Comparison between the acoustic fields of contra-rotating and conventional marine propellers in wetted conditions and thrust similitude

The acoustic fields of a contra-rotating propeller and isolated propellers producing the same overall thrust are compared at both design and off-design working conditions. The sound levels are reconstructed by using the Ffowcs Williams–Hawkings acoustic analogy, exploiting results of computations conducted on a cylindrical grid consisting of $4.6 \times {10}^9$ points and a large-eddy simulation technique. The analysis shows that, although the blades of the contra-rotating propeller are less loaded and produce less intense flow structures, the levels of radiated sound are reinforced, compared with the propellers working alone. This is due to the loading sound, originating from the pressure fluctuations on the surface of the blades of the propellers. The higher levels of linear sound are attributable to the interplay between the front and rear rotors of the contra-rotating system. This interaction is able to reinforce the unsteady component of the loads acting on the blades of the propellers and the resulting linear component of sound. While the shear occurring between the tip vortices shed by the front and rear rotors gives rise to a complex system of isolated vortex rings in the wake, increasing the quadrupole component of sound, these phenomena are balanced by the lower intensity of the vortices shed by the contra-rotating system.