A new automatic vortex (Liutex) core line identification method in boundary-layer transition

Conventional isosurface-based vortex visualization depends strongly on user-specified vortex-strength thresholds, which may hide weak vortices. Liutex core lines provide a line-based representation of vortex structure that is not dependent on a single global vortex-strength threshold and can display local vortex-strength variation along the vortex skeleton. However, an automatic extraction of Liutex core lines remains challenging in complex three-dimensional flows because real core lines usually pass through sub-grid locations, and redundant seed points may cause repeated or incorrect line generation. In this work, an automatic Liutex core line identification method is developed by combining a Liutex-based predictor–corrector procedure, two criteria for identifying Liutex core points, and a grouping algorithm for assembling these points into continuous core lines. The method is applied to both a micro-vortex-generator wake flow and a flat-plate boundary-layer transition flow. The results show that the method correctly captures the vortex structures in these two cases. Compared with conventional isosurface visualization, Liutex core lines can reveal weak structures that may be lost at a prescribed threshold while maintaining a clear representation of strong vortices. The method also reveals dual-core structures within hairpin vortex ring isosurfaces, providing new insight that the formation of hairpin vortices may be related to Kelvin–Helmholtz instability.