Reduction of fan noise in an anechoic chamber by reducing chamber wall induced inlet flow disturbances

Abstract

The difference between flight and ground static noise data has arisen in the past few years as a significant problem in fan jet engine noise testing. The additional noise for static testing has been attributed to inlet flow disturbances or turbulence interacting with the fan rotor. In an attempt to determine a possible source of inflow disturbances entering fans tested in the Lewis Research Center anechoic chamber the inflow field was studied using potential flow analyses. These potential flow calculations indicated that there was substantial flow over the wall directly behind the fan inlet. This flow near the wall anechoic wedges could produce significant inflow disturbances. Fan noise tests were run with various extensions added to the fan inlet ducting to move the inlet away from this back wall and thereby reduce the inlet flow disturbances. Significant noise reductions were observed with increased inlet length. Over 5‐dB reduction of the blade passage tone sound power level was observed between the shortest and longest inlet at 90% fan speed and the first overtone was reduced 9 dB. High‐frequency reductions in the broadband noise were also observed.