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Inferring Blade Rate Propeller Forces From Wind Tunnel Sound Power Measurements


 An extensive series of acoustic tests were conducted on an experimental propeller in the Naval Undersea Warfare Center Acoustic Wind Tunnel. The objective of the tests was to infer the fluctuating forces at blade rate frequency (BR) and its harmonics from sound power measurements, and validate an assumed radiation model. Ten microphones were used to spatially sample the acoustic field. Measurements were made at constant advance ratio for speeds of 6 to 52 m/s, in increments of 3 m/s. As expected, the BR sound power levels scaled as U6 and exhibited cos2θ (θ being the angle to the force) directivity, consistent with compact dipole theory. For (kD)2 > about 6.5 (k is the acoustic wavenumber, D the propeller diameter), the scaling shifted to U8, the mechanism being dominated by lateral quadrupole radiation (two dipoles separated by the diameter). At the second BR harmonic, sound power levels scaled significantly different than at BR. Possible mechanisms for this effect are discussed.