Subject to constant computational area, two expressions of optimal vertical gridlength with different accuracy and an expression of consistent vertical gridlength were presented in the case of a single wave. And the corresponding expression of consistent vertical gridlength in the multiwave case was derived-

Two-dimensional imcompressible viscous flows around a circular cylinder with periodic oncoming flow from infinity are investegated. By means of spectrum method, the stream functions are developed into Fourier series, so the stream functions depend on angle 0 continuousely. Numerical calculations are implemented for stream functions as well as vortices for the case K_c=4 and Re=200. The results manifeste that the method used here has the advantage of simplicity and saving computer time comparing with the method of fully discretiged N-S egs. and discretiged vortices method.