Modelisation des vibrations asynchrones d'aubes de compresseurs axiaux par la resonance de l'ecoulement de jeu.

摘要

The first step of the research consists of demonstrating the coupling mechanism causing the resonance of an impinging jet on a flexible plate. This demonstration, in essence, aims at verifying the first research hypothesis by demonstrating the resonance of an impinging jet at stream velocities where the resonance has never been observed before when the jet impinges on a flexible plate in vibration rather than on a rigid plate. The chosen approach is experimental. It consists of characterizing the unsteady pressure response of a jet impinging on a flexible plate, whose vibration is induced by a mechanical shaker.;The following step of the research consists of verifying the second hypothesis which stipulates that the resonance of the tip clearance flow, based on the same coupling mechanism as the resonant jet-on-flexible-plate, is the fundamental fluid-structure phenomenon behind asynchronous vibrations. The demonstration method is also experimental, though more complex since it involves high-speed rotation of the geometries and instrumentation. The rotor under study is installed in a test-section which isolates the effect of the tip clearance on the blades. Sensitivity studies are conducted with respect to the tip clearance, the blade aerodynamic loading, the inlet temperature and the compressor rotating speed. The results support the second research hypothesis and the critical asynchronous vibration conditions of occurrence are predicted from the proposed coupling mechanism.;Until this point, the resonances were predicted by an empirical model of a planar feedback wave travelling upstream towards the separation edge. The coupling mechanism between the mixing layer and the acoustic feedback was assumed to occur when a crest of the sound wave reached the separation edge. This assumption was based on a number of historic empirical observations on oscillating shear layers. The final part of the research aims at analytically capturing the coupling mechanism of a mixing layer to a source forcing located downstream in the high-speed flow stream. The coupling mechanism is fundamental to the resonances observed experimentally. The demonstration is made by capturing the spatial periodicity of the mixing layer resonance under stream forcing with respect to the downstream source location. This is the last research hypothesis and the demonstration method is analytical. It consists of solving the 2D problem of a mixing layer interface excited by an acoustic point source located downstream in the axial direction on the planar jet centerline. An analytical relationship linking the excitation frequency and the source axial location to the stream velocity is derived. The critical resonance conditions, predicted by the analytical solution are in good agreement with the resonances obtained in the two experiments presented above.;The main contributions of this last part of the work are to provide an analytical criterion for the spatial resonance of stream forced mixing layers along with a closed form prediction relationship for the critical source locations. The analysis captures the coupling mechanism of a mixing layer to a forcing source located downstream in the high-speed flow stream, which is fundamental to the experimentally observed resonances. Also, the field of fluid mechanics benefits from the research by being provided an analytical framework and a resolution method in the study of mixing layers forced response excited by a source located downstream in high-speed flow.