Abstract: An effective numerical method (fast vortex method) to solve unsteady N-S equations at high Reynolds number, combining Chorin's vortex scheme with the Vortex-in-Cell (VIC) method and the Fast Fourier Transforms (FFT), is presented in this paper. The scheme has successfully calculated the early stage flows of two dimensional circular cylinder which impulsively started from rest. The process of separation and evolution with time of the flow, the generation of the secondary vortex and the pair of the secondary vortex, and the radial velocity on the symmetry axis behind the cylinder, etc., are studied in detail. The comparison with the careful experimental data and other accurate numerical results is satisfactory. The program is also available for other problems of unsteady flow at high Reynolds number.

Key words: unsteady flow, cylindrical bodies, numerical calculation, numerical flow visualization