Nonlinear wake noise of an underwater propeller under nonuniform inflow

In this study, we investigate the nonlinear noise generated in the wake of an underwater propeller operating under nonuniform behind-hull inflow using large-eddy simulation coupled with the Ffowcs Williams and Hawkings acoustic analogy. The analysis characterizes the extent of the region in which nonlinear noise is influential relative to linear noise under nonuniform inflow, expands the current understanding of the localization of nonlinear noise sources, and interprets the propagation characteristics and spatial distribution of the resulting acoustic field based on the identified source localization. The streamwise evolution of nonlinear noise generation is correlated with energy transmission during azimuthally asymmetric wake destabilization, with the dominant content shifting from the blade-passing frequency tone toward broadband, azimuthally asymmetric radiation. The broadband noise is localized in and related to the short-wave destabilization of the hub vortex, whereas the asymmetry arises from stronger vortex intensity and more vigorous inter-vortex interactions in the upper half of the wake region. In the propeller disk plane, as the observation distance increases, the asymmetry of the noise radiation flips: the louder region alternates between the upper and lower sides of the propeller. This flip occurs because, within a specific localized region, the broadband noise sources in the lower-half wake become stronger than those in the upper half, which is an inversion relative to the distribution of the tonal sources.