Aligned and Unaligned Coherence: A New Diagnostic Tool

Core Noise Diagnostics of Turbofan Engine Noise Using Correlation and Coherence Functions

“…Shifting the signals by this time delay removes the coherence of random noise (identified frequently as broadband noise) but leaves the coherence of periodic functions (identified frequently as tones). Miles 46 found out that replacing ns with no in Eq. (5), where no is the number of overlapping segments, gives a good estimate of the coherence squared noise threshold, 'Y 2 nn (f ).…”

Spectral Separation of the Turbofan Engine Coherent Combustion Noise Component

“…Using the analysis parameters shown in Table 1, the segment length, T d = 1/16 is 62.5 ms (4096 samples). Consequently, if one calculates the coherence of the combustor pressure transducer signal, K(t) and a microphone signal M (t) using the measured time histories as measured one finds the coherence is that of random noise since the two signals are totally independent except at frequencies where tones are present as discussed by Miles (2006).…”

“…As discussed by Miles (2006) 11 tones can be identified using the aligned and unaligned coherence function. The unaligned coherence function has a value determined by the number of samples of two independent signals and the tones.…”

“…Recently similar coherence function analysis techniques were used to evaluate core noise from a Pratt & Whitney PW4098 by Miles (2006). 11 The data evaluated was acquired from a test conducted as part of the NASA Engine Validation of Noise Reduction Concepts (EVNRC) Program in 2001.…”

“…11 The data evaluated was acquired from a test conducted as part of the NASA Engine Validation of Noise Reduction Concepts (EVNRC) Program in 2001. In the time interval between these two sets of tests, that is, from 1980 to 2000, turbofan engines have changed with newer engines such as the Pratt & Whitney PW4098 and the Honeywell TECH977 being based on new materials and a longer history of experience in turbofan manufacturing, design, analysis, and optimization.…”