Adaptive flow control over a sphere using smart morphable skin*

Dimples on a sphere's surface are known to significantly reduce drag, but the optimal dimple depth varies with the Reynolds number. In this study, we devised an adaptive surface morphing strategy that adjusts dimple depth in response to changing flow velocity, minimizing drag over a sphere across a wide range of Reynolds numbers. We conducted systematic experiments for a Reynolds number range of Re = 6 x 10⁴ - 1.3 x 10⁵ and dimple depth ratios of  k/d = 0 - 2 x 10^-2 using simultaneous force and particle image velocimetry measurements in a subsonic wind tunnel. Our results indicate the existence of a critical optimal dimple depth ratio for a fixed Reynolds number. As the depth ratio increases, drag decreases monotonically until reaching a critical point, after which drag begins to increase. This behavior correlates with the upstream movement of the flow separation location. From these comprehensive experiments, we developed a model that relates the optimal dimple depth to the Reynolds number for minimizing drag. Implementing this model in our surface morphing strategy demonstrated real-time drag reduction of up to 50%, with the dimples automatically adjusting their size based on input flow velocity to achieve minimum drag.