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Interaction and scattered mode measurement in a mixed bypass/core duct with multiple rotating sources

The rotating rake mode measurement system was developed and implemented at the NASA Glenn Research Center in the 1990s to measure turbofan duct acoustic modes. The system is a radial rake with distributed microphones that is inserted into the duct and continuously rotates about the centerline. The rotating rake mode measurement system provides a complete map of the acoustic duct modes present in a ducted fan and has been used on a variety of test articles: from a low-speed concept test rig to full-scale production turbofan engines. Prior rotating rake mode measurement system measurements have occurred in environments with a single rotating source interacting with a stationary source to produce duct modes at the fan rotor harmonics. A recent test of a scale model high-speed research fan in the NASA 9 × 15 ft Low Speed Wind Tunnel had two rotating sources, a rotor fan and a booster fan, on the same shaft. The interaction of the two rotating sources generated scattering at the harmonic sum and difference tones, and modal scattering at a given fan harmonic. Rotating rake mode measurement system data were acquired from this high-speed fan model which provided a database to allow for validation of the duct mode analysis technique expansion and experimental evidence of the scattering phenomenon. In this paper, modes from the two different sources and associated stationary surfaces from a sample fan condition were reported and compared to analytical predictions, in order to validate the ability of the rotating rake mode measurement system to measure scattered modes. Sample modal content of the booster fan tone and its harmonics tones are presented as well as the sum and difference interaction between the rotor and booster fans.