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Collapsing Flip Fluids


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So I'm having an issue where when I have fluids inside an animated container, they wont hold their volume. As soon as the geometry moves they just begin to collapse in on themselves. I have reseeding turned off so I can see that all the points still exist, they just exist on top of each other.  If the geometry is not animated it will hold its volume. I have been running into this issue for a while now whenever I try to fill a container with fluid, anybody know what I may be doing wrong? Ill attach a simplified sample file as well. Thanks!


note - Changing the velocity type in the collisions tab of the fluid solver helps but the issue still happens.  Just slower.


Edited by mrhumbility
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Hey james,


I opened your scene and checked what was going wrong.

If you want to have a proper velocity collision, since your static object is transferred to volume, you have to change in the flip solver/volume motion/Collisions the velocity type to volume.


It will work for not so fast moving collider, but if your object is going to move faster, I know that the flip will start to compress the same way because of the pressure solve and I don<t know how to solve that.

I think the better way for fast moving collider is to slow it down for sim collision and retime your sim after.


Good luck



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  • 2 weeks later...

So it seems that animating the geometry on the OBJ level fixes the issue. What I have not been able to figure out it how to make Houdini think that animation is happening on the OBJ level for geometry that is being brought in pre animated. In my situation I have a cup that was animated in Maya and exported as an alembic file. I read this in and then use an object merge to bring the animated geometry into a new clean geo container. Then I read it into the DOP network, but the animation is not being read as OBJ level transformations. Any hints on how to do this?

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It's much simpler than all of this,

Mainly you're using the wrong kind of collider, delete your static rbd object, in the SOP level of your collision object compute the velocity on your points with a trail SOP then in the populate containers shelf use the source collision tool to create a collider for you're simulation. This will generate a collision and collisionvel.xyz volumes required to correctly calculate your collisions. This way you can also calculate colliders for cached geometry which is the most frequent use case anyway. If you need to do things at high speeed then smiply make sure you cache your collisions at substeps and increase dopsubsteps.


The RBD solvers > FLIP solver merging should only really be used for active RBDs to fluid two way interaction.


Please don't do the slow down sim and running overlength method, thats very expensive, physically innacurate and a bit of a hack.

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  • 1 month later...

Hey all,


So I spent a long time trying to use the collision volume to act as a container but I wasn't able to have the fluid source node create an accurate enough volume representation to behave the way I needed it to. A friend of mine helped me get this working though and as usual in my experience, it turned out to be a simple drop down. In the fluid solver in the volume motion > collisions tab the velocity type needed to be point instead of rigid. This allowed me to use rbd's for the collision type which was faster than the velocity volume method, and still have the fluids hold their separation. Here is a sample file of how I have it working now.


Edited by mrhumbility
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