[Sidefx-houdini-list] Minimum fields needed for a smoke or Pyro sim
jeff at sidefx.com
Mon Jan 9 13:08:53 EST 2012
You definitely need the vel vector fields as was mentioned before. If
you want to see smoke stuff, you also need the density field but not
mandatory. The density field is generally used as a mask inside the
solvers and as the visible result of smoke simulations.
If you don't use buoyancy, then you can toss the temperature field and
rely entirely on velocity momentum by manipulating the vel fields. Use a
Pump to inject velocity in to the sim if you wish.
As for confinement, if your solver doesn't use it or the parameters are
disabled or zero'ed out, toss it. Same goes for divergence.
I'd have a look at the Gas Project Non-Divergent DOP micro-solver as
this node is responsible for almost all (99.9%) of the fluid behaviour.
If you don't believe me, unlock the smoke or pyro solver from an
existing sim, find it and bypass it and see what you get. In the Pyro
Solver DOP, it's called project_1 and it's in the red colored box where
red = really really important.
Gas Project Non-Divergent step assumes that across the timestep that the
gas is non-compressible. The Gas Project Non-Divergence DOP will create
a corrective pressure field that is applied to the incoming vel fields
to remove any pressure by adjusting the neighboring voxels giving you
the nice swirly fluid behaviour. Everything in the Smoke and Pyro solver
mixes and matches various fields just to feed the Gas Project
Non-Divergent micro-solver with the few fields that it needs to
calculate the corrective pressure field. For the Gas Project Non
Divergent DOP, these are as follows with the default field names:
Velocity Field >>> vel
Surface Field >>> collision
Collision Field >>> collision
Goal Divergence Field >>> divergence
Pressure Field >>> pressure
If your simulation has no collision objects then get rid of collision
and collisionvel (which is used to modify vel earlier on in the
If you aren't using divergence to expand your simulation, get rid of it.
It is handy to know that the Pyro solver when set to enable combustion
will use heat and burn to map in to the divergence field to have the gas
expand (along with temperature). The various smoke presets don't use
divergence expansion that I know of so can be removed. Think of the
divergence field as a way of injecting pressure fields (expansion or
compression) in to your simulation and the Project Non-Divergent stage
will allow this to happen where the expansion field is non-zero but
careful as you can really mess up the sim (see the help for the Gas
Project Non Diverent DOP). Remember the Non-Divergent step exists to
remove any pressure fields with a counter-field that it calculates and
modifies the vel field accordingly. So the divergence field gives you a
bypass on the non-divergence step.
Pressure field is the result of the Non-Divergent step and is the
corrective field that is applied to the incoming vel fields. By default
it is kept around for you to inspect and use for whatever purposes. It
too can be deleted if not needed and is re-created each timestep so
don't bother modifying it thinking it will affect the simulation.
As an example exercise, if you use a default Billowy Smoke pyro shelf
preset on some geometry, with no other collision objects or other forces
applied, you can toss these fields on the Pyro Solver > Advanced > Clear
tab Additional parameter:
burn collision collisionvel confinement divergence fuel heat pressure
pump pumpindex rest sink source sourceindex
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