Interaction of Vellum system and Flip System
In Houdini's default flow, the grains of the Vellum system cannot interact and affect the particles of the Fluid system. But this problem can be solved by modifying the solver and adding a custom sub-solver
In the same dop, the bullet solver can directly interact with the flip solver by connecting to a merge node.
But the vellum and flip systems seem to be unable to interact directly in this way.
Therefore, it is necessary to manually add additional factors to the two systems to interact with each other.
The sop solver connected to the volume source of the flip system allows us to use the sop node in the dop to modify the attributes. Therefore, here, I used a volume rasterize attribute node to convert the v attribute of the flip particle into a vel field. It should be noted that although the sop solver is a sop environment, dop import will still update the solution results of each frame in dop for us, so the vel field generated here will also change in real time according to the results of the flip sim. With the vel field generated according to the flip particles, the pop advect by volumes node can be used to substitute the vel field into the simulation of the vellum system
In the vellum system, in order to make the vellum particles a collider that can be recognized by the flip system, the sop solver is also used. Here, through the vdb from particles node, particles can be turned into sdf field in real time. After that, the sdf field generated here is applied to the collision simulation of the flip through the static object, and the effect of the vellum particles affecting the flip particles can be obtained.
Note: In this method, due to the calculation order of the dop system, the sdf field generated by the vellum particles will be applied to the simulation with a lag of one frame. Therefore, errors may occur in some high-speed moving simulations