Dear David

The pressure coefficient at the end of a chimney is a combination the
Cp value on the considered position above the roof and on the Cp value
of the cowl which is used at the terminal of the chimney. For the second
the local vertical wind incident angle at the cowl position and the
ratio of the local wind speed to the air flow speed in the chimney has
an influence. It has also to be noted that the local wind speed at the
cowl position which has to be used as reference speed for the Cp value
of the cowl is different to the reference wind speed of the building.
These effects have been investigated in a case study and are presented
in the following paper:

A. Pfeiffer, V. Dorer, A. Weber, Modelling of cowl performance in
building simulation tools using experimental data and computational
fluid dynamics
Building and Environment 43/8 (2008) 1361-1372

In this paper CFD was used to determine Cp values above the roof of a
simple building geometry. The Cp above the roof is shown for one cowl
position. It varied between 0.05 an -0.45 depending on the wind
direction. The ratio of the local to the building reference speed varied
between 0.22 and 1.1 and the vertical wind incident angle between 75 and
120 degree. The latter influenced a variation of the Cp value of the
selected cowl of 0.4 starting from -0.8 up to 0.4 depending on the
ratio of the air velocity at the cowl position and velocity in the
shaft.

Best regards
Andreas Weber







Building Technologies
Empa - Materials Science & Technology
Ãœberlandstrasse 129
CH-8600 Dübendorf
Tel. ++41 ( 0)44 823 47 22
FAX ++41 (0)44 823 40 09
andreas.weber@...
www.empa.ch

>>> David Bradley <bradley@...> 20.01.2009 18:42 >>>
Colleagues,
I am attempting to model a clerestory building. On the lower roof
there are a number of solar chimneys that rise above the roof but not
so
high as the peak of the clerestory. I am struggling a bit with how to
estimate the wind pressure coefficients (Cp values) of the chimney tops

so that I can calculate the wind-driven part of their air flow. I have

used the TNO CpGenerator tool to obtain Cp values for the roof surfaces

themselves. Does anyone have any experience or suggestions as to how Cp

might vary for positions above the lower roof surface? Any suggestions

would be appreciated.
Kind regards,
David

PS if my explanation of the building configuration is unclear, please
let me know and I would be happy to email a sketch of what I am trying

to model.

--
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Thermal Energy System Specialists (TESS), LLC
David BRADLEY 22 N. Carroll Street - Suite 370
Partner Madison, WI 53703
USA
P: +1.608.274.2577
F: +1.608.278.1475
E-mail: bradley@...
Web Pages: http://www.tess-inc.com and http://www.trnsys.com

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