Acoustic mode measurements in the inlet of a model turbofan using a continuously rotating rake

Abstract

Comprehensive measurements of the spinning acoustic mode structure in the inlet of the Advanced Ducted Propeller (ADP) have been completed. These measurements were taken using a unique and previously untried method which was first proposed by T.G. Sofrin. A continuously rotating microphone system was employed. The ADP model was designed and built by Pratt & Whitney and tested in the NASA Lewis 9-by 15-Ft Anechoic Wind Tunnel. Three inlet configurations were tested with cuton and cutoff stator vane sets. The cutoff stator was designed to suppress all modes at the blade passing frequency. Rotating rake measurements indicate that several extraneous circumferential modes were active. The mode orders suggest that their source was an interaction between the rotor and small interruptions in the casing tip treatment. The cuton stator produced the expected circumferential modes plus higher levels of the unexpected modes seen with the cutoff stator.