Inlet acoustic mode measurements using a continuously rotating rake

Comprehensive measurements of the spinning acoustic mode structure in the inlet of the advanced ducted propeller were obtained using a unique method that was first proposed by Sofrin. A continuously rotating microphone system was employed. Three inlet configurations with cut-on as well as cut-off stator vane sets were tested. The cut-off stator was designed to suppress all modes at the blade passing frequency. Rotating rake measurements indicate that several extraneous circumferential modes, possibly due to the interaction between the rotor and small interruptions in the casing tip treatment, were present. The cut-on stator produced the expected circumferential modes plus higher levels of the unexpected modes seen with the cut-off stator. HE next generation of fan engines will likely employ a marriage of turbofan and propeller technologies to achieve significant noise and fuel consumption reductions. The ad- vanced ducted propeller (ADP) model used in this investi- gation was designed and built by Pratt and Whitney, a Di- vision of United Technologies, and tested in the NASA Lewis 9- by 15-ft Anechoic Wind Tunnel. Typical of propeller tech- nology, the ADP allows for the in-flight adjustment of the blade pitch angle. This provides reverse thrust and optimum performance over a wide range of conditions. The duct pro- vides the noise suppression advantage of a conventional fan engine. Since future engines are expected to use still higher bypass ratios, fan noise is likely to be the dominant engine source. One of the most important features of fan tone noise is its modal structure. Knowledge of these spinning modes helps to identify the generation mechanism, control duct propa- gation (thus, mode knowledge is needed for acoustic treat- ment design) and control far-field radiation. Previous at- tempts at direct mode measurements1'3 have faced formidable practical difficulties such as: very large axial and circumfer- ential arrays of wall microphones that are not practical for the short ducts of ultrahigh bypass engines, and radial mea- surements upstream of the fan that introduce a wake that interacts with the rotor, thus causing extraneous modes. A continuously rotating microphone technique first proposed by Sofrin4 overcomes the problem of wake-generate d modes, reduces the number of microphones and the duct length re- quired. This technique has been implemented for the first time in this investigation. Two important features of this tech- nique are as follows: