Noise levels for modern high-bypass-ratio
subsonic turbofans tend to be
aft dominated. That is, the highest flyover noise levels radiate
from the fan exit. Measuring fan inlet sound radiation without
aft radiation contamination requires selective suppression of
the aft noise. In NASA Lewis Research Center's 9- by 15-Foot Low-Speed
Wind Tunnel, an acoustic barrier was used to effectively isolate
the inlet noise field for a model of an advanced turbofan. This
proof-of-concept test was performed on a model turbofan manufactured
for NASA Lewis by the Allison Engine Company as part of the Advanced
Subsonic Technology program.
The 8-cm-thick acoustic barrier was constructed in sections that
were joined upon installation. These sections, which were composed
of a wood frame with typically 0.64-cm tempered fiberboard
skins, extended from the tunnel's floor to its ceiling and had
an axial length of 61 cm. On the fan side of the barrier just
downstream of the leading edge, the upstream section had an acoustic
treatment--a bulk absorber with a perforated metal skin. It had
a nominal full height and an axial length of 46 cm. In addition,
an elliptical leading edge was faired into the upstream barrier
section. The barrier was mounted on tracks on the tunnel floor
and ceiling at a sideline distance of 15 cm from the fan nacelle.
Tests were made with the barrier leading edge at the fan inlet
highlight plane and 15 cm further aft. The barrier extended downstream
essentially to the end of the treated tunnel test section.
In this photograph, the acoustic barrier is shown in the upstream
position. Acoustic data were taken with a translating microphone
on a 224-cm sideline. The directivity plot shows the aft fan noise
suppression due to the barrier along the 224-cm sideline. Data
are for one-third octave band levels at 6300 Hz, which correspond
to the second harmonic of the fan rotor-stator interaction tone.
Results are shown for the fan without the barrier present and
with the barrier leading edge at the two axial test locations.
Aerodynamic data indicate that the barrier does not influence
the performance of the fan.
Previous articleLast updated April 29, 1997
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