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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
361

Computational fluid dynamics and analytical modeling of supersonic retropropulsion flowfield structures across a wide range of potential vehicle configurations

Cordell, Christopher E. 13 January 2014 (has links)
For the past four decades, Mars missions have relied on Viking heritage technology for supersonic descent. Extending the use of propulsion, which is required for Mars subsonic deceleration, into the supersonic regime allows the ability to land larger payload masses. Wind tunnel and computational experiments on subscale supersonic retropropulsion models have shown a complex aerodynamic flow field characterized by the interaction of underexpanded jet plumes exhausting from nozzles on the vehicle with the supersonic freestream. Understanding the impact of vehicle and nozzle configuration on this interaction is critical for analyzing the performance of a supersonic retropropulsion system, as deceleration will have components provided by both the aerodynamic drag of the vehicle and thrust from the nozzles. This investigation focuses on the validity of steady state computational approaches to analyze supersonic retropropulsion flowfield structures and their effect on vehicle aerodynamics. Wind tunnel data for a single nozzle and a multiple nozzle configuration are used to validate a steady state, turbulent computational fluid dynamics approach to modeling supersonic retropropulsion. An analytic approximation to determine plume and bow shock structure in the flow field is also developed, enabling rapid assessment of flowfield structure for use in improved grid generation and as a configuration screening tool. Results for both the computational fluid dynamics and analytic approaches show good agreement with the experimental datasets. Potential limitations of the two methods are identified based on the comparisons with available data. Six additional geometries are defined to investigate the extensibility of the analytical model and determine the variation of supersonic retropropulsion performance with configuration. These validation geometries are split into two categories: three geometries with nozzles located on the vehicle forebody at varying nozzle cant angles, and three geometries with nozzles located on the vehicle aftbody at varying nozzle cant angles and number of nozzles. The forebody nozzle configurations show that nozzle cant angle is a significant driver in performance of a vehicle employing supersonic retropropulsion. Aerodynamic drag preservation for a given thrust level increases with increasing cant angle. However, increasing the cant angle reduces the contribution of thrust to deceleration. The tradeoff between these two contributions to the deceleration force is examined, noting that performance improvements are possible with modest nozzle cant angles. Static pitch stability characteristics are investigated for the lowest and highest cant angle configurations. The aftbody nozzle configuration results show that removing the plume flow from the region forward of the vehicle results in less interaction with the bow shock structure. This impacts aerodynamic performance, as the surface pressure remains relatively undisturbed for all thrust values examined. Static pitch stability characteristics for each of the aftbody nozzle configurations are investigated; noting that supersonic retropropulsion for these configurations exhibits a transition point from static stability to instability as a function of this center of mass location along the axis.
362

Total cavopulmonary hemodynamics and the single ventricle: functional relationships and translational possibilities / Total cavopulmonary connection hemodynamics and the single ventricle: functional relationships and translational possibilities

Haggerty, Christopher Mark 22 January 2012 (has links)
Single ventricle heart defects are a rare but serious form of congenital heart disease, which affect approximately 2000 children born in the United States each year. Staged surgical palliation, culminating with the “Fontan Procedure,” is typically required to achieve adequate supply of blood to both the pulmonary and systemic circulations while avoiding chronic ventricular volume overload. This surgery reroutes the systemic veins to the pulmonary arteries, forming a total cavopulmonary connection (TCPC), to completely bypass the right side of the heart and restore a series configuration to the cardiovascular circuits. Despite improved survival through this operative course in first weeks and years of life, Fontan patients are subjected gradual attrition and decreased life expectancy through a multitude of chronic complications. It is suspected that the adverse hemodynamics of this surgically altered physiology, including those specific to the surgical TCPC, play a role in determining patient outcome. However, the small and heterogeneous patient population has hindered decisive progress and there is still not a good understanding of the optimal care strategies on a patient-by-patient basis. In recent decades, advances in medical imaging and image-based computational fluid dynamics (CFD) have redefined the realm of possibility for studying complex biomedical phenomena. Combined, these methods provide the means to create and evaluate patient-specific models of a wide range of cardiovascular structures, including the TCPC, with high fidelity. Results from these models can then be used for a wide array of different analyses, such as identifying regions of flow separation or stagnation, calculating hemodynamic power loss, or quantifying local flow distribution patterns. Through significant effort from numerous past investigators, a robust set of validated computational and image processing tools has been assembled, along with the largest library of cardiac magnetic resonance (CMR) data of TCPC anatomy and flow. These tools are leveraged in this thesis to characterize the functional implications of TCPC power loss at an unprecedented scale: we report the largest CFD analysis of patient-specific TCPC hemodynamics to date with particular focus on identifying functional correlates. Combining these data with imaging-based analysis of ventricle function, we directly compare the CFD-derived hemodynamics to the performance of the single ventricle for the first time. Motivated by the physiologic significance of these findings, the same patient-specific CFD framework is used for the translational application of prospective surgery planning for hemodynamic optimization, including the first implementation of a novel TCPC connection design hypothesized to uniquely streamline the energetic performance. We conclude with a first look at the longitudinal evolution of patient functional status to begin understanding how factors such as TCPC hemodynamics contribute to poor long-term performance in these patients.
363

Hemodynamic investigation of the liver using magnetic resonance imaging and computational fluid dynamics

George, Stephanie Marie 02 July 2008 (has links)
Cirrhosis is a leading cause of death in the United States and has severe and costly complications. Because of the clinical significance of cirrhosis, it is important that noninvasive methods be developed to detect cirrhosis early and to monitor its progression with advancing liver disease. Previous studies on portal venous hemodynamics have been performed mainly with ultrasound with mixed results. Magnetic Resonance Imaging offers several advantages over ultrasound including acquisition of both high quality anatomical and hemodynamic information. Phase-Contrast MR was used to gather velocity data for the portal venous system. Methods were developed to perform registration, segmentation and isolation of the portal vein geometries and velocity data. Computational Fluid Dynamics was also employed to further investigate the flow within the portal vein. Velocity data for the portal vein, superior mesenteric vein, splenic vein and the right or left portal vein was acquired in varying numbers for both data sets. Even with the limited number of subjects a few parameters were significant. Patients with cirrhosis had a significantly increased portal vein area and a significantly decreased average velocity per liver volume and velocity variance. Patients with cirrhosis had a significantly increased splenic vein area and average flow rate per liver volume. While these results are preliminary due to small sample size, they are promising and require further investigation and more subjects including varying stages of disease.
364

Reynolds-Averaged Navier-Stokes Simulation around Mk 48 ADCAP Torpedoes

Austen Suqi (11845943) 18 December 2021 (has links)
<p>This work utilized Pointwise and Fluent to generate a two-dimensional axisymmetric model a Mk 48 torpedo, with the intention of informing methods to reduce the turbulence, and therefore hydrodynamic noise, of the torpedo’s wake. However, this work was unable to gather data on the unsteady nature of the turbulence expected around the torpedo due to Fluent providing unrealistic results when run using a transient solver. This work shows that the transient solver computed boundary layers greater than one order of magnitude smaller than expected, and in some cases there was no change in boundary layer thickness over the torpedo’s body. The work does contain steady state solutions that were validated by first performing a grid convergence study for a flat plate. The steady state results for the flat plate and torpedo both showed the expected growth for a turbulent boundary layer. Additionally, there was a high level of convergence with the Log-Law showing that the steady state data is valid. Future work should use a transient solver to determine the characteristics of the turbulence to resolve unsteady flow from vortex shedding, wake characteristics, and any broadband or narrowband noise to develop solutions to reduce the noise made by the Mk 48.</p>
365

Multi-regime Turbulent Combustion Modeling using Large Eddy Simulation/ Probability Density Function

Shashank Satyanarayana Kashyap (6945575) 14 August 2019 (has links)
Combustion research is at the forefront of development of clean and efficient IC engines, gas turbines, rocket propulsion systems etc. With the advent of faster computers and parallel programming, computational studies of turbulent combustion is increasing rapidly. Many turbulent combustion models have been previously developed based on certain underlying assumptions. One of the major assumptions of the models is the regime it can be used for: either premixed or non-premixed combustion. However in reality, combustion systems are multi-regime in nature, i.e.,\ co-existence of premixed and non-premixed modes. Thus, there is a need for development of multi-regime combustion models which closely follows the physics of combustion phenomena. Much of previous modeling efforts for multi-regime combustion was done using flamelet-type models. As a first, the current study uses the highly robust transported Probability Density Function (PDF) method coupled with Large Eddy Simulation (LES) to develop a multi-regime model. The model performance is tested for Sydney Flame L, a piloted methane-air turbulent flame. The concept of flame index is used to detect the extent of premixed and non-premixed combustion modes. The drawbacks of using the traditional flame index definition in the context of PDF method are identified. Necessary refinements to this definition, which are based on the species gradient magnitudes, are proposed for the multi-regime model development. This results in identifying a new model parameter beta which defines a gradient threshold for the calculation of flame index. A parametric study is done to determine a suitable value for beta, using which the multi-regime model performance is assessed for Flame L by comparing it against the widely used non-premixed PDF model for three mixing models: Modified Curl (MCurl), Interaction by Exchange with Mean (IEM) and Euclidean Minimum Spanning Trees (EMST). The multi-regime model shows a significant improvement in prediction of mean scalar quantities compared to the non-premixed PDF model when MCurl mixing model is used. Similar improvements are observed in the multi-regime model when IEM and EMST mixing models are used. The results show potential foundation for further multi-regime model development using PDF model.
366

Adaptation de la modélisation hybride eulérienne/lagrangienne stochastique de Code_Saturne à la dispersion atmosphérique de polluants à l’échelle micro-météorologique et comparaison à la méthode eulérienne / Adaptation of the hybrid Eulerian/Lagrangian stochastic model of the CFD code Code_Saturne to pollutant atmospheric dispersion at the micro-meteorological scale and comparison with the Eulerian method

Bahlali, Meïssam 19 October 2018 (has links)
Cette thèse s'inscrit dans un projet de modélisation numérique de la dispersion atmosphérique de polluants à travers le code de mécanique des fluides numérique Code_Saturne. L'objectif est de pouvoir simuler la dispersion atmosphérique de polluants en environnement complexe, c'est-à-dire autour de centrales, sites industriels ou en milieu urbain. Dans ce contexte, nous nous concentrons sur la modélisation de la dispersion des polluants à micro-échelle, c'est-à-dire pour des distances de l'ordre de quelques mètres à quelques kilomètres et correspondant à des échelles de temps de l'ordre de quelques dizaines de secondes à quelques dizaines de minutes : on parle de modélisation en champ proche. L’approche suivie dans ces travaux de recherche suit une formulation hybride eulérienne/lagrangienne, où les champs dynamiques moyens relatifs au fluide porteur (pression, vitesse, température, turbulence) sont calculés via une approche eulérienne et sont ensuite fournis au solveur lagrangien. Ce type de formulation est couramment utilisé dans la littérature atmosphérique pour son efficacité numérique. Le modèle lagrangien stochastique considéré dans nos travaux est le Simplified Langevin Model (SLM), développé par Pope (1985,2000). Ce modèle appartient aux méthodes communément appelées méthodes PDF (Probability Density Function), et, à notre connaissance, n'a pas été exploité auparavant dans le contexte de la dispersion atmosphérique. Premièrement, nous montrons que le SLM respecte le critère dit de mélange homogène (Thomson, 1987). Ce critère, essentiel pour juger de la bonne qualité d'un modèle lagrangien stochastique, correspond au fait que si des particules sont initialement uniformément réparties dans un fluide incompressible, alors elles doivent le rester. Nous vérifions le bon respect du critère de mélange homogène pour trois cas de turbulence inhomogène représentatifs d'une large gamme d'applications pratiques : une couche de mélange, un canal plan infini, ainsi qu'un cas de type atmosphérique mettant en jeu un obstacle au sein d'une couche limite neutre. Nous montrons que le bon respect du critère de mélange homogène réside simplement en la bonne introduction du terme de gradient de pression en tant que terme de dérive moyen dans le modèle de Langevin (Pope, 1987; Minier et al., 2014; Bahlali et al., 2018c). Nous discutons parallèlement de l'importance de la consistance entre champs eulériens et lagrangiens dans le cadre de telles formulations hybrides eulériennes/lagrangiennes. Ensuite, nous validons le modèle dans le cas d'un rejet de polluant ponctuel et continu, en conditions de vent uniforme et turbulence homogène. Dans ces conditions, nous disposons en effet d'une solution analytique nous permettant une vérification précise. Nous observons que dans ce cas, le modèle lagrangien discrimine bien les deux différents régimes de diffusion de champ proche et champ lointain, ce qui n'est pas le cas d'un modèle eulérien à viscosité turbulente (Bahlali et al., 2018b).Enfin, nous travaillons sur la validation du modèle sur plusieurs campagnes expérimentales en atmosphère réelle, en tenant compte de la stratification thermique de l'atmosphère et de la présence de bâtiments. Le premier programme expérimental considéré dans nos travaux concerne le site du SIRTA (Site Instrumental de Recherche par Télédétection Atmosphérique), dans la banlieue sud de Paris, et met en jeu une stratification stable de la couche limite atmosphérique. La seconde campagne étudiée est l'expérience MUST (Mock Urban Setting Test). Réalisée aux Etats-Unis, dans le désert de l'Utah, cette expérience a pour but de représenter une ville idéalisée, au travers d'un ensemble de lignées de conteneurs. Deux rejets ont été simulés et analysés, respectivement en conditions d'atmosphère neutre et stable (Bahlali et al., 2018a) / This Ph.D. thesis is part of a project that aims at modeling pollutant atmospheric dispersion with the Computational Fluid Dynamics code Code_Saturne. The objective is to simulate atmospheric dispersion of pollutants in a complex environment, that is to say around power plants, industrial sites or in urban areas. In this context, the focus is on modeling the dispersion at micro-scale, that is for distances of the order of a few meters to a few kilometers and corresponding to time scales of the order of a few tens of seconds to a few tens of minutes: this is also called the near field area. The approach followed in this thesis follows a hybrid Eulerian/Lagrangian formulation, where the mean dynamical fields relative to the carrier fluid (pressure, velocity, temperature, turbulence) are calculated through an Eulerian approach and are then provided to the Lagrangian solver. This type of formulation is commonly used in the atmospheric literature for its numerical efficiency. The Lagrangian stochastic model considered in our work is the Simplified Langevin Model (SLM), developed by Pope (1985,2000). This model belongs to the methods commonly referred to as PDF (Probability Density Function) methods, and, to our knowledge, has not been used before in the context of atmospheric dispersion. First, we show that the SLM meets the so-called well-mixed criterion (Thomson, 1987). This criterion, essential for any Lagrangian stochastic model to be regarded as acceptable, corresponds to the fact that if particles are initially uniformly distributed in an incompressible fluid, then they must remain so. We check the good respect of the well-mixed criterion for three cases of inhomogeneous turbulence representative of a wide range of practical applications: a mixing layer, an infinite plane channel, and an atmospheric-like case involving an obstacle within a neutral boundary layer. We show that the good respect of the well-mixed criterion lies simply in the good introduction of the pressure gradient term as the mean drift term in the Langevin model (Pope, 1987; Minier et al., 2014; Bahlali et al., 2018c). Also, we discuss the importance of consistency between Eulerian and Lagrangian fields in the framework of such Eulerian/Lagrangian hybrid formulations. Then, we validate the model in the case of continuous point source pollutant dispersion, under uniform wind and homogeneous turbulence. In these conditions, there is an analytical solution allowing a precise verification. We observe that in this case, the Lagrangian model discriminates well the two different near- and far-field diffusion regimes, which is not the case for an Eulerian model based on the eddy-viscosity hypothesis (Bahlali et al., 2018b).Finally, we work on the validation of the model on several experimental campaigns in real atmosphere, taking into account atmospheric thermal stratification and the presence of buildings. The first experimental program considered in our work has been conducted on the `SIRTA' site (Site Instrumental de Recherche par Télédétection Atmosphérique), in the southern suburb of Paris, and involves a stably stratified surface layer. The second campaign studied is the MUST (Mock Urban Setting Test) experiment. Conducted in the United States, in Utah's desert, this experiment aims at representing an idealized city, through several ranges of containers. Two cases are simulated and analyzed, respectively corresponding to neutral and stable atmospheric stratifications (Bahlali et al., 2018a)
367

Aération pour le décolmatage dans les bioréacteurs à membranes immergées pour le traitement des eaux usées : impact sur le milieu biologique et la filtration / Aeration for fouling limitation in submerged membrane bioreactors for wastewater treatment : impact on biological media and filtration

Braak, Etienne 08 November 2012 (has links)
Cette étude présente les travaux réalisés pour comprendre l'effet de l'aération sur le milieu biologique et sur la filtration dans les bioréacteurs à membranes immergées pour une gamme de paramètres opératoires proche de celles utilisées sur stations réelles. Notre démarche fait le lien entre paramètres opératoires (débit d'aération), hydrodynamique à l'échelle macroscopique (tailles et vitesses de bulles), hydrodynamique à l'échelle locale (contraintes de cisaillement) et propriétés du milieu biologique (taille de flocs et substances polymériques extracellulaires solubles). De moins bonnes performances de filtration à plus forte aération pourraient être expliquées par une plus grande déstructuration des boues sur le court terme. Par ailleurs tout effet d'évolution des boues sur le long terme en fonction des conditions d'aération a été écarté. / This work contributes to the knowledge on aeration for fouling prevention in submerged membrane bioreactors, which represents a great part of energy consumption of the process. More precisely it aims at estimating the impact of aeration on mixed liquor properties for operational parameters range close to those used in full scale plants. Our study links operational parameters (airflow rate), hydrodynamics at macroscopic scale (bubble sizeand velocity), hydrodynamics at local scale (shear stresses), biological media properties (floc size and soluble extracellular polymeric substances), and filtration performance (transmembrane pressure variations). Hydrodynamics characterisation of two phase flow in membrane module enabled to highlight differences between air/water and air/sludge hydrodynamics with 15-25 % lower bubble velocities in sludge but one order of magnitude higher shear stress (maximal values of 10 Pa). Controlled breakdown of biological media was performed by imposing constant shear stress (range 0,1-10 Pa) to mixed liquor samples. Increase of shear induced a decrease of floc size, and soluble extracellular polymeric substances release. The comparison with shear value obtained by simulation showed that stresses induced by aeration were in the range of mixed liquor destructuration. A pilot campaign showed that wastewater had a stronger impact on the long term on mixed liquor properties, and thus filtration performances, than aeration. However higher transmembrane pressure increase rate observed on pilot for higher airflow at similar wastewater quality could be explained by stronger breakage of biological agregates on short term
368

CFD optimisation of an oscillating water column wave energy converter

Horko, Michael January 2008 (has links)
Although oscillating water column type wave energy devices are nearing the stage of commercial exploitation, there is still much to be learnt about many facets of their hydrodynamic performance. This research uses the commercially available FLUENT computational fluid dynamics flow solver to model a complete OWC system in a two dimensional numerical wave tank. A key feature of the numerical modelling is the focus on the influence of the front wall geometry and in particular the effect of the front wall aperture shape on the hydrodynamic conversion efficiency. In order to validate the numerical modelling, a 1:12.5 scale experimental model has been tested in a wave tank under regular wave conditions. The effects of the front lip shape on the hydrodynamic efficiency are investigated both numerically and experimentally and the results compared. The results obtained show that with careful consideration of key modelling parameters as well as ensuring sufficient data resolution, there is good agreement between the two methods. The results of the testing have also illustrated that simple changes to the front wall aperture shape can provide marked improvements in the efficiency of energy capture for OWC type devices.
369

Analysis of handling stresses and breakage of thin crystalline silicon wafers

Brun, Xavier F. 08 September 2008 (has links)
Photovoltaic manufacturing is material intensive with the cost of crystalline silicon wafer, used as the substrate, representing 40% to 60% of the solar cell cost. Consequently, there is a growing trend to reduce the silicon wafer thickness leading to new technical challenges related to manufacturing. Specifically, wafer breakage during handling and/or transfer is a significant issue. Therefore improved methods for breakage-free handling are needed to address this problem. An important pre-requisite for realizing such methods is the need for fundamental understanding of the effect of handling device variables on the deformation, stresses, and fracture of crystalline silicon wafers. This knowledge is lacking for wafer handling devices including the Bernoulli gripper, which is an air flow nozzle based device. A computational fluid dynamics model of the air flow generated by a Bernoulli gripper has been developed. This model predicts the air flow, pressure distribution and lifting force generated by the gripper. For thin silicon wafers, the fluid model is combined with a finite element model to analyze the effects of wafer flexibility on the equilibrium pressure distribution, lifting force and handling stresses. The effect of wafer flexibility on the air pressure distribution is found to be increasingly significant at higher air flow rates. The model yields considerable insight into the relative effects of air flow induced vacuum and the direct impingement of air on the wafer on the air pressure distribution, lifting force, and handling stress. The latter effect is found to be especially significant when the wafer deformation is large. In addition to silicon wafers, the model can also be used to determine the lifting force and handling stress produced in other flexible materials. Finally, a systematic approach for the analysis of the total stress state (handling plus residual stresses) produced in crystalline silicon wafers and its impact on wafer breakage during handling is presented. Results confirm the capability of the approach to predict wafer breakage during handling given the crack size, location and fracture toughness. This methodology is general and can be applied to other thin wafer handling devices besides the Bernoulli gripper.
370

Modélisation numérique des écoulements pulmonaires / Numerical modeling of pulmonary flow

Elmi Robleh, Hassan 10 February 2012 (has links)
L’étude engagée dans cette thèse consiste à mettre en place une modélisation numérique fiable et complète du transport et du dépôt des particules dans un écoulement pulmonaire en se basant sur l’utilisation du code de calcul commercial CFD-ACE. Ce code intègre un solveur fluide qui résout les équations de Navier-Stokes incompressibles dans une formulation volumes finis. Le logiciel CFD-GEOM a été utilisé pour créer les surfaces en 3D de la géométrie générique du modèle de Weibel et ainsi générer le maillage non-structuré tétraèdrique en volumes finis. Dans le cadre de ce travail, il est supposé que le flux d’air est laminaire, stationnaire (ou instationnaire uniquement dans les modèles bronchiques) et incompressible ; les particules de diamètre 5μm sont sphériques et sans interaction. Le pourcentage global et local du dépôt des particules dans les poumons peut s’exprimer comme une efficacité de dépôt et se définit par le rapport entre le nombre de particules déposées dans une région donnée et le nombre total de particules admises initialement à l’entrée de la conduite. L’efficacité de dépôt dépend fortement du nombre de Stokes d’entrée, des conditions d’admission en termes de profil de vitesse du fluide (nombre de Reynolds d’entrée), de la distribution et des caractéristiques des particules. Nous avons donc modélisé avec succès les écoulements ainsi que le transport et le dépôt de particules dans des configurations simples (modèles de Weibel) et des configurations réalistes (poumons de rat et du lapin) et ce que l’on en peut dire c’est que la simulation, bien que coûteuse (surtout pour le dépôt des particules), ne présente pas de difficultés insurmontables. Par contre l’obtention d’une géométrie réaliste et la génération du maillage associé reste une étape délicate. / The study undertaken in this thesis is to develop a reliable and comprehensive numerical modeling of transport and deposition of particles in pulmonary flow based on the use of CFDACE computer code. This code includes a fluid solver that solves the Navier-Stokes in a finite volume formulation. The CFD-GEOM software was used to create 3D surfaces of the geometry of the generic model of Weibel and generate the unstructured tetrahedral finite volume mesh. As part of this work, it is assumed that the airflow is laminar, steady (unsteady only in bronchial models) ; the particles of diameter 5μm are spherical and noninteracting. The percentage of global and local particle deposition in the lungs can be expressed as a deposition efficiency and is defined as the ratio between the number of particles deposited in a given area and the total number of particles initially admitted to the entrance of lungs. The deposition efficiency depends strongly on the Stokes number of entry, the airflow fluid velocity profile (Reynolds number at the inlet), the distribution and characteristics of particles. We have successfully modeled the flow, the transport and deposition of particles in simple configurations (models of Weibel), realistic configurations (lungs of rats and rabbits) and we can conclude that the simulation, al though expensive in terms of computer memory & time (especially for particle deposition), does not present insurmountable difficulties. On the other hand, obtaining a realistic geometry and mesh generation main a challenge.

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