This paper describes laboratory experiments on the flow and stratification produced in two coupled rooms connected to the exterior by a single vent. The two rooms are connected by two openings one at high-level and one at low-level in the dividing wall. One ‘forced’ room has a buoyancy source while the outlet vent is in the ‘unforced’ room and is placed in such a way that the buoyancy in the room can potentially drive an exchange flow through it. The buoyancy source is also a source of volume, such as cool air pumped into a room from an overhead duct in which case the outlet is on the floor of the unforced room. We consider the effect of the size of the outlet vent on the resulting stratification in the two rooms. For a small vent only unidirectional flow occurs and, since no ambient fluid enters the rooms, the buoyancy in both rooms becomes uniform and asymptotes to the buoyancy of the source. Above a critical vent size a bidirectional flow is driven through the vent by the buoyancy forces. In this case ambient fluid enters the rooms and a steady state is reached when the buoyancy flux through the outlet vent equals that of the buoyancy source. Both rooms remain stratified with two-layer stratification in this case, with a mean density that decreases as the size of the outlet increases. The implications for ventilation are discussed.

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