This thesis deals with the study of hybrid RANS/LES methods for modeling of turbulent flow with a focus on the DES method and its modifications. The theoretical part focuses on the description of turbulent flow and classical methods for its modeling. The following describes the hybrid RANS/LES methods, their principles and categories. Finally, the DES method is described in detail together with its improvement in form of DDES and IDDES methods. The practical part is devoted to the testing of DES and DDES on benchmark problems. We describe here used software OpenFOAM and numerical methods used to discretize the equations. One part is devoted to grid generation. The DES and the DDES methods are tested on two benchmarks: flat plate with zero pressure gradient and backward facing step. The simulaton results are compared with experimental data, with a focus on good modeling of the velocity profile near wall, turbulent viscosity and skin friction coefficient. Powered by TCPDF (www.tcpdf.org)

Les calculs de CFD industriels pour les écoulements turbulents commencent par une phase complexe de réalisation de maillage (calculs de fond de cuve, de plénum supérieur ou d’assemblages combustibles par exemple dans le domaine nucléaire). Les premières contraintes prises en compte sont le plus souvent géométriques (complexité, détail, intuition ou retour d'expérience concernant les endroits « importants » où le maillage doit être raffiné). On doit cependant respecter des contraintes inhérentes aux modèles de turbulence RANS (Reynolds Averaged Navier Stokes) utilisés notamment la taille de la première cellule de calcul à la paroi. Si on utilise un modèle dit « Haut-Reynolds » (k- ε standard, SSG, …), on ne devrait trouver que des cellules de paroi ayant un centre à une distance adimensionnelle au moins égale à 20 pour pouvoir d’une part justifier l'utilisation de la loi « universelle » logarithmique pour la vitesse et d’autre part, ce qui souvent occulté, respecter le fait que ces modèles ne sont pas conçus pour des distances plus basses. En revanche, si on utilise un modèle dit « Bas-Reynolds » (BL-v²/k, EB-RSM, …), on devrait partout avoir des cellules de paroi ayant un centre à une distance adimensionnelle de la paroi très faible. Si ces modèles sont utilisés avec une partie des cellules en paroi ayant une distance adimensionnelle nettement supérieure, les résultats peuvent être catastrophiques (le calcul peut ou bien diverger ou bien donner des résultats avec une physique totalement fausse). Cette thèse propose le développement d'un nouveau modèle de turbulence avec lois de paroi adaptatives qui donne des résultats satisfaisants quelque soit le type de maillage utilisé, en particulier quand ce dernier contient à la fois des cellules dont le centre est à une distance « Bas-Reynolds » et « Haut-Reynolds ». Étant donné les écoulements complexes des configurations industrielles, ce nouveau modèle s'appuie sur l'utilisation d'un modèle du second ordre connu pour son bon comportement : le modèle EB-RSM. Ce modèle permet de reproduire l'anisotropie de la turbulence et comble certaines lacunes des modèles du premier ordre. Ce modèle est disponible dans Code_Saturne, code open source développé par EDF et au sein duquel les développements ont été réalisés. / CFD computations of turbulent flows always begin with a complex meshing process (upper plenum, fuel assembly in the nuclear industry for example). Geometrical constraints are the first ones to be satisfied (level of details, important zones to refine regarding “user experiences”). One has however to satisfy constraints that are inherent to the RANS model (Reynolds Averaged Navier Stokes) used for the computation. For example, if a « High-Reynolds » (k-ε standard, SSG, …) model is used one should only have wall cells with a dimensionless distance to the wall greater or equal to 20 to justify the use of the universal “law of the wall”. On the other hand, if a « Low-Reynolds » (BL-v²/k, EB-RSM, …) model is used, one should only find wall cells with a dimensionless distance to the wall below 1. If those models are used in an inappropriate way the results could be dramatic (computations can either diverge or give unphysical results). This thesis proposes the development of a new turbulence model with adaptive wall treatments that gives satisfactory results on all types of meshes. In particular, the model will be able to cope with meshes containing both « High-Reynolds » and « Low-Reynolds » wall cells. Given the complex flows encountered in the nuclear industry this thesis will use a model known for its good behavior: the EB-RSM model. This model is able to reproduce the anisotropy of the turbulence and give more satisfactory results than eddy viscosity models in different configurations. This model is available in Code_Saturne, an open source code developed at EDF. Al the developments are made in this code.

Cette thèse s'inscrit dans le cadre des énergies renouvelables au sein du programme HARVEST centré sur le développement d'un concept d'hydrolienne dérivé des turbines Darrieus et Gorlov. L'ajout d'un dispositif appelé carénage à la turbine permet à celle-ci d'extraire une portion de l'énergie cinétique du courant plus grande. Toutefois ce dernier peut favoriser la cavitation qui nuit à la turbine. Parmi les différents axes du programme, les travaux de thèse se situent dans cette problématique. En régime subcavitant et cavitant, l'analyse de l'hydrolienne a été menée suivant une approche numérique et expérimentale. Pour ce faire deux outils ont été mis en place. Du coté expérimental, le tunnel hydrodynamique du LEGI a été équipé d'une balance qui donne la mesure instantanée des forces et du couple qui s'exercent sur la turbine. Du coté numérique, les efforts ont été orientés sur l'amélioration et le développement du code de calcul universitaire, CAVKA. L'utilisation intensive de ces deux moyens, couplée à des modèles théoriques, a permis de mettre en évidence d'une part le fonctionnement de la turbine libre ou carénée et, d'autre part, les limites de fonctionnement vis-à-vis de la cavitation. / The general context of the present thesis is renewable energies within the HARVEST project, which consists in a water current turbine (WCT) development, inspired from the Darrieus and Gorlov geometries. The main advantage of the HARVEST WCT is the introduction of a channelling device, which allows extracting a bigger amount of the kinetic energy contained in the flowstream. However, the shrouding device can eventually increase cavitating risks, which generally damage the WCT itself and its performance. The main topic of this work is cavitation. The hydrodynamic behavior of the WCT is analyzed both numerically and experimentally, in non cavitating and cavitating conditions. For this analysis, two devices have been developed. On the one hand, the LEGI hydrodynamic channel is equipped with a measurement platform which provides the instantaneous and average measurements of two dimensional thrusts as well as the hydrodynamic torque. On the other hand, in the numerical domain, the work has been oriented to the improvement and the development of a CFD code, named Cavka. The intensive utilisation of these two devices, coupled to theoretical models, allow highlighting the functioning of the bare and shrouded WCTs and their limits in cavitating conditions.

Ce travail porte sur l’évaluation de la méthode ZDES pour la simulation de jets propulsifs. L’analyse se concentre sur le positionnement des cellules de chocs et le développement des couches de mélange d’une tuyère double-flux avec plug externe, typique des moteurs d’avions modernes. Les champs statistiques sont comparés aux résultats expérimentaux et discutés en termes de grandeurs moyennes, fluctuantes et dans le domaine fréquentiel. L’intérêt d’utiliser un schéma spatial peu dissipatif ainsi qu’une échelle de longueur sous-maille basée sur la vorticité locale est mis en évidence, notamment pour le dévelopement de la couche de mélange interne, et le mode 2 ("automatique") de la ZDES a démontré un comportement similaire au mode 1 ("manuel") dans les couches de mélange. Par ailleurs, la technique Random Flow Generation (RFG) mise en oeuvre afin de reproduire la turbulence amont existant au coeur des jets primaire et secondaire a permis d’accélérer la transition RANS-LES dans les deux couches de mélanges, plus conformément à l’expérience. La transition est d’autant plus rapide que le taux de turbulence est élevé et l’échelle de la turbulence injectée est petite. Le positionnement des cellules de choc est également amélioré, soulignant l’importance de prendre en compte la turbulence amont dans les simulations de jets. / In this thesis, the ZDES method is assessed for the simulation of propulsive jets. This work focuses on the shock-cell positioning and the mixing layer development of a dual-stream nozzle configuration with an external plug, typical of modern aircraft engines. Reynolds averaged data are discussed in terms of mean and fluctuating quantities as well as in the frequency domain and compared with experimental data. First, the advantage of using a low dissipative spatial scheme as well as a subgrid length scale based on the local vorticity is demonstrated, especially for the development of the core mixing layer. Besides, the "automatic" mode of ZDES (mode 2) is found to provide similar mixing layers as the user defined mode.Then, the use of the Random Flow Generation (RFG) technique at the inlet boundaries of the core and fan channels in order to reproduce the turbulence rate at the center of the nozzle ducts is shown to accelerate the RANS-to-LES transition in both external and internal mixing layers, which is in better agreement with the experimental results. The transition length is further reduced when the injected turbulent ratio is higher, but also when the injected turbulent length scale is smaller. Of interest, the shock-cell positioning in the fan jet is also improved using RFG, which emphasizes the importance of accounting for upstream turbulence for this type of simulations.

Modeling of complex flows involving the combined effects of flow transition and streamline curvature using two advanced turbulence models, one in the Reynolds-averaged Navier-Stokes (RANS) category and the other in the hybrid RANS-Large eddy simulation (LES) category is considered in this research effort. In the first part of the research, a new scalar eddy-viscosity model (EVM) is proposed, designed to exhibit physically correct responses to flow transition, streamline curvature, and system rotation effects. The four equation model developed herein is a curvature-sensitized version of a commercially available three-equation transition-sensitive model. The physical effects of rotation and curvature (RC) enter the model through the added transport equation, analogous to a transverse turbulent velocity scale. The eddy-viscosity has been redefined such that the proposed model is constrained to reduce to the original transition-sensitive model definition in nonrotating flows or in regions with negligible RC effects. In the second part of the research, the developed four-equation model is combined with a LES technique using a new hybrid modeling framework, dynamic hybrid RANS-LES. The new framework is highly generalized, allowing coupling of any desired LES model with any given RANS model and addresses several deficiencies inherent in most current hybrid models. In the present research effort, the DHRL model comprises of the proposed four-equation model for RANS component and the MILES scheme for LES component. Both the models were implemented into a commercial computational fluid dynamics (CFD) solver and tested on a number of engineering and generic flow problems. Results from both the RANS and hybrid models show successful resolution of the combined effects of transition and curvature with reasonable engineering accuracy, and for only a small increase in computational cost. In addition, results from the hybrid model indicate significant levels of turbulent fluctuations in the flowfield, improved accuracy compared to RANS models predictions, and are obtained at a significant reduction of computational cost compared to full LES models. The results suggest that the advanced turbulence modeling techniques presented in this research effort have potential as practical tools for solving low/high Re flows over blunt/curved bodies for the prediction of transition and RC effects.

No description available.

The comparison of aerodynamic characteristics for a combat aircraft studywas addressed in this work. The thesis is a feasibility study which reviewsthe workload and output quality efficiency of different numerical and experimentalmethods often used during conceptual aircraft design.For this reason the Vortex Lattice Method (VLM), Euler or Reynolds-Averaged-Navier-Stokes (RANS) simulations were compared to the moreheavier Large Eddy Simulation (LES) which also has the capability to capturealso more complex flow physics, such as those that occur, for example,at high angles of attack. To be able to crosscheck the numerical results,the same static alpha sweep tests were executed in a tunnel. Thereby itwas discovered that it was quite challenging to reach the same values in thewater tunnel as those previously calculated in computational fluid dynamics(CFD) due to different technical issues.However it could be shown that LES simulations can be today a suitabletool for conceptual aircraft design, as they offer much higher levels ofaccuracy and give the designer the possibility to check the new study at anearly stage along the border of the aircraft’s flight envelope.

The use of Large Eddy Simulation (LES) for modelling air flows in buildings is a growing area of Computational Fluid Dynamics (CFD). Compared to traditional CFD techniques, LES provides a more detailed approach to modelling turbulence in air. This offers the potential for more accurate modelling of low energy natural ventilation which is notoriously difficult to model using traditional CFD. Currently, very little is known about the performance of LES for modelling natural ventilation, and its computational intensity makes its practical use on desk top computers prohibitive. The objective of this work was to apply LES to a variety of natural ventilation strategies and to compile guidelines for practitioners on its performance, including the trade-off between accuracy and cost.

Reynolds average Navier-Stokes (RANS) modeling has established itself as a critical design tool in many engineering applications, thanks to its superior computational efficiency. The drawbacks of RANS models are well known, but not necessarily well understood: poor prediction of transition, non-equilibrium flows, mixing and heat transfer, to name the ones relevant to our study. In the present study, we use a direct numerical simulation (DNS) of a reciprocating channel flow driven by an oscillating pressure gradient to test several low- and high-Reynolds' RANS models. Temperature is introduced as a passive scalar to study heat transfer modeling. Low-Reynolds' models manage to capture the overall physics of wall shear and heat flux well, yet with some phase discrepancies, whereas high-Reynolds' models fail. We have derived an integral method for wall shear and wall heat flux analysis, which reveals the contributing terms for both metrics. This method shows that the qualitative agreement appears more serendipitous than driven by the ability of the models to capture the correct physics. The integral method is shown to be more insightful in the benchmarking of RANS models than the typical comparisons of statistical quantities. This method enables the identification of the sources of discrepancies in energy budget equations. For instance, in the wall heat flux, one model is shown to have an out of phase dynamic behavior when compared to the benchmark results, demonstrating a significant issue in the physics predicted by this model. Our study demonstrates that the integral method applied to RANS modeling yields information not previously available that should guide the derivation of physically more accurate models.

<div>Gas-turbine engines use film cooling to actively cool turbine components and keep thermal loads on the materials at acceptable levels for structural integrity and service life. The turbulent mixing between the film-cooling jet and the crossflow decreases the coolant temperature, which reduces the cooling performance. This turbulent mixing is sensitive to parameters such as density ratio (DR), blowing ratio (BR), velocity ratio (VR), and momentum-flux ratio (MR) and understanding the effects of these parameters on the turbulent mixing is critical for improving film cooling. </div><div><br></div><div>This research seeks to improve understanding by using large-eddy simulation (LES) as a tool to analyze the turbulence of film cooling. With this knowledge it is possible evaluate more fundamental turbulence modeling assumptions utilized by Reynolds-Averaged Navier-Stokes (RANS) approaches as they apply to film cooling. This analysis can provide insight regarding how to improve turbulence models.</div><div><br></div><div>The film-cooling problem studied involves the cooling of a flat plate, where the cooling jets issued from a plenum through one row of circular holes of diameter $D$ and length 4.7$D$ that are inclined at 35$^\circ$ relative to the plate. Parameters studied include BR = 0.5 - 1.3, DR = 1.1 - 2.1, VR = 0.3 - 0.9, and MR = 0.16 - 0.9. For LES, two different boundary layers upstream of the film-cooling hole were investigated - one in which a laminar boundary layer was tripped to become turbulent from near the leading edge of the flat plate, and another in which a mean turbulent BL is prescribed directly without any superimposed turbulent fluctuations. For RANS, two different turbulence models were investigated - realizable $k$-$\epsilon$ and $k$-$\omega$ shear-stress-transport (SST). The wall-resolved LES solutions generated are extensively verified and validated using analytical, DNS, and experimental measurements to ensure high quality. </div><div><br></div><div>LES results obtained show that having an upstream boundary layer that does not have turbulent fluctuations enhances the cooling effectiveness significantly at low VRs when compared to an upstream boundary layer that resolved the turbulent fluctuations. However, these differences diminish at higher VRs. Instantaneous flow reveals a bifurcation in the jet vorticity as it exits the hole at low VRs, one branch forming the shear-layer vortex, while the other forms the counter-rotating vortex pair. At higher VRs, the shear layer vorticity is found to reverse direction, changing the nature of the turbulence and the heat transfer. Results obtained also show the strength and structure of the turbulence in the film-cooling jet to be strongly correlated to VR. </div><div><br></div><div>RANS results obtained show the turbulent and thermal structure of the jets predicted by the two RANS models to differ considerably. However, both models are consistent in underpredicting the spread of the film-cooling jet. The counter-rotating vortex pair dominates the interaction of the jet and crossflow in the near-wall region, and neither RANS model could predict the strength and structure of this interaction. The gradient-diffusion and Boussinesq hypotheses were evaluated by using the LES data. Comparing LES and RANS results shows that $k$-$\epsilon$ tends to overpredict eddy viscosity, while SST tends to underpredict the eddy viscosity. Additionally, both models predict very low values of eddy viscosity near the wall which leads to incorrect Reynolds stresses. While regions of counter-gradient diffusion and stress-strain misalignment were identified in the near-wall region, further above the wall, the jet behaved according to the hypotheses.</div><div><br></div><div>The turbulence scaling when VR is fixed at 0.46 and 0.63 was investigated. The LES results show that separation and spreading of the film-cooling jet increase as BR, DR, and MR increase while VR remains constant. For a given VR, the LES predicts an absolute difference between the minimum adiabatic effectiveness of the lowest and highest MRs to be 2 to 5 times greater than those predicted by RANS. This is because RANS with either model cannot respond appropriately to changes in MR. However, RANS can correctly predict that adiabatic effectiveness decreases as VR increases. The LES results show the turbulent kinetic energy and Reynolds stresses near the film-cooling hole to change considerably with MRs at a constant VR, while turbulent heat flux changes negligibly. This suggests that while improved turbulence models for heat flux can improve RANS’ prediction of spreading, capturing trends, however, requires improved modeling of the Reynolds stresses.</div>