Gas field only in specified area. Stolen from the Sidefx site

1.  In the DOP network, add a SOP Scalar Field node after the Pyro object.
   
   • Turn on Use SOP Dimensions.
   • Set the SOP path to the path of the fog volume object (for example /obj/mask_object). Make sure Use Object Transform is on.
   • Set the Data Name to something indicating the purpose of this field, for example NoiseMask.
   This attaches the fog volume as field data on the Pyro object with the given name.
   
2. Create a Gas Turbulence node and connect it to the fourth ("Advection") input of the Pyro solver.
   
   • On the Bindings tab, set the Density Field to the name of the mask data, for example NoiseMask. The effect of the node is multiplied by the value of this field.
   • Use the controls on the Turbulence Settings tab to control the amount of turbulence added.
     
     
     
     
     Important thing to note - by default the control field will not be 100% in control.. You'll see some "leakage" of turbulence. This is because the Control Influence is set to 0.5 by default.. Whack it up to 1, and maybe tweak the remap-graph to fine tune the effect of the turb/whatever.