Добірка наукової літератури з теми "RANS numerical simulation"

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 industrielles<br>Internal 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

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-&epsilon<br>model, Shear Stress Transport k-&omega<br>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<br>and Shear Stress Transport k-&omega<br>) in terms of accuracy and Reynolds Stress Model in terms of convergence.

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.

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.

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

Dissertation (Ph.D.)--University of Toledo, 2004.<br>Typescript. "A dissertation [submitted] as partial fulfillment of the requirements of the Doctor of Philosophy degree in Engineering." Bibliography: leaves 323-340.

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.<br>Master of Science

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 présentés<br>The 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