How to set the smoke dop dissipation?

I'm not sure if it's precisely what you're after, but try the Cooling Rate under Forces on the Smoke Solver. Yank it up to 30 and your visible smoke should dissipate quickly.

Cheers,

J.C.

thanks, but the smoke can reach the top of fluid container, i hope that the smoke can increasingly disappear in the process of rising.

I poked around a bit, and nothing jumped out(btw, right click on the smoke solver, Allow Editing of Contents, and double click to dive in and there's lots of interesting and educational things in there, fairly logically laid out and labelled, once you grok the concept of DOPs being data attached to geometry).

When I think about it, dissipation in and of itself might not be something you simply raise, it's something that happens as a by-product of the density of the initial source(which you can control by adding a ‘density’ float attribute to the incoming data generating the smoke), the size of the smoke container, and other forces you may attach. Rate of dissipation seems closest tied to viscosity(under Forces), which is a rate of diffusion, but it's not as simple as ‘get higher, diffuse more’ by default. Certainly raising Viscosity didn't get me that, it's purpose is to average out neighbouring regions, which does sound related, though.

Someone may have a suggestion… I'm curious about this, however, so I'll take another look when I get a chance…

Cheers,

J.C.

Well, one simple way to do dissipation is just to scale density down by a small amount every frame.

So, in a typical “AutoDopNetwork” setup, just add a GasCalculate node and attach it to the last input of the SmokeSolver (the “post-solve” input). Set the source and destination fields both to density, then add a scale factor in the “Source Pre-Mult” parameter. You'll want this pretty close to 1, like .95 or something, but can play with it to taste.

If you look at the “cool_gas” node within the SmokeSolver as suggested above, you'll see it's doing the same thing, in that case just to temperature.

Hope that helps.

Thank you johner, this my hope

you can also do this in sops. Build a volume that ramps from 1 to 0 then use the volume mix node to multiply the two.

This approach gives you the ability to affect volumes without running the sim on changes.

jeff, that's a flexible way, thank you and everybody

mmlxx
jeff, that's a flexible way, thank you and everybody

Jeff's suggestion is indeed really nice from a flexibility standpoint. The only subtlety I would point out is that, by default, any forces you apply in your smoke sim will be scaled by the density at a given point. That's what it means that the “Density Force Mask” is set to “*” in the SmokeSolver node. So while you can absolutely scale the density post-sim if you're just trying to fix the render appearance of the smoke, if you really want your various forces to be affected by the dissipation you need to to do it in the sim somehow. You don't always need that, of course, but sometimes you might.