Abstract
The term “open rotor” refers to unducted counter-rotating dual rotors or propellers used for propulsion. The noise generated by an open rotor is very complicated and requires special techniques for its analysis. The determination of its tone and broadband components is vital for properly assessing the noise control parameters and also for validating open rotor noise prediction codes. The data analysis technique developed by Sree for processing raw acoustic data of open rotors has been modified to yield much better results of tone and broadband separation particularly for the case when the two rotor speeds are approximately the same. The modified algorithm is found to eliminate most or all of the “spikes” previously observed in the broadband spectra computed from the original algorithm. A full description of the modified algorithm and examples of improved results from its application are presented in this paper.
Highlights
Renewed interest in counter-rotating open rotor technology for aircraft propulsion application has prompted the development of advanced diagnostic tools for better design and improved acoustical performance
The acoustic spectra from open rotor systems are known to be dominated by a profusion of tones, the broadband component can be a significant contributor to the total noise level [5]
The determination of tone and broadband components of complex noise generated by open rotors is important for properly assessing the noise control parameters and for validating the open rotor noise prediction codes
Summary
Renewed interest in counter-rotating open rotor technology for aircraft propulsion application (see References [1,2,3,4]) has prompted the development of advanced diagnostic tools for better design and improved acoustical performance. In the data processing technique developed by Sree [1] the measured time series data is divided into uniform segments of a known number of samples and a cross-correlation operation is applied between two successive raw data segments to identify the phase shifts These phase shifts in raw open rotor acoustic data are usually random and they occur due to jitter or unsteadiness in shaft rotations and other extraneous effects. A new way of processing the open rotor data had to be devised to minimize the effects of phase shifts and thereby eliminate the spike levels in the broadband spectrum This continued search resulted in an improved method that involved a few modifications to the original technique. The emphasis will be on presenting improved results of tone and broadband separation rather than on technical details of experimentation and aeroacoustic characteristics of the open rotor models involved
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