The original votrex core alogrithm as described in Identification of Swirling Flow in 3-D Vector Fields is used on the tapered cylinder data by Jespersen and Levit.

Note: The method, by its nature, does not produce a contiguous line for the vortex core. This is due to two reasons; (1) for element types that are not tetrahedra the interpolant that describes point location within the cell is not linear. This means that if the core passes through these elements the line can display curvature. By subdividing pyramids, prisms, hexahedra and higher-order elements into tetrahedra for this operation produces a piecewise linear approximation of that curve. And (2) there is no guarantee that the line segments will meet up at shared faces between tetrahedra. This is because the eigenvector associated with the real eigenvalue will not be exactly the same in both neighbors, so when this vector is subtracted from the vector values at the shared nodes each tetrahedra sees a differing reduced velocity field for the face.

Data courtesy of NASA Ames Research Center.