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I'm on Blender 2.91 and I'm trying to make a simulation with Suzanne being pushed around by liquid. No idea how I'd approach that though. Obviously I'm making it a rigid body and a fluid obstacle, but what should I do from there? There's no FLIP two way coupling option, is there?

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  • $\begingroup$ Does this answer your question? Why does my Mantaflow waterwheel not work? $\endgroup$ Feb 12, 2021 at 13:33
  • $\begingroup$ It certainly would, except I'm not sure it applies as I'm using FLIP, not Mantaflow. $\endgroup$
    – Robot
    Feb 12, 2021 at 13:40
  • $\begingroup$ As far as I know, both mantaflow and FLIP fluids are implementations of the same simulation technique, so the answer should be valid for both. As a side note, a non-blender (but still FOSS) way to perform such simulations would probably be projectchrono.org $\endgroup$ Feb 12, 2021 at 14:18
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    $\begingroup$ The limitation of two-way coupling is less about the FLIP simulation technique itself, and more about how simulation systems are designed in Blender. Simulations in Blender are only able to be run separately and because of this, they are not able to coordinate and communicate forces with each other. Two-way coupling is possible with the FLIP simulation technique, but there is not a way to make this work between the separate Blender simulation systems at the moment due to limitations of the software. $\endgroup$
    – RLGUY
    Feb 12, 2021 at 17:01

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Well flip and manta dont support this.
However maybe you can fake it.
If you use a paint canvas for displacment you can mimic it a bit.
If the water moves wild your out of luck though.
There are some raindrops examples that might work.

Else you need some proffesional solver and an openvdb import (if it supports that) to render it in Blender.

btw there's a popular youtube channel 2 minute papers has lots of research about this topic, maybe through that you can find software that does it.

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  • $\begingroup$ I already watch tmp, their content is amazing. Does it really not support it? That's fine. Thanks for the help. $\endgroup$
    – Robot
    Feb 12, 2021 at 15:28
  • $\begingroup$ No its not supported i always wonder where those research papers end-up and why researched dont use Blender for it, after all education depends on community tax, why dont they do something back towards the opensource community (rarely people do this). $\endgroup$
    – Peter
    Feb 16, 2021 at 13:02
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Two-way coupling is not supported in the FLIP Fluids addon. This limitation is a due to how simulation systems in Blender work. Blender's simulation systems (fluid, rigidbody, softbody, cloth) are all run separately from each other and because of this, the separate systems are not able to coordinate or communicate with each other on how to transfer their forces to other systems.

Only one-way coupling is supported with Blender's simulations. Example: by running a rigid body simulation first and a fluid simulation second, rigid bodies will be able to push the fluid around, but the fluid will not be able to push the rigid bodies in return due to the rigid body simulation already being baked.

A common trick to work around this limitation is to 'fake' the effect by animating the rigid body objects by hand.

The FLIP Fluids addon presents a workflow for mixing rigid body simulation and manual animation to fake buoyancy effects in this tutorial: https://youtu.be/JyfMQ6RX6nA

Lawrence Jaeger shows another way to fake these effects using rigid body simulation and force fields in this tutorial: https://youtu.be/3EfgsciQ5rM

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