Wind Driven Cross-Flow Analysed as a Catchment Problem and as a Pressure Driven Flow

The wind approaching a building provided with openings has to make a choice; i.e. either to flow around the building or to flow through the openings. This selection procedure is dependent on both the external structure (shape) of the building and on its internal structure. In this paper the parameters controlling this selection procedure are explored. In order to clarify the basic process, simplified building models have been studied with a combination of wind tunnel studies, visualisation and CFD predictions. The results are interpreted in terms of concepts such as catchment and retardment area as introduced in an earlier paper by the second author. The effect of shape was considered by using a disk and a cylinder with the same diameter. The length of the cylinder was a little larger than its diameter. A solid disk is used as a reference followed by an analysis of various porosity values (opening area/facade area) in the range 0.25 to 25 %. In each case the openings were located opposite each other and the wind direction was applied perpendicular to the openings. It is concluded that the standard approach for the analysis of flow through building openings is to relate the flow to a pressure difference across the building envelope. A better approach might be to regard the flow through openings as a flow catchment problem. The flow rate is then expressed as a reference velocity times the catchment area. The results may then be conveniently expressed as catchment area divided by the opening area.