American Institute of Aeronautics and Astronautics
Bennett G., Fitzpatrick J., Noise Source Identification For Ducted Fan Systems, 13th AIAA/CEAS Aeroacoustics Conference, Rome, Italy, 22 May 2007, 2007
Understanding combustion noise source mechanisms, designing efficient acoustic liners
and optimising active control algorithms for noise reduction requires the identification of
the frequency and modal content of the combustion noise contribution. Coherence-based
noise source identification techniques have been developed which can be used to identify
the contribution of combustion noise to near and far-field acoustic measurements of aero-
engines. A number of existing identification techniques from the literature are implemented
and evaluated under controlled experimental conditions. An experimental rig was designed
and built to gain a fundamental physical understanding of the convection of noise through a
rotor/stator set-up. The identification techniques were applied to this rig and the pressure
field was separated into its constituent parts.
The underlying assumption with these identification techniques is that the propaga-
tion/convection path, from combustion can to measurement point, is a linear one. It is
shown with simulations that where the combustion noise propagates in a non-linear fashion
the identified contribution will be inaccurate. The experimental rig, consisting of a vane-
axial fan mounted in a duct, allows potential non-linear interaction mechanisms between
a convected sound source and the fan to be investigated. Tests carried out on the experi-
mental rig allowed a non-linear interaction tone, between the rotor BPF and a convected
tone, to be generated.
An experimental technique was developed which enabled the non-linear interaction be-
tween the convected sound source with the vane-axial fan to be detected and identified
when present. The technique was extended to allow the linear and non-linear acoustic
contributions to be separated. The capacity to decompose the coherent output power into
linear and non-linear components is a useful tool for the correct design of acoustic treatment
for core noise and for an accurate identification of noise source generating mechanisms.
The non-linear identification techniques, developed with the experimental rig, were ap-
plied to data from full scale turbo-fan engine tests. A Rolls-Royce engine was instrumented
with pressure transducers at the combustor can and in the hot-jet pipe, and microphones
were placed in the near-field. For a particular power setting, frequency scattering was seen
to occur between the combustion noise and the high pressure turbine which was measur-
able in the hot jet pipe after convection through the many turbine stages. The techniques
allowed the non-linear interaction to be successfully identified and linear and non-linear
coherent output powers to be determined.
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