Vortex-induced vibration and its effect on torsional flutter instability in the case of B/D=4 rectangular cylinder

For two-dimensional rectangular cylinder with the side-ratio of 4 (B/D=4, B: along-wind length, D: cross-wind length), two different types of vortex-induced excitation, which are motion-induced vortex excitation, i.e. shear layer instability related-vortex excitation, and Kármán vortex excitation, were observed in both heaving and torsional motion. Also, the role of vortex-generation on the torsional flutter instability was experimentally investigated by changing the rotation axis in the state without/with a splitter plate in the wake. The two different kinds of vortex-induced excitations was confirmed in the close reduced wind speed region, and the vortex-induced excitations could play an important role as triggering of torsional flutter instability when Scruton number is small enough. Furthermore, the flow pattern around the cylinder during torsional motion drastically changes between the low and high-reduced velocity.