Building a ventilator Edit
If you just want to blow gases in a particular direction, create a thin (zero cells thick) OBST and apply to it, via SURF_ID only, a SURF line that has the parameter POROUS=.TRUE. along with the other velocity parameters described above. This allows hot, smokey gases to pass through the obstruction, much like a free-standing fan. These obstructions are merely flat plates, by necessity. For example:
&SURF ID='blow', POROUS=.TRUE., VEL=5.0 / pushes towards +x direction &OBST XB=4.0,5.0,-0.2,-0.2,1.4,1.8 /
The velocity VEL associated with a POROUS surface represents the velocity in the positive or negative coordinate direction, as indicated by its sign.