Aerodynamic performance enhancement of ducted rotors with bio-inspired non-smooth structures: Mechanisms and flow control

Boundary layer separation on the duct inner wall and its complex coupling with the rotor tip leakage vortex (TLV) constitute the primary sources of aerodynamic blockage and energy dissipation in ducted rotor systems. This study investigates a passive flow control strategy utilizing biomimetic non-smooth structures to suppress separation and recover aerodynamic lift. An integrated optimization framework combining computational fluid dynamics, Kriging surrogate modeling, and genetic algorithms was employed to determine the optimal topological parameters of the surface modification. The optimized configuration, characterized by an array of elliptical concave pits, yielded a 2.12% increase in total lift and a 4% recovery in rotor lift compared to the baseline. Detailed flow physics analysis reveals that the lift enhancement stems from two governing mechanisms: “shear modulation” and the “breathing effect.” The alternating distribution of high-shear reattachment and low-shear recirculation zones within the pits functions as a virtual roughness, promoting momentum injection from the mainstream into the viscous sublayer to resist adverse pressure gradients. Simultaneously, the spatially alternating suction and ejection regions induced by the pit array decouple the TLV from wall secondary flows. This interaction fragments the coherent, large-scale separation ring into dissipative fine-scale turbulence, thereby eliminating the “virtual throat” effect inside the duct. Macroscopically, this topological reorganization manifests as a structural shift from a divergent, coherent wake to a fragmented, contracting streamtube in the far field, indicating a significant recovery of momentum flux and propulsion efficiency.