**2.1 Method of experiment**

Model experiments using a single circular cylinder or two arranged circular cylinders in tandem are carried out at a wave tank that has 27 m in length, 7 m in width and 1 m in water depth in the campus of Funabashi at CST of Nihon University. We cannot generate current so that forced oscillation tests in still water are carried.

An experimental method and concepts follow our own past model testing (Ikoma & Masuda et al., 2006, 2007). In this study, a single cylinder or double cylinders in tandem arrangement to inline direction are used.

Test models of a cylinder are made of acryl resin which is rigid. However the cylinder system is not fixed because elastic vibration is allowed in only cross-flow direction by attaching a flat spring on top of the cylinder. The flat spring does not allow inline movement of the cylinder. Inline movement is due to only forced oscillation by the oscillator. VIV occurs such like rolling motion around *x* axis which center is the flat spring.

Number of degrees of freedom is two in sway motion and roll motion. The sway in this experiment is horizontal displacement of center of gravity in *y* direction and rolling is rotation around *x* axis in a coordinate system of Fig. 1. Freedom of surge corresponding to *x* axis motion is allowed and decided due to forced oscillation by experimental operators.
