Velocity and stream direction bias in viscosity solver

[PavelBlend]

System Information

Blender Version: 2.79b (f4dc9f9d68b)
FLIP Fluids Version: 9.0.5.5
Operating System: Windows 64 bit
CPU: AMD A8 5600K
GFX: -
RAM: 8 GB

Describe the bug

If the fluid has a low viscosity and the source of the fluid is small, then the fluid particles change their direction.

To Reproduce

Run a simulation in this scene:
low_viscosity_test.zip

Expected Behaviour

Liquid should fall vertically down.

Actual Behaviour

Jet fluid changes direction.

Screenshots

[rlguy]

Hi PavelBlend,

Thanks for the report! I have noticed this too, but I am not sure what causes this. It seems like there is a bias in the velocity direction when there is not very much grid coverage of the liquid.

It could be possible that this is a limitation of the viscosity method/solver, but I will need to investigate this further to find out. The viscosity solver is quite complicated for me to modify, so this issue could take some time before I find a solution or answer to what the problem could be.

- Ryan

[rlguy]

My apologies that this issue has taken so long to fix. I have tried a few times in the past to fix this, but with no success until today.

Recently I took another try at this issue by re-writing the viscosity solver from the start. During this re-write, I had noticed a simple mistake in the code. Part of the solver requires shifting the offset of a grid position by half of a voxel width. The mistake I made was that I had applied this offset twice in different areas of the code, leading to a total offset of 1 voxel instead of half of a voxel. This is what caused the stream to always have a bias in the +X/+Y/+Z direction.

This issue has been fixed and will be available in the next experimental version, v9.0.9.15 (03-MAR-2021) or in the next stable version v1.0.9b (Release in early April).