Broadband indirect noise generation by accelerated vorticity

Abstract

In the framework of research on core noise in aero-engines the generation of broadband indirect noise is investigated in detail. Hereby, the noise generation due to turbulent velocity fluctuations accelerated in a transonic nozzle flow is in the focus. The approach follows the idea to link the turbulent velocity components, longitudinal and lateral, measured upstream of the nozzle to the downstream detected acoustic pressure fluctuations. An important aspect is the transition of the turbulent velocity fluctuations and their change during the acceleration process through the nozzle. A theoretical model is used to estimate this change in the turbulence quantities. The comparison between the measured acoustic field and the velocity fluctuations indicates a relation of the longitudinal velocity fluctuations to the acoustic pressure fluctuations in the range from 4 − 200Hz and of the lateral velocity fluctuations to the acoustic pressure fluctuations in the range from 201 − 4000Hz. This suggests the thesis that the change of the different turbulent velocity fluctuations generate sound in different frequency bands in the downstream propagating acoustic field.