A physical model to predict indirect noise in multicomponent non-isentropic nozzle flows*

Aircraft engine manufacturers are striving to make cleaner and less noisy engines. In aero-engine combustors, incomplete mixing and air cooling give rise to inhomogeneities in temperature and composition. These flow inhomogeneities accelerate downstream of the combustor in the nozzle guide vane and generate indirect noise. In this work, a quasi-one-dimensional model is mathematically developed and analyzed numerically to predict the indirect noise produced in non-isentropic nozzles. The physics-based model is compared with existing non-isentropic models, which make use of semi-empirical parameters. The results compare favorably with experimental data of the literature. It is found that friction significantly affects the gain/phase of the reflected and transmitted waves for compact and non-compact nozzles in the subsonic-choked regime. Furthermore, the effect of friction on thermoacoustics is studied, highlighting the sensitivity of thermoacoustic oscillations to the dissipation in the nozzle. The study opens new possibilities for the accurate prediction of indirect noise from nozzles.