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