Numerical Investigation of Entropy Noise and Its Acoustic Sources in Aero-Engines

In the present work, the generation and propagation of entropy noise was simulated applying a compressible three dimensional URANS CFD approach. To this end, a test rig, the so-called Entropy Wave Generator (EWG) was modeled. The EWG implies a non-reactive tube flow where entropy modes are induced to the air flow by a heating module. The generated temperature nonuniformities are accelerated in a convergent-divergent nozzle and excite entropy noise. Simulation results of pressure fluctuations and power spectra in the standard configuration as well as simulations of different operating points of the EWG are in very good agreement with measurements. Additionally, entropy noise was deduced for realistic gas turbine conditions with calculated sound pressure level above 160 dB, which evidences the relevance of entropy noise in gas turbines. The numerical investigation is completed by the analysis of the acoustic sources. For this purpose the acoustic sources caused by the acceleration of density inhomogeneities suggested by Dowling [1] were calculated. Thus, for the first time a method is introduced to locate the acoustic sources of entropy noise in the acceleration region.