# Terms about Fluid Sim ,Some Questions ?

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Hello , everyone,

I have a question about fluid sim , importance fluid term ?

I getting start to learn fluid using houdini ?

What Difference between incompressible fluid and compressible ?

What is Divergance  ??

What is fluid implicit particles , and the advantage of flip ?

Final question ,What  the best scientific resourses (books or any articles ) for effects artist ?

Edited by mohamedkamal
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You'll want to do some google/bing/duckduckgo research.  Most of your questions can be found answered through there.

As for papers check out all the SIGGRAPH papers, there's plenty.

and of course a sidefx tut

Edited by kleer001
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Thanks , I check now siggraph paper it's very useful ,

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This is my understanding of it all... I'm no programmer or maths expert, so some of this may be a tad naive and/or wrong :-P

Incompressible fluid is how most-if-not-all CG industry fluid sim solutions will model fluid - it presumes that in fluids like water and air, compressibility is negligable, and so leaves it out because, as I understand it, simulating it is mathematically far more complicated (for no real benefit unless you're doing it for a scientific application)

DIvergence is what you (usually) don't want in a fluid simulation... the pressure solve is the process of taking a divergent velocity field and iterating over it until the divergence is removed - basically, so that fluid can't move *through* other fluid... it always has to go around - so you end up with swirling motion.  With divergence, the fluid can grow/compress in volume.  That can actually be used as an artistic tool - most commonly in explosions - you can inject intentional divergence into the sim, and it'll expand rapidly.

Fluid Implicit Particles is the hybrid technique of using particles to represent the fluid, (as in classic SPH fluid) but splatting their velocities onto a voxel grid, running a pressure solve on that voxel grid (which at high particle counts is massively more efficient than attempting to pressure-solve the particles themselves), then using the resulting divergence-free velocity field to advect the original particles to their new positions.  The particles don't have any "awareness" of each other as they do in an SPH fluid, but should maintain distance from each other automatically due to the divergence-free advection.

In general terms, it takes the performance benefits of grid-based fluids, and the detail benefits of particles, and combines them.

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• 2 weeks later...

thanks Daniel

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