Lift generation on a sphere through asymmetric roughness using active surface morphing

Abstract

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 ($Re = U_{\infty} d/\nu$, where $U_{\infty}$ is the fluid velocity, $d$ is the sphere diameter, and $\nu$ is the fluid kinematic viscosity) ranging from $6\times10^4$ to $1.3\times10^5$, and dimple depth ratios ($k/d$, where $k$ is the dimple depth) from $0$ to $1\times10^{-2}$ using a smart morphable sphere with one smooth and one dimpled side. 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 the $Re$ at which lift generation begins and the maximum lift coefficient ($C_L$) achievable. The optimal $k/d$ for maximum lift varies with $Re$: 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 (PIV) 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. Overall, this study establishes the foundation for wake control over bluff bodies and paves the way for real-time manoeuvring applications.