Passive control of vortex-induced vibration of a sphere.

Although passive methods for controlling vortex-induced vibrations (VIV) are extensively studied for a circular cylinder, such methods remain unexplored for a basic three-dimensional bluff body, a sphere. In this study, we use a surface trip wire as a passive method to control sphere VIV. The effect of a surface trip is experimentally investigated for varying diameter (1.25 x 10$^{\mathrm{-2}} \quad \le \quad k/d \le $ 6.63 x10$^{\mathrm{-2}})$ and stream wise location ({\o} $=$ 20$^{\mathrm{o}}$ -70$^{\mathrm{o\thinspace }}$from the stagnation point) of the trip wire for a wide range of reduced velocities (3 $\le $U$^{\mathrm{\ast }}\le $20). It was found that the vibration amplitude decreases progressively with the increase in the stream wise location angle ({\o}) of the trip wire. The control was highly effective in mode II and mode III of the VIV response with maximum reduction of up to 97.8{\%} for {\o} $=$60$^{\mathrm{o.\thinspace }}$Interestingly, thicker trip wires (\textit{k/d \textgreater }1.25 x 10$^{\mathrm{-2}})^{\mathrm{\thinspace }}$were more effective in mode I, but showed a galloping response for higher reduced velocities. $^{\mathrm{\thinspace \thinspace }}$