Aerodynamic and Aeroacoustic Effects of Axial Clearance in a Wall-Penetrating Blade Ring Ducted Fan for Unmanned eVTOL Propulsion

Tip leakage degrades ducted fan performance and contributes to unsteady loading noise. This study investigates a wall-penetrating blade ring (WPBR) ducted fan for unmanned electric vertical takeoff and landing (eVTOL), replacing radial clearance with axial end face gaps. Six configurations of a 381 mm four-bladed rotor were evaluated at 5000 r/min under quasi-hover conditions: a conventional ducted fan, an internal blade ring rotor, and four WPBR cases with single-sided clearances of 0.8–2.0 mm. Sliding-mesh unsteady Reynolds-averaged Navier–Stokes simulations using the shear stress transport k-ω model were coupled with the Ffowcs Williams–Hawkings formulation. The WPBR formed a U-shaped cavity recirculation and redistributed the concentrated tip-region vortical structures. The 1.2 mm case retained 28.72 N of thrust, 2.1% above baseline, while reducing torque by 6.1% relative to the 0.8 mm case; its figure of merit remained lower. Its simulations predicted a reduction of 16.4 dB in the first blade-passing frequency level in the rotor plane and a predicted reduction of up to 15 dB in overall sound pressure level at 1 m. Thus, it represents a compromise among thrust, torque, and predicted acoustic performance rather than an aerodynamic optimum. A magnetically supported prototype operated up to 2000 r/min, demonstrating low-speed operability of the architecture for unmanned eVTOL propulsion.