Numerical investigation of ground effect on a toroidal propeller

In this study, a numerical investigation is conducted to examine propeller performance, flow physics, and aeroacoustic signatures of a toroidal propeller in ground effect (IGE). Delayed Detached Eddy Simulation (DDES) is employed to resolve propeller performance, flow structures, vortex formation, and surface pressure fluctuations across a range of normalized ground spacings. Aeroacoustic predictions are obtained using Ffowcs Williams–Hawkings (FW–H) acoustic analogy to quantify primary tonal noise, broadband noise, and overall sound pressure level (OASPL). The numerical results reveal that descreasing ground spacing leads to more thrust augmentation compared to two conventional propellers. Thrust, torque, and figure of merit (FM) all exhibit asymptotic trends with respect to ground spacing. Compared with experimental results, the numerical simulations predict propeller thrust more accurately than torque. Flowfield analysis indicates that this performance enhancement is primarily driven by ground-induced flow recirculation and a localized upflow underneath the propeller, rather than by the classical fountain-effect mechanism. Vorticity analysis further reveals increased coherence and confinement of the tip vortices at small ground spacings. From an aeroacoustic perspective, ground effect produces a pronounced modification to noise directivity. The primary tonal noise exhibits a quadrupole-like directivity pattern that weakens and nearly vanishes along the rotation axis at small ground spacings, consistent with enhanced destructive interference of coherent loading sources. In contrast, broadband noise and OASPL display predominantly dipole-like directivity and increase toward the rotation axis, driven by increased turbulence and unsteady flow interactions associated with the upflow.