Procedure for Separating Noise Sources in Measurements of Turbofan Engine Core Noise

The study of core noise from turbofan engines has become more important as noise from other sources like the fan and jet have been reduced. A multiple microphone and acoustic source modeling method to separate correlated and uncorrelated sources has been developed. The auto and cross spectrum in the frequency range below 1000 Hz is fitted with a noise propagation model based on a source couplet consisting of a single incoherent source with a single coherent source or a source triplet consisting of a single incoherent source with two coherent point sources. Examples are presented using data from a Pratt and Whitney PW4098 turbofan engine. The method works well. Nomenclature A amplitude B amplitude of a point source B Be resolution bandwidth,Hz.,Be = 1/Td = r/NP = 11.71875Hz co speed of sound, m/sec E expected value f frequency F[] two sided Fourier transform operator fc upper frequency limit,fc = 1/2t = r/2, Hz. (24000 Hz.) Gxx(f) auto power spectral density function defined for non-negative frequencies only (one-sided) Gxy(f) cross power spectral density function defined for non-negative frequencies only (one-sided) j positive imaginary square root of −1, √ −1 k wavenumber Ly number of frequencies, fc/f = N/2 ( 2048 ) MW molecular weight of air, 0.02897, kg mol