Hello,
I have a problem about this node's technic detail.
It said it use gradient of velocity to define scale/rotate,I'm not good at vector calculus but what does “gradient of velocity” mean? Is it divergence and curl(vorticity)? Or just three gradients ,for every component? ops: Thanks

Anyone?

You can think of the gradient of a velocity field as a shear detector. It returns to you the direction and magnitude of the surrounding velocity with respect to the current voxel's velocity in to a new field or geometry (vorticles in the case of the Gas Velocity Stretch DOP).

The Gas Velocity Stretch DOP is one key tool giving vorticles their effect.

In the context of Gas Velocity Stretch DOP, you want it to work in ares where there are shear forces in the fluid and then update the vorticles velocity (yes you need vorticles). The vorticles then mix in with the velocity from the previous timestep which gets passed in to the Project Non-Divergent DOP to give you various fluid behaviours.

The two DOPs inside both the Smoke and Pyro solvers doing the work with the vorticles are the Gas Velocity Stretch DOP and Gas Vorticle Forces DOP. The first figures out where the velocity shears are and updates the vorticle geometry's attributes. Next, the Gas Vorticle Forces DOP takes the vorticle attributes (v, N, up, orient) and modifies the vel field.


You could also have just google'd it.
“velocity gradient”

See the attached hip file.

Thankyou jeff,and Thanks for your example!
I google'd it,
it's just like ▽v term in Dv/Dt,right? hope i'm not misunderstand it.

Yep.