Prévision du bruit d'onde de choc d'un turboréacteur en régime transsonique par des méthodes analytiques et numériques

Abstract : Whereas the sound radiated from the inlet of turbofans is mainly due to rotor–stator interactions in approach flight, the shock waves (or N-waves) emitted by the rotor at transonic rotation speeds can be a dominant noise source during takeoff and climb. The study of N-waves needs to take account of two main processes: 1) the generation of N-waves for a perfect rotor (in which all blades are identical) and for a real rotor (considering small geometrical blade dispersion), and 2) the N-wave propagation through the inlet duct producing the blade passing harmonics for a perfect rotor, and the multiple pure tones (harmonics of the rotation frequency) for a real rotor. Several analytical and numerical approaches have been investigated for the past 40 years.This thesis first intends to relate the main propagation theories and to address the foremost MPT generation method hypotheses. The links between fluid dynamics equations and practical non-linear theories currently adopted are emphasized and discussed. In a second step, the main relevant semi-analytical methods are cross-checked by applying them to representative turbofan configurations. Moreover, a novel model of irregular N-wave generation based on geometrical considerations is investigated thanks to test data related to in-duct pressure signatures and blade stagger angle measurements during the engine operation. Then, a second part of the work investigates a numerical strategy based on elsA ONERA code, solving the full Euler’s equations (CAA approach). The objective is to prevent from the limitations of 2D analytical models and to take into account actual inlet geometry and realistic convection flow. Regular and non-regular shock waves are directly injected in a plane close to the fan and propagated through the inlet. These shock waves are injected through a non-reflective boundary condition which requires the conservative field. The initial shock description near the fan is provided either by a RANS computation or by experiment, or else from analytical model. As experiment or analytical signals only provide pressure signatures, a theory is set up to re-built the whole conservative field from the basis of a pressure shockwave. This injection method is firstly applied on an infinitely narrow annular duct and validated through the comparison with the McAlpine & Fisher analytical method. Then, the 3D propagation effects are pointed out by increasing the duct height. Finally, the CAA method is applied on actual intake geometry of modern turbofan demonstrators, and propagation of regular and irregular shock-waves are simulated. The numerical results are compared to RANS solutions and to available measurements.
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Johan Thisse. Prévision du bruit d'onde de choc d'un turboréacteur en régime transsonique par des méthodes analytiques et numériques. Acoustique [physics.class-ph]. Ecole nationale supérieure d'arts et métiers - ENSAM, 2015. Français. ⟨NNT : 2015ENAM0054⟩. ⟨tel-01315146⟩

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