Is Temperature affecting Smoke behaviour?

[BeneZ]

Hello,
I am wondering, if temperature has influence on Smoke behaviour? (e.g. buoyancy)
I tested some values but could not see a significant difference, but maybe I only tested something wrong

thanks

[Svetlin.Nikolov]

Hey,

You can think of it this way:
- Smoke has the Smoke Buoyancy option under Dynamics;
- Temperature Buoyancy is practically the Gravity parameter;

Both buoyancies affect the velocity grid channel - and so if you have both Temperature and Smoke in a voxel, the voxel's velocity is going to move both the Temperature and Smoke. Unlike FLIP particles, where you can e.g. tell the red particles to go left and the blue particles to go right and split them even if they occupy the same space, in fire/smoke grid simulations there is just a single velocity field which does not discriminated between Temperature and Smoke, so it would move both if they overlap. If they don't overlap though, there will be no surprises

Cheers!

[BeneZ]

Hi Svetlin,
thanks. Not sure if I understood There are now a bunch of questions:

- If you say "Temperature Buoyancy is practically the Gravity parameter"
1.) This means that the gravity "effect" is influenced by temperature?
2.) It means also that with gravity "off" temperature has no effect on velocity at all?
3.) How is the influence? which Temperature is "neutral" with standard gravity? A Temperature at 0 means gravity has no effect or negative effect?
4.) just to be sure (temperature is in Kelvin and 0°K is -273°C and 273°K is 0°C, right?)

[Svetlin.Nikolov]

Indeed, you got everything right - for Phoenix fire/smoke simulations, gravity affects only the voxels where the temperature is higher or lower than 300 (in Kelvins, which is the room temperature). Voxels above 300 will create upwards velocities, and voxels below 300 will create downwards velocities. The farther from 300 the Temperature goes, the stronger velocity will be created. If the Temperature channel is not simulated at all, the gravity will have no effect. The Gravity option is a multiplier, so if it's 1.0 as by default, it will use real-world scale and accelerations, 0.0 disables it, and you can also use negative values to inverse it, e.g. make liquids flow upwards

[BeneZ]

Svetlin.Nikolov
Hi Svetlin, I had issues with rendertime of this smoke yesterday. Every render takes 2.5times longer than needed and this a vast factor on working and testing time.
I posted under the Vray section but maybe it has to do with Phoenix:
https://forums.chaosgroup.com/forum/...-waste-of-time
Could you have a look at this? There is also a short screen recording uploaded as a zip.
Many thanks!!

[Svetlin.Nikolov]

Hey,

We have collected all render time optimization tips here, including when using V-Ray's Progressive sampler, please see if some of these make any difference: https://docs.chaosgroup.com/display/...ering+In-Depth

They should help you get quicker renders overall, and Alex H. from Support will help with the time until the render finishes - it's not a known issue to me at least, so let's see what he can find first.

Cheers!

[BeneZ]

In case it is not known and no solution, then look at this from a "marketing" side:
This is really a chance to get easily more out of Vray, if you are able to cut the "wasted" time off.
Some people pay 2000 bugs to get a threadripper 3990X that increases render speed by 50%.
And here you need only an absolute time control to increase render speed by 250%.

[Svetlin.Nikolov]

Um... absolutely, but since you are already in contact with a colleague, there is no point in more than one person looking at one case. Have you tried the optimization tips yet?

[BeneZ]

You are right, I just asked you in case you know this maybe somewhere from your deep Phoenix experience.

About the general Tips:
I will look at them with more time, but I did not see anything at first glance.
GI is anyway disabled.
Lighting is minimal, I will do another simple light test without domelight next time.
(The core problem is the missing ability to stop rendering and skip to the next frame. The sequence takes 3 hours to render and could be done in a bit more than 1 hour).

Cheers!

[Svetlin.Nikolov]

In short, try these
- enable Use Probabilistic Shading: https://docs.chaosgroup.com/display/...spheresettings and adjust the Samples as instructed here: https://docs.chaosgroup.com/display/...ticVolumetrics
- try turning off Volume Light Cache
- also try turning off Optimize Big Volumetric Grids

[BeneZ]

thanks, I will try..