SPH ParticleSource and collisions with a plane

[oporras]

Hi, I´m a new user of SOFA's GUI (Windows 10, SOFA 21.12.00). I would like to simulate the collision of fluid particles and irregular rigid surfaces. I've used the ParticleSource component of the SofaSphFluid plugin and a simple plane to start testing the idea as you can see in the attached XLM script of the scene. However I've found some problems:

1. Before choosing Animate, an unexpected particle appears in the position x=(0,0,0).
2. The particle mentioned in point 1 and the first particle generated by ParticleSource behave correctly falling and colliding with the plane but the others don´t fall and pass through the plane without bouncing and maintaining the initial velocity.

What am I doing wrong? Can you help me?

Test-SPH.txt

[hugtalbot]

hey @oporras

thanks for your question and welcome on SOFA Github Discussions!
The SPH model of SOFA has been implemented a long time ago and fluid mechanics remains limited in SOFA so far. It's good to see such profiles like yours taking a look.

I myself never used the SPHFluidForceField, however:

1. I guess this is due to the fact that in SOFA if the degrees of freedom (position of the particles) are undefined, the position vector is initialized with one entry in (0,0,0). Then, this particle moves due to the PlaneFF and new particles are finally populating the MechanicalObject when the ParticleSource starts acting.
2. Not sure, but I would say that the CollisionModel is not needed here

Let us know whether it helps!

[hugtalbot]

Did it help @oporras ?

[oporras]

Hi @hugtalbot , thanks for your answer.

As you can see in the attachment, I changed the script a little by adding another PlaneFF in order to better observe the behavior of the particles of the SPH model. I also included gravity="0 0 -9.81" inside the child node where the particles are defined. Finally, I removed the PointCollisionModel line.

My observations are:

1. The unexpected particle located in the origin of the coordinate system moves under the effect of gravity, collides with PlaneFFs and bounces and finally disappears. It’s a kind of ghost particle.
2. Only the first particle generated by ParticleSource suffers the effect of gravity and bounces against the planes. The other particles continue with constant velocity passing through the plane without bouncing although the graphical representation (red lines) of Force Field shows that the interaction between particles and the PlaneFF is detected.

So I continue in the same situation. Any other ideas?

Test-SPH.txt

[hugtalbot]

hey @oporras
sorry for the silence, I was away for a conference
I looked into the issue, I have the same here and it seems the bug has been introduced between SOFAv21.06 and v21.12 since v21.06 does not have it. We investigate it and keep you updated

[hugtalbot]

Dear @oporras

After investigation, the problem you raised is due to a bug in the UniformMass when topological changes occur.
I fixed it in the pull-request #2853, please feel free to review/test it.

For now, you can either use the SOFA v21.06 which is not having this issue, use the master branch of SOFA when the PR will be merged. Anyway, the future SOFA version v22.06 upcoming next June will include the fix.

Best,

Hugo

[hugtalbot]

It has now been merged, then solved!