Near wake flow dynamics beneath revolving beetle wings

Revolving-wing studies have typically characterized either leading-edge vortex (LEV) structure directly over the wing or integrated aerodynamic forces, largely using rigid, idealized wing models; the near-wake region itself, where momentum is actually transferred to the fluid, and its turbulence statistics and modal organization in the plane beneath the wing remains comparatively unexamined. We use particle image velocimetry to characterize the near-wake flow beneath revolving hindwings of the beetle (Batocera rufomaculata) at three angles of attack (20°, 30°, 40°), combining downwash footprint area, in-plane momentum flux, turbulence statistics, velocity spectra, and vorticity-based Proper Orthogonal Decomposition (POD). These measures converge on a single pattern: near-wake reorganization with angle of attack is non-monotonic, with footprint area, turbulence intensity, and spectral energy all peaking at the intermediate angle (30°) rather than scaling with loading. POD reveals a hierarchy of annular vortical structures that shift from radially compact at low loading, to diffuse at intermediate loading, to partially re-localized at high loading. These results indicate that the loading dependent wake reorganization of a real, morphologically complex insect wing; combining camber, corrugation, and flexibility, has no precedent in rigid flat-plate revolving-wing studies, with implications for rotor spacing in compact multi-rotor bio-inspired flight systems.