[AIVC, Air infiltration and ventilation glossary, 1992]:Also known as plug flow, and displacement flow, and is regarded as the most efficient form of ventilation. The ventilation air acts as a piston, which pushes the “old” air in the room in front of it without actually mixing. Therefore all of the air that reaches an arbitrary point from a small packet of fresh air at the inlet does so at the same time; this time is by definition, the local mean age of air at this point.(see local Mean Age of Air)