Velocity Problems in basic Flip Simulation

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Joined: Nov. 2013

I am having two issues with a pretty basic Flip Simulation (Shelf Tool Emit).

I am getting random velocity spikes inside the fluid as soon as the fluid settles. Little flashes with high velocities in small areas only for one frame. Gets very noticeable with the Particle Fluid Surface Preview, but also inside the DOP if you look carefully.

The other issue is the fluid behavior inside volume. When using the solvers boundaries, everything looks fine. But as soon as I use my own volume for collisions, the fluid becomes very jagged and I am getting also strange velocity spikes and particles sometimes escape the container.

Would be great if someone knows what's causing this. (Hip file attached below)

Edited by Fele - March 19, 2023 11:41:13

collisions.jpg (616.7 KB)
spikes.jpg (367.1 KB)
flip_problems.hiplc (2.3 MB)

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2412 posts
Joined: June 2008
One thing to try is supplying an alternate collision object. Instead of placing your FLIP source inside the collision object and inverting the SDF, try building walls around the source and running non-inverted. When I turn off Oversampling At Boundaries on the Reseeding tab, I notice the white velocity spikes go away. My guess is when old particles die in a voxel, and are reseeded, the new particles might intersect momentarily, generating a velocity boost that reveals itself as those white spikes.

ap_flip_problems.hiplc (2.3 MB)
flip_vel_issue.gif (1.3 MB)

Using Houdini Indie 19.5.
Ubuntu 64GB Ryzen 16 core.
nVidia 3050RTX 8BG RAM.
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665 posts
Joined: July 2005
If you don't need Reseeding turn this option off as it can definitely cause strange spikes in your simulation.
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