In order to understand the physical mechanism about the aeroelastic instability of bluff cylinders, the two-dimensional simulations have been mainly used for computational approach because the flow structure around the oscillating cylinder is expected to become two-dimensionalized due to the effects of the body motions. However, through the comparison with the experimental data, there is a little quantitative difference in the predicted values. We discuss the necessity of the three-dimensional simulation for the unsteady problems on oscillating rectangular cylinders in uniform flow. As the length of the cylinder and the grid space in the spanwise direction are varied, we investigate the relation between the unsteady three-dimensional flow structures around the oscillating cylinder and the predicted aerodynamic forces acting on the cylinder.
