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Numerical simulation and modelling of entropy noise in nozzle and turbine stator flows

Abstract : The investigation of combustion noise is motivated by its growing relative contribution to the noise emitted by modern turbofan engines overall, as well as its effect on low NOx emission combustor design. Entropy noise is a source of indirect combustion noise, which is generated by the acceleration of heterogeneities, in this case entropy, downstream of the combustion chamber. This study consists of the investigation of entropy noise in nozzle and turbine stator flow using both analytical and numerical methods. Nozzle flow is considered first. A Computational AeroAcoustics (CAA) reference case is built for the validation of an existing two-dimensional semi-analytical model developed under similar assumptions. The levels of entropy noise and acoustic scattering estimated using both methods are in good agreement. Two-dimensional effects on entropy noise are highlighted, notably by comparison with compact and 1D models. In addition, vorticity induced by the acceleration of entropy noise is evidenced, but it yields negligible vortex sound. Next, the focus is shifted to a 2D high-pressure turbine stator. The 2D model for nozzle flow is extended to this configuration, inheriting some of its main assumptions. Their investigation, using CAA in particular, sets the path for future developments and allows insight to be gained into the role of both vorticity and azimuthal variation of acoustics, which are neglected by the model. CAA also allows to characterise entropy noise generation in 2D stator flow under simplifying assumptions, using Euler and RANS mean flows. Further investigation is needed to validate the RANS case and to fully understand the effect of boundary layers on entropy noise generation. Finally, entropy noise is simulated using Zonal Detached Eddy Simulation (ZDES) in a stator channel in order to investigate 3D and viscous effects on entropy noise. The three-dimensionality of the flow is highlighted and acoustic signals are carefully post-processed, ensuring hydrodynamic perturbations are correctly filtered and boundary reflections are minimised. The closeness of noise levels obtained using CAA and ZDES suggest three-dimensional and viscous effects have a limited impact on the entropy noise generated in turbine stator flow.
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Ariane Emmanuelli. Numerical simulation and modelling of entropy noise in nozzle and turbine stator flows. Acoustics [physics.class-ph]. Université Paris-Saclay, 2019. English. ⟨NNT : 2019SACLC067⟩. ⟨tel-02502876⟩

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