Simulation scale

[lazza79]

Hi guys,

I have another question for you, this time about scale.

In some of the tutorials for FLIP fluids I've followed, they say that it is important to keep the scale of the simulation as close as possible to the reality.
In my case, I have a boat that is 100 meters long, should I try to scale the sim 1:1 or can I scale it down of half or 1/10th?

How should I change the solver's spatial scale and mass scale?

My main doubt is: should I use a 150m domain with bigger particle sep or a 15m domain with much smaller particle sep?

Thank you!
Cheers!
Andrea

[Sean-R]

If i remember correctly 1 houdini unit = 1m. I'd create a default box and compare it to your boat then adjust the spatial scale accordingly. In regards to the 150m/15m domain question it's really up to you, bare in mind a smaller domain will run much faster.

Edited May 12, 2016 by Sean-R

[lazza79]

2 minutes ago, Sean-R said:

If i remember correctly 1 houdini unit = 1m. I'd create a default box and compare it to your boat then adjust the spatial scale accordingly. In regards to the 150m/15m domain question it's really up to you, bare in mind a smaller domain will run much faster.

Hmmm, not really because obviously in a smaller domain I will boost up the particle separation, hence the running speed should be the same...

Yes, 1 houdini unit is 1meter. I already have the animation for the boat in the matching scale, I just need to decide how to run the sim.
Right now I am using 0.03 separation in a 1/10th of scale.

I scaled down the gravity as well but I didn't touch the spatial scale and mass scale.

Thanks,
Andrea

[fathom]

you can control the scene scale in houdini.  default is 1m, but you can change to other scales (preferences->hip file options).  if you do anything where you want to change what 1 unit represents, you should do it there BEFORE you place any dops.  when dops are placed, they check the scene scale and make adjustments to the default values based on the scale (like a gravity dop will be 9.8 meters or 980 cm).

now here's the major caveat: it doesn't always work correctly.  simple things will adjust, but more complex setups won't always (like shelf rigs).  your best bet really is to stay at meter scale and run at 1:1 or you're gonna have to chase down all sorts of random settings.

 

[lazza79]

Thank you fathom.

So I really need to upgrade my pc to 64gb of ram (right now I have 32)

Cheers!
Andrea

[fathom]

sim scale has more to do with the value of the forces involved than it does with the amount of data.  you can have a microscopic sim that has a much data as a battleship plowing thru the ocean.  simming at 1m just means that your units are in meters.  gravity is 9.8m, for example.  or a particle separation of .2 means each particle is 20cm in radius and your flip voxels will be around the same size.  sticking with real units then makes it easy to understand your sim.  is 20cm separation enough to capture the detail you need?  how many particles does that end up making?  how big of an area do you need to sim?  how deep?  that all drives your resolution which is different and really independent of the scene scale.