Mantaflow viscosity: Repeating emission from inflow causes initial emission to be twice as fast compared to proceeding emissions. #84280
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Reference: blender/blender#84280
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System Information
Operating system: Windows 10 Pro - 64 bit
Graphics card: AMD RX 5700XT
Blender Version
Broken: blender-2.92.0-c1740e9ad02d-windows64
Short description of error
When I tried to animate the inflow of turning it on and off, I noticed that the initial emission of liquid moves twice as fast, as if it is slamming to the ground, while the proceeding emissions moves slowly and being more stable.
While trying to find a fix, I managed to get a somewhat decent result with increasing the min and max time steps but nevertheless you still get weird behavior, and increasing the bake time for a simple simulation seems not ideal.
FLIP
viscosity - flip.mp4
APIC
viscosity - apic.mp4
Exact steps for others to reproduce the error
Open Blender, Add mesh -> Monkey -> Go to object -> Quick effects -> Quick liquid -> Click the domain and turn on 'Viscosity' and 'Diffusion' -> Click the monkey -> Change 'Flow behavior' to 'Inflow' -> animate the 'Use Flow' parameter to turn the emission of liquid on and off.
(Sample file)
sample.blend
Based on the default startup or an attached .blend file (as simple as possible).
Added subscriber: @Peeeynk
Also, I'm not an expert when it comes to this so if I am doing anything wrong please point it out.
Added subscribers: @sebbas, @lichtwerk
Changed status from 'Needs Triage' to: 'Confirmed'
Can confirm.
Dont see a reason why this should happen.
CC @sebbas
This is definitely a bug. I've been noticing this odd behavior too. Will look into it!
Just pushed an update (
0b711e6758) to fix the issue. Falling speeds from the sample file match much better now.But yes, in general I've been noticing better viscosity results with smaller timesteps too.
@Peeeynk Can you try it out the update?
@sebbas Thanks for the update, where can I find the update? Do I simply go to builder.blender.org/download ?
@Peeeynk Yep, that's the right place. The new build with the update will be available around midnight.
Changed status from 'Confirmed' to: 'Resolved'
Closing for now, but please reopen if there is still an issue.
Added subscriber: @CrossmindStudio
Hi
I am just testing the build from the 20th jan 2.92 beta version.
Enclosing a video viscosity velocity issue in initial frames.
Seems like the issue still persists.bandicam 2021-01-20 19-31-16-177.mp4 ,
I am seeing this with the example file from this report [which seems about right, no?]:
#84280.webm
Fix have reduced the effect but haven't eliminated it completely and it might look like there is no issue at low res such as 32
but to see the prominent effect, increase the resolution to at least 64, and substeps of the emitter to 2.
Please check the video below recreated with the same example file.
And in some cases its behaving quite weird as seen with default cube sample above.viscocity bug.mp4
Changed status from 'Resolved' to: 'Confirmed'
@sebbas: think you could have a look again?
Yes, you're absolutely right. This is still an issue. Will dig a bit more!
@CrossmindStudio So far, I have only seen the velocity issue when using a 'Geometry' flow object.
Let me know, if you see the issue with 'Inflow' objects too. Thx!
@sebbas Yes, issue is with inflow objects as well. Above examples are made with inflow object. Animation keys were set to 'use flow'
@CrossmindStudio Right, maybe more generally speaking: If you see it with a continuous inflow, that would be interesting to know.
A example showing continuous inflow using blender version blender-2.92.0-d5d8655ca15c
I used the same settings as @CrossmindStudio with 64 resolution for the domain and 2 substeps for the emitter. So far it is pretty stable, but once I keyframe the inflow property the bug happens.
test.mp4
Added subscriber: @vr_sebas
I found this problem using a more complex scene with colliders, and it appears that the viscosity is adding some thickness to the colliders, but as the simulation progresses the particles adjust and appear to even go through the collider.
I'm using 2.93 built on March 2nd, I can confirm this happens in 2.92
I've also found that activating adaptive timesteps increases the simulation unstabillity.
Mantabug2.mp4
Mantaflow (fluid simulator) has no active developer at the moment. Moving those bugs as known issues so at least other users can find their issues already reported.