Gas Turbine Core Noise Source Isolation by Internal-to-Far-Field Correlations

An auxiliary power unit (APU) was tested for exhaust noise in an anechoic chamber. Six internal and numerous near- and far-field microphones were employed. Extensive cross-correlation and coherence function analysis were performed. The combustor was found to be one of the dominant sources of exhaust noise in the far field below 400 Hz. Additional noise generation around 375 and 600 Hz was apparent between the combustor exit and the turbine exit, which may be entropy noise or flow noise generated in the turbine inlet torus. The mixing process between the cooling air and the exhaust flow (which takes place a short distance upstream of the nozzle exit) was also identified as an important source of low-frequency noise.