Entropy noise modelling in 2D choked nozzle flows

Abstract

Entropy noise is produced when temperature fluctuations (entropy spots) are accelerated by the mean flow, through a nozzle or a turbine stage for instance. When it propagates outside the engine it contributes to community noise and may generate thermoacoustic instabilities when reflected back towards the combustor, hence the need for its modelling. Among all the inviscid models proposed in the literature, only the one developed by ONERA takes into account the 2D nature of the mean flow and the radial deformation of the entropy waves through the nozzle, which plays a crucial role in noise generation (Emmanuelli et al., J. Sound Vib., vol. 472, 2020, pp. 115163). This model has been validated in the subsonic regime and is extended in the present work to 2D supercritical configurations, without and with a normal shock in the diffuser. Modelled transfer functions are validated by comparison with reference data obtained with computational aeroacoustics simulations and excellent agreement is found between the simulations and the model. The contribution of the shear dispersion of the entropy wave to noise generation is evidenced and the failure of the quasi-1D models, which do not account for the radial deformation of the entropy fluctuations, is illustrated. Noise scattering through the nozzle is also investigated. The 2D model is found to correctly recover the simulated transmitted and reflected acoustic waves through the nozzle. Quasi-1D solutions are also found to collapse with the reference simulations, which indicates that 2D mean flow effects are negligible for the propagation of the acoustic waves through the nozzle.