Re: [EnergyPlus_Support] Calculation of angular properties

["Michael J. Witte" <mjwitte@xxxxxxxx>]

On 1 Apr 2005, at 12:44, Fábyo Luiz Pereira wrote:

> 
>    I have some doubts about the calculation of angular properties for uncoated
> glass. This section is part of the engineering document and since I'm using an
> previous version of EnergyPlus (1.1.1.012), I will describe the equations and
> show its numbers used in my manual version. The "Calculation of Angular
> Properties" sub-section is located in the EngineeringDoc.pdf, chapter
> "Daylighting and Window Calculations", section "Window Calculation Module".

I would encourage you to upgrade to version 1.2.1.022 to gain the benefit of 
bug fixes, new features and documentation changes.

> 
>    In this sub-section, on "Angular Properties of Uncoated Glass" description,
> the first two equations (1.87 and 1.88) relates the angular function of
> transmittance and reflectance ("T" and "R") with the angular function of
> transmissivity and reflectivity ("tau" and "ro"). I have two questions:
> 
> 1 - The "phi" angle is the incidence angle, but it is not clear what angle is
> the "phi'"? It is the refraction angle inside the glass?

Yes, "phi'" is the angle inside the glass.

> 
> 2 - I concluded that terms with the -ivity ending (transmissivity,
> reflectivity) are refered as material properties (no matter physical features),
> while terms with the ?ance ending (transmittance, reflectance) are refered to
> samples because in these equations are functions of thickness. This is correct?
> 

That appears to be correct for the purpose of the equations you reference.  
Note that trasmissivity (tau) is defined for those equations as 1-rho 
(reflectivity).

>    Finally, in equation 1.90, that gives the spectral transmissivity as
> function of the spectral reflectivity: tau(phi) = 1 - ro(phi), the third doubt
> is:
> 
> 3 - Where is the spectral absorptivity term "alpha(phi)"?

As you stated above, it appears that transmissivity is not a function of 
thickness.  Transmissivity is used here as a property of a single air/glass 
interface.  At a single interface, there can be reflection but no absorption.  
Thus, tau=1 - ro.

Mike


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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support      
EnergyPlus-Support@xxxxxxxx




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