Two Pyro smoke questions

[catchyid]

Hi,

(1) I am doing a building collapse simulation, and I want to emit smoke "outwards" from the building while it collapses (i.e. debris/dust/smoke)... I understand that if I turn on "consumption model", I can increase "gas" released to create this effect (i.e. add more gas/smoke  to increase smoke pressure and make it expand outwards very rapidly). However, I don't need fire and hence turned off the consumption mode, and I am not sure how to do that?

(2) I understand that If I want the smoke to go through the walls windows, then I need to use collision volumes (i.e. create volume source and set its type to collision and add it pyro solver input), however this seems slow as I have many pieces in the building and creating a good volume out of it is very time consuming (and from my initial tests, Houdini did not produce a right volume, i.e. it got confused what's inside and what's outside, got lots of extra volumes, ...). So my question: Is there a better way to do that (i.e. to make it faster), or I have to do it this way?

Thanks,

[dimovfx]

1. try with pump preset on the source volume DOP.

2. here is scene file with collision setup using vdb volumes. it's faster and more accurate. intended to be used with source volume DOP with collision preset.

 

hope this helps.

cheers.

collisionVolume_alex.hip

[celd]

the combustion model's "gas released" parameter introduces divergence into the simulation, you can see it when you look at the field visualisation

source it into the simulation just like any other field, as Alexander mentioned

[catchyid]

Thanks guys for your reply...sorry to bug you with more questions:

-attached is a scene where I use Source Volume DOP once as "Pump" and once as "Expand". It seems that "Expand" is doing what I need, however Alexander mentioned "Pump", so am I doing something wrong? i.e. why "Pump" does not work?

-A Source Volume configured as "Expand" adds "Divergence" to the simulation (as cled mentioned), so I thought it's like a pressure because when it's added it makes the smoke expands. However, I found another field called "Pressure" (Smoke Object DOP --> Fields defines both "Divergence" and "Pressure"). My question: what's the difference between "Divergence" and "Pressure"?.

-"Pump" adds: "pump" attribute which is scalar, and "vel" which is a vector. I could not find in the documentation what "pump" attribute is? http://www.sidefx.com/docs/houdini/shelf/pumpfromobjects. It seems it calculates "vel" just a difference between neighboring points, but I am not sure how?!

Thanks

pump_vs_expand.hipnc

[celd]

I will try to answer this with various levels of knowledge I have about your questions:

 

Pump preset introduces velocity into the simulation, you should always look into "SOP to DOP binding" tab, that's where you can see what is actually going on. As you can see in your hip file, it is trying to use a field called "pumpvel" and add it into simulations "vel" field. There is a second field, scalar field, called "pump", which is being added into "pump" field, I believe this scalar field does not actually do anything, however in Masks tab you can see its being used to mask out the vel field, so that the vel field gets multiplied by the pump field, effectively masking it. You can see how these fields are created on the SOP level.

The pressure field is the one that you cannot visualize in the smoke object. It is an internal field that pushes against the velocity to make the simulation divergence free. Pressure solve is one of the steps of calculating the solver equation.

Now the expand preset does indeed do what you originally wanted to do, that is, add divergence. I admit I am not entirely sure how it fits into the whole picture, but in the end it is not too important to know more than that it introduces divergence into the sim to make it expand.

 

Hope this helps and if anyone wants to fill in the blanks - please do.

 

 

[catchyid]

Hi Filip,

First sorry for this late reply, I got distracted couple days. Second, thanks a lot for your input, it's really appreciated. I still need to test more scenarios and re-process your input to figure out how things relate to each other. In the mean time, I think the best policy is to use the "default" presets and compose things together to create more complex behaviors. In the long run, I think it's best to study some vector calculus (like divergence, curl, ...) + some basic fluid dynamics to have a better sense of how to control Pyro simulation (but I think for basic stuff, one should not do that).

Once more, thanks for your help and feedback