Scale issue with fire sim

[markus_s_cg]

Hi, I am trying to set up a fire simulation/smoke simulation following your Ballistic missile tutorial. I only need the smoke sim, not the liquid, but it's a great starting point.
Submarine Launched Ballistic Missile - Phoenix FD 4 for 3ds Max - Chaos Help

My 3ds max scene is already rigged and animated, and my system units are set to 1 unit = 1 CM, but the tutorial for the smoke simulation is recommending to use 1 unit = 1 meter, and the example files use this scale. If I try to rescale to cm the sim does not behave similarly.
3dsmax does not play nice if I try to rescale the scene to new world units (the rigging breaks).

What settings do I need to change if I want to use the tutorial as a template but in a different scale?
Edit: If I change the scene scale from 1 to 0.01, is that all I have to do?

[markus_s_cg]

For some reason, it seems like I have to reduce the velocity in the fire source from 40 meters (or 4000 cm) to 40 cm. Then I get similar-looking results.

EDIT: I kept the temperature at 3500, but I had to increase cooling from 3 to 300

[cory_holm]

Hi,

You are correct that the Scene Scale would be the parameter you’d want to adjust, if you cannot change the System Units.

However, in this case, since the original scene was set to 1 unit = 1 meter, and you’re switching to 1 unit = 1 cm, then you would want to increase the Scene Scale to 100. This is because your simulator grid will be smaller when switched from 1 meter to 1 cm (or 0.01 meter), so you can increase the Scene Scale 100x to compensate for this.

I ran a quick test using the latest Phoenix nightly, and you can see below that the resulting sim behavior is comparable, even though the sim grid is 1/100 the size. I did not change any other settings, I just hid the ship geometry.

Here is the original scene in Meters:


Here is the new scene in Centimeters, with Scene Scale = 100:



Note that the sim behavior will be similar, but it will not be exactly the same.

I’ve also included screenshots of my Simulator Grid settings, showing the original in meters vs the cm version with Scene Scale = 100.

Sim Grid settings for the original scene in Meters:


Sim Grid settings for the new scene in Centimeters:


I hope that helps! Let me know if you have any further questions for now

[markus_s_cg]

Here is the new scene in Centimeters, with Scene Scale = 100:

Yes, figured out I got the scene scale backwards, 100 worked, not 0,01 hehe, silly me! But I also had to change velocity for the fire source
from 40 meters to 40 cm and cooling from 3 to 300. Seems like those values might be in system units or something, not display units.
Maybe I should upgrade to a new nightly build. I'm running 4.41.02

[cory_holm]

Hello again,

My apologies for the delay in getting back to you on this! I am wondering if I could get some more information about your new scene's setup.

I tested the tutorial scene file again today using the latest Phoenix nightly, and I was once again able to recreate the same simulation result by changing only the Scene Scale to 100.

Regarding the Outgoing Velocity for the Fire Source, you are correct that it should be 40 cm. In the original scene though, this parameter value changed automatically from 40 meters to 40 cm when I changed my System Unit Scale from 1 unit = 1 meter to 1 unit = 1 centimeter, and so I did not need to tweak the Source parameters manually.

I am not sure why in your scene you would need to increase the Cooling to achieve the same result. It might help to confirm that you're able to reproduce the same result with the original tutorial scene first, using the latest nightly, and I can provide the steps I followed below.

In the original tutorial scene, make sure the System Unit Scale is set to 1 unit = 1 meter, then run the Fire/Smoke simulation, and then you can render a frame and save it for comparison, e.g. frame 89. Then you can save a separate scene, and change the Fire/Smoke sim's output path for the caches to a new destination. Then, change the System Unit Scale to 1 unit = 1 centimeter, set the Fire/Smoke sim's Scene Scale = 100, then run the Fire/Smoke sim again. Render the same frame, e.g. frame 89, and then compare the two results. Please let me know if you're seeing something that is noticeably different!

Otherwise, if that is producing a comparable result, it would help if you can share more details about the specific steps you're taking to set up the simulation in your new scene, and what you observed increasing the Cooling did to help achieve the original result. Also, you can feel free to attach the scene or send it to our support team if there is something confidential. With some more information, I will do my best to provide clarification about what is going on