So I am working on a project that has a somewhat large flat river with some whitewater and churning where it meets clusters of rocks but otherwise needs to be pretty flat with some nice small scale surface detail (the new jungle book movie has a similar look in this way). After several tests I keep getting something that is too “billowy” or roaring rapids and that lacks fine surface detail. I am almost wondering if FLIP is the best way to approach this. Any ideas?

Show us what it looks like? I'm pretty sure you will get what you want through FLIP. FLIP usually doesn't work for macro liquids or sheeting generally.

So here is the look I am going for. Notice lots of fine surface detail mixed with churning and whitewater that happens when you are near rocks.

https://upload.wikimedia.org/wikipedia/commons/4/44/Thredbo-River_Crackenback_rocks.jpg [upload.wikimedia.org]

Attached is what I am currently getting. Notice it is boiling far too much and lacks fine detail in larger areas.

You could try setting up dual rest fields for the mesh to add in detail:

http://fx-td.com/?p=329 [fx-td.com] This example isn't the best, but check out the end of the video where it shows the dual rest field showing a noise pattern that moves with the fluid.

Something else you can consider is adding in small rocks and such at the bottom of the river to break up the flow. Even if it's moving slowly in that area because it's mostly flat, anything extra can likely help add in some more natural drag and unevenness in the sim.

So after some tweaking and adding more collision geometry to the river bed it is starting to look a lot better. Thanks for your advice.

I have been considering using an advected texture or rest position to add some smaller noise to the surface of the water but haven't done anything like this before. Any tips on how to do the dual restfield or dual uvs method? Thanks!

Glad to hear it's working better.

There's a file set up with dual rest fields on that link I gave you.

Oh, thanks I seemed to have missed the link on my first read through. Great example file by the way.

Thanks. I've actually been meaning to make it look much nicer by applying to another river scene I have lying around. I hope it helps.

FLIP has a tendency to boil as there is a bit of error when mapping the point attributes to grids and back again. Adds dramatic hyper-motion to FLIP simulations that many users/directors like. H15 will have a different method that is more accurate, targeted specifically for slower moving fluids with more accurate motion and results with a bit of overhead (time not memory).


As for this in H14, have you tried increasing the Velocity Smoothing parameter in the Volume Motion tab on the FLIP Solver? It defaults to 0.2 but increasing slightly may help adding a bit more control to the simulation. This is actually a blend between the FLIP method and the PIC method. PIC allows more of the velocity grid motion to be carried in to the final velocity which tends to be smoother.

Thanks for all the tips. After digging around in your file, Solitude, I was able to set up a system that generates the rest fields in the same simulation as the flip and seems to work pretty well for what I need. Thanks for demystifying the process with that file!

One last question I have is tips for how to shade the water and foam so that it feels like a largish river. Right now I am finding that my water either ends up feeling “thick” or too clear. Any shader tricks that help sell scale?

The default shelf setups puts down a uniform volume shader on a copy of the water mesh. If you increase the density that will help with seeing through too much and a bit of murkiness to it. If you add whitewater to it, just having the foam and bubbles will help scale too.

Setting the particle separation to the maximum your hardware allows and dialing down the Velocity Smoothing to 0.001 got me the fine details in this project.

https://vimeo.com/130328238 [vimeo.com]

Simple, but very effective.

Cheers,

Bernd

Very nice simulations!

Totally agree with large physical simulations and just cranking it to the max.

With FLIP you go low res to set up and prove things work, then dial down the particle separation to the point where your machine just barely stays out of swap and go with sim in background caching from SOPs to disk. Experience helps here with your machine specs.

Good for playing with the smoothing working for your sim. Many don't know about how effective this can be.
In H15 I find the swirly kernel to also be pretty good at calming down the chatter but is a bit more expensive.