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Mantaflow - liquid sim - strange behaviour
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System Information
Operating system: Windows 7 Pro 64-bit
Graphics card: Nvidia GeForce GTX 1060 6gig

Blender Version
Broken: 2.90.0 Alpha - 2020 - 06 - 27 - hash 59d2dd2237ce
Worked: - - -

Short description of error

Mantaflow liquid not behaving as expected.

Liquid streams out of in-flow object, splashes down to bottom of domain, but then rises up unexpectedly to form what appears to be a moving layer upon the surface of an invisible liquid.

Normally, I would expect the liquid to fill the domain volume from the bottom... up to the top surface of the liquid particles.

While I think this is an interesting effect in its own way, it is certainly not the behaviour that most users would expect to achieve.

Please see attached .blend file and video.

Event Timeline

Strange volume behaviour appears to have already been spotted and corrected in the following release of 2.90.0 alpha - 2020-06-28 - hash b21ba5e57974

Philipp Oeser (lichtwerk) claimed this task.

This looks like it can be closed then?
(feel free to comment again if issues persist though, can always reopen in that case)

I spoke too soon. The problem appears to still be here. Latest build: Blender 2.90.0 - June 30 - hash a3e6b7c2ce06

Some other observations:

Odd mesh behaviour: When rebaking the simulation with the mesh, Blender will sometimes simply place one large mesh around the domain for the first frame, then starting the sim calculation correctly with the following frame. (see screenshot below).

Clumsy interface: When changing any parameters of the inflow objects, Blender does not sense this and will not automatically rebake the simulation. The user has to force this, but in the 'replay' mode there is no "free data" button so the user has to force this by making some sort of alteration to the domain parameters (even if entering exactly the same information into one of the fields). I'm sure this is confusing to some users.

Philipp Oeser (lichtwerk) reopened this task as Needs Triage.Jul 1 2020, 9:52 AM

Another piece of information that may prove useful to the programmer(s):

On a hunch I decided to reduce the Fluid Flow field weight setting down to 0.200 and the attached video shows the result.

Germano Cavalcante (mano-wii) changed the task status from Needs Triage to Needs Information from User.Jul 2 2020, 3:12 PM

I cannot reproduce this with either the current development version of Blender.

Please try the latest daily build:

If the problem persists, please give us more clear instructions on how to reproduce it from scratch.

To me it also looks like this is fixed already. Not sure which commit actually did that though.

@Ignatz (ignatz) Yes, the issue with flow objects not resetting the simulation is a known issue. This should probably be noted somewhere (or even better, fixed :)

Now using version 2.90.0 alpha - 2020-07-01 - hash 52b125a790e3

Tried about ten different times with various inflow objects at different positions, angles and inflow volumes, but I can no longer recreate the problem.

Cannot figure out what has changed, but if the problem doesn't show up it is just fine by me. :-)

That's great to hear! Thanks for reporting, closing for now!