Passive increase in driving force in vortex-induced vibration of a semi-hollow cylinder for Reynolds number 200

We present a semi-hollow body as an effective strategy to increase the driving force for vortex-induced vibrations (VIVs) of a circular cylinder. The hollow-body concept is evaluated numerically at Reynolds number Re = 200 and in a range of reduced velocity 3 ≤ Ured ≤ 8 with a mass-spring system released to vibrate in the transverse direction. Our numerical solutions reveal that, compared with solid-cylinder counterparts, the net transverse force is increased significantly through the semi-hollow body. The transverse force acting on the inner surface is found to be developed as a consequence of semi-confined flows driven by the cylinder oscillation. Furthermore, it is shown that the inner force has a phase difference with respect to the force acting on the outer surface. Based on a systematic force analysis, the appreciable increase in the transverse force is attributed to the constructive interference between the inner and outer forces.