Liquid sim: phantom velocities

[tysonibele]

I have a very basic liquid simulation....a collider mesh with a bumpy surface holding some liquid. Around the edges of the mesh I get lots of phantom velocities injected into the liquid sim for no apparent reason. They manifest as bubbly areas on the liquid surface that never go away.

I've had this issue with PhoenixFD for years, never really found a solution -- figured I'd finally post about it. I'm using 3dsmax 2017 and PhoenixFD 3.1.

Attached is an example scene file with the problem, and a playblast showing it in action. I would expect the surface to eventually even out, but it continues to boil and bubble forever. This is just a demonstration scene...in my actual production scene the perpetual boiling is much, much worse.

Any ideas?

[tysonibele]

It's worth noting that I just tested things in the latest stable build (3.12) and the problem still occurs.

[tysonibele]

Here's a playblast looking up from underneath the fluid, with the collider hidden and the fluid mesh hidden as well. You can see there are various velocity hotspots that don't ever go away, that are continually injecting velocities into the sim which are resulting in the surface boiling as the forces propagate upwards.

[Svetlin.Nikolov]

Hey, this might just be related to an issue with border conditions I'm looking at right now. Thank you very much for the scene!

Will check it and get back to you when I know more...

Cheers!

[tysonibele]

Cool

Also, here's a final playblast I made from the provided scene file, isolating just the bottom layer of particles closest to the collider that the other liquid is contained inside. You can see there are hotspots where some wild stuff is happening for seemingly no reason, which would explain all of the velocity artifacts.

[Svetlin.Nikolov]

Hey, so just a temporary workaround until the ocean border comditions are changed - set the bottom wall to Jammed.

[tysonibele]

Hey, tried that but it doesn't seem to have an effect (well, it had some effect...the sim is a bit different, but it didn't fix the problem)....I set the bottom wall to jammed, set the side walls to jammed too, set all walls jammed, and still get the crazy mystery velocities.

[Svetlin.Nikolov]

Ah, sorry, maybe this would work only with my latest changes. So I'll ping you when the changes are in the nightly builds - I hope it won't take more than 4-5 days.

Cheers!

[tysonibele]

Ok, thanks!

[Svetlin.Nikolov]

Hey, so in tomorrow's nightlies this should not happen anymore and you don't need to change the setup in any way. There are still some disturbances of the liquid surface (which you can fix by raising the Scene Scale or the Steps per Frame), as well as some grid velocity being emitted where the liquid contacts the underwater geometry, and I will look at these separately.

Cheers!

[tysonibele]

Thanks, I'll check it out.

Although....

some grid velocity being emitted where the liquid contacts the underwater geometry

That's what's causing the boiling...

Either way, I'll try tomorrow's nightly.

[Svetlin.Nikolov]

That's right, they did, but it happened because they were en route to the surface aided by a self supporting process that should not be able to happen.

Cheers!

[tysonibele]

A year later and this issue still hasn't been fixed (happens in latest 3.x and 4.x builds), and the phantom velocities are making it impossible to simulate stable fluids inside a collider. Will this be addressed at some point?

[Paul Oblomov]

Heh, my lovely boiling Great to see, it is still here *sarcasm*

[Svetlin.Nikolov]

Um, my suggestions from above still hold - raising the Scene Scale and/or raising the Steps Per Frame would reduce these effects. Here's the difference between scale 1 and 10:





and how the mesh looks with the larger scale:



The reason for the issue is some funky magic left from the original FLIP implementation that I have not reworked yet. It's creating incorrect supporting pressures into voxels that are partially covered by solid geometry.

It's always getting offset by other changes or fixes since this one has a workaround and is gonna take extra time to make sure everything is okay after...

Cheers!