The fluid mechanics of CV

Cross Ventilation (CV) is common in many naturally ventilated buildings, with air flowing through windows, open doorways and large internal apertures across rooms and corridors in the building. In these cases significant conservation of inflow momentum occurs with the inflow traveling freely across the room towards the outlet. Because of the high momentum conservation, CV strategies are often used when there is need for high ventilation airflow rates. CV flows may be driven by wind or a mechanical ventilation system. Typically CV flows occur in wind-driven ventilation systems with inflow through operable windows.

As ventilation air flows across the room, heat transfer between the airflow, the room surfaces and the internal heat sources occurs and the airflow temperature changes between inlet and outlet, reflecting energy conservation. Heat transfer between air and room surfaces is important as its magnitude has considerable influence in the indoor temperature as well as in determining the success of any night cooling system.

Recirculating flow

Schematic representations of the two basic airflow patterns that can occur in CV are shown in the figures on the right. Any cross-ventilated room will have an airflow pattern that is either similar to one of the two geometries shown (with or without recirculation regions), or a combination of the two with both recirculation and inlet flow attaching to a lateral surface or the ceiling.

The simplest flow configuration, with no recirculation regions, commonly occurs in corridors and long spaces whose inlet aperture area is similar to the room cross-sectional area. In this case, the flow occupies the full cross section of the room and the transport of pollutants and momentum is unidirectional, similar to turbulent flow in a channel. The flow velocity profile across the channel is approximately flat as a result of the high degree of mixing that is characteristic of turbulent flows. The average airflow velocity in the cross section can be obtained approximately by dividing the flow rate by the cross sectional area of the space.

A more complex airflow pattern occurs when the inlet aperture area is an order of magnitude smaller than the cross sectional area of the room AR=W.H, where W is the width and H is the height of the room. For the case shown in the computation fluid dynamics (CFD) simulation on the right, AR=4.H2. In this case, the main CV region in the core of the room entrains air from the adjacent regions and forms recirculation regions that ensure mass conservation, with air moving in the opposite direction to the core jet flow. In these room geometries, when the inlet is located close to the center of the inlet surface, most of the contact between ventilation flow and the internal surfaces and, therefore, most of the heat transfer occurs in the recirculation regions that occupy the majority of the room volume.