The S-shape inlet has been widely used in advanced military aircraft due to their advantage of reducing radar signature.However,the curvature of the inlet usually causes different kinds of intake distortion at the aerodynamic interface plane (AIP).Among them,the swirl distortion has been seriously concerned because of its great impact on the performance and stability of aero-engines.There is still no universal criterion for assessing the stability of compressors in the condition of strong swirl distortion.As an approach of assessing the swirl intensity and pattern,vortex identification method may be used as an auxiliary method for stability analysis.In this paper,numerical and experimental investigations on different S-ducts were carried out.The axial vorticity component and Q criterion were used to analyze the quantitative correlation between geometry and swirl intensity.It was found that there is a relatively strong correlation between the geometry,the axial vorticity component and the Q criterion.The present investigation may provide a quick reconstruction method to model the effect of S-ducts for compressor stability prediction.