps de calcul, ce modèle étant un bon candidat pour être implémenté dans des codes CFD traitant des problèmes d'incendie. Dans un second temps, un modèle de formation/oxydation des suies semi-détaillé, considérant l'acétylène et le benzène comme précurseurs, a été validé dans des flammes de diffusion laminaires de type coflow sur une large gamme d'hydrocarbures (C1-C3) et pour différentes conditions. Ensuite, le FSCK et le modèle de formation/destruction ont été appliqués pour simuler des feux de nappe de méthane et de propane aux échelles du laboratoire et intermédiaire. Les structures de flamme prédites ainsi que les flux radiatif transférés au milieu environnant ont montré un bon accord avec les résultats expérimentaux disponibles. Finalement, les interactions entre le rayonnement et la turbulence ont été quantifiées. / The radiative heat transfer plays an important role in fire problems since it is the dominant mode of heat transfer between flames and surroundings. It controls the pyrolysis, and therefore the heat release rate, and the growth rate of the fire. In the present work a numerical study of buoyant diffusion flames is carried out, with the main objective of modelling the thermal radiative transfer and the soot formation/destruction processes. In a first step, different radiative property models were tested in benchmark configurations. It was found that the FSCK coupled with the Modest and Riazzi mixing scheme was the best compromise in terms of accuracy and computational requirements, and was a good candidate to be implemented in CFD codes dealing with fire problems. In a second step, a semi-empirical soot model, considering acetylene and benzene as precursor species for soot nucleation, was validated in laminar coflow diffusion flames over a wide range of hydrocarbons (C1-C3) and conditions. In addition, the optically-thin approximation was found to produce large discrepancies in the upper part of these small laminar flames. Reliable predictions of soot volume fractions require the use of an advanced radiation model. Then the FSCK and the semi-empirical soot model were applied to simulate laboratory-scale and intermediate-scale pool fires of methane and propane. Predicted flame structures as well as the radiant heat flux transferred to the surroundings were found to be in good agreement with the available experimental data. Finally, the interaction between radiation and turbulence was quantified.

La dynamique des plasmas de fusion par confinement magnétique dans la configuration Reversed Field Pinch (RFP) est ´étudiée en utilisant la description magnétohydrodynamique (MHD) incompressible. Une méthode pseudo-spectrale et une technique de pénalisation en volume sont utilisées pour résoudre le système d’équations dans un cylindre. Les simulations numériques montrent que la pression joue un rôle important dans la dynamique des RFP et ne peut pas être négligée. Ainsi, ß n’est plus le paramètre principal pour décrire la dynamique des RFPs mais plutôt ß’ ∇, un nouveau paramètre qui équivaut le rapport du module de gradient de pression et le module de la force de Lorentz. A un autre niveau, l’effet du changement de la section poloïdale du RFP sur la dynamique est étudié. Les simulations des écoulements RFP ayant le même nombre de Lundquist et des sections différentes (circulaire et elliptique), montrent une grande différence dans les spectres et la diffusion turbulente radiale. Finalement, les écoulements RFP sont utilisés pour étudier l’effet dynamo. Les résultats obtenus montrent que les écoulements RFP sont capables d’amplifier un champ magnétique passif qui aura une tendance à être plus non-linéaire que le champ magnétique du RFP dans les régimes turbulents. / The dynamics of magnetic fusion plasmas in the Reversed Field Pinch (RFP) configuration are studied using an incompressible magnetohydrodynamics (MHD) description. A pseudospectral method combined with a volume penalization method are used to resolve the governing equations in a straight cylinder. Numerical simulations show that the pressure effects on the RFP dynamics cannot be neglected, and thus the _ parameter is not adequate to characterize the importance of pressure in the dynamics. A new parameter, _0r , which is the ratio of the pressure gradient’s magnitude to the Lorentz force’s magnitude, is proposed to be the proper parameter to describe the RFP dynamics. Another investigated influence on the RFP dynamics is the shaping of the poloidal cross-section. Simulations of flows with the same Lundquist number and different cross-sections (circular and elliptic) show a clear change in the spectral behaviour, as well as in the radial turbulent diffusion. Finally, the RFP flows are used to study the dynamo effect. Numerical results show that RFP flows are capable of amplifying a seed magnetic field, which will have tendency to be more nonlinear than the RFP magnetic field in the turbulent regime.

Ce travail de thèse est consacré à l'étude des écoulements diphasiques dispersés turbulents gaz/gouttes et plus particulièrement à la modélisation du phénomène de dépôt de gouttes en canal horizontal, dont la compréhension et la prédiction sont essentielles pour de nombreuses applications industrielles. Les gouttes sont supposées de taille plus petite que les échelles de longueur caractéristiques de l'écoulement de gaz turbulent, avec une masse volumique grande devant celle de la phase continue, les forces qui agissent sur les gouttes se limitent ainsi à la traînée, à la poussée d'Archimède et à la gravité. Le taux de présence de la phase dispersée est suffisamment important pour tenir compte de l'influence des gouttes sur la turbulence du gaz (couplage à deux sens), mais suffisamment faible pour pouvoir négliger les collisions entre les gouttes. En écoulement horizontal, le dépôt des gouttes en paroi est piloté par deux mécanismes principaux qui agissent en parallèle : la gravité et la diffusion turbulente/vol libre. Cette physique du dépôt est déclinée en deux volets, avec une première étude à l'échelle 3D locale et une seconde étude à l'échelle système 1D. Dans chacune de ces approches, un modèle pour la vitesse de dépôt de gouttes en paroi est développé, puis validé par comparaison à des données expérimentales. Le modèle de dépôt local, établi sous l'hypothèse d'un film liquide infiniment mince et absorbant, est implanté dans le code de simulation numérique NEPTUNE_CFD, puis validé par comparaison aux données expérimentales de Namie & Ueda, qui étudient le dépôt des gouttes en canal horizontal. Une analyse des équations de transport des principales grandeurs moyennes de l'écoulement, ainsi que des transferts d'énergies entre phases, est menée afin de mettre en évidence les phénomènes de couplage et leurs influences sur la turbulence de la phase continue. Le modèle unidimensionnel, développé dans le cadre d'un besoin industriel, est implanté dans le code CATHARE-3 et est confronté aux données de l'expérience REGARD du CEA Grenoble. / Droplets dispersion and deposition in turbulent duct flows are important processes, occurring in numerous environmental and industrial applications. This work is devoted to the study of gas-droplets flows and, more particularly, the objective is to improve the droplets deposition modeling in horizontal flows. Droplets are supposed to be smaller than the Kolmogorov scale, with a density large compared to the density of the gas phase. Under these assumptions, the motion of a droplet is considered to be governed by the drag force, the buoyancy force, and the gravity. Dilute incompressible and isothermal gas-droplets flows are studied, so inter-particle collisions are neglected but two-way coupling is retained, which means that modulation of turbulence by the particles is accounted for. In horizontal flow, droplets reach the wall under the actions of the gravitational settling and the turbulent diffusion. Two approaches will be used in developing this deposition physics with a first study at the 3D local scale and a second one at the 1D scale, realized for an industrial need. For each case, a model is developed for the mean deposition velocity of the droplets, with is implemented in a numerical simulation tool and then validated by comparison to experimental data. The local deposition model is established under the assumptions that the liquid film is extremely thin and perfectly absorbing and is implemented in the Neptune_CFD code. The experience carried out by Namie & Ueda, which consist in small droplets deposition from a turbulent dispersed flow in a horizontal rectangular duct, is simulated. An analysis of the interphase transfer terms in the kinetic energy equations shows the interactions between the dispersed phase and the continuous one and the impact of these phenomena on the turbulence of the gas phase is pointed out. The 1D deposition model is developed for the CATHARE-3 code and experimental data from the REGARD facility of the CEA Grenoble are used for validation.

La dispersion atmosphérique de substances dangereuses est un évènement susceptible d’entrainer de graves conséquences. Sa modélisation est primordiale pour anticiper des situations accidentelles. L’objectif de ce travail fut de développer un modèle opérationnel, à la fois rapide et précis, prenant en compte la dispersion en champ proche sur un site industriel. L’approche développée s’appuie sur des modèles issus de l’intelligence artificielle : les réseaux de neurones et les automates cellulaires. L’utilisation des réseaux de neurones requiert l’apprentissage d’une base de données de dispersion : des simulations CFD k-ϵ dans ce travail. Différents paramètres sont évalués lors de l’apprentissage : échantillonnage et architecture du réseau. Trois méthodologies sont développées :La première méthode permet d’estimer la dispersion continue en champ libre, par réseaux de neurones seuls.La deuxième méthode utilise le réseau de neurones en tant que règle de transition de l’automate cellulaire pour le suivi de l’évolution d’une bouffée en champ libre.La troisième méthode sépare la problématique : le calcul de l’écoulement est effectué par les réseaux de neurones alors que le calcul de la dispersion est réalisé par la résolution de l’équation d’advection diffusion pour le suivi de l’évolution d’un nuage autour d’un obstacle cylindrique. La simulation de cas tests non-appris avec des simulations CFD permet de valider les méthodes développées. Les temps de calcul mis en œuvre pour réaliser la dispersion sont en accord avec la cinétique d’une situation de crise. L’application à des données réelles doit être développée dans la perspective de rendre les modèles opérationnels. / Atmospheric dispersion of hazardous materials is an event that could lead to serious consequences. Atmospheric dispersion is studied in particular in this work. Modeling of atmospheric dispersion is an important tool to anticipate industrial accidents. The objective of this work was to develop a model that is both fast and accurate, considering the dispersion in the near field on an industrial site. The approach developed is based on models from artificial intelligence: neural networks and cellular automata. Using neural networks requires training a database typical of the phenomenon, CFD k-ϵ simulations in this work. Training the neural network is carried out by identifying the important parameters: database sampling and network architecture. Three methodologies are developed:The first method estimates the continuous dispersion in free field by neural networks.The second method uses the neural network as a transition rule of the cellular automaton to estimate puff evolution in the free field.The third method divides the problem: the flow calculation is performed by the neural network and the calculation of the dispersion is realized by solving the advection diffusion equation to estimate the evolution of a cloud around a cylindrical obstacle. For the three methods, assessment of the generalization capabilities of the neural network has been validated on a test database and on unlearned cases. A comparison between developed method and CFD simulations is done on unlearned cases in order to validate them. Simulations computing time are low according to crisis duration. Application to real data should be developed to make these models operational.

Nano-particle aerosols are one of the major types of air pollutants in the urban indoor and outdoor environments. Therefore, determination of mechanisms of formation, dispersion, evolution, and transformation of combustion aerosols near the major source of this type of air pollution - busy roads and road networks - is one of the most essential and urgent goals. This Thesis addresses this particular direction of research by filling in gaps in the existing physical understanding of aerosol behaviour and evolution. The applicability of the Gaussian plume model to combustion aerosols near busy roads is discussed and used for the numerical analysis of aerosol dispersion. New methods of determination of emission factors from the average fleet on a road and from different types of vehicles are developed. Strong and fast evolution processes in combustion aerosols near busy roads are discovered experimentally, interpreted, modelled, and statistically analysed. A new major mechanism of aerosol evolution based on the intensive thermal fragmentation of nano-particles is proposed, discussed and modelled. A comprehensive interpretation of mutual transformations of particle modes, a strong maximum of the total number concentration at an optimal distance from the road, increase of the proportion of small nano-particles far from the road is suggested. Modelling of the new mechanism is developed on the basis of the theory of turbulent diffusion, kinetic equations, and theory of stochastic evaporation/degradation processes. Several new powerful statistical methods of analysis are developed for comprehensive data analysis in the presence of strong turbulent mixing and stochastic fluctuations of environmental factors and parameters. These methods are based upon the moving average approach, multi-variate and canonical correlation analyses. As a result, an important new physical insight into the relationships/interactions between particle modes, atmospheric parameters and traffic conditions is presented. In particular, a new definition of particle modes as groups of particles with similar diameters, characterised by strong mutual correlations, is introduced. Likely sources of different particle modes near a busy road are identified and investigated. Strong anti-correlations between some of the particle modes are discovered and interpreted using the derived fragmentation theorem. The results obtained in this thesis will be important for accurate prediction of aerosol pollution levels in the outdoor and indoor environments, for the reliable determination of human exposure and impact of transport emissions on the environment on local and possibly global scales. This work will also be important for the development of reliable and scientifically-based national and international standards for nano-particle emissions.