The equation between secondary flow and vorticity was established through kinematic analysis.One method of digital visualization for secondary flow in curved passage was presented.Secondary flow velocity can be obtained from cross section normal vorticity.Some advantages of this method are below:the equation describes secondary flow as vortices.The influence of potential flow is eliminated.Quantizing secondary flow field can be figured out.More complicated secondary structure is observed.The visualization is more accurate than representation of velocity vector
and doesn ’t depend on the normal direction of cross section.