New omega vortex identification method
A new vortex identification criterion called O method is proposed based on the ideas that vorticity overtakes deformation in vortex. The comparison with other vortex identification methods like Q criterion and l2 method is conducted and the advantages of the new method can be summarized as follows: (1) the method is able to capture vortex well and very easy to perform; (2) the physical meaning of O is clear while the interpretations of iso surface values of Q and l2 chosen to visualize vortices are obscure; (3) being different from Q and l2 iso surface visualization which requires wildly various thresholds to capture the vortex structure properly, O is pretty universal and does not need much adjustment in different cases and the iso surfaces of O=0.52 can always capture the vortices properly in all the cases at different time steps, which we investigated; (4) both strong and weak vortices can be captured well simultaneously while improper Q and l2 threshold may lead to strong vortex capture while weak vortices are lost or weak vortices are captured but strong vortices are smeared; (5) O=0.52 is a quantity to approximately define the vortex boundary. Note that, to calculate O, the length and velocity must be used in the non dimensional form. From our direct numerical simulation, it is found that the vorticity direction is very different from the vortex rotation direction in general 3 D vortical flow, the Helmholtz velocity decomposition is reviewed and vorticity is proposed to be further decomposed to vortical vorticity and non vortical vorticity.