Vortex Axis Contour Analysis Through a New Axis-Line Connectivity Algorithm for VATIP

The recent development of the VATIP (vortex axis tracking by iterative propagation) method allows the vortex axes to be extracted from turbulent flow fields and individually analyzed and quantified. The existing VATIP algorithm, however, only generates groups of vortex axis points for individual vortex objects. This study proposed an improved algorithm that connects those vortex axis points into organized vortex axis lines, allowing for enhanced vortex geometric analysis. This connecting algorithm first labels and orders all vortex axis points while they are being collected into each vortex segment during the iterative search process. Then, points of different segments belonging to the axis line of the same vortex can be merged and renumbered sequentially to form the skeleton of that vortex. The new algorithm can be applied to measure several geometric features of the vortices, such as their contour length, number of branches, and curvature, some of which only become possible after axis points are connected into vortex axis lines.