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Mantaflow Initial velocity bugged?
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System Information
Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: AMD Mobility Radeon HD 5000 Series ATI Technologies Inc. 4.5.13399 Core Profile Context 15.200.1062.1004

Blender Version
Broken: version: 2.90 (sub 4), branch: master, commit date: 2020-05-28 18:34, hash: rB2deb6c517787
Worked: (newest version of Blender that worked as expected)

Short description of error
Enabling initial velocity seems to fix the problem.
Simple scene, some obstacles, an inflow item (sphere)
With non-alterated gravity force, 0 force field in the scene, the flow seems to go on the left.
Activating the flow initial velocity, it goes straight down (as expected)

Initial velocity non activated

Initial velocity activated

Exact steps for others to reproduce the error
Just try to activate and deactivate the initial velocity in the inflow object (sphere)

Event Timeline

I can reproduce the issue on Windows 10 Blender 2.90 today build.
Viewing from the top it's like if it emits particles in a vortex manner, and the effect is like increasing exponentially over time.
Maybe @Sebastián Barschkis (sebbas) could give a look?

Richard Antalik (ISS) changed the task status from Needs Triage to Confirmed.Jun 2 2020, 2:40 AM

The effect is more visible with a smaller inflow object. I seems to be caused by an uneven distribution of cells in the inflow object. If that is the correct solution, then this is probably more a known limitation that should be documented and not necessarily a bug.

I don't think so. In my opinion it should work with every inflow object with every shape. Mantaflow simply takes the volume, the cells inside the volume are considered inflows, the halfcells that are in the rim are not considered. That's perfect. Increasing the resolution you can almost fully the inflow shape. But the inflow path should be the same for 1 cell as well as 10000 cells. Am I wrong?