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[EnergyPlus_Support] Re: Cp: wind pressure coefficient
Lixing,
Do you know the aspect ratio of the lowrise building investigated by
Swami and Chandra?
In regards to the Walker/Wilson equation, it was empirically developed
using data from 2 studies: one having an aspect ratio of unity, the
other study appears to be Swami and Chandra which is why I ask the
quesiton above.
A word of caution on the Walker/Wilson equation. I believe a logic
statement is needed to avoid an error with the cos(theta)^(3/4) term
when cos(theta) < 0. To recreate the Walker/Wilson curve fit, I had to
use the absolute value of cos(theta) and then multiple the entire term
by -1 whenever cos(theta) < 0.
Lessons learned,
j bush
university of kentucky
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Lixing Gu <gu@...> wrote:
>
> 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@...
> 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@... 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@...
> 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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> Yahoo! Groups Links
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