Time-Resolved Flowfield and Acoustic Measurements of a Ducted and Unducted Rotor

The impact of adding a duct to an [Formula: see text] diameter, triple-bladed, electrically driven rotor operating up to [Formula: see text] at static conditions is investigated experimentally to identify the source mechanisms governing the measured acoustic differences. An unducted rotor is characterized by strong, coherent tip vorticity that interacts with each blade passing, generating high-amplitude harmonic tones. The addition of the duct results in tonal broadening and an increase of the primary and harmonic tone amplitudes. Up to a 15 dB increase in broadband noise across the entire frequency range is measured. High-speed particle imaging velocimetry up to 6.4 kHz is employed to acquire phase-locked and ensemble-averaged measurements of the velocity, vorticity, and turbulence fields. While it is often theorized that a duct can shield lateral acoustic radiation, this study finds that the ducted rotor is louder across all operating regimes, and the noise increases with tip Mach number. The ducted rotor dissipates the coherent tip vorticity into incoherent turbulence intensity due to the interaction of the vortices with the duct internal walls, contributing to a strong broadband noise increase and tonal broadening. The ducted rotor broadband noise increase is due to the rotor interacting with a more turbulent inflow condition and increased turbulence intensity throughout the plume shear layer.