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1
Kim, Su Jin. "3D numerical simulation of turbulent open-channel flow through vegetation." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/42892.
Повний текст джерелаАнотація:
A comprehensive understanding of the hydrodynamics in vegetated open-channels and flow-vegetation interaction is of high interest to researchers and practitioners alike for instance in the content of river and coastal restoration schemes. The focus of this study was to investigate the effect of the presence of vegetation on flow resistance, turbulence statistics, and the instantaneous flow in open channels by performing three-dimensional computational-fluid-dynamics (CFD) simulations.
Firstly, fully developed turbulent flow in fully-vegetated channel was analyzed by employing the method of high-resolution Large-Eddy Simulation (LES). Flow through a staggered array of rigid, emergent cylinders was simulated and the LES was validated through experiments. After validation, numerical simulations were performed at an extended parameter range of two different cylinder Reynolds numbers (ReD = 500 and 1340) and three different vegetation densities (φ = 0.016, 0.063, and 0.251). Flow structures and statistics were analyzed on the instantaneous flow and the effect of the vegetation density and cylinder Reynolds number was assessed. Moreover, drag forces exerted on the cylinders were calculated explicitly, and the effect of both ReD and φ on the drag coefficient was quantified. Secondly, two new alternative simulation strategies, a RANS based strategy with a vegetative closure model and a low-resolution Large-Eddy Simulation, were devised. They were evaluated by simulating several experimental cases with diverse conditions of the cylinder arrangement (i.e., staggered vs. random distribution), vegetation densities (φ = 0.016, 0.022, 0.063, 0.087, 0.091, 0.150, and 0.251), and cylinder Reynolds number (ReD = 170 - 1700). For the RANS based strategy, the importance of a-priori knowledge was assessed, and for the low-resolution LES, the efficiency and accuracy was demonstrated. Finally, a numerical strategy based on a porosity approach was developed and applied to open-channel flow through a natural plant. The simulated velocities were compared with experimentally acquired ones and results showed reasonable agreement.
The results obtained in this research contribute to the understanding of fundamental mechanism of flow-vegetation interaction in vegetated open-channels, resolving turbulent flow-vegetation interaction explicitly. In addition, the new numerical strategies developed as part of this research are expected to allow describing the behavior of turbulent flow through artificial and natural vegetation with high efficiency and accuracy.
2
Afailal, Al Hassan. "Numerical simulation of non-reactive aerodynamics in Internal Combustion Engines using a hybrid RANS/LES approach." Thesis, Pau, 2020. http://www.theses.fr/2020PAUU3028.
Повний текст джерелаАнотація:
L'aérodynamique interne est un élément fondamental pour améliorer la combustion dans les moteurs à allumage commandé. Une meilleure maitrise des écoulements internes est permise grâce aux outils de simulation CFD qui sont de plus en plus utilisés dans le processus de développement des moteurs à allumage commandé. Cette thèse avait pour objectif d’étendre l'approche hybride RANS/LES-temporelle dite HTLES, initialement dédiée pour des écoulements statistiquement stationnaires, aux écoulements moteurs avec des parois mobiles et des modes opératoires cycliques, puis de la valider dans des configurations représentatives des écoulements moteurs. Cette approche vise à modéliser les régions proches parois par approche statistique RANS et tend continûment vers la LES temporelle loin des parois si la discrétisation spatiale et temporelle est suffisamment résolue. Le formalisme temporel permet une hybridation RANS/LES consistante dans un écoulement statistiquement stationnaire, les deux méthodes se basant sur des opérateurs temporels (respectivement la moyenne temporelle et le filtrage temporel). Une première amélioration de l’approche HTLES a été proposée en ajoutant une fonction de protection qui impose le mode RANS dans la région proche paroi, indépendamment de la discrétisation locale (spatiale et temporelle). Dans les écoulements cycliques, l’approche HTLES modélise les échelles turbulentes non-résolues en se basant sur des moyennes de phase des grandeurs résolues qui sont inconnues lors de la simulation. La moyenne glissante exponentielle (EWA) a été utilisée afin d’approximer ces moyennes de phase. Une formule pour définir la largeur de la moyenne glissante a été proposée de sorte que les fluctuations turbulentes (hautes fréquences) soient filtrées des quantités résolues, tout en conservant les composantes cycliques (basses fréquences). Cette approche a été implémentée dans le code de calcul industriel CONVERGE CFD. Elle a d'abord été validée dans deux configurations stationnaires : un canal plan infini et un banc volute. A cet effet, les résultats ont été comparés aux données de référence et aux résultats RANS et LES. Dans les régions proches parois où le maillage est sous résolu pour la LES, EWA-HTLES a mieux prédit l’écoulement grâce à l'utilisation du mode RANS, permettant une meilleure prédiction des pertes de charge. La résolution des grandes échelles dans la région centrale a permis d'obtenir des prédictions aussi précises qu’une simulation LES en termes de vitesses moyennes et des fluctuations. La validation de l'EWA-HTLES a également été effectuée dans deux configurations moteurs : le tumble compressé et le moteur Darmstadt, tous deux présentant des caractéristiques aérodynamiques typiques aux moteurs à allumage commandé telles que la génération et la compression du mouvement de tumble et la variabilité cyclique. Pour chaque configuration, un nombre total de 40 cycles consécutifs simulés à l'aide de EWA-HTLES a été utilisé pour calculer les deux premiers moments statistiques. Les résultats ont été comparés aux données de la PIV, et aux résultats donnés par les simulations RANS et LES. Les résultats ont montré que le modèle développé arrive à contrôler correctement la transition RANS-LES dans des configurations complexes avec des conditions d'écoulement non stationnaires et des déformations géométriques importantes, assurant le mode RANS aux parois et la LES au centre du cylindre. La résolution des grandes échelles a permis une bonne prédiction des phénomènes instationnaires, particulièrement l'évolution des caractéristiques du mouvement de tumble et des phénomènes associés aux variabilités cycliques, tels que l'augmentation locale de vitesses fluctuantes. Les résultats de l'EWA-HTLES sont similaires à ceux prédits par la LES et meilleurs que ceux donnés par les simulations RANS. Ces résultats montrent des perspectives encourageantes pour l'application de cette méthode dans de nombreuses configurations industriellesInternal aerodynamics is a key element for improving the combustion efficiency in Spark-Ignition (SI) engines. Within this context, CFD tools are increasingly used to investigate in-cylinder flows and to support the design of fuel-efficient engines. The present research aimed at extending and validating a non-zonal hybrid Reynolds-Averaged Navier-Stokes / Temporal Large-Eddy Simulation (HTLES) approach, initially formulated for stationary flows, to cyclic SI engine flows with moving walls. The aim was to model the near-wall regions and coarse mesh regions in RANS, while solving the turbulent scales in core regions with sufficient mesh resolution using temporal LES, in a seamless approach with no a priori user input. HTLES was retained as it proposed a consistent hybridization combining time-averaging in RANS regions with temporal filtering in TLES.A first development consisted in implementing a smooth shielding function that enforces the RANS mode in near-wall regions, regardless of the local temporal and spatial resolution. The extension of HTLES to cyclic flows was then achieved via the formulation of a method allowing approximating the phase averages of resolved flow quantities based on an Exponentially Weighted Average (EWA). A dynamic expression for the width of the weighted average was proposed, in order to ensure that the high frequency turbulent fluctuations be filtered out from the resolved quantities, while keeping the low frequency cyclic components of the flow variables. The resulting EWA-HTLES model was implemented in the commercial CONVERGE CFD code. The developed EWA-HTLES model was first applied to the simulation of two steady flow configurations: a minimal turbulent channel and a steady flow rig. Predictions were confronted with reference data, as well as with those from RANS and LES. All simulations relied on the use of standard wall laws and coarse grids at walls. Imposing the RANS mode at walls yielded EWA-HTLES predictions of pressure losses much closer to DNS and experimental findings than with LES. At the same time, it allowed yielding results in terms of mean and RMS velocities s in the core regions of the same quality than LES, and superior to RANS.Finally, EWA-HTLES was applied to the simulation of two cyclic flows representative of SI engines: the compressed tumble and the Darmstadt single-cylinder pentroof 4valve engine. For each configuration, a total number of 40 consecutive cycles were simulated. The results were confronted to PIV data, and to RANS and LES predictions obtained using the same numerical set-up. It was shown that EWA-HTLES successfully drives the RANS-to-LES transition in such complex configurations exhibiting unsteady flow features and important cyclic geometrical deformations. It switched from the RANS mode at the walls to LES in the core region of the cylinder, allowing a better prediction of unsteady phenomena including the evolution of the overall tumble characteristics and phenomena associated to cyclic variability. The EWA-HTLES results were shown to be comparable to those predicted by LES, and superior to RANS.The performed developments and obtained results open encouraging perspectives for the application of this hybrid RANS/LES method in industrial configurations involving non-stationary conditions and in particular moving boundaries
3
Gorgulu, Ilhan. "Numerical Simulation Of Turbine Internal Cooling And Conjugate Heat Transfer Problems With Rans-based Turbulance Models." Master's thesis, METU, 2012. http://etd.lib.metu.edu.tr/upload/12615000/index.pdf.
Повний текст джерелаАнотація:
The present study considers the numerical simulation of the different flow characteristics involved in the conjugate heat transfer analysis of an internally cooled gas turbine blade. Conjugate simulations require full coupling of convective heat transfer in fluid regions to the heat diffusion in solid regions. Therefore, accurate prediction of heat transfer quantities on both external and internal surfaces has the uppermost importance and highly connected with the performance of the employed turbulence models. The complex flow on both surfaces of the internally cooled turbine blades is caused from the boundary layer laminar-to-turbulence transition, shock wave interaction with boundary layer, high streamline curvature and sequential flow separation. In order to discover the performances of different turbulence models on these flow types, analyses have been conducted on five different experimental studies each concerned with different flow and heat transfer characteristics. Each experimental study has been examined with four different turbulence models available in the commercial software (ANSYS FLUENT13.0) to decide most suitable RANS-based turbulence model. The Realizable k-&epsilonmodel, Shear Stress Transport k-&omega
model, Reynolds Stress Model and V2-f model, which became increasingly popular during the last few years, have been used at the numerical simulations. According to conducted analyses, despite a few unreasonable predictions, in the majority of the numerical simulations, V2-f model outperforms other first-order turbulence models (Realizable k-&epsilon
and Shear Stress Transport k-&omega
) in terms of accuracy and Reynolds Stress Model in terms of convergence.
4
Tristanto, Indi Himawan. "A mesh transparent numerical method for large-eddy simulation of compressible turbulent flows." Thesis, Loughborough University, 2004. https://dspace.lboro.ac.uk/2134/12128.
Повний текст джерелаАнотація:
A Large Eddy-Simulation code, based on a mesh transparent algorithm, for hybrid unstructured meshes is presented to deal with complex geometries that are often found in engineering flow problems. While tetrahedral elements are very effective in dealing with complex geometry, excessive numerical diffusion often affects results. Thus, prismatic or hexahedral elements are preferable in regions where turbulence structures are important. A second order reconstruction methodology is used since an investigation of a higher order method based upon Lele's compact scheme has shown this to be impractical on general unstructured meshes. The convective fluxes are treated with the Roe scheme that has been modified by introducing a variable scaling to the dissipation matrix to obtain a nearly second order accurate centred scheme in statistically smooth flow, whilst retaining the high resolution TVD behaviour across a shock discontinuity. The code has been parallelised using MPI to ensure portability. The base numerical scheme has been validated for steady flow computations over complex geometries using inviscid and RANS forms of the governing equations. The extension of the numerical scheme to unsteady turbulent flows and the complete LES code have been validated for the interaction of a shock with a laminar mixing layer, a Mach 0.9 turbulent round jet and a fully developed turbulent pipe flow. The mixing layer and round jet computations indicate that, for similar mesh resolution of the shear layer, the present code exhibits results comparable to previously published work using a higher order scheme on a structured mesh. The unstructured meshes have a significantly smaller total number of nodes since tetrahedral elements are used to fill to the far field region. The pipe flow results show that the present code is capable of producing the correct flow features. Finally, the code has been applied to the LES computation of the impingement of a highly under-expanded jet that produces plate shock oscillation. Comparison with other workers' experiments indicates good qualitative agreement for the major features of the flow. However, in this preliminary computation the computed frequency is somewhat lower than that of experimental measurements.
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Manickam, Bhuvaneswaran [Verfasser]. "Numerical Modelling and Simulation of Hydrogen Enriched Premixed Turbulent Flames with RANS and LES Approaches / Bhuvaneswaran Manickam." München : Verlag Dr. Hut, 2012. http://d-nb.info/1022535161/34.
Повний текст джерела
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Nikolaou, Zacharias M. "Study of multi-component fuel premixed combustion using direct numerical simulation." Thesis, University of Cambridge, 2014. https://www.repository.cam.ac.uk/handle/1810/245278.
Повний текст джерелаАнотація:
Fossil fuel reserves are projected to be decreasing, and emission regulations are becoming more stringent due to increasing atmospheric pollution. Alternative fuels for power generation in industrial gas turbines are thus required able to meet the above demands. Examples of such fuels are synthetic gas, blast furnace gas and coke oven gas. A common characteristic of these fuels is that they are multi-component fuels, whose composition varies greatly depending on their production process. This implies that their combustion characteristics will also vary significantly. Thus, accurate and yet flexible enough combustion sub-models are required for such fuels, which are used during the design stage, to ensure optimum performance during practical operating conditions. Most combustion sub-model development and validation is based on Direct Numerical Simulation (DNS) studies. DNS however is computationally expensive. This, has so far limited DNS to single-component fuels such as methane and hydrogen. Furthermore, the majority of DNS conducted to date used one-step chemistry in 3D, and skeletal chemistry in 2D only. The need for 3D DNS using skeletal chemistry is thus apparent. In this study, an accurate reduced chemical mechanism suitable for multi-component fuel-air combustion is developed from a skeletal mechanism. Three-dimensional DNS of a freely propagating turbulent premixed flame is then conducted using both mechanisms to shed some light into the flame structure and turbulence-scalar interaction of such multi-component fuel flames. It is found that for the multi-component fuel flame heat is released over a wider temperature range contrary to a methane flame. This, results from the presence of individual species reactions zones which do not all overlap. The performance of the reduced mechanism is also validated using the DNS data. Results suggest it to be a good substitute of the skeletal mechanism, resulting in significant time and memory savings. The flame markers commonly used to visualize heat release rate in laser diagnostics are found to be inadequate for the multi-component fuel flame, and alternative markers are proposed. Finally, some popular mean reaction rate closures are tested for the multi-component fuel flame. Significant differences are observed between the models’ performance at the highest turbulence level considered in this study. These arise from the chemical complexity of the fuel, and further parametric studies using skeletal chemistry DNS would be useful for the refinement of the models.
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Sinha, Nityanand. "Towards RANS Parameterization of Vertical Mixing by Langmuir Turbulence in Shallow Coastal Shelves." Scholar Commons, 2013. http://scholarcommons.usf.edu/etd/4945.
Повний текст джерелаАнотація:
Langmuir turbulence in the upper ocean is generated by the interaction between the wind-driven shear current and the Stokes drift velocity induced by surface gravity waves. In homogenous (neutrally stratified) shallow water, the largest scales of Langmuir turbulence are characterized by full-depth Langmuir circulation (LC). LC consists of parallel counter-rotating vortices aligned roughly in the direction of the wind. In shallow coastal shelves, LC has been observed engulfing the entire water column, interacting with the boundary layer and serving as an important mechanism for sediment re-suspension.
In this research, large-eddy simulations (LES) of Langmuir turbulence with full-depth LC in a wind-driven shear current have revealed deviations from classical log-layer dynamics in the surface and bottom of the water column. For example, mixing due to full-depth LC induces a large wake region eroding the classical bottom (bed) log-law velocity profile. Meanwhile, near the surface, Stokes drift shear serves to intensify small scale eddies leading to enhanced mixing and disruption of the surface velocity log-law.
The modified surface and bottom log-layer dynamics induced by Langmuir turbulence and full-depth LC have important implications on Reynolds-averaged Navier-Stokes simulations (RANSS) of the general coastal ocean circulation. Turbulence models in RANSS are typically calibrated under the assumption of log-layer dynamics, which could potentially be invalid during occurrence of Langmuir turbulence and associated full-depth LC. A K-Profile Parameterization (KPP) of the Reynolds shear stress in RANSS is introduced capturing the basic mechanisms by which shallow water Langmuir turbulence and full-depth LC impact the mean flow. Single water column RANS simulations with the new parameterization are presented showing good agreement with LES
8
Khosravi, Rahmani Ramin. "THREE-DIMENSIONAL NUMERICAL SIMULATION AND PERFORMANCE STUDY OF AN INDUSTRIAL HELICAL STATIC MIXER." See Full Text at OhioLINK ETD Center (Requires Adobe Acrobat Reader for viewing), 2004. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=toledo1103149825.
Повний текст джерелаАнотація:
Dissertation (Ph.D.)--University of Toledo, 2004.Typescript. "A dissertation [submitted] as partial fulfillment of the requirements of the Doctor of Philosophy degree in Engineering." Bibliography: leaves 323-340.
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Kumar, Vivek Mohan. "3D Numerical Simulation to Determine Liner Wall Heat Transfer and Flow through a Radial Swirler of an Annular Turbine Combustor." Thesis, Virginia Tech, 2013. http://hdl.handle.net/10919/51949.
Повний текст джерелаАнотація:
RANS models in CFD are used to predict the liner wall heat transfer characteristics of a gas turbine annular combustor with radial swirlers, over a Reynolds number range from 50,000 to 840,000. A three dimensional hybrid mesh of around twenty five million cells is created for a periodic section of an annular combustor with a single radial swirler. Different turbulence models are tested and it is found that the RNG k-e model with swirl correction gives the best comparisons with experiments. The Swirl number is shown to be an important factor in the behavior of the resulting flow field. The swirl flow entering the combustor expands and impinges on the combustor walls, resulting in a peak in heat transfer coefficient. The peak Nusselt number is found to be quite insensitive to the Reynolds number only increasing from 1850 at Re=50,000 to 2200 at Re=840,000, indicating a strong dependence on the Swirl number which remains constant at 0.8 on entry to the combustor. Thus the peak augmentation ratio calculated with respect to a turbulent pipe flow decreases with Reynolds number. As the Reynolds number increases from 50,000 to 840,000, not only does the peak augmentation ratio decrease but it also diffuses out, such that at Re=840,000, the augmentation profiles at the combustor walls are quite uniform once the swirl flow impinges on the walls. It is surmised with some evidence that as the Reynolds number increases, a high tangential velocity persists in the vicinity of the combustor walls downstream of impingement, maintaining a near constant value of the heat transfer coefficient. The computed and experimental heat transfer augmentation ratios at low Reynolds numbers are within 30-40% of each other.Master of Science
10
Zanette, Jerônimo. "Hydroliennes à flux transverse : contribution à l’analyse de l’interaction fluide-structure." Grenoble INPG, 2010. http://www.theses.fr/2010INPG0161.
Повний текст джерелаАнотація:
Cette thèse a été réalisée dans le cadre du Projet HARVEST, programme d'études initié au Laboratoire LEGI de Grenoble visant la production d'électricité à partir d'un concept original d'hydrolienne. Au sein du Projet HARVEST, ce travail constitue une contribution à l'analyse de l'interaction fluide-structure, appuyée sur des outils de simulation numérique disponibles. Une démarche progressive a été mise en place. L'étude porte ainsi tout d'abord sur des configurations bidimensionnelles représentant une coupe transversale de la géométrie réelle. Des géométries tridimensionnelles simplifiées, incluant quelques composants de la turbine, sont ensuite analysées. Enfin, dans la dernière partie de ce manuscrit, le rendement hydrodynamique et les caractéristiques mécaniques d'une géométrie complète de turbine à flux transverse sont présentés. Ce mémoire est clôturé par des conclusions d'ordre méthodologique et technologique des travaux présentésThe general context of the present study is the HARVEST Project, research program initiated at LEGI Laboratory in Grenoble devoted to the development of an original concept of cross-flow water turbine allowing to harness the kinetic energy of rivers and oceans streams. Within the HARVEST Project, this thesis is an important contribution to the analysis of fluid-structure interaction, based on available numerical simulation tools. A gradual approach was implemented. The study is primarily performed on two-dimensional configuration representing a cross section of the real geometry. Simplified three-dimensional geometries, including some components of the turbine, are analyzed after. Finally, in the last part of this manuscript, the hydrodynamic performance and mechanical characteristics of a complete geometry of cross-flow water turbine are presented. This thesis is concluded with methodological and technological considerations
11
Gougeon, Pierre. "Interactions aérodynamiques entre une turbine haute pression et le premier distributeur basse pression." Thesis, Ecully, Ecole centrale de Lyon, 2014. http://www.theses.fr/2014ECDL0026/document.
Повний текст джерелаАнотація:
L’amélioration des performances des turboréacteurs actuels est un enjeu crucial dans un contexte de contraintes économiques et environnementales fortes. Au sein du turboréacteur, le canal inter-turbines, localisé à l’interface entre la turbine Haute Pression (HP) et le premier distributeur Basse Pression (BP), est le siège d’écoulements très complexes. Ainsi, les structures aérodynamiques issues de la turbine HP (sillages, tourbillons et ondes de choc) interagissent fortement entre elles et impactent l’écoulement du distributeur BP, engendrant ainsi des pertes de rendement de l’ensemble de la configuration. Ce travail de thèse s’attache à étudier les phénomènes d’interactions aérodynamiques entre une turbine HP et le premier distributeur BP et à analyser les mécanismes à l’origine des pertes aérodynamiques dans le distributeur BP. Une campagne expérimentale antérieure, réalisée sur un banc d’essai comprenant une turbine HP couplée à un distributeur BP, avait permis de recueillir des mesures de l’écoulement dans des plans situés dans le canal inter-turbines et à l’aval du distributeur BP. En lien avec ces résultats expérimentaux, les simulations numériques menées dans cette étude avec le logiciel elsA s’attachent à restituer précisément la nature tridimensionnelle, instationnaire et turbulente de l’écoulement au sein de cette même configuration. Ces travaux se développent alors en trois étapes principales. Dans un premier temps, une étude stationnaire avec traitement plan de mélange permet de comprendre et quantifier les aspects généraux de l’écoulement. Une évaluation de l’effet de la modélisation turbulente RANS (Reynolds-Averaged Navier-Stokes) et du schéma numérique spatial sur les structures aérodynamiques présentes dans la configuration est réalisée. Dans un deuxième temps, une modélisation turbulente avancée de type ZDES (Zonal Detached-Eddy Simulation) est employée pour la résolution de l’écoulement dans le distributeur BP. Les structures aérodynamiques instationnaires issues de la roue HP amont sont modélisées par une condition limite à l’entrée du domaine de calcul. L’approche ZDES est comparée à une approche Unsteady RANS (URANS) sur la même configuration. La formation et la dissipation des sillages et des tourbillons est significativement différente entre les deux modélisations, ce qui impacte de manière importante la génération des pertes aérodynamiques. Enfin, des simulations URANS de plusieurs configurations permettent de mieux comprendre les effets d’interaction entre les différentes rangées d’aubes. Ainsi, les approches instationnaires chorochroniques prenant en compte un seul rotor et un seul stator évaluent des effets instationnaires importants dans le canal inter-turbines. Ces approches conduisent à la mise en oeuvre d’un calcul sur une configuration multipassages-chorochronique prenant en compte les deux stators et le rotor afin de modéliser complètement les interactions déterministes existantes. Afin de quantifier celles-ci avec précision, une décomposition modale du champ instationnaire est mise en place. Les niveaux d’interactions liées aux différentes roues sont alors quantifiés et l’impact sur les pertes aérodynamiques est évaluéImproving the performance of current aeronautical turbines is an important issue in a context of severe economical and environmental constraints. In a turbofan, the inter-turbine channel which is located between the High-Pressure (HP) turbine and the first Low Pressure (LP) vane is characterized by a complex flow. Therefore aerodynamic structures coming from the HP turbine (wakes, vortices and showkwaves) strongly interact between each other and affect the LP vane flow field. This generates efficiency losses of the overall configuration. This PhD thesis aims at studying the aerodynamic phenomena between a HP turbine and the first LP vane and at analyzing the mechanisms creating aerodynamic losses. A previous experimental campaign, which was carried out on a facility including a HP turbine coupled to a LP vane, enabled to gather flow field measurements in planes located in the inter-turbine channel and downstream of the LP vane. In comparison with these experimental data, the numerical simulations done with elsA software intend to reproduce accurately the 3D, unsteady and turbulent nature of the flow within this configuration. The work can be divided into three mains steps. As a first step, steady simulations with a sliding mesh treatment enable to understand the general aspects of the flow. An assessment of the effects of RANS (Reynolds-Averaged Navier-Stokes) turbulent predictions and of spatial numerical schemes on the aerodynamic structures present in the configuration is carried out. As a second step, the advanced turbulence approach ZDES (Zonal Detached-Eddy Simulation) is considered for the LP vane flow prediction. The unsteady aerodynamic structures coming from the upstream HP rotor are set as an inlet boundary condition of the computational domain. The ZDES approach is compared to a URANS (Unsteady RANS) approach on the same computational domain. The generation and dissipation of the wakes and vortices are significantly different on the two simulations, and thus impact the creation of aerodynamic losses. Finally, URANS simulations enable to better understand the interaction effects between the different blade rows. First, the unsteady phase-lagged approaches that take into account a single rotor and stator assess the important unsteady effects in the inter-turbine channel. They finally lead to the implementation of a multipassages phase-lagged computation that takes into account the two stators and the rotor in order to model all the existing determinist interactions. In order to quantify them accurately, a modal decomposition of the unsteady flow field is set up. The interaction levels linked to the different blade rows are therefore quantified and the impact of the aerodynamic losses is evaluated
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Szubert, Damien. "Physics and modelling of unsteady turbulent flows around aerodynamic and hydrodynamic structures at high Reynold number by numerical simulation." Phd thesis, Toulouse, INPT, 2015. http://oatao.univ-toulouse.fr/15129/2/szubert_1.pdf.
Повний текст джерелаАнотація:
This thesis aims at analysing the predictive capabilities of statistical URANS and hybrid RANS-LES methods to model complex flows at high Reynolds numbers and carrying out a physical analysis of the near-region turbulence and coherent structures. This study handles configurations included in the European research programmes ATAAC (Advanced Turbulent Simulation for Aerodynamics Application Challenges) and TFAST (Transition Location Effect on Shock Wave Boundary Layer Interaction). First, the detached flow in a configuration of a tandem of cylinders, positionned behind one another, is investigated at Reynolds number 166000. A static case, corresponding to the layout of the support of a landing gear, is initially considered. The fluid-structure interaction is then studied in a dynamic case where the downstream cylinder, situated in the wake of the upstream one, is given one degree of freedom in translation in the crosswise direction. A parametric study of the structural parameters is carried out to identify the various regimes of interaction. Secondly, the physics of the transonic buffet is studied by means of time-frequency analysis and proper orthogonal decomposition (POD), in the Mach number range 0.70–0.75. The interactions between the main shock wave, the alternately detached boundary layer and the vortices developing in the wake are analysed. A stochastic forcing, based on reinjection of synthetic turbulence in the transport equations of kinetic energy and dissipation rate by using POD reconstruction, has been introduced in the so-called organised-eddy simulation (OES) approach. This method introduces an upscale turbulence modelling, acting as an eddy-blocking mechanism able to capture thin shear-layer and turbulent/non-turbulent interfaces around the body. This method highly improves the aerodynamic forces prediction and opens new ensemble-averaged approaches able to model the coherent and random processes at high Reynolds number. Finally, the shock-wave/boundary-layer interaction (SWBLI) is investigated in the case of an oblique shock wave at Mach number 1.7 in order to contribute to the so-called "laminar wing design" studies at European level. The performance of statistical URANS and hybrid RANS-LES models is analysed with comparison, with experimental results, of integral boundary-layer values (displacement and momentum thicknesses) and wall quantities (friction coefficient). The influence of a transitional boundary layer on the SWBLI is featured.
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Dominguez, Bermudez Favio Enrique. "Simulation numérique de parcs d'hydroliennes à axe vertical carénées par une approche de type cylindre actif." Thesis, Université Grenoble Alpes (ComUE), 2016. http://www.theses.fr/2016GREAI020.
Повний текст джерелаАнотація:
La récupération, grâce aux hydroliennes, de l’énergie cinétique de courants marins et fluviaux constitue une source d’énergie renouvelable considérable et prédictible. La simulation fine, par une description statistique instationnaire de type URANS, de l’écoulement autour d’une hydrolienne isolée à axe vertical, bi-rotor et munie d’un carénage (hydrolienne de type HARVEST) donne accès à une estimation précise de la puissance produite. Cependant, le coût élevé de cette approche URANS la rend inadaptée à la simulation d’un parc de machines. Une analyse de la littérature conduit à retenir un modèle basse-fidélité de type Blade Element Momentum (BEM) pour décrire à moindre coût l’effet du rotor de la turbine sur l’écoulement, dans le contexte d’une description 2D (coupe horizontale). La performance de l’hydrolienne est alors prédite par un calcul RANS incluant des termes sources distribués dans un anneau rotor virtuel et conservant le maillage des parties fixes (carénage). Ces termes sources sont construits grâce à une procédure originale exploitant les conditions locales de l’écoulement en amont des cellules du rotor virtuel et le débit de l’écoulement traversant l’hydrolienne. Les coefficients hydrodynamiques utilisés pour le calcul des termes sources BEM-RANS sont construits une fois pour toutes en exploitant une série de simulations URANS préliminaires ; ils intègrent les effets du carénage et le fonctionnement de chaque rotor à une vitesse de rotation optimale (maximisant la puissance produite) grâce au système de régulation de l’hydrolienne. Le modèle BEM-RANS développé est validé par comparaison avec des simulations URANS de référence : il fournit une estimation fiable de la puissance produite (erreur de quelques % par rapport à l’approche URANS) pour un coût réduit de plusieurs ordres de grandeur. Ce modèle est appliqué à l'analyse de la puissance produite par une rangée d’hydroliennes HARVEST dans un canal pour différents facteurs de blocage et d’espacement latéral ainsi qu’à une ferme marine composée de trois hydroliennesThe capture, thanks to hydrokinetic turbines, of the kinetic energy generated by sea and river currents provides a significant and predictable source of renewable energy. The detailed simulation, using an unsteady statistical description of URANS type, of the flow around an isolated water turbine of HARVEST type (cross flow vertical axis ducted water turbine) provides an accurate estimate of the power output. However, the cost of the URANS approach is much too expensive to be applied to a farm of several turbines. A review of the literature leads to select a low-fidelity model of Blade Element Momentum (BEM) type to describe at a reduced cost the rotor effect on the flow, in a 2D context (horizontal cross-section). The turbine performance is then predicted using a steady RANS simulation including source terms distributed within a virtual rotor ring and preserving the mesh of the turbine fixed parts (duct). These source terms are derived using an original procedure which exploits both the local flow conditions upstream of the virtual rotor cells and the flow rate through the turbine. The hydrodynamic coefficients used to compute the BEM-RANS source terms are built once for all from a series of preliminary URANS simulations; they include the effects of the duct on the flow and the rotor operating at optimal rotational speed (maximizing the power output) thanks to the turbine regulation system. The BEM-RANS model is validated against reference URANS simulations: it provides a reliable prediction for the power output (within a few % of the URANS results) at a computational cost which is lowered by several orders of magnitude. This model is applied to the analysis of the power produced by a row of Vertical Axis Water Turbines in a channel for various values of the blockage ratio and lateral spacing as well as to a 3-machine sea farm
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Bénichou, Emmanuel. "Analyse numérique des instabilités aérodynamiques dans un compresseur centrifuge de nouvelle génération." Thesis, Ecully, Ecole centrale de Lyon, 2015. http://www.theses.fr/2015ECDL0046.
Повний текст джерелаАнотація:
L’étude effectuée au cours de cette thèse a permis de caractériser numériquement les instabilités d’origine aérodynamique rencontrées dans un compresseur centrifuge dessiné par Turbomeca. Ce compresseur est composé d’une roue directrice d’entrée, d’un rouet centrifuge, d’un diffuseur radial et de redresseurs axiaux. Le module expérimental, dénommé Turbocel, sera accueilli au LMFA courant 2016. Le contenu de cette étude repose donc exclusivement sur des résultats numériques dont certains sont cependant comparés à des résultats expérimentaux partiels obtenus par Turbomeca sur une configuration proche. _ Le fonctionnement du compresseur est analysé à différentes vitesses de rotation, à partir de simulations RANS et URANS menées avec le code elsA. Du point de vue de la méthodologie, deux points importants sont à retenir :- Du fait du caractère transsonique de l’écoulement dans le rouet et le diffuseur radial à haut régime de rotation, les simulations RANS stationnaires ne permettent pas d’accéder à une description satisfaisante des phénomènes physiques. Cela est dû à l’utilisation d’un plan de mélange aux différentes interfaces rotor-stator qui a pour effet d’empêcher les ondes de choc de remonter à l’amont, et qui affecte tant la physique de l’écoulement que l’étendue de la plage de fonctionnement stable.- En-dessous d’un certain débit, les calculs URANS sur période machine révèlent que le comportement de l’étage n’obéit plus à la périodicité spatio-temporelle mono-canal. Une plage instable est alors obtenue à toutes les iso-vitesses simulées. A bas régime de rotation, une autre plage stable existe lorsque le compresseur est suffisamment vanné. L’étage retrouve alors une périodicité spatio-temporelle, à condition d’étendre le domaine de calcul dans le stator à deux canaux inter-aubes. En ce qui concerne les limites de stabilité de Turbocel, différentes évolutions sont décrites selon la vitesse de rotation considérée :- A haut régime de rotation, une basse fréquence commence à émerger près du point de rendement maximal et son intensité ne fait qu’augmenter jusqu.au pompage.- A bas régime, une signature basse fréquence comparable se manifeste près du point de rendement maximal mais disparaît passé un certain vannage, et n’est donc présente que sur une plage de débit délimitée. La seconde zone stable peut alors être numériquement parcourue jusqu.au pompage proprement dit. La signature basse fréquence est imputée à l’instauration d’une recirculation dans l’inducteur qui une fois établie est quasi-stationnaire. Les résultats numériques mettent en évidence que la source d’instabilité sévère sur Turbocel provient du diffuseur aubé. En fonction du point de fonctionnement, ce composant adopte des comportements différents, entre lesquels une certaine continuité existe, et ses performances chutent progressivement lorsque le débit diminue. Au final, les domaines de stabilité de l’étage de compression peuvent être reliés au type d’écoulement qui se développe dans le diffuseur radial, et apparaissent dictés par le diffuseur semi-lisse à haut régime de rotation. Enfin, afin d’étendre les plages de fonctionnement stable, une stratégie de contrôle basée sur l’aspiration de couche limite dans le diffuseur aubé a également été déterminée dans le cadre de cette thèse. Son évaluation fera l’objet d’études ultérieures sur TurbocelThe present study aims at characterizing the aerodynamic instabilities involved in a centrifugal compressor designed by Turbomeca, by means of numerical simulation. This compressor is composed of inlet guide vanes, a centrifugal impeller, a radial vaned diffuser and axial outlet guide vanes. The test module, named Turbocel, will be delivered to the LMFA in 2016. Thus, the results presented in this manuscript are only based on CFD, although some of them are compared to experimental results obtained by Turbomeca on a close configuration.RANS and URANS simulations are performed for several rotational speeds, using the elsA software.Two methodological key points are to be emphasized:- As the flow in both the impeller and the radial diffuser is transonic at high rotational speed, steady RANS simulations cannot provide a satisfactory description of the physical phenomena taking place. This can be explained by the use of the mixing plane approach which prevents shock waves to extend upstream the rotor-stator interfaces, and which impacts the flow field predicted as well as the prediction of the stable operating range.- Below a given massflow rate, URANS simulations covering the spatial period of the compressor prove that the stage behavior does not obey to the single passage spatio-temporal periodicity anymore. An unstable operating range then appears at all the simulated rotational speeds. At low rotational speed, another stable range is however obtained if the compressor is further throttled’ A new periodicity arises on this massflow range, provided that the stator domain is extended to two neighboring blade passages. Concerning the stability domains of Turbocel, different evolutions are obtained depending on the rotational speed:- At high rotational speed, a low frequency phenomenon starts to develop near the peak efficiency point and its intensity keeps increasing until surge happens.- At low rotational speed, a low frequency signature also appears near the peak efficiency point, but it then vanishes when the compressor is further throttled, so that only a restricted operating range exhibits this instability. It then gives rise to a second stable operating range which can be described numerically, ending with surge itself. The low frequency signature is attributed to the enhancement of a flow recirculation in the inducer which, once fully established, is quasi-steady. The numerical results underline that the source of severe instability in the compressor comes from the vaned diffuser. Depending on the operating point, this component can adopt different behaviors, between which a relative continuity exists, and its performances decrease when the massflow rate decresases. The overall stage performances prove that at high rotational speed, the global stability is driven by the semi-vaneless diffuser and depends on the flow developing in the radial diffuser. Finally, in order to extend the stable operating range of the compressor, a flow control strategy based on boundary layer suction has also been determined in the diffuser. Its impact on the performances of Turbocel will be deeply studied later on
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Grossi, Fernando. "Physique et modélisation d’interactions instationnaires onde de choc/couche limite autour de profils d’aile transsoniques par simulation numérique." Thesis, Toulouse, INPT, 2014. http://www.theses.fr/2014INPT0015/document.
Повний текст джерелаАнотація:
L’interaction onde de choc/couche limite en écoulement transsonique autour de profils aérodynamiques est étudiée numériquement utilisant différentes classes de modélisation de la turbulence. Les approches utilisées sont celles de modèles URANS et de méthodes hybrides RANS-LES. L’emploi d’une correction de compressibilité pour les fermetures à une équation est aussi évalué. Premièrement, la séparation intermittente induite par le choc sur un profil supercritique en conditions d’incidence proches de l’angle critique d’apparition du tremblement est analysée. Suite à des simulations URANS, la modélisation statistique la mieux adaptée est étudiée et utilisée dans l’approche DDES (Delayed Detached-Eddy Simulation). L’étude de la topologie de l’écoulement, des pressions pariétales et champs de vitesse statistiques montrent que les principales caractéristiques de l’oscillation auto-entretenue du choc sont capturées par les simulations. De plus, la DDES prédit des fluctuations secondaires de l’écoulement qui n’apparaissent pas en URANS. L’étude de l’interface instationnaire RANS-LES montre que la DDES évite le MSD (modeled stress depletion) pour les phases de l’écoulement attaché ou séparé. Le problème de la ‘zone grise’ et de son influence sur les résultats est considéré. Les conclusions de l’étude sur le profil supercritique est ensuite appliquées à l’étude numérique d’un profil transsonique laminaire. Dans ce contexte, l’effet de la position de la transition de la couche limite sur les caractéristiques de deux régimes d’interaction choc/couche limite sélectionnés est étudié. En conditions de tremblement, les simulations montrent une forte influence du point de transition sur l’amplitude du mouvement du choc et sur l’instationnarité globale de l’écoulementShock wave/boundary layer interactions arising in the transonic flow over airfoils are studied numerically using different levels of turbulence modeling. The simulations employ standard URANS models suitable for aerodynamics and hybrid RANS-LES methods. The use of a compressibility correction for one-equation closures is also considered. First, the intermittent shock-induced separation occurring over a supercritical airfoil at an angle of attack close to the buffet onset boundary is investigated. After a set of URANS computations, a scale-resolving simulation is performed using the best statistical approach in the context of a Delayed Detached-Eddy Simulation (DDES). The analysis of the flow topology and of the statistical wall-pressure distributions and velocity fields show that the main features of the self-sustained shock-wave oscillation are predicted by the simulations. The DDES also captures secondary flow fluctuations which are not predicted by URANS. An examination of the unsteady RANS-LES interface shows that the DDES successfully prevents modeled-stress depletion whether the flow is attached or separated. The gray area issue and its impact on the results are also addressed. The conclusions from the supercritical airfoil simulations are then applied to the numerical study of a laminar transonic profile. Following a preliminary characterization of the airfoil aerodynamics, the effect of the boundary layer transition location on the properties of two selected shock wave/boundary layer interaction regimes is assessed. In transonic buffet conditions, the simulations indicate a strong dependence of the shock-wave motion amplitude and of the global flow unsteadiness on the tripping location
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Uribe, Cédric. "Développement d'une approche ZDES à deux équations de transport et application turbomachines." Thesis, Sorbonne université, 2018. http://www.theses.fr/2018SORUS252.
Повний текст джерелаАнотація:
Afin d’améliorer les caractéristiques d’une turbomachine, il est capital de prévoir correctement les écoulements secondaires et/ou transitionnels s’y développant. Ces écoulements peuvent être à l’origine de la réduction de ses performances et de son domaine de fonctionnement. À titre d’exemple la présence de tourbillons de jeu ou de décollements de coin sur les aubages de compresseur H.P. engendre des pertes de pression totale favorisant l’apparition d’instabilités axiales (pompage). Les multiples processus de transition laminaire-turbulent modulent la charge thermique subie par les aubes de turbine et conditionnent donc leur durée de vie. La nature généralement fortement instationnaire, décollée et transitionnelle de ces écoulements rend leur prévision délicate voire imprécise avec les modélisations de la turbulence utilisées en conception (approche RANS). En réponse à ces défis une solution est l’approche hybride RANS/LES dite ZDES (Zonal Detached Eddy Simulation (Deck 2012)) dans ses modes de fonctionnement 0, 1 et 2 : les couches limites sont traitées par une approche RANS dans leur entière épaisseur afin de s’y affranchir du coût excessif d’une approche LES (résolution des grandes échelles de la turbulence) qui est seulement utilisée - si nécessaire - sur le reste du domaine de calcul pour une prévision haute fidélité des écoulements décollés, l’interface entre les deux approches étant continue. Afin que cette approche soit compatible avec la majorité des modèles de transition laminaire-turbulent, en particulier en aérodynamique interne, ces travaux en proposent diverses variantes non plus basées sur le modèle de turbulence de Spalart et Allmaras (approche ZDES SA) mais sur le modèle k − ω SST de Menter (approche ZDES SST). Au regard d’une décomposition en problématiques majeures de leurs performances, ils démontrent la similitude comportementale de ces deux approches sur des cas d’écoulement génériques (couche de mélange, marche descendante, barreau cylindrique) peu exigeants vis-à-vis de leur modèle de turbulence sous-jacent au sein des régions RANS. Ils démontrent également l’avantage de la ZDES SST sur la ZDES SA sur un cas d’intérêt industriel (rotor de compresseur axial H.P.) autrement plus exigeant et évaluent diverses voies d’amélioration pour l’approche ZDES. Enfin ils enrichissent cette démarche d’évaluation d’un nouveau cas générique d’écoulement (bosse axisymétrique) permettant d’isoler la problématique de la prévision des décollements de couche limite turbulente en l’absence de singularité géométriqueAccurate secondary and transitional flows predictions are essential to turbomachinery improvements. They cause additional losses and operating domain reduction. For example tip-leakage vortex flow or corner separation flow in H.P. compressor lead to total pressure losses and promote axial instabilities (surge). Various laminar-turbulent transition processes regulate turbine blade thermal load and thus turbine blades life. Usual CFD turbulence modellings (RANS method) fail to cope with such strongly fluctuating, separated and transitional flows. In response to these challenges, one solution relies in the hybrid RANS/LES method called ZDES (Zonal Detached Eddy Simulation (Deck 2012)) in its operating modes 0, 1 and 2 : boundary layers are modelled following a RANS method in their whole thickness for avoiding their expensive resolution following an LES method (resolution of the large turbulence scales) which is only used - if needed - far from the wall for high fidelity prediction of separated flows thanks to continuous RANS/LES interfaces. In view to allow ZDES method to be compatible with the majority of laminar-turbulent transition models applied for internal flows, this work proposes numerous ZDES’s variants no more based on Spalart et Allmaras turbulence model (ZDES SA method) but on Menter k − ω SST turbulence model (ZDES SST method). With regards to a deconstruction of their performances in major issues, this work demonstrates the behavioral similarity of those two methods on common flow test cases (mixing layer, backward-facing step, circular cylinder) undemanding for their underlying turbulence model in RANS regions. This work also demonstrates the advantage of ZDES SST over ZDES SA for an industrial relevant test case (high-pressure axial compressor rotor) much more demanding. It assesses various ways of improvement for ZDES method. Finally, it supplements this validation process with a new common test case (axisymetric hill) allowing to isolate the issue of turbulent boundary layer separation prediction in the absence of geometrical singularity
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Renard, Nicolas. "Simulations numériques avancées et analyses physiques de couches limites turbulentes à grand nombre de Reynolds." Thesis, Paris 6, 2016. http://www.theses.fr/2016PA066041/document.
Повний текст джерелаАнотація:
Mieux comprendre les spécificités de la dynamique des couches limites à grand nombre de Reynolds malgré les contraintes métrologiques et son coût de simulation numérique est crucial. A titre d'exemple, cette dynamique peut déterminer plus de la moitié de la traînée d'un avion en croisière. Décrire la turbulence pariétale peut guider la résolution numérique d'une partie des fluctuations à un coût maîtrisé par des stratégies WMLES (simulation des grandes échelles avec modèle de paroi). Les présentes analyses physiques de couches limites turbulentes incompressibles à gradient de pression nul et à grand nombre de Reynolds s'appuient sur des simulations numériques avancées. Après validation d'une base de données, le frottement moyen pariétal est décomposé selon l'identité FIK (Fukagata et al. (2002)), dont l'application malgré le développement spatial est discutée. Une analyse spectrale montre que les grandes échelles (\lambda_x > \delta) contribuent à environ la moitié du frottement vers Re_\theta = 10^4. Les limitations de l'identité FIK motivent la dérivation d'une décomposition physique de la génération du frottement dont le comportement asymptotique est alors relié à la production d'énergie cinétique turbulente dans la zone logarithmique. Pour mieux reconstruire les spectres spatiaux, une nouvelle méthode d'estimation de la vitesse de convection turbulente en fonction de la longueur d'onde des fluctuations, adaptée au développement spatial et à des signaux temporels de durée finie, est dérivée, interprétée et évaluée à Re_\theta = 13000. Certaines des conclusions éclairent des modifications d'une stratégie WMLES, le mode III de la méthode ZDESBetter understanding the specificities of the dynamics of high-Reynolds number boundary layers despite metrological constraints and its numerical simulation cost is crucial. For instance, this dynamics can determine more than half of the drag of a cruising aircraft. Describing wall turbulence can guide the numerical resolution of some of the fluctuations at a limited cost by WMLES strategies (wall-modelled large eddy simulation). The present physical analyses of zero-pressure gradient incompressible turbulent boundary layers at high Reynolds number rely on advanced numerical simulations. After validating a database, mean skin friction is decomposed by means of the FIK identity (Fukagata et al. (2002)), whose application despite the spatial growth is discussed. A spectral analysis shows that the large scales (\lambda_x > \delta) contribute approximately half of the friction near Re_\theta = 10^4. The limitations of the FIK identity motivate the derivation of a physical decomposition of the generation of friction whose asymptotic behaviour is then related to turbulent kinetic energy production in the logarithmic layer. In order to better reconstruct spatial spectra, a new method to estimate the turbulent convection velocity as a function of the wavelength of the fluctuations, adapted to spatial growth and to temporal signals of finite duration, is derived, interpreted, and assessed at Re_\theta = 13000. Some of the conclusions enlighten modifications to a WMLES strategy, mode III of the ZDES method
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Keshmiri, Amir. "Thermal-hydraulic analysis of gas-cooled reactor core flows." Thesis, University of Manchester, 2010. https://www.research.manchester.ac.uk/portal/en/theses/thermalhydraulic-analysis-of-gascooled-reactor-core-flows(29335acf-a397-4b8c-8217-fd2ee0d26967).html.
Повний текст джерелаАнотація:
In this thesis a numerical study has been undertaken to investigate turbulent flow and heat transfer in a number of flow problems, representing the gas-cooled reactor core flows. The first part of the research consisted of a meticulous assessment of various advanced RANS models of fluid turbulence against experimental and numerical data for buoyancy-modified mixed convection flows, such flows being representative of low-flow-rate flows in the cores of nuclear reactors, both presently-operating Advanced Gas-cooled Reactors (AGRs) and proposed ‘Generation IV’ designs. For this part of the project, an in-house code (‘CONVERT’), a commercial CFD package (‘STAR-CD’) and an industrial code (‘Code_Saturne’) were used to generate results. Wide variations in turbulence model performance were identified. Comparison with the DNS data showed that the Launder-Sharma model best captures the phenomenon of heat transfer impairment that occurs in the ascending flow case; v^2-f formulations also performed well. The k-omega-SST model was found to be in the poorest agreement with the data. Cross-code comparison was also carried out and satisfactory agreement was found between the results.The research described above concerned flow in smooth passages; a second distinct contribution made in this thesis concerned the thermal-hydraulic performance of rib-roughened surfaces, these being representative of the fuel elements employed in the UK fleet of AGRs. All computations in this part of the study were undertaken using STAR-CD. This part of the research took four continuous and four discrete design factors into consideration including the effects of rib profile, rib height-to-channel height ratio, rib width-to-height ratio, rib pitch-to-height ratio, and Reynolds number. For each design factor, the optimum configuration was identified using the ‘efficiency index’. Through comparison with experimental data, the performance of different RANS turbulence models was also assessed. Of the four models, the v^2-f was found to be in the best agreement with the experimental data as, to a somewhat lesser degree were the results of the k-omega-SST model. The k-epsilon and Suga models, however, performed poorly. Structured and unstructured meshes were also compared, where some discrepancies were found, especially in the heat transfer results. The final stage of the study involved a simulation of a simplified 3-dimensional representation of an AGR fuel element using a 30 degree sector configuration. The v^2-f model was employed and comparison was made against the results of a 2D rib-roughened channel in order to assess the validity and relevance of the precursor 2D simulations of rib-roughened channels. It was shown that although a 2D approach is extremely useful and economical for ‘parametric studies’, it does not provide an accurate representation of a 3D fuel element configuration, especially for the velocity and pressure coefficient distributions, where large discrepancies were found between the results of the 2D channel and azimuthal planes of the 3D configuration.
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Riera, William. "Evaluation of the ZDES method on an axial compressor : analysis of the effects of upstream wake and throttle on the tip-leakage flow." Thesis, Ecully, Ecole centrale de Lyon, 2014. http://www.theses.fr/2014ECDL0030/document.
Повний текст джерелаАнотація:
L’écoulement de jeu dans les compresseurs axiaux est étudié à l’aide de la Zonal Detached Eddy Simulation (ZDES). L’objectif consiste à évaluer la capacité de méthodes hybrides URANS/LES à simuler l’écoulement de jeu d’un compresseur axial réaliste afin de mieux comprendre la physique de cet écoulement, notamment son comportement au vannage ainsi que l’effet de sillages venant du stator amont sur le rotor aval. Après avoir choisi la méthode hybride ZDES, un banc d’essai numérique est défini afin de simuler le premier rotor du compresseur de recherche CREATE. Ce banc a la particularité de pouvoir prendre en compte les effets instationnaires venant de la roue directrice d’entrée (RDE), notamment son sillage ainsi que les tourbillons générés en pied et en tête. Basé sur des critères de maillage ZDES, il est utilisé pour évaluer cette méthode comparativement aux méthodes classiques RANS et URANS. La ZDES est validée par étape jusqu’à une analyse spectrale de l’écoulement de jeu se basant sur des données expérimentales. Elle s’est révélée capable de capturer plus précisément l’intensité et la position des phénomènes instationnaires rencontrés en tête du rotor, notamment le tourbillon de jeu. Les densités spectrales de puissance analysées montrent que cela est dû en partie à une meilleure prise en compte du transfert d’énergie des grandes vers les petites structures de l’écoulement avant leur dissipation. De plus, l’écart entre les approches s’accentue lorsque le tourbillon de jeu traverse le choc en tête. Proche pompage, les effets d’interaction entre le choc, le tourbillon de jeu, la couche limite carter et le tourbillon venant de la tête de la RDE sont amplifiés. Le décollement de la couche limite carter s’accentue et une inversion locale de l’écoulement est observée. De plus, le tourbillon de jeu s’élargit et est dévié vers la pale adjacente, ce qui intensifie le phénomène de double écoulement de jeu. L’interaction du tourbillon venant de la tête de la RDE avec le choc et le tourbillon de jeu du rotor est ensuite étudiée au point de dessin. Un battement du tourbillon de jeu est rencontré lors de l’interaction de ce tourbillon avec le tourbillon de tête de la RDE, ce qui diminue le double écoulement de jeuThe tip-leakage flow in axial compressors is studied with the Zonal Detached Eddy Simulation (ZDES). This study aims at evaluating the capability of hybrid URANS/LES methods to simulate the tip-leakage flow within a realistic axial compressor in order to better understand the involved physics, especially the behaviour of the flow near surge and the effects of stator wakes on the downstream rotor. Once the ZDES method is chosen, a numerical test bench is defined to simulate the first rotor of the research compressor CREATE. This bench takes into account the unsteady effects of the Inlet Guide Vane (IGV), such as its wake as well as vortices generated at the IGV hub and tip. It is based upon ZDES meshing criteria and is used to evaluate this method compared to classic RANS and URANS approaches. A method validation is carried out up to a spectral analysis compared to experimental data. The ZDES is capable to capture more accurately the intensity and position of the unsteady phenomena encountered in the tip region, especially the tip-leakage vortex. The power spectral densities highlight that this partly originates from a better capture of the energy transfer from large to small structures until their dissipation. The discrepancy between the methods is accentuated as the tip-leakage vortex crosses the shock. Near the surge line, the interactions between the shock, the tip-leakage vortex, the boundary layer developing on the shroud and the vortex generated by the IGV tip are amplified. The boundary layer on the shroud separates earlier and a local flow inversion occurs. Besides, the tip-leakage vortex widens and is deflected toward the adjacent blade. This strengthens the double leakage. At the design operating point, the interaction of the IGV tip vortex with the shock and the rotor tip vortex is studied. A vortex flutter is observed as the IGV tip vortex arrives on the rotor blade and stretches the rotor tip vortex. This phenomenon decreases the double leakage
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Sanchez, Marc. "Etude des extracteurs d'air hybrides éoliens : conception de géométries et analyse des écoulements." Thesis, Perpignan, 2015. http://www.theses.fr/2015PERP0040/document.
Повний текст джерелаАнотація:
Ce travail de thèse concerne l'étude d'extracteurs d'air hybrides éoliens. Il se décompose en des investigations amont et appliquées. Dans la partie amont, des simulations fines ont été effectuées en conduite carrée avec et sans rotation, pour des nombres de Reynolds turbulents de l'ordre de 600, afin d'analyser l'impact de la rotation sur la turbulence. Elles ont montré que la rotation rompt la symétrie de l'écoulement. La partie appliquée est dédiée à la conception d'une nouvelle géométrie d'extracteur d'air. Cette géométrie a été proposée à partir de l'analyse de simulations RANS. Ses performances ont été confirmées par des mesures expérimentales sur banc d'essais. Les tests en soufflerie d'un système de captage d'énergie éolienne, conçu pour l'extracteur, ont mis en évidence son adéquation au régime de fonctionnement de l'extracteur. Les essais expérimentaux de l'extracteur complet, montrent que le système de captage apporte une part significative de l'énergie. Des essais en soufflerie ont permis d'observer le comportement global de l'extracteurThis PhD work concerns the study of hybrid air extractors. It is composed of upstream and applied investigations. In the upstream part, fine simulations are realized in square duct flow with and without rotation to analyse the impact of rotation on turbulence. It is found that rotation removes symmetry property of the flow with turbulent Reynolds number of 600. The applied part is dedicated to the conception of a new air extractor geometry. This geometry is proposed from the analyse of RANS simulations. Its performances are confirmed by experimental measurements on test rig. Wind tunnel tests of a wind power capturing system, designed for the extractor, show a good adequation to the operating regime of the extractor. Experimental investigations on the complete air extractor, show the wind power capturing system brings a significant part of the energy. Wind tunnel tests allow to observe the complete air extractor behaviour
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Longo, Riccardo. "Advanced turbulence models for the simulation of air pollutants dispersion in urban area." Doctoral thesis, Universite Libre de Bruxelles, 2020. https://dipot.ulb.ac.be/dspace/bitstream/2013/312254/3/thesis.pdf.
Повний текст джерелаАнотація:
NOWADAYS, a number of studies keep on demonstrating the existence of a strong relation between high concentrations of particulate matter (PM) and the prevalence of human morbidity and mortality. Large particles can be filtered in the nose or in the throat, while fine particles (about10 micrometer) can settle in the bronchi and lungs, leading to more serious consequences. According to Karagulian et al. the major sources of urban air pollution are traffic (25%), combustion and agriculture (22%), domestic fuel burning (20%), natural dust (18%) and industrial activities (15%).As a consequence, the detailed study of dispersion phenomena within the urban canopy becomes a target of great interest. To this end, Computational Fluid Dynamics (CFD) can be successfully employed to predict turbulence and dispersion patterns, accounting for a detailed characterization of the pollutant sources, complex obstacles and atmospheric stability classes.Despite being intrinsically different phenomena, turbulence and dispersion are closely related. It is universally accepted that, to reach accurate prediction of the concentration field, it is necessary to properly reproduce the turbulence one. For this reason, the present PhD thesis is split into two main Sections: one focused on turbulence modelling and the subsequent, centered on the dispersion modelling.Thanks to its good compromise between accuracy of results and calculation time, Reynolds-averaged Navier-Stokes (RANS) still represents a valid alternative to more resource-demanding methods. However, focusing on the models’ performance in urban studies, Large Eddy Simulation (LES) generally outperforms RANS results, even if the former is at least one order of magnitude more expensive. Stemming from this consideration, the aim of this work is to propose a variety of approaches meant to solve some of the major limitations linked to standard RANS simulation and to further improve its accuracy in disturbed flow fields, without renouncing to its intrinsic feasibility. The proposed models are suitable for the urban context, being capable of automatically switching from a formulation proper for undisturbed flow fields to one suitable for disturbed areas. For neutral homogeneous atmospheric boundary layer (ABL), a comprehensive approach is adopted, solving the issue of the erroneous stream-wise gradients affecting the turbulent profiles and able to correctly represent the various roughness elements. Around obstacles, more performing closures are employed. The transition between the two treatments is achieved through the definition of a Building Influence Area (BIA). The finalgoal is to offer more affordable alternatives to LES simulations without sacrificing a good grade of accuracy.Focusing on the dispersion modelling framework, there exists a number of parameters which have to be properly specified. In particular, the definition of the turbulent Schmidt number Sct, expressing the ratio of turbulent viscosity to turbulent mass diffusivity, is imperative. Despite its relevance, the literature does not report a clear guideline on the definition of this quantity. Nevertheless, the importance of Sct with respect to dispersion is undoubted and further demonstrated in the works of different authors. For atmospheric boundary layer flows, typical constant values range between 0.2 and 1.3. As a matter of fact, the local variability of Sct is supported by experimental evidence and by direct numerical simulations (DNS). These observations further suggest that the turbulent Schmidt number should be prescribed as a dynamic variable. Following these observations a variable turbulent Schmidt number formulation is proposed in this work. The latter stems from the same hypothesis of the variable formulation developed by Gorlé et al. Moreover, the relevant uncertain model parameters are optimized through uncertainty quantification (UQ). This formulation further increased the accuracy of the predictions, and was successfully verified by Di Bernardino et al. However, the turbulent Schmidt number resulting from this formulation is still intrinsically linked to the turbulence model employed, i.e. to the Cμ coefficient. To overcome this constraint, the nature and the dependencies of Sct were further analyzed through correlation studies and employing principal component analysis (PCA) on data obtained through the proposed ABL RANS model. Subsequently, the same data-driven technique was employed based on the high-fidelity outcomes of a delayed Detached Eddy Simulation (dDES) to derive a generalized turbulentSchmidt number formulation. The latter can be employed within a wide range of turbulence models, without limiting its variability.Doctorat en Sciences de l'ingénieur et technologie
info:eu-repo/semantics/nonPublished
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Benyoucef, Farid. "Amélioration de la prévision des écoulements turbulents par une approche URANS avancée." Thesis, Toulouse, ISAE, 2013. http://www.theses.fr/2013ESAE0014/document.
Повний текст джерелаАнотація:
Ces travaux de recherche ont pour but d’évaluer la méthode dite de la "Simulation auxEchelles Adaptées" (SAS pour Scale-Adaptive Simulation). Cette approche coïncide avec uneapproche RANS classique dans les zones pariétales attachées et adapte le niveau de viscositéturbulente dans les zones décollées pour y permettre une résolution partielle des structures turbulentes.Dans une première partie, une analyse théorique du modèle SAS original a été menéeet a permis de développer une correction visant à favoriser l’adaptation du niveau de viscositéturbulente dans les zones sièges d’instabilités de type Kelvin-Helmholtz. Le modèle ainsi corrigéest nommé SAS-αL. Les modèles SAS et SAS-αL ont été implantés dans le code de calculNavier-Stokes elsA de l’ONERA. À l’issue de cette étape, trois cas académiques d’écoulementsturbulents instationnaires, cylindre à grand nombre de Reynolds, marche descendante et cavitétranssonique, ont été simulés grâce aux trois modèles de turbulence SST, SAS et SAS-αL. Outreune comparaison aux bases de données expérimentales disponibles, une attention particulièrea été portée à l’influence de paramètres numériques tels que des schémas numériques d’ordreélevé. Enfin, afin d’étudier la viabilité de l’approche SAS dans un contexte industriel, les troismodèles de turbulence ont été testés sur une configuration issue de l’industrie aéronautique etcorrespondant à la sortie d’air chaud d’un système de dégivrage des nacelles d’avion. La comparaisondes prévisions obtenues avec les modèles SST, SAS et SAS-αL aux données expérimentalesobtenues à l’ONERA a permis de montrer un gain de précision grâce à l’emploi de l’approcheSAS et ce pour un coût de calcul compatible avec un cycle de conception industrielleThis research work is meant to assess an upgraded URANS approach, namely the Scale-Adaptive Simulation (SAS). This method is similar to a conventional RANS approach (namelythe SSTmodel) in attached areas and is able to adapt the eddy-viscosity level in detached areas toensure the resolution, at least partially, of the turbulent structures. In a first part of this researchwork, an improvement of the SAS approach is suggestedto allowa better sensitivity of themodelto instabilities such as Kelvin-Helmholtz ones. This "improved" model is referred to as SAS-αLmodel. Both SAS and SAS-αL models were implemented in the ONERA Navier-Stokes solverelsA and both of themaswell as the SSTmodelwere tested on academic test cases : a cylinder in acrossflowat a high Reynolds number, a backward-facing step flowcorresponding to theDriver&Seegmiller experiment and the transonic flow over the M219 cavity experimentally investigatedby de Henshaw. The influence of the numerical parameters was deeply investigated and particularattention was paid to the high-order space-discretization schemes effects. The reliabilityof the SAS approach in an industrial framework was assessed on an aeronautic configurationnamely a nacelle de-icing device. Comparisons between the threemodels (SST, SAS and SAS-αL)and an experimental database available at ONERA - The French Aerospace Lab have shown thebetter accuracy of the SAS approach as well as the high potential of the SAS-αL model
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Menier, Victorien. "Numerical methods and mesh adaptation for reliable rans simulations." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066481/document.
Повний текст джерелаАнотація:
Cette thèse porte sur la prédiction haute-fidélité de phénomènes visqueux turbulents modélisés par les équations Reynolds-Averaged Navier-Stokes (RANS). Si l’adaptation de maillage a été appliquée avec succès aux simulations non-visqueuses comme la prédiction du bang sonique ou la propagation d’explosion, prouver que ces méthodes s’étendent et s’appliquent également aux simulations RANS avec le même succès reste un problème ouvert. Dans ce contexte, cette thèse traite des problématiques relatives aux méthodes numériques (solveur de mécanique des fluides) et aux stratégies d’adaptation de maillage. Pour les méthodes numériques, nous avons implémenté un modèle de turbulence dans notre solveur et nous avons conduit une étude de vérification et validation en deux et trois dimensions avec comparaisons à l’expérience. Des bons résultats ont été obtenus sur un ensemble de cas tests, notamment sur le calcul de la traînée pour des géométries complexes. Nous avons également amélioré la robustesse et la rapidité de convergence du solveur, grâce à une intégration en temps implicite, et grâce à une procédure d’accélération multigrille. En ce qui concerne les stratégies d’adaptation de maillage, nous avons couplé les méthodes multigrilles à la boucle d’adaptation dans le but de bénéficier des propriétés de convergence du multigrille, et ainsi, améliorer la robustesse du processus et le temps CPU des simulations. Nous avons également développé un algorithme de génération de maillage en parallèle. Celui-ci permet de générer des maillages anisotropes adaptés d’un milliard d’éléments en moins de 20 minutes sur 120 coeurs de calcul. Enfin, nous avons proposé une procédure pour générer automatiquement des maillages anisotropes adaptés quasi-structurés pour les couches limitesThis thesis deals with the high-fidelity prediction of viscous turbulent flows modelized by the Reynolds-Averaged Navier-Stokes (RANS) equations. If mesh adaptation has been successfully applied to inviscid simulations like the sonic boom prediction or the blast propagation, demonstrating that these methods are also well-suited for 3D RANS simulations remains a challenge. This thesis addresses research issues that arise in this context, which are related to both numerical methods (flow solver) and mesh adaptation strategies. For the numerical methods, we have implemented a turbulence model in our in-house flow solver and carried out its verification & validation study. Accurate results were obtained for a representative set of test cases, including the drag prediction workshop. Additional developments have been done to improve the robustness and the convergence speed of the flow solver. They include the implementation of an implicit time integration and of a multigrid acceleration procedure. As regards mesh adaptation, we have coupled the adaptive process to multigrid in order to benefit from its convergence properties and thus improve the robustness while preventing losses of computational effort. We also have devised a parallel mesh generation algorithm. We are able to generate anisotropic adapted meshes containing around one billion elements in less than 20min on 120 cores. Finally, we introduced a procedure to automatically generate anisotropic adapted quasi-structured meshes in boundary layer regions
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Davis, Mallory. "Numerical Simulation of Unsteady Hydrodynamics in the Lower Mississippi River." ScholarWorks@UNO, 2010. http://scholarworks.uno.edu/td/1126.
Повний текст джерелаАнотація:
Alterations along the Mississippi River, such as dams and levees, have greatly reduced the amount of freshwater and sediment that reaches the Louisiana coastal area. Several freshwater and sediment diversions have been proposed to combat the associated land loss problem. To aid in this restoration effort a 1-D numerical model was calibrated, validated, and used to predict the response of the river to certain stimuli, such as proposed diversions, channel closures, channel modifications, and relative sea level rise. This study utilized HEC-RAS 4.0, a 1-D mobile-bed numerical model, which was calibrated using a discharge hydrograph at Tarbert Landing and a stage hydrograph at the Gulf of Mexico, to calculate the hydrodynamics of the river. The model showed that RSLR will decrease the capacity of the Lower Mississippi River to carry bed material. The stage at Carrollton Gage is not significantly impacted by large scale diversions
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Ahmed, Dhafar Ibrahim. "Experimental and numerical study of model gravity currents in coastal environment : bottom gravity currents." Thesis, Brest, 2017. http://www.theses.fr/2017BRES0060/document.
Повний текст джерелаАнотація:
Le but de ce travail de recherche est de contribuer à une meilleure compréhension de la dynamique de propagation et de la miscibilité de jets gravitaires au-dessous d’un liquide ambiant. Des expériences ont été réalisées en laboratoire à l’aide d’une plateforme expérimentale constituée d’un bassin parallélépipédique contenant de l’eau douce et d’un canal d’injection de section rectangulaire de jets gravitaires de concentration constante initiale fixée. Les calculs mathématiques et numériques sont basés sur les modèles RANS (Reynolds-Averaged Navier Stokes equations), k-ε (K-epsilon) et DCE (Diffusion-Convective Equation) de la fraction volumique de l’eau salée pour décrire la propagation et le mélange du jet gravitaire. L’évolution du front du jet obtenue expérimentalement est utilisée pour valider le modèle numérique. Par ailleurs, la comparaison des résultats obtenus sur l’écoulement moyen (z⁄z0.5 =U/Umax) avec ceux des études 2D expérimentales et numériques antérieures ont montré des similarités. La simulation numérique des champs hydrodynamiques montre que la vitesse maximale est atteinte à la position 0.18 z0.5, où z0.5 est la hauteur d’eau pour laquelle la vitesse moyenne u est égale à la moitié de la vitesse maximale UmaxThe aim of this investigation is to contribute to a better understanding of the propagation dynamics and the mixing process of dense gravity currents. The Laboratory experiments proceeded with a fixed initial gravity current concentration in one experimental set-up. The gravity currents are injected using a rectangular injection channel into a rectangular basin containing the ambient lighter liquid. The injection studied is said in unsteady state volume, as the Reynolds number lies in the range 1111 - 3889. The experiments provided the evolution of the boundary interface of the jet, and it is used to validate the numerical model. The numerical model depends on the Reynolds-Averaged Navier Stokes equations (RANS). The k-ε (K-epsilon) and the Diffusion-Convective Equation (DCE) of the saline water volume fraction were used to model the mixing and the propagation of the gravity current jet. On the other hand, comparison of the mean flow (z⁄z0.5 =U/Umax) with previous two-dimensional numerical simulations and experimental measurements have shown similarities. The numerical simulations of the hydrodynamic fields indicate that the velocity maximum at 0.18 z0.5, where z0.5 is the height at which the mean velocity u is the half of the maximum velocity Umax
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Menchaca, Roa Ane. "Analyse numérique des hydroliennes à axe vertical munies d'un carénage." Thesis, Grenoble, 2011. http://www.theses.fr/2011GRENI050/document.
Повний текст джерелаАнотація:
Cette thèse s'inscrit dans le cadre des énergies renouvelables au sein du programme HARVEST du laboratoire LEGI, qui consiste à développer un concept d'hydrolienne de type Darrieus. L'hydrolienne peut être équipée d'un dispositif appelé carénage afin de transformer une portion plus grande de l'énergie cinétique contenue dans le courant d'eau en électricité. Les travaux présentés se sont focalisés sur ces systèmes de carénage, autour de trois axes : l'explication du principe de fonctionnement hydrodynamique du carénage, la quantification des performances de l'hydrolienne carénée et la mise en évidence des grandeurs géométriques clés du carénage permettant d'améliorer ou d'optimiser la performance du système. Toutes les études ont été réalisées à l'aide des calculs RANS 2D et des données expérimentales mises à disposition et, comparées aux résultats obtenus pour une hydrolienne non-carénéeThe general context of the present thesis is renewable energies within the HARVEST program initialized at LEGI laboratory, which consists in developing a Darrieus-type water current turbine (WCT). The WTC can be equipped with a channelling device which allows transforming a bigger amount of the kinetic energy contained in the flowstream into electricity. The present work is focused on the channelling devices. Studies concern three main topics: the explanation of the channelling device hydrodynamic functioning, the evaluation of the performance of the shrouded WCT and the revealing of the system geometrical parameters which allow its improvement or optimisation. All studies have been carried out by 2D RANS calculations and available experimental data, and have been compared to bare WTC results
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Enayati, Hooman. "NUMERICAL FLOW AND THERMAL SIMULATIONS OF NATURAL CONVECTION FLOW IN LATERALLY-HEATED CYLINDRICAL ENCLOSURES FOR CRYSTAL GROWTH." University of Akron / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=akron1561542061969173.
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Schreiber, Johannes. "Investigation of experimental and numerical methods, and analysis of stator clocking and instabilities in a high-speed multistage compressor." Thesis, Lyon, 2016. http://www.theses.fr/2016LYSEC063/document.
Повний текст джерелаАнотація:
Les études expérimentales et numériques suivantes visent à la compréhension profonde de l’écoulement se développant dans le compresseur haute-vitesse axial de 3.5 étages CREATE, étudié sur un banc d’essai de 2 MW au Laboratoire de Mécanique des Fluides et Acoustique (LMFA) à Lyon, France. Ce travail a trois objectifs principaux : D’abord, une description globale de l’écoulement avec une identification des limites aux méthodes d’exploration utilisées ; Ensuite, la caractérisation de l’effet du clocking stator-stator dans un compresseur à haute-vitesse ; Troisièmement, l’identification des instabilités à faibles débits pour confirmer les études sur les compresseurs à basse-vitesse et contribuer à plus de compréhension.Il est montré qu’une mauvaise interprétation des données de performance stationnaire se fait facilement en raison des contraintes de mesure et des coefficients de correction sont proposés. À certains endroits dans le compresseur, des limites aux méthodes d’exploration (expérimentales et numériques) de l’écoulement sont identifiées. Cette identification va permettre la poursuite du développement des méthodes. Les principales erreurs de prédiction des simulations concernent la surestimation du blocage induit par l’écoulement de jeu et l’augmentation de pression. En outre, les mesures fournies par les sondes de pression pneumatique surestiment la pression statique en amont des stators. Cette erreur est probablement provoquée par l’interaction entre le champ potentiel du stator et la sonde elle-même. De plus, l’anémométrie Doppler laser surestime la vitesse en aval des stators. Le transport des sillages du rotor à travers des stators n’est pas correctement capturé avec les particules d’ensemencement.Le clocking a seulement un petit effet global dans la bande d’incertitude de mesure dans ce compresseur. Plusieurs contributions à ce faible effet de clocking sont identifiées par l’analyse du transport des structures d’écoulement : Le mélange circonférentiel du sillage de stator et la déformation des sillages le long de leur trajet dans l’écoulement. L’effet local du clocking dépend de la hauteur de veine en raison de la variation de la forme des aubages et du transport des sillages. Des effets positifs et négatifs sont présentés, qui globalement se compensent dans ce compresseur. Les instabilités dans ce compresseur dépendent du point de fonctionnement et des méthodes d’exploration de l’écoulement. Aux points de fonctionnement stables et à la vitesse nominale du compresseur, les résultats numériques montrent une perturbation tournante dans les rotors 2 et 3, alors que les mesures montrent une perturbation tournante que dans le premier rotor et seulement à basse vitesse du compresseur. Dans les deux cas, les perturbations montrent des caractéristiques semblables. Une étude numérique permet d’exclure l’influence des interactions rotor-stator sur la perturbation tournante et met en évidence sa source. Des nouvelles connaissances sur le comportement stable et la périodicité du rotating instability (mesuré) sont dérivées contrairement au comportement instable suggéré par la dénomination et la littérature. Il est montré que cette perturbation évolue en cellule de décrochage tournante à l’approche de la limite de stabilité. A la vitesse nominale du compresseur, une entrée en instabilités de type spike est identifiée expérimentalement. Une description précise de l’apparition brutale du spike et sa différence par rapport à une cellule de décollement tournant sont présentéesThe following experimental and numerical investigations aim at the deep understanding of the flow field in the 3.5 stages high-speed axial compressor CREATE, studied on a 2 MW test rig at the Laboratory of Fluid Mechanics and Acoustics (LMFA) in Lyon, France. This work focuses on three major objectives: Firstly, a global description of the flow field with an identification of limitations to the used exploration methods; Secondly, the characterization of the effect of stator-stator clocking in a high-speed compressor; Thirdly, the identification of instabilities arising at low mass flow rates for confirming studies on low-speed compressors and giving new insights.This work demonstrates that a mis-interpretation of steady performance data occurs easily due to measurement constraints and correction coefficients are proposed. At certain locations in the compressor, the flow field exploration (experimental and numerical) methods are identified to be challenged. This identification will initiate further development of the methods. The main mis-predictions of the simulations concern the over-prediction of the blockage induced by the tip leakage flow and eventually an over-predicted pressure rise. Furthermore, the measurements provided by the pneumatic pressure probes over-estimate the static pressure upstream of the stators. This error is induced by the interaction between the stator potential field and the probe it-self. In addition, the laser Doppler anemometry method over-estimates the velocity downstream the stators. The transport of the rotor wakes through the stators might not be correctly captured with the seeding particles in this high-speed compressor.The investigation of the stator clocking reveals only a small global effect within the measurement uncertainty band. Several contributions to the weak effect of clocking are identified by analysis of the flow structure transport, namely the time-mean mixing out of the stator wakes and the deformation of wakes along their flow path. The local effect of clocking depends on the span-height because of the variation of the circumferential position of the stator wakes and the stator blade shape over the span-height. Local possible positive and negative effects of clocking are identified and are shown to be almost in balance in this compressor. Furthermore, this work demonstrates that the unsteadiness in the flow field is not linked conclusively to the stator clocking.In this compressor, the arising instabilities depend on the operating point and flow field exploration methods. At stable operating points and nominal compressor speed, the numerical results reveal a rotating disturbance in the rotors 2 and 3, whereas the measurements show a rotating disturbance only in the first rotor and only at part speed. In both cases the disturbance exhibits rotating instability like characteristics. An exhaustive numerical study allows to exclude the commonly assumed influence of rotor-stator interactions on the rotating disturbance and pinpoints its source. New insights into the stable behavior and periodicity of the measured rotating instability are derived contrary to the unstable behavior suggested by the naming and literature. This disturbance is shown to evolve into rotating stall cells when approaching the stability limit. At nominal compressor speed, a spike type surge inception is identified I n the measured field. A precise description of the abrupt onset of the spike cell and its difference to a rotating stall cell are presented
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Finke, Justin David. "Monte Carlo/Fokker-Planck simulations of Accretion Phenomena and Optical Spectra of BL Lacertae Objects." Ohio University / OhioLINK, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1181833339.
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Drbušková, Magdaléna. "Numerická analýza smršťování vybraných silikátových kompozitů." Master's thesis, Vysoké učení technické v Brně. Fakulta stavební, 2014. http://www.nusl.cz/ntk/nusl-226798.
Повний текст джерелаАнотація:
The thesis is divided into two main parts. In the first theoretical part is described the problems of shrinking including a comparison of Czech standard and Model Code 2010, Vol. 1. The second practical part of the master`s thesis is focused on the numerical analysis shrinkage primarily on the initial stage of this process. The experimentally obtained data are set approximations of the relative deformation using ShrCeC. Subsequently the numerical simulation of shrinkage of selected silicate specimens using a computer applications SpatiDist and FyDiK 2D. The real test specimens are modelled as two-component composite consisting of cement paste and aggregates. The result is a parametric study takes into account the influence of type and size of grain aggregate.
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Lundberg, Johan. "On the Search for High-Energy Neutrinos : Analysis of data from AMANDA-II." Doctoral thesis, Uppsala University, Department of Physics and Astronomy, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-8906.
Повний текст джерелаАнотація:
A search for a diffuse flux of cosmic neutrinos with energies in excess of 1014 eV was performed using two years of AMANDA-II data, collected in 2003 and 2004. A 20% evenly distributed sub-sample of experimental data was used to verify the detector description and the analysis cuts. A very good agreement between this 20% sample and the background simulations was observed. The analysis was optimised for discovery, to a relatively low price in limit setting power. The background estimate for the livetime of the examined 80% sample is 0.035 ± 68% events with an additional 41% systematical uncertainty.
The total neutrino flux needed for a 5σ discovery to be made with 50% probability was estimated to 3.4 ∙ 10-7 E-2 GeV s-1 sr-1 cm-2 equally distributed over the three flavours, taking statistical and systematic uncertainties in the background expectation and the signal efficiency into account. No experimental events survived the final discriminator cut. Hence, no ultra-high energy neutrino candidates were found in the examined sample. A 90% upper limit is placed on the total ultra-high energy neutrino flux at 2.8 ∙ 10-7 E-2 GeV s-1 sr-1 cm-2, taking both systematical and statistical uncertainties into account. The energy range in which 90% of the simulated E-2 signal is contained is 2.94 ∙ 1014 eV to 1.54 ∙ 1018 eV (central interval), assuming an equal distribution over the neutrino flavours at the Earth. The final acceptance is distributed as 48% electron neutrinos, 27% muon neutrinos, and 25% tau neutrinos.
A set of models for the production of neutrinos in active galactic nuclei that predict spectra deviating from E-2 was excluded.
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Maschio, Célio. "Desenvolvimento de metodos para avaliação de processos de filtração por meio de simulação por meio de simulação numerica e tomografia de raios-x." [s.n.], 2001. http://repositorio.unicamp.br/jspui/handle/REPOSIP/262896.
Повний текст джерелаАнотація:
Orientador: Antonio Celso Fonseca de ArrudaTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica
Made available in DSpace on 2018-07-31T14:51:24Z (GMT). No. of bitstreams: 1 Maschio_Celio_D.pdf: 13410400 bytes, checksum: bcad1a44963fabb3c498533b4663302a (MD5) Previous issue date: 2001
Resumo: O objetivo deste trabalho foi o desenvolvimento de metodologias para o estudo de filtros e processos de filtração, utilizando a tomografia de raios-X, técnicas de processamento digital de imagens e simulação numérica. Os ensaios convencionais utilizados pelos fabricantes são realizados através da medida de parâmetros, tais como pressão e vazão, em pontos localizados a montante e a jusante do filtro. Estes ensaios não são eficientes na caracterização de defeitos e não revelam como ocorre o processo de saturação no interior do filtro. Neste trabalho, a tomografia de raios-X foi utilizada para duas finalidades básicas: estudar a distribuição de contaminantes e analisar defeitos no interior de elementos filtrantes. Os modelos de filtração encontrados na literatura não levam em conta o efeito do acúmulo de contaminantes no desempenho do filtro ao longo do processo de filtração. Neste trabalho, foi desenvolvido um modelo acoplando-se as equações de filtração com as equações fenomenológicas (lei de Darcy e equação da continuidade), de tal forma que o acúmulo de partículas no meio poroso (filtro) fosse considerado na simulação do processo de filtração. Os resultados demonstraram que o processo de saturação no interior dos elementos analisados não ocorre de forma homogênea, ou seja, o acúmulo de partículas é predominante em determinadas regiões. Em geral, há formação de canais preferenciais e o espaço interno do filtro não é totalmente utilizado na captura de partículas. Demonstrou-se também que, em alguns casos, o comportamento do filtro não é coerente com a especificação do fabricante. Foi possível, por fim, a utilização dos dados experimentais, obtidos via tomografia, para a validação do modelo teórico desenvolvido
Abstract: The development of methods for analysis of filters and filtration process, using X-ray computerized tomography, digital image processing and numerical simulation, was the objective of this work. Tests conventionally used by filter manufacturers are made through the measurement ofparameters, such as flow and pressure, at upstream and downstream ofthe filter. These tests are not efficient in the characterization of defects and do not reveal how the saturation process occurs within the filter. In this work, X-ray computerized tomography was used for two basic purposes: to evaluate contaminant distribution and to detect defects within the interior of the filter elements. The filtration models found in the literature do not consider the effect of contaminant concentration on the filter efficiency during the filtration processo In this work, filtration equations and phenomenological equations (Darcy's law and continuity equation) were coupled and a model that takes into account the contaminant accumulation on the filter performance was developed. The results demonstrated that the saturation process within the analyzed filters is not homogeneous, that is, the accumulation of partic1es is predominant in some regions. Generally, there are preferential channels and the interior of the filter elements is not totally utilized. In some cases, it was also demonstrated that the filter behavior does not agree with the manufacturer specifications. Finally, it was possible to use the experimental data obtained with X-ray computerized tomography, in order to validate the theoretical developed model
Doutorado
Materiais e Processos de Fabricação
Doutor em Engenharia Mecânica
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Ferrand, Gilles. "Etude de l'accélération des rayons cosmiques par les ondes de choc des restes de supernovae dans les superbulles galactiques." Toulouse 3, 2007. http://thesesups.ups-tlse.fr/165/.
Повний текст джерелаАнотація:
Dans cette thèse nous étudions l'accélération des rayons cosmiques (RC), ces particules très énergétiques qui emplissent l'univers. Il est admis que les RC galactiques sont produits par accélération diffusive par onde de choc dans les restes de supernovae. La théorie linéaire explique la formation de spectres en loi de puissance, mais elle doit être modifiée du fait de la rétroaction des RC. Nous nous concentrons sur l'accélération répétée par chocs successifs, qui durcit les spectres, et qui dépend du transport des rayons cosmiques entre les chocs. Pour cette étude nous avons développé un outil numérique qui couple l'évolution hydrodynamique du plasma et le transport cinétique des RC. Nous l'avons validé grâce à des résultats déjà connus. Pour résoudre toutes les échelles induites par la dépendance en énergie du coefficient de diffusion des RC nous avons implémenté une technique de grille adaptative. Pour réduire le temps de calcul nous avons aussi parallélisé notre code, dans la dimension d'énergie. Cela nous permet de présenter les premières simulations de l'accélération non-linéaire par chocs multiples. Nous appliquons notre outil aux superbulles, les vastes structures chaudes et peu denses entourant les associations OB, car c'est probablement là que la plupart des supernovae explose en fait -- ce qui induit des modifications substantielles du modèle standard de production des RC galactiques. Plus précisément nous avons commencé à explorer les effets de chocs multiples, par une étude du rôle de RC pré-existants en amont d'une onde de choc. Pour finir nous passons en revue l'émission haute énergie des superbulles dans l'optique d'une production efficace de RCIn this thesis we study the acceleration of cosmic-rays, high-energy particles pervading the Universe. Galactic cosmic-rays are believed to be produced by diffusive shock acceleration in supernovae remnants. The linear theory explains the formation of power-law spectra, but it has to be amended because of the back-reaction of cosmic-rays. We focus our attention on repeated acceleration by successive shocks, which hardens the spectra, and relies on the transport of cosmic-rays between the shocks. For this study we have developped a numerical tool which couples the hydrodynamical evolution of the plasma with the kinetic transport of the cosmic-rays. We have validated it against previous results. To resolve all the space- and time-scales induced by the energy-dependent diffusion of cosmic-rays we have implemented an adaptive mesh refinement technique. To save more computational power we have also parallelized our code, in the energy dimension. This enables us to present the first numerical simulations of non-linear acceleration by multiple shocks. We apply our tool to superbubbles, the big hot tenuous structures surrounding OB associations, as this is probably the place where most supernovae explode -- leading to substantial modifications of the standard model of the production of Galactic cosmic-rays. More precisely we have begun to investigate the effects of multiple shocks, through studying the role of pre-existing cosmic-rays upstream of a blast wave. Finally we review the radiation from superbubbles with a view to efficient cosmic-ray production
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Zhang, J., J. Zheng, D.-S. Jeng, and Yakun Guo. "Numerical simulation of solitary wave propagation over a steady current." 2014. http://hdl.handle.net/10454/10205.
Повний текст джерелаАнотація:
YesA two-dimensional numerical model is developed to study the propagation of a solitary wave in the presence of a steady current flow. The numerical model is based on the Reynolds-averaged Navier-Stokes (RANS) equations with a k-ε turbulence closure scheme and an internal wave-maker method. To capture the air-water interface, the volume of fluid (VOF) method is used in the numerical simulation. The current flow is initialized by imposing a steady inlet velocity on one computational domain end and a constant pressure outlet on the other end. The desired wave is generated by an internal wave-maker. The propagation of a solitary wave travelling with a following/opposing current is simulated. The effects of the current velocity on the solitary wave motion are investigated. The results show that the solitary wave has a smaller wave height, larger wave width and higher travelling speed after interacting with a following current. Contrariwise, the solitary wave becomes higher with a smaller wave width and lower travelling speed with an opposing current. The regression equations for predicting the wave height, wave width and travelling speed of the resulting solitary wave are for practical engineering applications. The impacts of current flow on the induced velocity and the turbulent kinetic energy (TKE) of a solitary wave are also investigated.
National Natural Science Foundation of China Grant #51209083, #51137002 and #41176073, the Natural Science Foundation of Jiangsu Province (China) Grant #BK2011026, the 111 Project under Grant No. B12032, the Fundamental Research Funds for the Central University, China (2013B31614), and the Carnegie Trust for Scottish Universities
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Hsieh, Kun-Jung. "A Hybrid Numerical Simulation Approach for Turbulent Flows over Building-Like Obstacles." Thesis, 2008. http://hdl.handle.net/10012/4125.
Повний текст джерелаАнотація:
Computational fluid dynamics (CFD) has been widely applied to simulate turbulent flows in an urban environment. The two basic methodologies in CFD that have been applied here are a Reynolds-averaged Navier-Stokes (RANS) modeling and a large-eddy simulation (LES). The nature of the flow in a built-up urban area consisting of an arbitrary aggregation of buildings is dominated by unsteady large-scale turbulent structures. Recognizing that RANS is unable to correctly capture these turbulent structures while LES is associated with high computational costs, a hybrid RANS/LES methodology that combines the computational efficiency of RANS with the predictive accuracy of LES can be a promising simulation approach for the application to urban flows.
In the non-zonal approach of hybrid RANS/LES methodology, a single generalized turbulence model is used in the entire computational domain. This model can function as a RANS turbulence closure model or as a LES subgrid scale model, depending on the local grid resolution or flow properties. A variant of non-zonal approaches, referred as partially resolved numerical simulation (PRNS) in this study, obtains the generalized turbulence model from the rescaling of a conventional RANS model through the incorporation of a resolution control function (F_R). The resolution control function F_R is used to characterize the degree of modeling required to represent the unresolved scales of motion.
A new generalized functional form for F_R in PRNS is proposed in this thesis. The predictive performance of PRNS is compared with unsteady RANS (URANS) and LES computations, for a plane channel flow, and for fully-developed and developing flows over a matrix of cubes resembling a group of buildings. It is demonstrated that PRNS behaves similarly to LES, in terms of the predictions of the mean flow and turbulence, but outperforms URANS in general. This indicates PRNS is a promising approach for the simulation of complex turbulent flows in an urban environment.
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Chang, Shu-Hao. "Numerical simulation of steady and unsteady cavitating flows inside water-jets." Thesis, 2012. http://hdl.handle.net/2152/ETD-UT-2012-08-6310.
Повний текст джерелаАнотація:
A numerical panel method based on the potential flow theory has been refined and
applied to the simulations of steady and unsteady cavitating flows inside water-jet pumps.
The potential flow inside the water-jet is solved simultaneously in order to take the
interaction of all geometries (blades, hub and casing) into account. The integral equation
and boundary conditions for the water-jet problem are formulated and solved by
distributing constant dipoles and sources on blades, hub and shroud surfaces, and
constant dipoles in the trailing wake sheets behind the rotor (or stator) blades. The
interaction between the rotor and stator is carried out based on an iterative procedure by
considering the circumferentially averaged velocities induced on each one by the other.
The present numerical scheme is coupled with a 2-D axisymmetric version of the
Reynolds Averaged Navier-Stokes (RANS) solver to evaluate the pressure rise on the
shroud and simulate viscous flow fields inside the pump.
A tip gap model based on a 2-D orifice equation derived from Bernoulli’s obstruction
theory is implemented in the present method to analyze the clearance effect between the
blade tip and the shroud inner wall in a global sense. The reduction of the flow from
losses in the orifice can be defined in terms of an empirically determined discharge
coefficient (CQ) representing the relationship between the flow rate and the pressure
difference across the gap because of the viscous effect in the tip gap region.
The simulations of the rotor/stator interaction, the prediction of partial and super
cavitation on the rotor blade and their effects on the hydrodynamic performance
including the thrust/torque breakdown of a water-jet pump are presented. The predicted
results, including the power coefficient (P*), head coefficient (H*), pump efficiency (η),
thrust and torque coefficients (KT and KQ), as well as the cavity patterns are compared
and validated against the experimental data from a series of on the ONR AxWJ-2 pump
at NSWCCD.text
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Soares, António José Espadinha Vieira. "Numerical simulation of vegetated flows using RANS equations coupled with a porous media approach in OpenFOAM." Master's thesis, 2017. http://hdl.handle.net/10362/39373.
Повний текст джерелаАнотація:
The purpose of this Masters thesis is to study the use of an alternative formulation of a submerged vegetation layer in open channel flow, using a porous medium instead. The turbulent flow is modelled with the use of Reynolds-Averaged Navier-Stokes (RANS) equations using the open-source toolbox Open source Field Operation And Manipulation (OpenFOAM). The porous medium is defined according to the Darcy-Forchheimer model which requires the determination of the intrinsic permeability and passability coefficients. These parameters are estimated from the average geometric properties of the vegetation elements in a method validated in previously published studies. This method elimi- nates the ad hoc calibration required in the use of global drag coefficients in the RANS equations traditionally used to take into account the effects of submerged vegetation on open-channel flow. The use of this methodology has been previously partially validated using commercial numeric simulation codes. However, these tend to be costly solutions and very limited in their customizability. This study seeks to partially reproduce work done on the commercial code ANSYS-CFX in an open-source code environment as well as to try and conduct numeric studies of other open-channel flow experiments with a range of varied submerged vegetation parameters (ergo, porosity values) so as to not only test the robustness of the numeric code but of this methodology as well. This work’s methodology also took on a more simplified numerical solution approach by use of a less robust algorithm (with no time derivative) than previously conducted studies. It was then possible to further understand the types of phenomena present in this type of flow and the required theoretical considerations which should be taken into account so as to produce valid computational study results.
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(8674956), Sai Lakshminarayanan Balakrishnan. "Numerical Simulations of Spatially Developing Mixing Layers." Thesis, 2020.
Знайти повний текст джерелаАнотація:
Turbulent mixing layers have been researched for many years. Currently, research is focused on studying compressible mixing layers because of their widespread applications in high-speed flight systems. While the effect of compressibility on the shear layer growth rate is well established, there is a lack of consensus over its effect on the turbulent stresses and hence warrants additional research in this area. Computational studies on compressible shear layers could provide a deep cognizance of the dynamics of fluid structures present in these flow fields which in turn would be viable for understanding the effects of compressibility on such flows. However, performing a Direct Numerical Simulation (DNS) of a highly compressible shear layer with experimental flow conditions is extremely expensive, especially when resolving the boundary layers that lead into the mixing section. The attractive alternative is to use Large Eddy Simulation (LES), as it possesses the potential to resolve the flow physics at a reasonable computational cost. Therefore the current work deals with developing a methodology to perform LES of a compressible mixing layer with experimental flow conditions, with resolving the boundary layers that lead into the mixing section through a wall model. The wall model approach, as opposed to a wall resolved simulation, greatly reduces the computational cost associated with the boundary layer regions, especially when using an explicit time-stepping scheme. An in house LES solver which has been used previously for performing simulations of jets, has been chosen for this purpose. The solver is first verified and validated for mixing layer flows by performing simulations of laminar and incompressible turbulent mixing layer flows and comparing the results with the literature. Following this, LES of a compressible mixing layer at a convective Mach number of 0.53 is performed. The inflow profiles for the LES are taken from a precursor RANS solution based on the k-ε and RSM turbulence models. The results of the LES present good agreement with the reference experiment for the upstream boundary layer properties, the mean velocity profile of the shear layer and the shear layer growth rate. The turbulent stresses, however, have been found to be underpredicted. The anisotropy of the normal Reynolds stresses have been found to be in good agreement with the literature. Based on the present results, suggestions for future work are also discussed.
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Reuß, Silvia. "A Grid-Adaptive Algebraic Hybrid RANS/LES Method." Doctoral thesis, 2015. http://hdl.handle.net/11858/00-1735-0000-0028-8717-2.
Повний текст джерела
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Allamaprabhu, Yaravintelimath. "Turbulence Modeling for Predicting Flow Separation in Rocket Nozzles." Thesis, 2014. http://hdl.handle.net/2005/3046.
Повний текст джерелаАнотація:
Convergent-Divergent (C-D) nozzles are used in rocket engines to produce thrust as a reaction to the acceleration of hot combustion chamber gases in the opposite direction. To maximize the engine performance at high altitudes, large area ratio, bell-shaped or contoured nozzles are used. At lower altitudes, the exit pressure of these nozzles is lower than the ambient pressure. During this over-expanded condition, the nozzle-internal flow adapts to the ambient pressure through an oblique shock. But the boundary layer inside the divergent portion of the nozzle is unable to withstand the pressure rise associated with the shock, and consequently flow separation is induced.
Numerical simulation of separated flows in rocket nozzles is challenging because the existing turbulence models are unable to correctly predict shock-induced flow separation. The present thesis addresses this problem. Axisymmetric, steady-state, Reynolds-Averaged Navier-Stokes (RANS) simulations of a conical nozzle and three sub-scale contoured nozzles were carried out to numerically predict flow separation in over-expanded rocket nozzles at different nozzle pressure ratios (NPR). The conical nozzle is the JPL 45◦-15◦ and the contoured nozzles are the VAC-S1, the DLR-PAR and the VAC-S6-short. The commercial CFD code ANSYS FLUENT 13 was first validated for simulation of separated cold gas flows in the VAC-S1 nozzle. Some modeling issues in the numerical simulations of flow separation in rocket nozzles were determined. It is recognized that compressibility correction, nozzle-lip thickness and upstream-extension of the external domain are the sources of uncertainty, besides turbulence modeling.
In high-speed turbulent flows, compressibility is known to affect dissipation rate of turbulence kinetic energy. As a consequence, a reduction in the spreading rate of supersonic mixing layers occurs. Whereas, the standard turbulence models are developed and calibrated for incompressible flows and hence, do not account for this effect. ANSYS FLUENT uses the compressibility correction proposed by Wilcox [1] which modifies the turbulence dissipation terms based on turbulent Mach number. This, as shown in this thesis, may not be appropriate to the prediction of flow separation in rocket nozzles. Simulation results of the standard SST model, with and without the compressibility correction, are compared with the experimental data at NPR=22 for the DLR-PAR nozzle. Compressibility correction is found to cause under-prediction of separation location and hence its use in the prediction of flow separation is not recommended.
In the literature, computational domains for the simulation of DLR subscale nozzles have thick nozzle-lips whereas for the VAC subscale nozzles they have no nozzle-lip. Effect of nozzle-lip thickness on flow separation is studied in the DLR-PAR nozzle by varying its nozzle-lip thickness. It is found that nozzle-lip thickness significantly influences both separation location and post-separation pressure recovery by means of the recirculation bubbles formed at the nozzle-lip.
Usually, experimental values of free stream turbulence are unknown. So conventionally, to minimize solution dependence on the boundary conditions specified for the ambient flow, the computational domain external to the nozzle is extended in the upstream direction. Its effect on flow separation is studied in the DLR-PAR nozzle through simulations conducted with and without this domain extension. No considerable effect on separation location and pressure recovery is found.
The two eddy-viscosity based turbulence models, Spalart-Allmaras (SA) model and Shear Stress Transport (SST) model, are well known to predict separation location better than other eddy-viscosity models, but with moderate success. Their performances, in terms of predicting separation location and post-separation wall pressure distribution, were compared with each other and evaluated against experimental data for the conical and two contoured nozzles. It is found that they fail to predict the separation location correctly, exhibiting sensitivity to the range of NPRs and to the type of nozzle.
Depending on NPR, the SST model either under-predicts or over-predicts Free Shock Separation (FSS). Moreover, it also fails to capture Restricted Shock Separation (RSS). With compressibility correction, it under-predicts separation at all NPRs to a greater extent. Even though RSS is captured by using compressibility correction, the transition from FSS to RSS is over-predicted [2]. Early efforts by few researchers to improve predictions of nozzle flow separation by realizability corrections to turbulence models have not been successful, especially in terms of capturing both the separation types.
Therefore, causes of turbulence modeling failure in predicting nozzle flow separation correctly were further investigated. It is learnt that limiting of the shear stress inside boundary layer, due to Bradshaw’s assumption, and over-prediction of jet spreading rate are the causes of SST model’s failure in predicting nozzle flow separation correctly. Based on this physical reasoning, values of the a 1 parameter and the two diffusion coefficients σk,2 and σω,2 were empirically modified to match the predicted wall pressure distributions with experimental data of the DLR-PAR and the VAC-S6-short nozzles. The results confirm that accurate prediction of flow separation in rocket nozzles indeed depends on the correct prediction of spreading rate of the supersonic separation-jet. It is demonstrated that accurate RANS simulation of flow separation in rocket nozzles over a wide range of NPRs is feasible by modified values of the diffusion coefficients in turbulence model.
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Crimp, Steven Jeffrey. "Simulating sea-surface temperature effects on Southern African rainfall using a mesoscale numerical model." Thesis, 1996. https://hdl.handle.net/10539/24293.
Повний текст джерелаАнотація:
Dissertation submitted to the Faculty of Science, University of the Witwatersrand, for
completion of the Degree of' Master of ScienceThe atmospheric response of the Colorado State University Regional Atmospheric Modelling System (RAMS) to sea-surface temperature anomaliesis investigated. A period of four days was chosen from 21 to 24 January 1981, where focus was placed on the development and dissipation of a tropical-temperate trough across Southern Africa. Previous experimenting this mesoscalenumerical model have detemined the kinematic, moisture, and thermodynamic nature of these synoptic features. The research in this dissertation focuses specifically on the sensitivity of the numerical model's simulated responses to positive sea-surface temperature anomalies. Three separate experiments were devised, in which positive anomalous temperatures were added to the ocean surface north of Madagascar (in the tropical Indian Ocean), at the region of the Agulhas Current retroflection, and along the tropical African west coast (in the Northern Benguela and Angola currents). The circulation aspects of each sensitivity test were investigated through the comparison of simulated variables such as vapour and cloud mixing ratios, temperature, streamlines and vertical velocity, with the same variables created by a control simulation. The results indicate that for the first sensitivity test, (the Madagascar anomaly), cyclogenesis was initiated over the area of modified sea temperatures which resulted in a marginal decrease in continental precipitation. The second sensitivity test (over the Agulhas retroflection) produced a much smaller simulated response to the addition of anomalously warm sea temperatures than the tropical Indian Ocean anomaly. Instability and precipitation values increased over the anomalously warm retroflection region, and were slowly transferred along the westerly wave perturbation and the South African east coast. The third sensitivity experiment showed a predominantly localised simulated increase in precipitation over Gabon and the Congo, with the slow southward progression of other simulated circulation differences taking place. The small perturbations in each of the simulated meteorological responses are consistent with the expected climate response to anomalously warm sea-surface temperatures in those areas.
AC 2018