[Sidefx-houdini-list] Minimum fields needed for a smoke or Pyro sim

Pablo Giménez pablogipi at gmail.com
Mon Jan 9 13:17:44 EST 2012

Thanks a lot Jeff!!!
This is what I was looking for.

2012/1/9 Jeff Wagner <jeff at sidefx.com>

> 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
> That's it.
> If your simulation has no collision objects then get rid of collision and
> collisionvel (which is used to modify vel earlier on in the simulation
> set-up).
> 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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Un saludo
Best Regards
Pablo Giménez

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