Modern turbomachines are designed towards thinner, lighter and highly loaded blades. This gives rise to increased sensitivity to flow induced vibrations such as flutter, which leads to structure failure in a short period of time if not sufficiently damped. Although numerical tools are more and more reliable, flutter prediction still depends on a large degree on simplified models. In addition, the critical nature of flutter, resulting in poor well-documented real cases in the open literature, and the lack of experimental database typical of engine flows make its apprehension even more challenging.  In that context, the present thesis is dedicated to study flutter in recent turbines through aerodynamic analysis of subsonic or supersonic flows in response to a prescribed vibratory mode of the structure. The objective is to highlight some mechanisms potentially responsible for flutter in order to be in better position when designing blades. The strategy consists in leading both experimental and numerical investigations.  The experimental part is based on a worldwide unique annular turbine sector cascade employed for measuring the aeroelastic response by means of the aerodynamic influence coefficient technique. The cascade comprises seven low pressure gas turbine blades one of which can oscillate in a controlled way as a rigid body. Aeroelastic responses are measured at various mechanical and aerodynamic parameters: pure and combined modeshapes, reduced frequency, Mach number, incidence angle. In addition to turbulence level measurements, the database aims at assessing the influence of these parameters on the aerodynamic damping, at validating the linear combination principle and at providing input for numerical tools. The numerical part is based on unsteady computations linearized in the frequency domain and performed in the traveling wave mode. The focus is put on two industrial space turbines: 2D computations are performed on an integrally bladed disk, also called blisk; its very low viscous material damping results in complex motions with combined modes and extremely high reduced frequency. The blisk operates at low subsonic conditions without strong non-linearities. Although the blades have been predicted aeroelastically stable, an original methodology based on elementary decompositions of the blade motion is presented to identify the destabilizing movements. The results suggest that the so-called classical flutter is surprisingly prone to occur. Moreover, the aerodynamic damping has been found extremely sensitive to the interblade phase angle and cut-on/cut-off conditions. 3D computations are then performed on a supersonic turbine, which features shock waves and boundary layer separation. In contrast, the blade motion is of elementary nature, i.e. purely axial. The blades have been predicted aeroelastically unstable for backward traveling waves and stable for forward traveling waves. The low reduced frequencies allow quasi-steady analysis, which still account for flutter mechanisms: the shock wave motion establishes the boundary between stable and unstable configurations. / <p>QC 20111209</p>

Modern turbomachines are designed towards thinner, lighter and highly loaded blades. This gives rise to increased sensitivity to flow induced vibrations such as flutter, which leads to structure failure in a short period of time if not sufficiently damped. Although numerical tools are more and more reliable, flutter prediction still depends on a large degree on simplified models. In addition, the critical nature of flutter, resulting in poor welldocumented real cases in the open literature, and the lack of experimental database typical of engine flows make its apprehension even more challenging. In that context, the present thesis is dedicated to study flutter in recent turbines through aerodynamic analysis of subsonic or supersonic flows in response to a prescribed vibratory mode of the structure. The objective is to highlight some mechanisms potentially responsible for flutter in order to be in better position when designing blades. The strategy consists in leading both experimental and numerical investigations. The experimental part is based on a worldwide unique annular turbine sector cascade employed for measuring the aeroelastic response by means of the aerodynamic influence coefficient technique. The cascade comprises seven low pressure gas turbine blades one of which can oscillate in a controlled way as a rigid body. Aeroelastic responses are measured at various mechanical and aerodynamic parameters: pure and combined modeshapes, reduced frequency, Mach number, incidence angle. In addition to turbulence level measurements, the database aims at assessing the influence of these parameters on the aerodynamic damping, at validating the linear combination principle and at providing input for numerical tools. The numerical part is based on unsteady computations linearized in the frequency domain and performed in the traveling wave mode. The focus is put on two industrial space turbines: 2D computations are performed on an integrally bladed disk, also called blisk; its very low viscous material damping results in complex motions with combined modes and extremely high reduced frequency. The blisk operates at low subsonic conditions without strong non-linearities. Although the blades have been predicted aeroelastically stable, an original methodology based on elementary decompositions of the blade motion is presented to identify the destabilizing movements. The results suggest that the so-called classical flutter is surprisingly prone to occur. Moreover, the aerodynamic damping has been found extremely sensitive to the interblade phase angle and cut-on/cut-off conditions.* 3D computations are then performed on a supersonic turbine, which features shockwaves and boundary layer separation. In contrast, the blade motion is of elementary nature, i.e. purely axial. The blades have been predicted aeroelastically unstable for backward traveling waves and stable for forward traveling waves. The low reduced frequencies allow quasi-steady analysis, which still account for flutter mechanisms: the shock wave motion establishes the boundary between stable and unstable configurations.

L’intégration des énergies renouvelables sur le réseau électrique entraîne de nouveaux besoins chez les exploitants de centrales hydroélectriques. Ainsi, les fabricants de turbines hydrauliques doivent garantir des plages de fonctionnement de plus en plus étendues, afin d’assurer une plus grande flexibilité d’utilisation des machines. Pour les turbines Francis, un fonctionnement en-dehors des conditions nominales implique potentiellement une augmentation des sollicitations mécaniques, notamment à faible débit (charge partielle). Cette hausse est due à l’apparition de phénomènes hydrauliques dans l’écoulement, dont notamment les vortex inter-aubes. Pour garantir des plages de fonctionnement étendues à leurs clients, les fabricants se doivent de maîtriser l’impact de telles conditions d’opération sur la durée de vie de leurs machines. Il est donc nécessaire de mieux comprendre l’écoulement complexe dans la turbine et son impact mécanique à charge partielle. Dans ce contexte, cette thèse a une double approche expérimentale et numérique. L’analyse se base sur des mesures et des observations réalisées lors d’essais sur modèles réduits. Elles ont permis de corréler les phénomènes hydrauliques observés et l’évolution des fluctuations de pression et des déformations dynamiques mesurées, pour différents points de fonctionnement. Ces résultats ont notamment été utilisés pour estimer le phénomène de fatigue lors du fonctionnement continu d’une turbine à très faible charge. La simulation numérique des fluides (CFD) a également été utilisée pour mieux comprendre les mécanismes impliqués dans la formation des vortex inter-aubes, et pour prédire le chargement dynamique qui s’exerce sur la roue à charge partielle. La validation des résultats numériques est basée sur la comparaison avec les données expérimentales issues des précédents essais sur modèles réduits. / The integration of renewable energies into the electricity grid brings new needs for hydro power plant operators in terms of how they are operated. Consequently, hydraulic turbine manufacturers are required to extend their machine’s operating range in order to increase their flexibility. In the case of Francis turbines, dynamic stresses could increase in off-design conditions due to several hydraulic phenomena that appear in the flow, especially at partial load. One of them is the development of inter-blade vortices in the runner. In order to guarantee an extended operating range manufacturers have to control the impact of such operating conditions on their turbines lifetime. Therefore, a better understanding of complex partial load flows and their mechanical impact on the turbines is needed. In this context, this thesis uses both experimental and numerical approaches. Reduced scale model turbines were tested in order to correlate hydraulic phenomena observed in the flow and the evolution of pressure and strain fluctuations for different operating points. The results were then used to estimate the turbine fatigue in partial load conditions. Computational Fluid Dynamics was also used to better understand the formation of inter-blade vortices and to predict the dynamic loading on the runner at partial load. These numerical results were validated by comparison with the experimental data from the previous test rig measurements and observation campaigns.

Les aubes des turbomachines modernes sont de plus en plus fines, légères et chargées aérodynamiquement. Cette tendance accroît l'apparition de phénomènes aéroélastiques tel que le flottement qui conduit à la rupture si l'amortissement est insuffisant. Bien que les outils numériques soient de plus en plus robustes, la fiabilité de sa prédiction demeure insuffisante. La nature critique du phénomène et le manque de données expérimentales pour des écoulements typiques de l'industrie encouragent des travaux de recherche. Dans ce contexte, la présente thèse est dédiée à l'étude du flottement dans des configurations récentes de turbine à travers l'analyse aérodynamique des écoulements subsoniques ou supersoniques soumis à un mode de structure vibratoire imposé. L'objectif est de fournir des éléments de compréhension des mécanismes potentiellement générateurs de flottement pour une meilleure intégration lors de la conception des aubes. L’approche consiste à mener des travaux expérimentaux et numériques. La partie expérimentale s'appuie sur un secteur de grille annulaire constitué de sept aubes dont une peut osciller de manière contrôlée. Les fluctuations de pressions instationnaires générées sont mesurées avec la technique dite des coefficients d'influence pour différents paramètres mécaniques et aérodynamiques : déformées modales pures et combinées, fréquence réduite, nombre de Mach, angle d'incidence. Complétée par des mesures de niveau de turbulence, la base de données vise à évaluer l'influence de ces paramètres sur la réponse aéroélastique, à valider le principe de superposition linéaire et à soutenir les codes numériques. La partie numérique se base sur des calculs instationnaires linéarisés dans le domaine fréquentiel en utilisant la technique dite des "ondes propagatives" (traveling wave mode).Deux cas de turbines spatiales industrielles sont étudiés.• Des calculs 2D sont réalisés sur une turbine monobloc ou blisk. L'amortissement mécanique quasi-nul entraîne des déformées complexes avec couplage de modes et des fréquences réduites très élevées. Bien que les aubes soient prédites stables, une méthodologie basée sur des décompositions géométriques élémentaires est présentée afin d'identifier les contributions déstabilisantes. Les résultats aboutissent étonnamment aux conclusions de la théorie du flottement classique : la torsion est une source potentielle d'instabilité. De plus, le coefficient d'amortissement aérodynamique a été trouvé extrêmement sensible au déphasage interaube et aux fréquences de coupure (modes cut-on/cut-off).• Des calculs 3D sont ensuite réalisés sur une turbine supersonique. L'écoulement présente des ondes de chocs avec décollement de la couche limite et le mouvement de l'aube est de nature élémentaire, i.e. purement axial. Les aubes ont été prédites instables pour les modes rétrogrades et stables pour les modes propagatifs. En dépit des fortes hypothèses, des analyses quasi-stationnaires rendent compte des mécanismes de flottement : la phase entre le mouvement du choc et l'excitation établit la frontière entre configurations stable et instable. / Modern turbomachines are designed towards thinner, lighter and highly loaded blades. This gives rise to increased sensitivity to flow induced vibrations such as flutter, which leads to structure failure in a short period of time if not sufficiently damped. Although numerical tools are more and more reliable, flutter prediction still depends on a large degree on simplified models. In addition, the critical nature of flutter, resulting in poor welldocumented real cases in the open literature, and the lack of experimental database typical of engine flows make its apprehension even more challenging. In that context, the present thesis is dedicated to study flutter in recent turbines through aerodynamic analysis of subsonic or supersonic flows in response to a prescribed vibratory mode of the structure. The objective is to highlight some mechanisms potentially responsible for flutter in order to be in better position when designing blades. The strategy consists in leading both experimental and numerical investigations. The experimental part is based on a worldwide unique annular turbine sector cascade employed for measuring the aeroelastic response by means of the aerodynamic influence coefficient technique. The cascade comprises seven low pressure gas turbine blades one of which can oscillate in a controlled way as a rigid body. Aeroelastic responses are measured at various mechanical and aerodynamic parameters: pure and combined modeshapes, reduced frequency, Mach number, incidence angle. In addition to turbulence level measurements, the database aims at assessing the influence of these parameters on the aerodynamic damping, at validating the linear combination principle and at providing input for numerical tools. The numerical part is based on unsteady computations linearized in the frequency domain and performed in the traveling wave mode. The focus is put on two industrial space turbines: 2D computations are performed on an integrally bladed disk, also called blisk; its very low viscous material damping results in complex motions with combined modes and extremely high reduced frequency. The blisk operates at low subsonic conditions without strong non-linearities. Although the blades have been predicted aeroelastically stable, an original methodology based on elementary decompositions of the blade motion is presented to identify the destabilizing movements. The results suggest that the so-called classical flutter is surprisingly prone to occur. Moreover, the aerodynamic damping has been found extremely sensitive to the interblade phase angle and cut-on/cut-off conditions.• 3D computations are then performed on a supersonic turbine, which features shockwaves and boundary layer separation. In contrast, the blade motion is of elementary nature, i.e. purely axial. The blades have been predicted aeroelastically unstable for backward traveling waves and stable for forward traveling waves. The low reduced frequencies allow quasi-steady analysis, which still account for flutter mechanisms: the shock wave motion establishes the boundary between stable and unstable configurations.