Objective This study investigates the effects of increasing the blade number of a toroidal propeller from three to five on cavitation inception, hydrodynamic performance, and underwater radiated noise. Particular attention is paid to distinguishing configuration-related acoustic changes from those associated with different thrust levels.
Method Three models were examined: the David Taylor Model Basin (DTMB) P4119 propeller and three- and five-bladed toroidal propellers. The five-bladed design retained the diameter, hub ratio, airfoil sections, closed-loop arrangement, and principal pitch and chord distributions of the three-bladed design, while its total blade area increased with blade number. A steady Reynolds-averaged Navier–Stokes calculation provided initial conditions for a transient sliding-mesh large eddy simulation. The Smagorinsky–Lilly subgrid-scale model, Schnerr–Sauer cavitation model, and Ffowcs Williams–Hawkings acoustic analogy were coupled to evaluate cavitation, flow-induced pressure fluctuations, and sound radiation. Open-water hydrodynamic predictions were checked against DTMB P4119 experimental data, and grid-convergence indices were evaluated. Comparisons were performed at equal rotational speed and equal thrust. Spectra and overall sound pressure levels were assessed at three radial and three downstream axial locations.
Results The pressure-side and suction-side cavitation inception speeds of the five-bladed propeller reached 1 050 and 1 480 r/min, respectively, representing increases of 75% and approximately 34% over the three-bladed design. At 1 100 and 1 200 r/min, its vapor-volume coefficients were lower by 72.38% and 51.42%. At 600 r/min and an advance coefficient of 0.833, thrust rose from approximately 245 to 295 N, an increase of 20.4%. For an equal thrust of 245 N, the required speed fell from 600 to 547 r/min, but torque increased from 17.4 to 17.8 N·m and propulsive efficiency declined from 56.94% to 55.66%. Under equal-thrust conditions, overall sound pressure levels at P1 and P3 decreased by 3.385 and 8.589 dB, whereas those at P2 and the downstream points P4–P6 increased by 6.409 and 5.832–6.550 dB, respectively. The spectral changes were direction-dependent and involved broadband components as well as blade-passing-frequency responses.
Conclusion Increasing blade number in this toroidal configuration redistributes circumferential loading, delays cavitation, and improves thrust capacity, but does not provide uniform noise reduction or an efficiency gain. Elevated broadband noise near the blade-tip-radius measurement point and in the axial wake merits further optimization. Because blade area also changes, the results characterize this practical configuration modification rather than an isolated blade-number effect.
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