Baseline acoustic levels of the NASA active noise control fan rig

Abstract

Extensive measurements of the spinning acoustic mode structure in the NASA 48-in. Active Noise Control Fan (ANCF) test rig have been taken. A continuously rotating microphone rake system with a least-squares data reduction technique was employed to measure these modes in the inlet and exhaust. Far-field directivity patterns in an anechoic environment were also measured at matched corrected rotor speeds. Several vane counts and spacings were tested over a range of rotor speeds. The Eversman finite element radiation code was run with the measured in-duct modes as input and the computed far-field results were compared to the experimentally measured directivity pattern. The experimental data show that inlet spinning mode measurements can be made very accurately. Exhaust mode measurements may have wake interference, but the least-squares reduction does a good job of rejecting the nonacoustic pressure. The Eversman radiation code accurately extrapolates the far-field levels and directivity pattern when all in-duct modes are included. (Author)