Recommended Scene Scale for Fluid Simulations

[jgreen3895]

Hey Guys,

Hopefully this is a fairly quick and easy question for someone more experienced with Phoenix FD and liquid simulations, but I was just wondering if there is a general rule of thumb for scene scale and setting up a simulation with fluids. I've noticed that a lot of tutorials mention that they've scaled the scene up by several times the actual size for the benefit of the simulation, but they don't really say why they do this. Is there some correlation between the scale of the scene, the "scene scale" under grid settings", "steps per frame", etc.?

I am just trying to understand if it's generally better to work via real world scale (even if it's a pretty small size simulation) or if it makes sense to typically scale up the scene objects prior to running a simulation and if so why, how much should the scale up be, etc.?

Also, is there a general rule of thumb for what the total cells should be for final quality simulations?

Thanks!

[georgi.zhekov]

Hey,

You can find more about the scene scale here - https://docs.chaosgroup.com/display/...idGrid-General

Generally start with real world scale and adjust it from here. If you need your simulation to behave slower and with more impact - increase the scene scale if you want the opposite - decrease it. There is no rule of thumb, just try different settings and see what works best, this applies to the grid resolution as well.

[jgreen3895]

Hey Georgi,

Thanks for the response. I've read the documentation and I, for the most part, understand the "Scene Scale" setting. I was also getting at the fact that I've seen many tutorials mention that they have literally scaled up the objects that will be used for a simulation. For example, dropping a Phone in some water to create a splash effect. People mention that the phone size is not real-world scale, but has been scaled up several times for the purpose of creating a more efficient simulation.

It seems like some people prefer scaling up some of there assets (especially if they are small objects) because they claim it improves the performance of a Phoenix FD simulation. Basically, implying that if the simulation is at real-world scale, and that real-world scale happens to be fairly small, it might be a good idea to scale up your objects by 3 - 4 times for the benefit of the simulation. Like they won't need as many Steps per Frame or something. Does anyone have any comments on this?

[georgi.zhekov]

If you adjust the scene scale value from inside of Phoenix's options there is no need to change the scale of your geometry. In some cases larger scene scale can get away with less steps per frame indeed.

[Svetlin.Nikolov]

Hey,

Simulating with larger scale makes the fluid in the simulation move slower, and this is pretty much the culprit. The gravity pulls the liquid down more slowly or moves the temperature of a fire simulation up more slowly. Having quickly moving fluid or obstacles requires more Steps per Frame for the simulation so it can keep up with the motion, and more steps per frame are slower to simulate. However, in order for a drop of liquid to travel the same distance while falling, you could either use 10 frames with 5 SPF at scene scale 1, or 50 frames with 1 SPF at scene scale 5 and both simulations will take roughly the same time, except that one of the two will save 10 cache files and the other will save 50 and you will have a slower animation. There is no performance gain in using higher scale, you will just get a longer, slow motion animation.

Cheers!