Mats Sandberg
Bibliographic info:
The International Journal of Ventilation, Vol. 2 N°4, March 2004, pp 409-418, 14 Fig., 6 Ref.

The mechanism of cross ventilation is dealt with in this paper. The results are obtained by a combination of wind tunnel studies and CFD predictions using a Reynolds stress model as the turbulence model. All buildings have been exposed to a uniform velocity field and therefore the reference flow rate for an opening is equal to the velocity multiplied by the opening area. The openings were located at or close to the position of the stagnation point on the corresponding sealed building. The view taken in the paper is that the flow is generated by an interaction between the atmospheric boundary layer flow approaching the building and the building itself. The process starts with the undisturbed boundary layer flow and ends when the boundary layer has been fully restored. The paper begins with scrutinising the basic assumptions behind the orifice equation, the most common model for relating a pressure differential to a flow. This pressure differential is usually taken from the pressure coefficients recorded on a sealed building. The applicability of pressure coefficients from sealed bodies, as a way to predict the flow rate, is dealt with. An important difference between a sealed building and one provided with an opening is that in the latter case the flow has a choice i.e to flow through the opening or pass around the building. This choice leads to the formation of a flow tube passing through the opening. It starts in the undisturbed boundary layer, with the catchment area and, close to the building, occurs the retardment area. The size of the retardment area and its state (i.e. relation between static and dynamic pressure) convey information about the type of flow. A new phenomenon i.e. a catchment effect or an attractor effect has been identified, which, at certain conditions, generates a flow through an opening which is larger than the reference flow rate.