Influence of the Interaction between Rotor and Nozzle on the Acoustic Signature of a Ducted Propeller in Wetted Conditions

Results of computations using a Large Eddy Simulation (LES) technique on a cylindrical grid consisting of 3.5 billion points are utilized to reconstruct the acoustic signature of ducted and non-ducted propellers by means of the Ffowcs Williams & Hawkings (FWH) acoustic analogy and an array of 2,160 virtual hydrophones. The results of the present study demonstrate that the sound radiated by the ducted propeller is more intense than the one from its open rotor operating alone without nozzle. This is especially attributable to the loading sound from the rotor of the ducted propeller rather than to the additional contribution of the loading sound from the nozzle. Although the overall load is transferred in part to the accelerating nozzle, producing a positive thrust, the unsteady component of the loads experienced by the rotor of the ducted propeller is indeed increased, as a result of its interaction with the nozzle. In contrast, the non-linear component of sound from the ducted propeller is diminished, due to the lower intensity of its wake structures, but this component is actually able to affect significantly only the acoustic near field in the downstream direction.