Flow-induced vibration often becomes a cause that damaged in a structure and causes a serious accident like the case of “Monju”. Therefore, the research of mechanism of flow-induced vibration is very important subject. Moreover, many structures have three-dimensional shape with sharp edge in span-wise direction. Therefore we investigated the three-dimensional flow structure about flow-induced vibration of a circular cylinder with finite span-wise length by numerical simulation. An analysis object is three kind of span-wise length, L=2.5D, 5D and 8D, where D is a diameter of circular cylinder. The range of simulated reduced speed is 1.5 to 4.0, in which shows 'in-line oscillation'. A reduced mass-damping parameter sets to zero. We assumed that the flow field is unsteady and laminar because Reynolds number which is defined by uniform flow velocity U and D, is 103. We inspected simulated results in detail and investigated the relation between the fluctuation of fluid forces and flow pattern. As a result, at small span-wise length, because cylinder is largely affected by the flow, which rolls up from the tip, vortex shedding is restrained. On the other hand, as larger span-wise length, the flow interference between tip and wake, so the flow pattern and flluid forces affected to cylinder is more complex.
