[Question] Buoyancy is not applied correctly

[NAMHAUK]

Question

I implemented a fluid using particles in Isaac Lab. I want to float a lightweight object on top of the fluid and place a character on top of it.

However, even though I made the lightweight object large enough for the buoyancy equation, it does not support the weight of the character and sinks. Below I have attached a video of my run in Isaac Sim.

board.webm

Execution Environment

rigid object
size: 3.0×1.0×0.2
density: 10

fluid density: 1000
character mass: 45kg

buoyancy (when half submerged): 1000×V(0.3)×9.8 = 2940
character + rigid object weight: 45kg×9.8+V(3×1×0.2)×10×9.8 = 499.8

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Thinking I may have weighed the character incorrectly, I created a cube instead of a character and applied the same weight as the character and it sank equally.

When I gave the density difference, I confirmed that the object floats on top of the water and that it is buoyant, but the magnitude of the buoyancy seems to be different from the theoretical value, so I ask why this is happening.

[RandomOakForest]

Thank you for posting this. Could you elaborate on what "density difference" helped to make the object float? Steps to repro can help us figure this out. I will move this post to our Discussions section for the team to follow up.

[NAMHAUK]

Density difference is shown below. Except for density, the rest are default values.
particle: 1000
rigid object: 10
character mass: 45kg

[RandomOakForest]

You probably just need a bigger and taller white cube, as to make more of it to be submerged, thus displacing more liquid and getting more force to float. Alternatively, having a thicker liquid should hold the current cube.

[NAMHAUK]

I've already tried increasing the size of the rigid cube or increasing the particle density to make it float.

My goal is to have a character float on a thin board (like a surfing board) like in real life, and I don't want to increase the size of the board or use a fluid with a different density than water.

As stated in the original post, theoretically the buoyancy is calculated to be sufficient, so I'm wondering why IsaacSim doesn't show enough buoyancy.

[NAMHAUK]

Is there a confirmed solution or is this an issue that will be addressed in a future release?