Description and application of a 2D-axisymmetric model for entropy noise in nozzle flows

Entropy noise corresponds to acoustic fluctuations generated inside gas-turbine engines when temperature (entropy) spots are accelerated by the mean flow. This type of noise currently faces growing interest because of its contribution to global aero-engine noise as well as its impact on combustion instabilities. In the present article, a two-dimensional semi-analytical model is developed to predict entropy noise in nozzle flow. It complements the reference model of Marble and Candel (“Acoustic disturbance from gas non-uniformities convected through a nozzle”, J. Sound Vib., 55, pp. 225–243, 1977) and its more recent extensions which remain one-dimensional, by taking the radial variations of the flow into account in the noise generation computation. Validations are performed by comparison with computational aeroacoustics simulations with either entropy or acoustic excitations in a subsonic nozzle. Very good agreement is observed between the numerical simulations and the 2D model for all the frequencies considered. The role of entropy wave distortion in noise generation is demonstrated by comparison with a 1D model which fails to reproduce the simulated results for medium to large frequencies.