| How much noise did the fan make,
and how can we reduce it? They sound like straightforward
questions. The trouble is, it is uncommon to run a fan
rotor alone. Measuring noise from just the rotor calls
for a special wind tunnel test in which the engine components
normally installed are taken out of the way. If you remove
the vanes and struts downstream of the fan, though, what
holds up the nacelle?
Engineers at NASA Glenn took up the challenges of
this test. They chose to support the nacelle by external
struts attached to an actuated table inside the wall
of the 9’ x 15’ Low Speed Wind Tunnel.
A control system automatically adjusted the position
of the nacelle to keep the rotor centered during the
test. Far field noise measurements were not contaminated
by the additional hardware for this test since the
struts were located on side of the nacelle opposite
the tunnel traversing microphone system.
In 2000 and again in 2002, NASA Glenn collaborated
with General Electric to study the acoustic performance
of the GE90 fan. By eliminating the noise created by
the interaction of the rotor wakes with the downstream
vanes and struts, engineers could better study the
effects of fan shape, tip gap, and nacelle nozzle design.
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