Modeling of Aft-Emitted Tonal Fan Noise in Isolated and Installed Configurations

The paper presents an initial methodology for low-cost modeling of aft-emitted tonal fan noise in isolated and installed configurations. The theoretical development is done jointly with experimental measurements in an anechoic facility. The model comprises linear coherent partial fields prescribed on a cylindrical surface near the edge of the shear layer formed at the fan exit. The partial fields possess the same axial wavenumbers and azimuthal modes as the aft-propagating duct modes that are cut-on. The shapes of the partial fields are parameterized and the parameter vector is determined by least-squares minimization of the difference between modeled and experimental cross-spectral densities in the far field. In installed arrangements, the emission from the parameterized source is used as the incident field in the computation of scattering using the boundary element method. The experiment utilized a small-scale ducted fan with rotor tip Mach number of 0.59. Microphone measurements with high spatial resolution were enabled by the combination of fixed sensors with one scanning sensor. The recording of a tachometer enabled filtering of the signals into tonal and broadband components, with the focus here on the second harmonic of the blade pass frequency. Integration of a flat plate enabled measurements in a canonical shielded configuration. The results indicate an extended, wavepacket-like nature of the noise source with emission similar to Mach wave radiation from supersonic jets. The model captures the overall directivity of the experimental spectrum and its modification by insertion of the plate.

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