On the lift generation over a sphere using asymmetric roughness

This study investigates a novel phenomenon of lift generation around a sphere by pneumatically manipulating its surface topology with an asymmetric roughness distribution. A comprehensive series of systematic experiments were conducted for Reynolds numbers (Re=U∞d/ν, where U∞ is the free-stream velocity, d is the sphere diameter, and ν is the fluid kinematic viscosity) ranging from 6×104 to 1.3×105, and dimple depth ratios (k/d, where k is the dimple depth) from 0 to 1×10−2 using a smart morphable sphere. The findings show that an asymmetrically rough sphere can generate lift forces up to 80% of the drag, comparable to those produced by the Magnus effect. The dimple depth ratio affects both Re at which lift generation begins and the maximum lift coefficient (CL) achievable. The optimal k/d for maximum lift varies with Re: deeper dimples are needed at low Re, while shallower dimples are more effective at 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 revealed that dimples delay flow separation on the rough side, while the smooth side remains unchanged, resulting in asymmetric boundary layer separation, leading to wake deflection and lift generation. Beyond the critical k/d, the flow separation location moved upstream, increasing the size of the rear wake, reducing wake deflection, and thus decreasing lift. This study is the first to report how manipulating surface topography can generate lift forces comparable to those produced by the Magnus effect, offering a new mechanism for aerodynamic control. These findings could have significant implications for the maneuvering capabilities of unmanned underwater vehicles, where fine control of lift is essential for agility.