The curtain or barrier is formed by {HEAD ~ a region} of {TAIL ~ flowing conditioned air} directed across the opening 
of
The curtain or barrier is formed by { HEAD ~ a region } of { TAIL ~ flowing conditioned air } directed across the opening
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH