The interaction noise that radiates from ducted contra-rotating (CR) fans are theoretically characterized using Green’s analytical functions for lined and unlined circular ducts containing uniform mean flow. The derived models can be used to analyze the spectral characteristics and directivity patterns of noise in various modes. The axial wavenumbers determining modal propagation are treated numerically with the specified boundary conditions. The solved results show that for each cut-on mode, an imaginary part is added to the axial wavenumber, while for the cutoff mode, the already-existing imaginary part is increased. Therefore, it is indicated that both types of modes will attenuate as they propagate along the lined casing, whereas the modes at higher frequencies experience more noise attenuations than those at lower frequencies, which is experimentally verified. Meanwhile, the discrepancy between predicted and experimental results in terms of noise reduction can also be found and is probably due to near-field source modification resulting from the recasting of the tip-clearance flow. Furthermore, the modal directivity patterns of the interaction tone at 2,332 Hz have been investigated. In theory, the lined casing makes the directivity pattern thinner and lean more to the rotation axis than the one with the unlined casing. For both types of casings, the variation trends in experiments are accurately captured by theoretical predictions. The current study is anticipated to improve the understanding of the underlying mechanisms of interaction noise as well as provide an essential reference for devising noise control methods for ducted CR fans.
Analyzing Interaction Noise from a Small Contra-Rotating Fan Enclosed with Lined and Unlined Casings
Journal of Aerospace Engineering ; 34 , 4
2021-04-21
Article (Journal)
Electronic Resource
Unknown
Analytical solution of sound propagation in lined or unlined circular ducts with laminar mean flow
Online Contents | 1993
|Contra-Rotating Open Rotor Tone Noise Prediction
AIAA | 2014
|