Evaluation of Mesh Characteristics for Scale-Resolving Simulation of Incompressible Flows

RÉSUMÉ L'objectif principal de cette recherche est la simulation d'écoulements similaires à ceux rencontrés à l'intérieur des aspirateurs des turbines hydrauliques lorsque celles-ci fonctionnent hors des conditions nominales. L'importance de cette application réside dans le fait que les turbines doivent souvent être exploitées dans une gamme étendue de conditions de fonctionnement, y compris des conditions hors du point du rendement maximum. Ceci s'explique par le fait que l'hydroélectricité joue un rôle important en tant que source flexible d'alimentation en énergie pour le réseau électrique. L'énergie hydro-électrique est particulièrement importante dans la mesure où des sources d'énergie intermittentes telles que l'énergie solaire et éolienne font désormais partie du marché. Cependant, élargir les gammes de conditions de fonctionnement rend plus cruciale l'analyse des contraintes fluctuantes. Celles-ci peuvent en effet entraîner des instabilités, des défaillances mécaniques du système et également des oscillations de puissance spontanées sur le réseau. Par conséquent, la compréhension et l'atténuation du comportement instable des turbines hydrauliques est centrale. Les approches SRS (Scale Resolving Simulation) telles que les LES et DES ont suscité beaucoup d'intérêt au cours de la dernière décennie pour une compréhension plus complète du comportement opérationnel instable des turbines hydrauliques. Cet intérêt s'explique par leur capacité à résoudre une partie de l'écoulement turbulent. Cependant, pour certains écoulements industriels, comme ceux à charge partielle, à charge partielle profonde ou à vide, pour lesquels les données expérimentales sont insuffisantes pour une compréhension approfondie des phénomènes, la fiabilité des simulations numériques en termes de dépendance au maillage est toujours un problème en suspens. Les études de vérification en LES sont également très difficiles, car les erreurs de discrétisation numérique et de modélisation des échelles sont toutes deux influencées par la résolution du maillage. Un examen approfondi de la littérature montre que les résultats SRS des différentes conditions de fonctionnement des turbines hydrauliques sont encore assez limités et qu'il n'y a pas de consensus sur l'exigence de résolution pour ces études. Par conséquent, le but de cette recherche est de développer un cadre fiable pour la validation et la vérification des études SRS, et plus particulièrement les études LES, afin qu'elles puissent être utilisées pour l'analyse des phénomènes d'écoulement dans les aspirateurs et les roues des turbines hydrauliques, pour des conditions de fonctionnement hors conception. Plusieurs critères de résolution pour l'analyse LES ont été identifiés dans la littérature et leur applicabilité et leur sensibilité sont examinées. Deux principaux cas test sont considérés dans cette recherche: l'écoulement turbulent dans un canal et un cas d'expansion soudaine. Dans cette étude, nous n'irons pas plus loin dans les applications aux turbines hydrauliques, mais celles-ci bénéficieront à terme des résultats des recherches en cours. Les résultats montrent que l'autocorrélation entre deux points est plus sensible à la résolution du maillage que le spectre énergétique. De plus, dans le cas d'une expansion soudaine, la résolution du maillage a un effet énorme sur les résultats et jusqu'à présent, nous n'avons pas capté de comportement de convergence asymptotique dans les résultats de RMS des fluctuations de vitesse et d'autocorrélation en deux points. Ce cas, qui représente un comportement d'écoulement complexe, nécessite d'autres études de résolution de maillage.----------ABSTRACT The central aim of this research is on the simulation of flows similar to the ones which occur inside hydraulic turbine draft-tubes at off-design operating conditions. The importance of this application is due to the fact that hydroturbines often need to be operated over an extended range of operating conditions including off-design conditions, since hydropower plays a significant role as a flexible source of energy supply to the electric network. This significance is due to the integration of non-dispatchable sources of energy such as solar and wind power. This range of operating conditions, however, makes the investigation of fluctuating stresses more crucial. Load fluctuations lead to instability, system mechanical failure and also to spontaneous power swings to the grid. Consequently, understanding and mitigating unsteady operational behavior of hydro turbines is very crucial. SRS approaches such as LES and DES have received more interests in the recent decade for understanding and mitigating unsteady operational behavior of hydro turbines. This interest is due to the ability of these methods to resolve part of turbulent flow. However, for some industrial flows, where there is no adequate experimental data for deep understanding of the flow physics, such as the ones which happen at part load, deep part load and speed no-load operation of hydraulic turbines, the reliability of numerical simulations in terms of their grid-dependency is still an open question. Verification studies in LES are also very challenging, since errors in numerical discretization and also subgrid-scale-model are both influenced by grid resolution. Comprehensive examination of the literature shows that the SRS of different operating condition of the hydraulic turbines is still quite limited and that there is no consensus on the resolution requirement of SRS studies. Therefore, the goal of this research is to develop a reliable framework for validation and verification of SRS , specially LES, so that it can be applied for the investigation of flow phenomena in hydraulic turbines draft-tubes and runners at their off-design operating conditions. Several resolution criteria for LES analysis have been identified in the literature and their applicability and the level of insight which they put into our analysis are scrutinized. Two main test cases are considered in this research, turbulent channel flow and a case of sudden expansion. In this study we will not further go to the real applications and simulations in hydraulic turbines. Hydraulic turbines will eventually benefit from the results of the current research. The results show that two-point autocorrelation is more sensitive to mesh resolution that energy spectra. In addition, for the case of sudden expansion, the mesh resolution has a tremendous effect on the results and so far, we did not capture an asymptotic converging behaviour in the results of RMS of velocity fluctuations and two-point autocorrelation. This case, which represents complex flow behaviour, needs further mesh resolution studies

Flow Control of Transonic Airfoils using Optimum Suction and Injection Parameters

In this paper, the application of the surface mass transfer optimization in shock wave-boundary layer interaction control at off-design conditions of transonic aircraft wing is presented. The suction or injection parameters include for example its position on the airfoil, its angle, the length of the hole and the rate of the injected or sucked flow. The optimization process is carried out using an efficient Genetic Algorithm (GA) method. The compressible viscous flow equations in Reynolds Averaged form are solved together with a two-equation k-epsilon turbulence model to accurately compute the objective function. Four different objective functions are introduced including maximum lift to drag ratio, minimum drag coefficient, maximum lift to drag ratio with no drag increment and minimum drag coefficient with no lift decrement. Effectiveness of each objective function is examined by comparing the optimum results in terms of the flow control parameters and flow characteristics

On the evaluation of mesh resolution for large-eddy simulation of internal flows using openfoam

ABSTRACT: The central aim of this paper is to use OpenFOAM for the assessment of mesh resolution requirements for large-eddy simulation (LES) of flows similar to the ones which occur inside the draft-tube of hydraulic turbines at off-design operating conditions. The importance of this study is related to the fact that hydraulic turbines often need to be operated over an extended range of operating conditions, which makes the investigation of fluctuating stresses crucial. Scale-resolving simulation (SRS) approaches, such as LES and detached-eddy simulation (DES), have received more interests in the recent decade for understanding and mitigating unsteady operational behavior of hydro turbines. This interest is due to their ability to resolve a larger part of turbulent flows. However, verification studies in LES are very challenging, since errors in numerical discretization, but also subgrid-scale (SGS) models, are both influenced by grid resolution. A comprehensive examination of the literature shows that SRS for different operating conditions of hydraulic turbines is still quite limited and that there is no consensus on mesh resolution requirement for SRS studies. Therefore, the goal of this research is to develop a reliable framework for the validation and verification of SRS, especially LES, so that it can be applied for the investigation of flow phenomena inside hydraulic turbine draft-tube and runner at their off-design operating conditions. Two academic test cases are considered in this research, a turbulent channel flow and a case of sudden expansion. The sudden expansion test case resembles the flow inside the draft-tube of hydraulic turbines at part load. In this study, we concentrate on these academic test cases, but it is expected that hydraulic turbine flow simulations will eventually benefit from the results of the current research. The results show that two-point autocorrelation is more sensitive to mesh resolution than energy spectra. In addition, for the case of sudden expansion, the mesh resolution has a tremendous effect on the results, and, so far, we have not capture an asymptotic converging behavior in the results of Root Mean Square (RMS) of velocity fluctuations and two-point autocorrelation. This case, which represents complex flow behavior, needs further mesh resolution studies

On the Evaluation of Mesh Resolution for Large-Eddy Simulation of Internal Flows Using Openfoam

The central aim of this paper is to use OpenFOAM for the assessment of mesh resolution requirements for large-eddy simulation (LES) of flows similar to the ones which occur inside the draft-tube of hydraulic turbines at off-design operating conditions. The importance of this study is related to the fact that hydraulic turbines often need to be operated over an extended range of operating conditions, which makes the investigation of fluctuating stresses crucial. Scale-resolving simulation (SRS) approaches, such as LES and detached-eddy simulation (DES), have received more interests in the recent decade for understanding and mitigating unsteady operational behavior of hydro turbines. This interest is due to their ability to resolve a larger part of turbulent flows. However, verification studies in LES are very challenging, since errors in numerical discretization, but also subgrid-scale (SGS) models, are both influenced by grid resolution. A comprehensive examination of the literature shows that SRS for different operating conditions of hydraulic turbines is still quite limited and that there is no consensus on mesh resolution requirement for SRS studies. Therefore, the goal of this research is to develop a reliable framework for the validation and verification of SRS, especially LES, so that it can be applied for the investigation of flow phenomena inside hydraulic turbine draft-tube and runner at their off-design operating conditions. Two academic test cases are considered in this research, a turbulent channel flow and a case of sudden expansion. The sudden expansion test case resembles the flow inside the draft-tube of hydraulic turbines at part load. In this study, we concentrate on these academic test cases, but it is expected that hydraulic turbine flow simulations will eventually benefit from the results of the current research. The results show that two-point autocorrelation is more sensitive to mesh resolution than energy spectra. In addition, for the case of sudden expansion, the mesh resolution has a tremendous effect on the results, and, so far, we have not capture an asymptotic converging behavior in the results of Root Mean Square (RMS) of velocity fluctuations and two-point autocorrelation. This case, which represents complex flow behavior, needs further mesh resolution studies