I am planning to create a destruction sim of a cliff where the top breaks away and cascades down the side. The real world scale of the cliff is about 1,000 feet (300m). Should I use this same scale in Houdini, i.e. use a cliff model that is 300 units in Scale, given that the default Houdini scale is 1 unit = 1 meter? As I understand it, the scale is important since it affects the look of the sim, e.g. the time it takes for the pieces to hit the base of the cliff. Is it as simple as that?

I tried a sim at this scale and the time it took for an object to fall from 300 metres was roughly accurate. The only issue seems to be that since everything is scaled up, this can have an affect on processing -- presumably on dust creation using pyro sim?

Would just slowing down the sim by half after rendering at half scale be a common technique to save processing?

Or could Gravity Force just be reduced by half to simulate the same result with a half-scaled model?

adjusting gravity and shaping forces is common when simulating at scales where 1unit != 1m
I do it mostly for precision reasons when I need to sim something tiny or enormous

however your scene with 300m clif seems reasonable to sim at 1unit = 1meter so you can stick to that (unless its not convenient for you, for example if you have already tweaked look you like at different scale, etc)

it shouldn't affect your processing time assuming you are adjusting voxel size and particle separation according to your scale
in other words if you are simming smoke that ends up to be M*N*O voxels large, it should take roughly the same amount of time to sim regardless of the size of the voxels

Thanks tamte.

If I scaled down the cliff by a factor of 10, would I simply scale down the gravity and other forces used by 10 also?

How do you adjust voxel size and particle separation according to scale?

litote
If I scaled down the cliff by a factor of 10, would I simply scale down the gravity and other forces used by 10 also?

yes, you would scale them by the same factor

litote
How do you adjust voxel size and particle separation according to scale?

same way
but this is always arbitrary, since its common to adjust voxel size for pyro in such a way that you have enough resolution, so you can start with larger voxel size and decrease as you need for the amount of detail you are after

Thanks

How do you mean adjust particle separation to match scale? Can you give me an example of what sim you would do this for and how to adjust? Do you mean there is a way to create distance between particles? I presume particle separation relate to fluid sims?