Three-dimensional visualizations of open fan noise fields
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Fan noise is a dominant component of noise fields radiated from high-speed turbomachinery such as turbofan engines. For the purpose of understanding fan noise generation and propagation characteristics, a Nearfield Acoustical Holography (NAH) procedure is applied to visualize the three-dimensional sound fields radiated from a 4-blade test fan operating at 4.3, 4.7, and 5.1 kRPM. Here, a generalized planar NAH description including a partial field decomposition technique is described. The fan radiates strong tonal noise components at the 1st Blade Passing Frequency (BPF) and its higher harmonics. NAH reconstruction frequencies are selected at the 1st, 2nd, and 4th BPFs. It is shown that reconstructed sound fields can be used to identify the sound source locations and radiation patterns of the fan. For example, at the 1st BPF, a composite sound source that consists of 8 monopoles and rotates at the fan rotation speed is identified at the leading and trailing edges of blade tips. The latter finding leads to the modeling of the fan noise source as a combination of monopoles that rotates at the fan speed. At the 2nd and 4th BPFs, the sound sources of monopole type are mainly identified at the blade surfaces and edges. The reconstructed supersonic sound intensity fields indicate that the farfield noise radiations from the fan can be modeled by using a single off-centered monopole. The total sound power level calculated from the reconstructed active sound intensity fields on the source surface is dominant at the 1st BPF and decreases as the BPF increases: e.g., the total sound power levels are 96, 85, and 72 dB for the 1st, 2nd, and 4th BPFs, respectively. © 2012 Institute of Noise Control Engineering.
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