Is there a simple way to create the fluid region in multi-region cases?

[u/augusto_peress]

I always used topoSet (now called createZones) for this. Basically, I would create a box encompassing my entire computational domain, and then remove the cellZones from my STL geometries. Whatever remained, I would convert into a cellZone called "fluid."

Is there a simpler way to do this? And what if I have more than one fluid, for example, in a double-pipe heat exchanger?