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