Turbofan Duct Mode Measurements Using a Continuously Rotating Microphone Rake

Abstract

An experimental measurement system was developed and implemented at the NASA Glenn Research Center in the 1990s to measure fan duct acoustic modes. The system is a radial array of microphones inserted into the duct that continuously rotates about the duct centerline. This Rotating Rake provides a complete map of the acoustic duct modes at fan harmonics in a ducted fan. It has been used on a variety of test articles: from a low-speed, concept test fan rig, to a full-scale production turbofan engine. The Rotating Rake has been critical in developing and evaluating a number of noise reduction concepts as well as providing experimental databases for verification of several aero-acoustic codes.