[David81]

I understand, through reading some literatures, that

First, the streamwise (x) velocity gradient in wall-normal (y) direction forms the spanwise vortices. This is because the difference in the streamwise velocity produces a rotating shear force in the spanwise (z) direction. This is my understanding, if it is wrong please correct me.

Then, I also read that these spanwise vortices are lifted upwards and stretched. This led to the existence of counter-rotating streamwise vortices. The point I cannot understand is that how can the spanwise vortices create the resulting streamwise vortices?

Could anyone please explain this mechanism? Thank you very much in advance.

[FMDenaro]

That can be demonstrated from the Helmholtz equation written in the main reference system for grad v. The stretching term is responsible of the alignement...