Free-vibration tests of a cantilevered rectangular prism with a side ratio of less than D/H = 0.5 (D, depth of the rectangular prism in the flow direction; H, height of a rectangular prism) that experiences low-speed galloping at a lower reduced velocity than the resonant reduced velocity were conducted in a water tunnel. We also carried out a power generation experiment using an iron-gallium alloy, which is a magnetostrictive material, to investigate the possibility of harvesting energy from a flow-induced transverse vibration. The effects of the side ratio of the rectangular prism and the configuration of a fin that was fitted on the back of a rectangular prism to increase the flexural rigidity on the response amplitude were investigated. The vibration of a rectangular prism with D/H = 0.1 occurred at a lower reduced velocity than that of the other rectangular prisms. This prism has the largest increment rate of the non-dimensional response amplitude ηrms for reduced velocity Vr, and the vibration has an uniform amplitude for each reduced velocity. The reduced velocity at the 20% non-dimensional response amplitude of a rectangular prism Vr0.2 decreases linearly with decreasing side ratio D/H. The response amplitude of the rectangular prism with D/H = 0.2 is only slightly dampened for all values of the reduced mass-damping parameter Cn.
The values of Vr0.2 of the rectangular prism with a fin increased as the depth ratio of the fin increased. The response amplitude of the rectangular prism with/without a fin was found to be related to the second moment of area of the prism. The energy harvester using magnetostrictive material and a rectangular prism with D/H = 0.2 generates a maximum electric power of 8.8 mW.