The
Tutorials - Disclaimer and Important Notes (Updated June 2008)
These tutorials are presented as is, without warranty that they are
applicable to a particular situation. They relate to classical theory as
applied to airflow through openings.
The following should be noted:
·
Wind Induced Flow
through Openings: If an opening is ‘large,’ kinetic energy is not
fully converted to induce a surface pressure. Instead the wind simply
drives air under its own momentum through the opening. Some research
suggests that this breakdown occurs immediately an opening is present but
a porosity figure of up to 30% of the wall surface area has been suggested
as satisfactory for classical theory to be acceptable. In practice
momentum driven flow will be greater than classically driven flow.
·
Discharge
Coefficient: The discharge coefficient will depend on many factors
including the geometry of the opening and the direction of attack of the
wind. The value suggested in this tutorial is a guideline figure within
the classical range for an orifice opening in which wind is normal to the
opening. The literature reveals an increasing amount of discharge
coefficients for a wider range of conditions. Such information has been
published in the International Journal of Ventilation.
·
Single Sided Wind
Driven Ventilation: The wind induced pressure distribution over a
single side varies continuously, both spatially and over time. Thus, at
anytime, two openings on the same wall surface will be subject to
different and fluctuating pressures. Airflow will therefore be induced
between one opening and the other. If this pressure distribution is known
then, in theory, the guidelines presented in these tutorials can be
applied to the case of wind driven single sided ventilation. Often,
however, pressure data are averaged over time and space such that the
pressure distribution on a single side is presented as a uniform value. In
this case the method presented does not ‘see’ the contribution made by
non uniformities in wind induced pressure on a single surface. In a cross
flow network, this does not usually matter because the contribution to
ventilation made by single sided wind driven ventilation is usually
relatively small. The theory presented in these tutorials therefore
assumes that, for wind driven ventilation, openings are applied to more
than one surface. Stack (temperature) driven ventilation is not influenced
by this restriction and a single sided approach can be applied.
In all cases these tutorials will be amended as and when suitable
practical guidance becomes available.
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