Natural ventilation through a single opening - the effects of headwind.

The airflow between a warm room and cool exterior can be significantly affected by an external headwind. Pollutant concentrations within the space depend on the relative sizes of the wind and the undisturbed stack driven flow. Two scenarios are described. Firstly, a space is filled initially with buoyant polluted air. The space is then naturally ventilated through a single opening. In the "no wind" case, a gravity current of external air flows into the space. All the polluted air is expelled from the room. At high wind speeds the turbulence associated with the headwind produces mixing just inside the doorway. Under some conditions, ventilation levels are reduced. The second scenario considered is the natural ventilation of a space containing a continuous source of buoyant pollutant. For weak headwinds, fresh external air flows into the room and the pollutant concentration in that lower layer remains close to zero. High headwind speeds again generate doorway mixing. Air flowing into the space becomes contaminated with pollutant These flows were studied experimentally using small-scale saline modelling techniques. Simple mathematical models are presented which agree closely with the experimental results. In both the transient and continuous cases, an increase in the headwind could lead to reduction in ventilation and an increase in internal pollutant levels. Natural ventilation through a single opening is not necessarily enhanced by wind.