[MachZero]

In a recent discussion with a non expert, a colleague and I tried to explain why what we are visualizing when we display ISO surfaces of vorticity and q-criterion. We each had a very different way of explaining it, and realized we may not have a good intrinsic feel.

Can people please give there interpretation of "what am I looking at" when a bluff wake is being depicted by isosurfaces of vorticity or q-criterion?

[Svetlana]

I might suggest this resource for an introduction:

J. Jeong and F. Hussain. On the Identification of a Vortex. J. Fluid Mechanics, 285:69-94, 1995.

[FMDenaro]

Originally Posted by Светлана

I might suggest this resource for an introduction:

J. Jeong and F. Hussain. On the Identification of a Vortex. J. Fluid Mechanics, 285:69-94, 1995.

This is a very good reference. I remember also some paragraph in the book of Lesieur.

My comment is that the vorticity field is not necessarily a representation of a vortical structure. Think about the simple case of the laminar flow in a channel, you have vorticity and no vortical structures.

[leflix]

Hi,

Vorticity represents in a flow some regions where you have some shear. But vortices and regions near wall are regions where you have shear. Q-criterion display regions where you have only vortices. So if your aim is to vizualize vortices in a flow, Q-criterion is preferable.
In a 3D domain isosurface of a scalar is a surface where on all of its points, this scalar has the same value. In 2D you use isocontours which are lines where all points on these lines have the same value. This is a common way to vizualize graphically some data.