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Houdini Flip Stepping Issue on Inclined Surface

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Cola8
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Hello, I got into an issue with stepping of fluids on an inclined surface. I used an angled groundplane but I also tried using my own vdb collision and it was still the same result.

Are there any ideas to solve this issue?

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stepping_issue.png (365.9 KB)

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evanrudefx
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Cola8
Hello, I got into an issue with stepping of fluids on an inclined surface. I used an angled groundplane but I also tried using my own vdb collision and it was still the same result.
Image Not Found


Are there any ideas to solve this issue?

Hello,
My guess is it has to do with 3 things:
1. sloped surface
2. the flip sim is thin (very shallow)
3. the volume operations of the flip sim are not high res enough.

The fip sim isn't just particle based, there are volume operations involved. By default the grid scale (on the flip object) is set to 2. If you are doing a low res sim, you might need to set the grid scale closer to 1 for higher res volumes. I would probably try 1.5. If you check the visualization on the flip object for one of the volume operations (like pressure) you will likely see it's too low res. Grid scale of 2 usually works fine when simming at higher resolutions. Let me know if decreasing the grid scale fixes it.
Edited by evanrudefx - March 19, 2026 13:53:17

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gridscale.JPG (61.1 KB)

Thanks,

Evan
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Cola8
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Hello Evan,

Thank you for replying to my post.

I've given a try to play around with the grid scale, where I lowered it to value of 1 and still have the stair pattern on a flat sloped surface. I feel like it might even have become worse in quantity of step pattern, might be wrong tho.

I'll add more images to make it more clear. I also changed the experiment a bit so I can experiment a bit faster so it looks a bit a different now.




I will play around more, if I will find a fix that works for me I will post it here.

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stepping_issue_2.png (282.9 KB)
stepping_issue_3.png (245.0 KB)
stepping_issue_4.png (492.1 KB)

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Cola8
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So if I go quite low such as 0.5 grid scale it starts to look alright, just that it's super slow
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evanrudefx
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Cola8
So if I go quite low such as 0.5 grid scale it starts to look alright, just that it's super slow

Yea, 0.5 is too low. If you can post the hip file (make sure to freeze any geo being loaded from disk) I can take a look.If you can't post the hip, maybe a simplified version with the same issue. I tried re-creating the issue myself, but I can't.
Edited by evanrudefx - March 20, 2026 11:32:52
Thanks,

Evan
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Cola8
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Image Not Found


I recreated the same sim in a new file, here is the hip.

As I said gridscale helps but I need to go eventually really low to 0.5.

To see it properly you can wait till frame 45 or even further maybe frame 70 with the lower gridscale.

Check it out if you have time!

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Houdini_Forum_v001.hiplc (950.4 KB)

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evanrudefx
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In this case, it seems to be a bug (or limitation) with viscosity enabled. I have seen a few other post that suggest this too, like this:

https://forums.odforce.net/topic/30629-flip-viscosity-collision-object-grid-stepping-artifacts/ [forums.odforce.net]

Your particle separation was too low, but then you had reseed adding in lots of particles. I got better results by setting particle separation to 0.01 (which also improves the resolution of the volumes). After doing that I adjusted the particles per voxel in reseed to be less. Then in the flip solver under volume motion then collisions, setting the surface extrapolation to a low value. That gave me the best results, but it didn't get rid of the issue. I even tried re-creating the setup using the sop flip (just to test it) and it still was broken (see photo below). Honestly I would report to sidefx.

Attachments:
step_visc.JPG (223.4 KB)

Thanks,

Evan
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Cola8
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Thank you for looking into this! I will try to report it.
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