声学
消声室
话筒
导管(解剖学)
波形
物理
方位角
马赫数
光学
工程类
声压
航空航天工程
机械
雷达
医学
病理
摘要
The paper presents advances in 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 waveforms prescribed on a cylindrical surface downstream of the fan exit. A generic shape for the waveform is proposed based on a simple analytical treatment of the sound field radiated from the fan duct. The waveforms possess the same azimuthal orders as the aft-propagating duct modes that are cut-on. In addition, the initial axial wavenumber, at the duct exit, matches closely that of the internal acoustic field. Downstream of the fan exit the axial wavenumber increases until the propagation speed equals the speed of sound. The shapes of the waveforms are parameterized and the parameter vector is determined by least-squares minimization of the difference between the 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 model captures the overall directivity of the experimental spectrum and its modification by insertion of the plate.
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