Past exam of the mathematics course of the University of Cambridge / 2025 / iii / Paper 345 / 3 / f

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.