Dans la conception des turbomachines modernes, la prédiction des phénomènes aéroélastiques est devenue un point clé. La tendance à réduire la masse et à augmenter la charge des composants aérodynamiques accroit le risque de rupture. Dans un tel contexte, la compréhension et la bonne prédiction des diverses instabilités constituent un enjeu industriel et scientifique majeur. Le présent travail de recherche a pour objectif d’améliorer la prédiction des phénomènes instationnaires intervenant dans les problèmes d’aéroélasticité en turbomachines. Cette thèse est plus particulièrement axée sur la simulation de l’interaction onde de choc/couche limite. Le support d’étude est une tuyère transsonique présentant un écoulement avec des zones décollées. L’oscillation forcée de l’onde de choc est simulée grâce à une méthode de petites perturbations instationnaires couplée avec une hypothèse de turbulence variable. Cette approche est validée par comparaison avec des mesures. Elle permet une prédiction tout à fait satisfaisante du premier harmonique de pression sur la paroi de la tuyère. Ce travail a montré la nécessité de linéariser le modèle de turbulence. Le besoin de dériver le modèle de turbulence nous a amené à investiguer la modélisation faite pour prédire l’interaction onde de choc/couche limite. Un modèle de turbulence à deux équations complété par une équation de « retard » est implémenté afin de capter un déséquilibre de la turbulence. Les résultats obtenus en tuyère sont cohérents avec la théorie mais une surproduction d’énergie turbulente en présence de bord d’attaque rend le modèle inefficace pour des configurations de turbomachines. Au final, l’introduction d’un limiteur de viscosité turbulente dans un modèle de turbulence à deux équations s’avère donner de bons résultats. La méthode de dérivation du modèle est alors présentée sur le modèle de Wilcox proposé en 2008. Enfin, la technique de linéarisation est étendue à la problématique aéroélastique. Une approche de couplage fluide-structure faible est adoptée. L’oscillation structurelle des aubages suivant les modes propres est considérée mais en laissant la fréquence évoluer au cours du couplage. La nouvelle méthode utilisée s’appuie sur la construction d’un méta-modèle du comportement dynamique du fluide afin de résoudre directement le système fluide-structure couplé. Cette technique est validée sur une configuration de grille annulaire de turbine en haut subsonique et présente l’avantage d’un temps de calcul réduit. / In modern turbomachinery design, predicting aerolastic phenomena has become a key point. The development of highly loaded components, while reducing their weight, increases the risk of failure. In this context, good understanding and prediction of various instabilities are a major industrial and scientific challenge. This research work aims to improve the prediction of unsteady phenomena involved in turbomachinery aeroelasticity. This study focuses especially on the simulation of shock wave/boundary layer interaction. To begin with, a transonic nozzle separated flow is investigated. Forced oscillation of the shock wave system is simulated through a small unsteady perturbation method combined with the assumption of variable turbulence. This approach is validated against exprimental measurements. The first harmonic of pressure on the wall of the nozzle is predicted quite satisfactorily. The need to linearize the turbulence model was shown of high importance. Deriving the turbulence model, leads us to investigate the turbulence modeling performed to predict the shockwave/boundary layer interaction. A two equations turbulence model supplemented by a "time-lagged" equation is implemented to capture non-equilibrium effects of turbulence. All achieved results for a nozzle are consistent with theory, but overproduction of turbulent kinetic energy at leading edge makes the model useless for turbomachinery configurations. However, the introduction of an eddy viscosity stress limiter inside a two-equation turbulence model proves to give good results. The derivation method is thus presented on such a model, precisely on Wilcox model proposed in 2008. Finally, the linearization technique is extended to aeroelastic problems. A loose fluid-structure coupling strategy is adopted. The structural oscillation of the blades is considered for eigen-modes but frequency is free to change during coupling resolution. The new approach is based on the building of a meta-model to describe the fluid dynamic behavior in order to solve directly the coupled fluid-structure system. This technique is validated on a standard high subsonic turbine configuration and takes advantage of a reduced computation time.

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>

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.

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.