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Wind Driven Cross-Flow
Analysed as a Catchment Problem and as a Pressure Driven Flow
Jan Jensen
True1, Mats Sandberg2, Per Heiselberg3
and Peter V. Nielsen4
1, 3
and 4Hybrid Ventilation Centre, Aalborg University
Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
2Centre
for Built Environment, University of Gävle
Södra Sjötullsgatan 3, 801 76 Gävle, Sweden
Abstract
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.
Key words: natural ventilation, cross ventilation, openings,
pressure distribution, catchment area.
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