Reconstruction of Noise Source in a Ducted Fan Using a Generalized Nearfield Acoustical Holography

The identification of the propulsion noise of turbofan engines plays an important role in the design of low-noise aircraft. The noise generation mechanisms of a typical turbofan engine are very complicated and it is not practical, if not impossible, to identify these noise sources efficiently and accurately using numerical or experimental techniques alone. In addition, a major practical concern for the measurement of acoustic pressure inside the duct of a turbofan is the placement of microphones and their supporting frames which will change the flow conditions under normal operational conditions. The measurement of acoustic pressures on the surface of the duct using surface-mounted microphones eliminates this undesirable effect. In this paper, a generalized acoustical holography (GAH) method that is capable of estimating aeroacoustic sources using surface sound pressure is developed. An indirect boundary element formulation is derived to generate the transfer matrix that relates aeroacoustic sources (monopoles and dipoles) to the measured surface sound pressure field. The surface sound pressure on the duct is measured and utilized to reconstruct the noise source in the ducted fan. The validity of the integrated numerical/experiment method is demonstrated by the good correlation of the measured sound pressure at verification locations with the pressure computed from reconstructed sources.