Hi,
Does anyone know why certain FLIP fluid attributes like viscosity are so large. When others are more manageable like surface Extrapolation ? Surely it would be better to have values all in a more manageable range.

Rob

while I haven't investigated if the units relate to real world units, they certainly feel that way for me

in real world
density is in around 1000 kg/m3 which is default for FLIP
viscosity of honey is for example 2000-10000 cP
viscosity of peanut butter is around 250000 cP
so for me ranges in which FLIP works are quite logical for me

but you can keep attributes in 0-1 range if you will since they can act as multipliers to those high values set on FLIP object, so I see no problem controlling them in both ways

I would certainly not like if houdini would express values in purely arbitrary ranges. Some connection to commonly used units is always welcomed (although since I came to UK where metric system is not very spread, I realized that no matter which units you use in your software, some people will not find them logical)

Viscosity is specified in units of dynamic viscosity, or kg / (m·s), so it does tend to have high values. In particular it will be divided by density, so the same viscosity value will have more of an effect on less dense fluid.

If you're trying to do a variable-viscosity effect like melting, you might want to set a high viscosity value on the FLIP Object's itself via the Physical tab, then use 0-1 viscosity values on the FLIP particle attribute, with the Viscosity attribute Mix Method set to multiply.

If you're trying to do a variable-viscosity effect like melting, you might want to set a high viscosity value on the FLIP Object's itself via the Physical tab, then use 0-1 viscosity values on the FLIP particle attribute, with the Viscosity attribute Mix Method set to multiply.

Now why didn't I think of that. Thanks for the info gents !

in real world
density is in around 1000 kg/m3 which is default for FLIP
viscosity of honey is for example 2000-10000 cP
viscosity of peanut butter is around 250000 cP
so for me ranges in which FLIP works are quite logical for me

Where on earth did you dig that info up from melting peanut butter now that's a thought hehe



Rob

Melt with FLIP test. The johner concept works although it will be interesting to push the limits I start with 20000 for viscosity + gravity. As the surface falls FLIP does struggle to hold itself together.

Rob

Here's another variation, melting by transfering low viscosity by a metaball.

circusmonkey
As the surface falls FLIP does struggle to hold itself together.

The section on viscosity [sidefx.com] in the docs has some info that might help keeping the viscous fluid together. Some of the things that work well for inviscid fluids, like reseeding and automatic detection of ballistic particles, can be less useful or even counter-productive with highly viscous fluids. This issue can crop up as well with high viscosity values:

The FLIP Solver computes the number of substeps to calculate based on velocity. While the viscosity solver should always be stable, it will be more accurate with smaller substeps. A slow moving, highly viscous sim might need more substeps than the 1 that the FLIP Solver will give it based on its velocity. Increase Minimum Substeps until you get sufficient viscosity.

Unfortunately the viscosity solve is fairly expensive, so increasing minsubsteps can be painful but necessary sometimes.

eetu
Here's another variation, melting by transfering low viscosity by a metaball.

Cool!

Hi johner,
yes I had noted and implemented some of the tips ! and Ian has pointed me to your thread on elasticity. Very interesting otl !

Is there any way to get more detail from objects like the default teapot. The point distribution when viewed as a guide always has a uneven distribution of points. Would it even be possible to do more in sops and feed data in to create better surface representation.

rob