Hydroacoustic characteristics of a Kaplan-series propeller: Near and far-field analysis using numerical and experimental methods

The Sound generated by ships is a crucial component of the ocean soundscape, with propeller noise being a significant contributor. Among various propeller types, Kaplan-series propellers are widely used on specific vessels, such as tugs, yet their noise characteristics have received less attention. This study aims to fill this knowledge gap through a comprehensive numerical and experimental investigation of the underwater radiated noise of a Kaplan-series propeller. A hybrid numerical framework coupling improved delayed detached eddy simulation to resolve turbulent flow structures responsible for broadband noise, with the Ffowcs Williams–Hawkings acoustic analogy is employed. The framework is first benchmarked against the standard David Taylor model basin 4119 propeller, bounding the low-frequency (<200 Hz) sound pressure level deviation to under 4 dB. Upon application to the Kaplan propeller at 600 rpm, analysis extracts specific physical mechanisms governing its acoustic signature. Numerical results reveal an anisotropic directivity pattern; specifically, unsteady in-plane forces, inherent to the heavily loaded, low-skew blade geometry, are identified as dominant dipole sources, concentrating maximum noise radiation exactly at the 90° rotational plane. Furthermore, hydroacoustic mapping quantitatively confirms that near-to-far-field noise attenuation is governed strictly by spherical geometric spreading. Experimental validation confirms the accurate numerical prediction of the fundamental tonal frequency; however, it also highlights differences in broadband noise levels, attributed to the reverberant tank environment, contrasting with free-field numerical assumptions. Ultimately, extracted directivity maps and identification of dominant dipole mechanisms establish a rigorous physical baseline for optimizing on-board sonar placement and implementing targeted trailing-edge noise mitigation strategies in Kaplan-propelled vessels.