Does feedback scale in flip solver bring in the correct buoyancy?

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I was creating a fluid simulation with rbd object.

I set the density of flip object and rbd object the same and set the feedback scale in flip solver to 1.

The rbd object is put inside the flip object and as the both objects share the same density, the rbd object should not moves upwards. However it does.

So I am not sure what goes wrong.
Thx for any help~

Attachments:
buoyancy.hiplc (2.1 MB)

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I think I may understand the feedback scale in a wrong way.

That parameter decide the reverse force while the fluid hit collision.

To get the buoyancy force, I need to use buoyancy node.
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FLIP feedback is really sensitive to particle density, if you turn down your Feedback Scale value you can balance it out so your box remains in position. But if you increase your sim resolution you'll need to re-tweak the value.
Once it's balanced, if you reduce your RBD density you'll see buoyancy-like behavior. Seems to work best to also use the Buoyancy Force DOP to augment the FLIP feedback behavior.
–Dave
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Thx Dave.

I am using the Buoyancy Force now. It turns out to be extremely difficult to find a balanced parameter combination. And it's also pretty unstable.(If I put an RBD inside the flip object, it may float and fall back into flip and never float again)

Since I am trying to achieve the effect of bubbles in water, I am now using a large buoyancy force with Gas Limit Particle to limit the RBD on the surface.
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Ah nice! Haven't tested out Gas Limit Particle, will check it out. I wonder if “POP Float By Volumes” might be useful too?
–Dave
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Just tested the “POP Float By Volumes”.

That's a better version for bouyancy.

The problem is, as the RBD does not have energy loss, it will bounce up and down at the boundary instead of floating on the surface like a boat.. I tried to use “POP Drag” but its move is stiff.

If I use the “Gas Limit Particles”, the movement is pretty good. And RBD could also be constrained at the boundary. However “Gas Limit Particles” could only use the “bounding box” of a volume, instead of the volume shape.

So…haven't found a perfect way.
Edited by fengshuochange - Aug. 22, 2019 06:10:26
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Hmm… yeah, I'm not sure if there's an ideal approach to what you're describing. One thing to remember is that Pop Drag should only be used on Bullet Packed RBD's, which don't work well with FLIP…
–Dave
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Can you post an updated .hip or video example?
–Dave
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Hi Dave.
Sry for the delay.
Here is the .hip file.

The problem about Pop Float by Volume last time does not exist. Maybe last time I made some mistake on connecting nodes.

But here is a new one, the Pop Float by Volume works pretty good with Pop Object while interacting with Flip object while not so good
with RBD Packed Object.
I find a way to solve this, which is to import the Flip Object into SOP and create volume from that, then import it back to DOP. But the problem is that takes too much time.

And the limitation of Gas Limit Particles is also included.

Thx so much for the advice and help.

Attachments:
pop_float_by_volume and gas_limit_particles.hip (828.3 KB)

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