An Alternative View on the
Theory
of Cross-Ventilation
Mats Sandberg
University of Gävle, KTH Research School, 801 76 Gävle, Sweden
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Abstract
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 it’s 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.
Key words: Cross
ventilation, large openings, flow tube, catchment area , retardment area.
