Lift generation on a sphere using active surface morphing

This study investigates the generation of lift on a sphere by pneumatically manipulating its surface topology with an asymmetric roughness distribution using morphable dimples. The experiments were conducted at Reynolds numbers of 6×10⁴ ≤ Re ≤ 1.3×10⁵ and dimple depth ratios of 0 ≤ k/d ≤ 1×10^-2. A generation of lift forces up to 80% of the drag was found, comparable to those produced by the Magnus Effect. The dimple depth ratio affects both the Re at which lift generation begins and the maximum lift coefficient C_L achievable. The optimal k/d for maximum lift varies with Re where low Re requires deeper dimples and shallower dimples for high Re. For a fixed Re, increasing k/d monotonically increases lift until a critical k/d is reached, beyond which lift decreases. Particle Image Velocimetry (PIV) revealed a delay in flow separation on the dimpled side while the smooth side remains unchanged, resulting in asymmetric boundary layer separation that leads to lift generation. Beyond the critical k/d, the flow separation location moves upstream as the rear wake increases. This study establishes the foundation for flow separation control over bluff bodies and paves the way for maneuvering applications.