Locating and Quantifying Broadband Fan Sources Using In-Duct Microphones

In-duct beamforming techniques have been developed for locating broadband noise sources on a low-speed fan and quantifying the acoustic power in the inlet and aft fan ducts. The NASA Glenn Research Center’s Advanced Noise Control Fan was used as a test bed. Several of the blades were modified to provide a broadband source to evaluate the efficacy of the in-duct beamforming technique. Phased arrays consisting of rings and line arrays of microphones were employed. For the imaging, the data were mathematically resampled in the frame of reference of the rotating fan. For both the imaging and power measurement steps, array steering vectors were computed using annular duct modal expansions, selected subsets of the cross spectral matrix elements were used, and the DAMAS and CLEAN-SC deconvolution algorithms were applied. Introduction Significant reduction in aircraft noise is required to meet ongoing noise regulation in the USA and Europe. Since the turbofan engine is a large contributor to aircraft noise, any overall reduction in aircraft noise must include engine noise reduction. In order to efficiently achieve noise reduction, detailed understanding of the physics of noise source generation is required. The NASA focus area—Fundamental Aeronautics, Subsonic: Fixed Wing Program—emphasizes developing technologies for diagnostics of noise for subsonic aircraft. The work described here is intended to advance the state of the art for imaging broadband fan noise of turbofan engines using microphones that are flush mounted in the outer wall of the bypass flow path of the nacelle. The recordings are post-processed to form images of broadband noise sources on the rotating blades and to measure the modal amplitudes and the sound power in the inlet and aft fan ducts. The blade acoustic images and mode maps are intended to help clarify the mechanism for fan broadband noise generation and to identify any problem areas on a given set of blades. The sound power measurement may be a convenient and more-precise substitute for far field microphones.