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