FLIP not being very flippy.

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So I got back into H after seeing the progress with v12 and now playing with the FLIP solver. I have been using Realflow for all liquids up to this point and im missing the nice tumbling or flipping behavior from viscous fluids that I get with realflow, surface tension and drag daemons.

Currently H fluid sticks together, collides, etc looks like fluid, but when sections fall off, they tend to fly in a purely ballistic trajectory, with no conservation of angular momentum. Do i need more voxel resolution or something? Or do i have to post process this with a POP solver to flip particles around clump centroids manually?

thanks!
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Currently the default FLIP solver misses surface tension because the main focus is for large scale effects, but you can add your own surface tension solver for volume or particle stage, it's easy to implement, you need to push or pull the particles relative to the surface curvature as a scale factor for the force and use the surface gradient for direction.

Hope that this helps!!!
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it does, can you give me a rough explanation for the gradient force? i havent been using the surface fields much yet. I am assuming that the surface tension that is already in houdini is more for smoothing out imperfections rather than pushing particles along surface?
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Here is my surface tension solver for volume stage.

The default surface tension node works, but is very slow.

Attachments:
DOP_aeb_surfacetension.otl (12.7 KB)

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thank you so much! I am testing this right now, its kind of hard to tell if its working on this particular shot, but i will get back to you as i test it on more shots!

Thank you so much for your help, im looking at the asset you gave me and learning.

thanks again.
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on another note, i saw your awesome path forces asset. How did you get the visualizations to work like that? I would love to learn that too.

thanks!
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Here http://www.sidefx.com/index.php?option=com_forum&Itemid=172&page=viewtopic&t=21558 [sidefx.com] you can find two versions of the tool, but the last one is a more compact version, still work in progress now with VDB primitives.
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wow thanks for the links.

So i have been using your surface tensions node, its definitelly working and definitelly fast. But im also realizing that the tumbling look of liquids isnt jsut surface tension, also there is drag. What makes the fluids rotate is the drag on leading particles, which causes trailing particles to pile up and deform the droplet if there is more compressive force than the surface tension can hold back.

so would it be possible to also ad a little bit of a drag onto the flip particles but only on the particles nearest to the leading surface field? maybe there is already existing gas solvers i should be looking at?
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actually upon closer investigation your surface tension does impart some tumbling. Sorry, I had allot of unresolved flip particles which were concealing and reducing the apparent effects of your microsolver. After tuning sub-steps and sampling distances, i got the particles to stay resolved (its a pretty high speed sim).

thank you for your help !
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actually upon closer investigation your surface tension does impart some tumbling. Sorry, I had allot of unresolved flip particles which were concealing and reducing the apparent effects of your microsolver. After tuning sub-steps and sampling distances, i got the particles to stay resolved (its a pretty high speed sim).

As you may know, you can turn off the unresolved behavior altogether by setting the Under-Resolved Particles to No Detection. For a high surface tension look you might want to do that, coupled with increasing the Particle Radius scale by a bit, say to 1.5-1.7 or so.
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The default surface tension node works, but is very slow.
The good news is the GasSurfaceTension DOP was multithreaded and optimized for 12.5. The bad news is there was a bug in it that made it give incorrect forces. This is fixed in tomorrow's build (12.5.384), so I'd encourage you to give that a try as well.
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Thanks for the info Johner, I'll give a try!!!
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Thanks for the info Johner, I'll give a try!!!

Great, let us know how it goes! Just to be clear, the fix didn't make tonight's build, so it's “tomorrow” as in Thursday, I believe.
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Thanks for the info Johner, I'll give a try!!!

Great, let us know how it goes! Just to be clear, the fix didn't make tonight's build, so it's “tomorrow” as in Thursday, I believe.

It works now a lot faster!!!! Thanks for the update!!!
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