<p>A new theory for predicting the windprofile over a canopy has been evaluated. The theory was first presented by Harman and Finnigan (2007). The theory relies on the forming of a mixing-layer above the canopy, due to different mean wind in and above the canopy. Characteristics from both mixing-layer and Monin Obukhov similarity theory have been used to develop the governingequations that give the wind profile. The theory has been used to calculate wind profiles for sixdifferent atmospheric stabilities. In order to evaluate the theory, profiles from the theory have beencompared to measurements from Jädraås forest, Sweden. Profiles from Monin Obukhov similarity theory were also used for comparison.In general the mixing-layer theory gives better results than Monin Obukhov similarity theory. Agreement with measurements is good in neutral conditions, but fails when the atmospheric stability is altered, especially in convective conditions. This is believed to be due to the canopy lacking in thickness. The mean wind speed is systematically underestimated and this is also believed to be caused by insufficient thickness of the canopy. A correction for this behaviour is proposed. The theory gives higher values of the mean wind speed in convective conditions with the correction and the calculated values of mean wind speed are closer to the measurements.</p>

Afin de répondre aux enjeux environnementaux, les fabricants de turbine à gaz ont mis au point de nouveaux concepts de chambre de combustion plus propres et moins consommateurs. Ces technologies sont cependant plus sensibles aux instabilités de combustion, un couplage entre acoustique et flamme pouvant conduire à des niveaux dangereux de fluctuations de pression et de dégagement de chaleur. Les solveurs de Helmholtz sont une méthode numérique efficace pour prédire ces instabilités de combustion. Ils reposent sur la description d'un fluide non visqueux au repos, dont le comportement acoustique est régi par une équation d'Helmholtz thermoacoustique, résolue dans le domaine fréquentiel comme un problème aux valeurs propres. Le couplage flamme/acoustique est modélisé par une fonction de transfert du premier ordre entre les perturbations de dégagement de chaleur et la vitesse acoustique en un point de référence. Bien que performants, les solveurs de Helmholtz négligent l'interaction entre acoustique et vorticité aux coins, car celle-ci dépend d'effets visqueux. Cette interaction pourrait fortement amortir l'acoustique d'une chambre de combustion et la négliger revient à faire des prédictions trop pessimistes voire erronées. Par conséquent, une méthodologie a été mise au point afin d'inclure dans un solveur de Helmholtz l'effet d'interactions complexes entre acoustique et écoulement. Ces interactions étant compactes, elles sont modélisées par des matrices 2x2 et ajoutées au solveur comme des paires de conditions limites : les conditions limites de matrice. Grâce à cette méthodologie, les fréquences et modes d'une configuration académique non-réactive sont correctement calculées en présence de deux éléments où une telle interaction est forte: un orifice et un tourbilloneur. Afin d'être applicable aux chambres industrielles, deux extensions sont nécessaires. Premièrement, les surfaces de matrices doivent pouvoir être non-planes, afin de s'adapter aux géométries industrielles complexes. Pour cela, une procédure d'ajustement a été mise en place. La matrice est mesurée sur des surfaces planes et des transformations nondissipatives lui sont appliquées afin de la déplacer sur les surfaces non planes. Ces transformations peuvent être déterminées analytiquement ou calculées avec un solveur de propagation acoustique. Le deuxième point concerne le point de référence du modèle de flamme. En effet, celui-ci est souvent choisi à l'intérieur de l'injecteur ce qui pose problème si celui-ci est retiré du domaine de calcul et remplacé par sa matrice. Dans cette thèse, le point de référence est remplacé par une surface de référence. La méthodologie étendue est validée sur des configurations académiques puis appliquée à une chambre annulaire de Safran. Cette nouvelle méthodologie permet de constater que l'interaction écoulement/acoustique au niveau des trous de dilution et des injecteurs joue un effet important sur la stabilité de la chambre mais aussi sur la structure des modes. Les premiers résultats avec une surface de référence pour la flamme sont encourageants. / Environmental concerns have motivated turbine engine manufacturers to create new combustor designs with reduced fuel consumption and pollutant emissions. These designs are however more sensitive to a mechanism known as combustion instabilities, a coupling between flame and acoustics that can generate dangerous levels of heat release and pressure fluctuations. Combustion instabilities can be predicted at an attractive cost by Helmholtz solvers. These solvers describe the acoustic behavior of an inviscid fluid at rest with a thermoacoustic Helmholtz equation, that can be solved in the frequency domain as an eigenvalue problem. The flame/acoustics coupling is modeled, often with a first order transfer function relating heat release fluctuations to the acoustic velocity at a reference point. One limitation of Helmholtz solvers is that they cannot account for the interaction between acoustics and vorticity at sharp edges. Indeed, this interaction relies on viscous processes at the tip of the edge and is suspected to play a strong damping role in a combustor. Neglecting it results in overly pessimistic stability predictions but can also affect the spatial structure of the unstable modes. In this thesis, a methodology was developed to include the effect of complex flow-acoustic interactions into a Helmholtz solver. It takes advantage of the compactness of these interactions and models them as 2-port matrices, introduced in the Helmholtz solver as a pair of coupled boundary conditions: the Matrix Boundary Conditions. This methodology correctly predicts the frequencies and mode shapes of a nonreactive academic configuration with either an orifice or a swirler, two elements where flowacoustic interactions are important. For industrial combustors, the matrix methodology must be extended for two reasons. First, industrial geometries are complex, and the Matrix Boundary Conditions must be applied to non-plane surfaces. This limitation is overcome thanks to an adjustment procedure. The matrix data on non-plane surfaces is obtained from the well-defined data on plane surfaces, by applying non-dissipative transformations determined either analytically or from an acoustics propagation solver. Second, the reference point of the flame/acoustics model is often chosen inside the injector and a new reference location must be defined if the injector is removed and replaced by its equivalent matrix. In this work, the reference point is replaced by a reference surface, chosen as the upstream matrix surface of the injector. The extended matrix methodology is successfully validated on academic configurations. It is then applied to study the stability of an annular combustor from Safran. Compared to standard Helmholtz computations, it is found that complex flow-acoustic features at dilution holes and injectors play an important role on the combustor stability and mode shapes. First encouraging results are obtained with surfacebased flame models.

A new theory for predicting the windprofile over a canopy has been evaluated. The theory was first presented by Harman and Finnigan (2007). The theory relies on the forming of a mixing-layer above the canopy, due to different mean wind in and above the canopy. Characteristics from both mixing-layer and Monin Obukhov similarity theory have been used to develop the governingequations that give the wind profile. The theory has been used to calculate wind profiles for sixdifferent atmospheric stabilities. In order to evaluate the theory, profiles from the theory have beencompared to measurements from Jädraås forest, Sweden. Profiles from Monin Obukhov similarity theory were also used for comparison.In general the mixing-layer theory gives better results than Monin Obukhov similarity theory. Agreement with measurements is good in neutral conditions, but fails when the atmospheric stability is altered, especially in convective conditions. This is believed to be due to the canopy lacking in thickness. The mean wind speed is systematically underestimated and this is also believed to be caused by insufficient thickness of the canopy. A correction for this behaviour is proposed. The theory gives higher values of the mean wind speed in convective conditions with the correction and the calculated values of mean wind speed are closer to the measurements.