Hello

I have a very simplified sim to show you what my problem is.
two simple flip volume sources, I want one of them to ignore the collision plane.
I couldn't do it in flip sop as well, yeah more good news.

I really appreciate any help you can provide.

I found a not quite trusty way to do this.
but It works strangely.
using just surface collision instead of volume...
"popcollisionignor" did the job.

popcollision ignore will just ignore particle collisions, which in flip are just secondary to volume collisions and purely optional, they don't contribute to fluid motion, just help to keep advected particles not pass through colliders

pressure projection needs proper volume collisions

if you want to make some region of volume collisions "transparent" (the transparency option in Collision tab) you can zero out "collisionweights" field around particles you don't want to collide

keep in mind that volume vel field is solved for the whole sim and all particles will use it so there is no specific way to separate that some particles will properly collide with volume colliders ehile other will pass through in the same region
since all particles are advected by the same field

if however you do tweak "collisionweights" make sure that you either keep your particle collision ignore settings to reflect the same behavior or set particle collisions to None

An enlightening explanation like always.

thank you very much, Tomas!