  Influenced by the fact that vorticity represents rotation for rigid body,
people believe it also works for fluid flow. However, the theoretical
predictions by vorticity do not match experiment results, which drove
scientists to look for better methods to describe vortex. According to Dr. Liu
classification, all methods applied to detect vortex can be categorized into
three generations. The vorticity-based method is classified as the first
generation. Methods relying on eigenvalues of velocity gradient tensor are
considered as the second generation. Although so many methods appeared, people
still believe vorticity is vortex since vorticity theory looks perfect in math,
and all other methods are only scalars and unable to indicate swirl direction.
Recently, Dr. Liu innovated a new vortex identification method called Liutex.
Liutex, a vector quantity, which is regarded as the third-generation method,
not only overcomes all previous methods drawbacks, but also has a clear
physical meaning. The direction of Liutex represents the swirl axis of
rotation, and its strength is equal to twice of angular speed. In this paper,
we did a correlation analysis between vorticity, Q, Lambda ci, Lambda 2 methods
and Liutex based on a DNS case of boundary layer transition. The results show
that the correlation between vorticity and Liutex is minimal in strong shear
region, which demonstrates the idea that using vorticity to detect vortex lacks
a scientific foundation; in other words, vorticity is not vortex.
