摘要
Influenced by the fact that vorticity represents rotation for rigid body,people believe this idea also works for fluid flow.However,the vortex predictions by vorticity do not match experimental results,which drove scientists to look for more appropriate methods to identify vortex.All vortex identification methods 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.People still believe vorticity is vortex since vorticity theory looks correct in mathematics,but all other methods are only scalars and unable to indicate the swirl direction.Recently,a new vortex identification method called Liutex is innovated.It is regarded as the third-generation method,not only overcoming all previous methods’drawbacks but also having a clear physical meaning.The direction of Liutex represents the swirl axis of rotation,and its strength is equal to twice the angular speed.In this paper,we did a correlation analysis between vorticity,Q,λ_(ci),λ_(2)methods and Liutex based on a direct numerical simulation(DNS)case of boundary layer transition.The results show that the correlation between vorticity and Liutex is very small or even negative in strong shear regions,which demonstrates that using vorticity to detect vortex lacks scientific foundation and vorticity is not appropriate to represent vortex.The correlation analysis also shows that the second generation is contaminated too by shear and thus is not accurate to identify the vortex structure.
