View Full Version : Fluid dissipates with high-speed velocities

01 January 2010, 01:05 PM
Hey guys.

I have noticed that when I simulate in maya Fluids with high-speed velocities, it dissipates fast without having any dissipation on at all, does anybody know how to deal with this?

Higher resolution? More density? Higher subsampling? I have tried all these, but what works best for me is different approaches from different times/simulations.

Just wanted to hear your thoughts on it, and if anyone has a good workaround to suggest.

Thanks guys

01 January 2010, 08:14 PM
Have a play with the opacity ramp can usually solve that problem wiothout adding overheads to your simulation.

02 February 2010, 08:12 AM
Hi Jeremy. Thanks for the fast reply.

Yeah, I've tried with the opacity scale, but I usually need it to work at the "end" of the opacity, but not at the beginning, so working with the opacity hasn't worked for me yet.

Here's an example what I've encountered: Say I want to emit a high-speed fluid into a, for example, pipe leading into an engine, and then I want the fluid to stop there and roll around for a bit, and then when I open a vent, it goes out a different pipe.
What happens for me, is that the fluid dissipates when I use high velocities, it gets better when I do more substeps, but still it doesn't cut it. Of course I can "cheat" and put in some extra desity with an extra emitter, but I was more wondering in more of a theoretical way of how maya works.


02 February 2010, 08:43 AM
Yeah Ive found that increasing the substeps while caching definately helps, Ive gone as low as .1 for a small rocket thats moving too fast per-frame/voxel and the density seems to hang around for a long time.

Is your problem a density thing or is it a velocity thing?

Are your velocity vectors are becoming unstable and breaking up the density?? Maybe not.
Also its been noted that having High Detail Solve on velocity with velocity swirl creates unstable velocities.

Does your density break up into little voxels and disappear or does it actually dissipate smoothly?

02 February 2010, 09:27 AM
Yeah, it's more dissipating smoothly, so I guess it comes from more from density than velocity, although I find it only beeing affected when I work in high velocities.

And, no High Detail Solve, because of making it more unstable.

So generally speaking, It dissolves smoothly, like I had any dissolve on, but it's not.

The hunt continues :-)

02 February 2010, 10:01 PM
Fast divergent velocities on the fluid with the standard solver will tend to diffuse, while with the high detail solve can cause a blowup. By making the timestep smaller you can deal with such issues. Currently the simplest way of doing this is cache the fluid with more sub frame sampling. Note that the higher the resolution of the fluid the more sub samples are required per frame (this is one of the big problems with fluid dynamics in general).

For avoiding blowup with high detail solve also try to avoid having extreme velocities in any voxels (just one high speed voxel can cause the blowup).

02 February 2010, 10:15 AM
Aaaah, yes that's exactly what I've noticed, so I guess turning on high detail solve with high subsampling is the way to go? I haven't even start to think of the way of using high detail solve, because of the unstability, but I guess that if you would go with higher substeps, then maybe it could work? I will have to do some test :-).

Thanks Duncan

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