Flight Noise Simulation in an Anechoic Chamber

Abstract

A widely recognized problem in the jet engine industry is the discrepancy between inflight measurements of fan noise as compared to static tests. This discrepancy consists of blade passing frequency tones, caused by ingested turbulence and flow distortions that appear in the static tests but do not appear in flight. An intensive effort has been carried out to devise means by which an anechoic chamber could be employed to yield fan noise data of the type that one obtains in flight. To reduce flow distortions and turbulence caused by wakes, a streamlined inlet is used to quiet the air entering the fan. Inlet casing suction surfaces are incorporated to allow removal of the turbulent boundary layer. Reduction of the midstream turbulence and flow distortion is accomplished by means of a honeycomb and screen combination. This effort has succeeded in reducing the ingested turbulence, to the point where reductions in the acoustic power at blade passing frequency are as high as 18 dB for subsonic tip speeds. Turbulence mapping of the inlet has confirmed that the tone reductions are due to a reduction in turbulence, as the low frequency (large scale) streamwise and transverse turbulent velocities have been reduced by up to four times and ten times, respectively.