An improved phased array method for estimating free-field engine core noise spectra from test cell measurements on short cowl engines

摘要

Following a previous study on separating core noise radiation from the jet mixing noise of a long cowl engine using an inverse phased array method (AFINDS), this paper describes further development of the same method in combination with an Actran-based radiation model in order to separate out core noise from aft fan noise radiated from the bypass nozzle of a short cowl engine. The Actran model describes the in-duct modal propagation from the aft fan and core noise and the radiation through the coaxial exhaust flows out into the near-field microphone locations of the phased array. The model accounts for linear convection and refraction effects, plane and edge scattering and both geometric and acoustic near-field effects but not reflections from the test cell surfaces. Simple source models are used to combine the modal radiation amplitudes, namely a uniform axial dipole distribution for the aft fan source and equal energy per mode for the core noise source. AFINDS then uses the Actran-based cross-spectral matrices for the aft fan & core together with a reverberant field model, to determine the aft fan and core noise source levels. Simulation results are presented which suggest that the aft fan and core sources - the nozzle planes being less than a metre apart - can be separated out and their spectra determined with acceptable accuracy down to 100 Hz, and application to measured engine data acquired in a test cell has produced encouraging results.