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The energy-conserving full-depth lock-exchange flow is symmetric between the cold lower current and warm upper return current, soPart (b) then gives $u_2=\sqrt{g^{\prime }H}$ and the laboratory front speedDuring one opening, the cold current displaces the volumeThe same volume of warm air exits in the upper layer.

Mechanical mixing destroys the sharp density interface that supports efficient displacement ventilation. It entrains warm air into the incoming cold current and cold air into the outgoing current, reducing both reduced gravity and the heat removed per exchanged volume. Stopping fans or other mixing while the door is open therefore preserves the two-layer lock-exchange flow and improves cooling efficiency.