In this paper, unsteady flowfield past a square cylinder is predicted by Large Eddy Simulation (LES). Results of two-dimensional (2D) and three-dimensional (3D) computations are compared with those of previous experiments. It is confirmed that the results of the 3D computation correspond very well with those of experiments, while the results of the 2D computation include some significant discrepancies. The structural difference between the 2D and 3D computations was clarified by comparing distributions of mean vorticity, vorticity fluctuations and the turbulence terms in the vorticity equations. It became clear that vortex-stretching, which is essentially three-dimensional and is not reproduced in the 2D computation, plays a significant role in the unsteady flow phenomena, in particular in the flow near the side face of the cylinder.
