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In this study, the new concept of vortex core line based on Liutex definitionis applied to demonstrate that vortex ring is not part of the Lambda-vortex andthe generation of ring like vortex is formed separately from the Lambda-vortex.The proper orthogonal decomposition is also applied to analyze the KelvinHelmholtz instability happening in hairpin ring areas of flow transition on theflat plate to understand the ring like vortex formation. The new identificationmethod named modified Omega-Liutex method is efficiently used to visualize andobserve the shapes of vortex structures in 3D. The streamwise vortex structurecharacteristics can be found in POD Mode1 as the mean flow. The other POD modesare in spanwise structures and have the fluctuation motions, which are inducedby KH instability. Moreover, the result shows that fluctuated POD modes are inpairs and share the same characteristics such as amplitudes, mode shapes andtime evolutions. The vortex core and POD results confirm that the Lambda-vortexis not self deformed to a hairpin vortex, but it is formed by the KHinstability during the formation of Lambda vortex and hairpin vortex inboundary layer flow transition.

Vortex Core and POD Analysis on Hairpin Vortex Formation in Flow Transition