NASA Glenn develops methods to identify turbofan engine noise sources

NASA Glenn researchers have developed instrumentation to measure airflow and sound from turbofan engines. The agency uses particle image velocimetry, hot wire anemometry, Raman spectroscopy, microphones in phased arrays and continuously rotating rakes.

NASA Glenn engineers pioneered procedures using rotating microphones in the 1990s to measure the modal content of engine tones. The Rotating Rake Mode Measurement System provides data that cannot be measured with stationary microphones. Engineers use the data to design advanced liners and tuned active noise control systems.

Rotating Microphone Rakes have been used by NASA Glenn researchers in low speed proof of concept tests in the Advanced Noise Control Fan Rig and the 9 by 15 Foot Low Noise Wind Tunnel. The rakes have also been used in full scale turbofan engine ground tests.

The agency has an Array48 microphone system that has been used to locate noise sources in scale model jet flows, a counter rotating open rotor fan model and a full scale F404 military engine. Microphones in 1D, 2D or 3D arrangements are processed to pinpoint noise sources when properly handled.

Through NASA funding, a method was developed to use microphones embedded in the fan duct of an engine to determine the location of noise sources on the rotating blades.

Researchers use hotwire probes and Laser Doppler Velocimeters to measure air velocity upstream, downstream and between the rotor and the stator of experimental propulsors. Velocity measurements visualize disturbances created by the rotor. Hotwire and LDV data are used to validate calculations that estimate noise from turbofan engines.

The global aviation noise reduction community relies on data from NASA GRC experiments to validate theory and noise prediction software. NASA Glenn Research Center is located at Lewis Field in Cleveland and the Neil Armstrong Test Facility is in Sandusky, Ohio.