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Re: [EnergyPlus_Support] Cp: wind pressure coefficient
Ana:
The EnergyPlus equation is different from the equation 19 on page 27.6 in
2005. By assuming the equation in 2001 is the same as 2005, my observation
focuses on the difference between the 2005 equation and EnergyPlus equation.
The 2005 equation uses Cp values at 0, 90, 180 and 270 to calculate Cp value
at any wind angel. The equation was developed by Walker and Wilson (1994).
The EnergyPlus equation calculates Cp with a given angel only and does not
require any given Cp values at 4 angels. It is obvious these two equations
serve different purposes. The 2005 equation uses given Cp values to
calculate Cp and does not require any aspect ratio by assuming a rectangular
building (my guess). The EnergyPlus equation requires the aspect ratio as an
independent variable, by assuming building shape has an impact on wind
pressure. Since I have not read the paper, I don't the how Walker and Wilson
developed the 2005 equation. However, I do know how Swami and Chandra
developed the equation based on curve fit, because they are my colleagues.
It is very common for authors to use different independent variables to
develop empirical equations, since the equation development does not require
real physics. It is better for you to ask why to the authors.
Thanks for your interest.
Lixing Gu
Florida Solar Energy Center
1679 Clearlake Road
Cocoa, FL 32922
Phone: (321) 638-1411
Fax: (321) 638-1439
----- Original Message -----
From: "Ana Paula Melo" <apaula_melo@xxxxxxxxx>
To: <EnergyPlus_Support@xxxxxxxxxxxxxxx>
Sent: Monday, February 25, 2008 12:48 PM
Subject: Re: [EnergyPlus_Support] Cp: wind pressure coefficient
Hi,
I know that the EnergyPlus uses the formula of Sawmi and Chandra (1988) to
calculate wind pressure coefficient values. But, this equation has a G value
(Natural log of the ratio of the width of the wall under consideration to
the width of the adjacent wall) which I could not find in the ASHRAE
Fundamentals Handbook 2001 - Cap26 - page 26.6 - equation 19 and also in
ASHRAE Fundamentals Handbook 2005 - Cap27 -page 27.6 - equation 19.
Why does EnergyPlus use this G value and the ASHRAE equations not???
Thanks for all,
Ana Paula.
Lixing Gu <gu@xxxxxxxxxxxx> wrote: Ana:
EnergyPlus uses the formula of Sawmi and Chandra (1988), which is the
equation in the Engineering Reference on page 387. The equation uses G. I
compared the equation with the original paper and found both are the same.
I tried to find the equation in Ch 26 of 2001 HOF and Ch 27 of 2005 HOF and
could not locate the equation. Please let me know what specific equation you
refer, equation number and page number.
Thanks.
Lixing
----- Original Message -----
From: "Ana Paula Melo" <apaula_melo@xxxxxxxxx>
To: "EnergyPlus Support" <energyplus_support@xxxxxxxxxxxxxxx>
Sent: Friday, February 22, 2008 8:50 AM
Subject: [EnergyPlus_Support] Cp: wind pressure coefficient
Hi,
I would like to know about the Cp = Wind surface pressure coefficient.
I read in the Enguneering Reference that the EnergyPlus uses the formula
of Sawmi and Chandra (1988) to calculate wind pressure coefficient values.
The equation has a G value (Natural log of the ratio of the width of the
wall under consideration to the width of the adjacent wall).
I looked at ASHRAE Fundamentals Handbook 2001 - Cap26 and I noticed that
the formula to calculate wind pressure coefficient values is different.
Also, I looked at ASHRAE Fundamentals Handbook 2005 - Cap27 and it is
different too as it does not have this G value.
What is the real formula that is used by EnergyPlus and Why???
Thanks for all,
Ana Paula.
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