Fundamental indirect noise generation by interactions between entropy, vorticity and acoustic waves in the context of aero engine applications

In aero engines indirect noise is released by the acceleration of entropy and vorticity waves in the turbine, which are in turn created mainly further upstream in the combustor by unsteady combustion. Recent studies show that these interactions contribute substantially to the total emitted core noise. Consequently a detailed knowledge of the sources of entropy and vorticity waves and their interactions with mean flow and acoustics is essential for the efficient development of technologies to reduce indirect noise. In this study, a generic convergent-divergent nozzle configuration is considered as a simplified model of the transonic turbine vane flow to study entropy noise as well as the acoustic scattering behavior. A two step numerical approach is applied consisting of RANS mean flow simulations and succeeding acoustic simulations in frequency domain based on linearized Navier-Stokes equations. Source terms for both waves are deduced from linearized entropy and vorticity equation and evaluated spatially. Important interactions between entropy, vorticity and acoustic waves and the mean flow are highlighted and confirmed by an energy analysis. This analysis can be applied to real aero engine combustors and turbines and enables to quickly identify the impact of the different source mechanisms on indirect noise.