Looks like a tricky problem!
I will play around with it as well, but there are some smart guys in this thread already, so not sure if I will be able to solve that specific issue.
Sometimes a little bit of shader displacement (along the gradient, tracking with the rest position) might be enough to help hide the problem too - that's more of a fix than a real solution though.
Apart from that it does look like banding of some sort, I've had that happen when the container gets resampled and the resolutions between containers are not the same. It might be good to analyze the velocity in that region.
Also the confinement exagerates the eddies/rotational vortex movement. But if that is too strong it can cause density to collapse in on itself. (Kinda like a black hole). The pressure field should then work as a counterforce to prevent this from happening as that region would cause a zone of high pressure.
I would try to visualize the various fields in that area, specifically your velocity and pressure fields.
– it might be that the confinement is messing with the areas of low velocity.
– or it is a sampling issue. I know for some things only half the res of your fields is used. For example the density field for a resize-able container gets sampled at half res. There might be other areas within the solver where this is happening, but I don't know by heart.
– I would be curious to see if you could make it go away by resampling the field in a vdb and filtering it a bit.
If all else fails, send a pm to Jeff Lait (jlait) or support .