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Re: [EnergyPlus_Support] Re: Cp: wind pressure coefficient



Ana:

The Swami and Chandra´s equation does not have the item of cos(theta)^(3/4). 
The equation is listed in the Engineering Reference on page 387.

Lixing Gu

----- Original Message ----- 
From: "Ana Paula Melo" <apaula_melo@xxxxxxxxx>
To: <EnergyPlus_Support@xxxxxxxxxxxxxxx>
Sent: Wednesday, March 12, 2008 10:17 AM
Subject: Re: [EnergyPlus_Support] Re: Cp: wind pressure coefficient


Hi,

  I would like to know the answer of the question in the e.mail sent by 
James Bush.

  I am having some troubles using the Swami and Chandra´s equation.

  When the eqaution asks for cos(theta)^(3/4) I do not know what value I 
should use....positive or negative????

  Thanks for all,

  Ana Paula.


energyplususer <james.bush@xxxxxxxxxxx> wrote:
          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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> The primary EnergyPlus web site is found at:
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The primary EnergyPlus web site is found at:
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http://groups.yahoo.com/group/EnergyPlus_Support/

Attachments are not allowed -- please post any files to the appropriate 
folder in the Files area of the Support Web Site.

EnergyPlus Documentation is searchable.  Open EPlusMainMenu.pdf under the 
Documentation link and press the "search" button.

Yahoo! Groups Links






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The group web site is:
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Attachments are not allowed -- please post any files to the appropriate folder in the Files area of the Support Web Site.

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