In FLIP we can set surface tension for fluid.

In MPM, I can't find where the surface tension is. The "viscous" behavior's parameters look like this:



It's a bit confusing... where is surface tension? And why is stiffness a thing for fluid? The document says stiffness is Young's Modulus, but fluid should has zero Young's Modulus, shouldn't it? Why the default value is more than 100000?

Wikipedia [en.wikipedia.org]:

a very soft material (such as a fluid) would deform without force, and would have zero Young's modulus.

https://www.sidefx.com/forum/topic/96931/?page=1#post-426235 [www.sidefx.com]

Hopefully there will be detailed info in the upcoming masterclass

You are right that fluids like water have no resistance to deformation and only oppose pressure changes. To get this behavior, pick the liquid constitutive model "Behavior" or pick the "Water" material preset on the MPM Source.

You are currently looking at the "Viscous" (viscoelastic) "Behavior" (constitutive model) which can oppose deformation when viscosity is used. This model is defined using two lamé parameters that can be derived from the Young's Modulus and Poisson's Ratio which is what we are doing here to simplify the UI/UX. If viscosity is set to 0, "Stiffness" (Young's Modulus) will behave a lot like the "Incompressibility" (Bulk Modulus) parameter of the liquid "Behavior" (constitutive model).

Long story short, you should be able to start from the "Honey" preset and only change the viscosity without caring too much about the other parameters.

AlexandreSV
You are right that fluids like water have no resistance to deformation and only oppose pressure changes. To get this behavior, pick the liquid constitutive model "Behavior" or pick the "Water" material preset on the MPM Source.

You are currently looking at the "Viscous" (viscoelastic) "Behavior" (constitutive model) which can oppose deformation when viscosity is used. This model is defined using two lamé parameters that can be derived from the Young's Modulus and Poisson's Ratio which is what we are doing here to simplify the UI/UX. If viscosity is set to 0, "Stiffness" (Young's Modulus) will behave a lot like the "Incompressibility" (Bulk Modulus) parameter of the liquid "Behavior" (constitutive model).

Long story short, you should be able to start from the "Honey" preset and only change the viscosity without caring too much about the other parameters.

Thank for your answer. I suppose it makes sense... but I still don't know how to control the surface tension. I know honey doesn't have a very high value of surface tension in real life. I just wonder if it's a controllable parameter in MPM.

kodra
Thank for your answer. I suppose it makes sense... but I still don't know how to control the surface tension. I know honey doesn't have a very high value of surface tension in real life. I just wonder if it's a controllable parameter in MPM.

Surface tension is currently not implemented in MPM, but it is being looked at.

For material surface tension, you could add peridynamic surface layer, but fluid FLIP/APIC and other variants are fluid specific "MPM" solvers since MPM is a generalization of FLIP into a continuum solver. Hopefully adding bonds (vellum constraints) to the MPM solver you can get necessary data to shape anisotropic surfacing on the VDBs (https://faculty.cc.gatech.edu/~turk/my_papers/sph_surfaces.pdf) (https://github.com/dariaq/anisotropic_kernels)



I also saw this PBD-MPM video that is pretty cool and could help too.