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[TRNSYS-users] view factor to sky

Dear all,
We're doing research on solar shading and are comparing new techniques with old techniques, such as overhangs above the window and obstructions next or in front of windows. Therefore, we make use of type 34 and type 68, and connect the reduced total and reduced beam radiation to the specific shaded surfaces of the building. But I was wondering weither I have to change the view factors of the shaded windows in TRNBUILD aswell. Which influence does this parameter have ? Changing this the whole time would be hard since it is a parameter in the bui-file and you can't connect it to an output in trnsys.
And how do you calcute them ? With type 34 it is easy; you can take output 6 (view factor to the sky), but for type 68 I'm not sure; I thoughed to take output 4 (fraction of diffuse visible for surface). Since I'm using the isotropic sky model for diffuse radiation, this should be the same, I thoughed.
However, when I put in no obstruction for a vertical wall looking to the east, so with a input-file like 
"   090    ! Opening slope
  -90    ! Opening azimuth (0 = facing Equator, East is negative)
-180.0 -157.5 -135.0 -112.5 -90.0 -67.5 -45.0 -22.5 0.0 22.5 45.0 67.5 90.0 112.5 135.0 157.5 ! View angles - ALWAYS (-180:step:180-step) 00.000 ! Obstruction height for opening 1, view angle 1 (i.e. between -180 and -157.5 degrees or N and NNE for Northern Hemisphere) 00.000 ! Obstruction height for opening 1, view angle 2 (i.e. between -157.5 and -135 degrees or NNE and NE for Northern Hemisphere) 00.000 ! Obstruction height for opening 1, view angle 3 (i.e. between -135 and -112.5 degrees or NE and ENE for Northern Hemisphere) 00.000 ! Obstruction height for opening 1, view angle 4 (i.e. between -112.5 and -90 degrees or ENE and E for Northern Hemisphere) 00.000 ! Obstruction height for opening 1, view angle 5 (i.e. between -90 and -67.5 degrees or E and ESE for Northern Hemisphere) 00.000 ! Obstruction height for opening 1, view angle 6 (i.e. between -67.5 and -45 degrees or ESE and SE for Northern Hemisphere) 00.000 ! Obstruction height for opening 1, view angle 7 (i.e. between -45 and -22.5 degrees or SE and SSE for Northern Hemisphere) 00.000 ! Obstruction height for opening 1, view angle 8 (i.e. between -22.5 and 0 degrees or SSE and S for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 9 (i.e. between 0 and 22.5 degrees or S and SSW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 10 (i.e. between 22.5 and 45 degrees or SSW and SW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 11 (i.e. between 45 and 67.5 degrees or SW and WSW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 12 (i.e. between 67.5 and 90 degrees or WSW and W for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 13 (i.e. between 90 and 112.5 degrees or W and WNW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 14 (i.e. between 112.5 and 135 degrees or WNW and NW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 15 (i.e. between 135 and 157.5 degrees or NW and NNW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 16 (i.e. between 157.5 and 180 degrees or NNW and N for Northern Hemisphere)" 

I get a diffuse fraction of 0.8333 instead of 1 or 0.5.

When I put in an obstruction from East to North, with
"   090    ! Opening slope
  -90    ! Opening azimuth (0 = facing Equator, East is negative)
-180.0 -157.5 -135.0 -112.5 -90.0 -67.5 -45.0 -22.5 0.0 22.5 45.0 67.5 90.0 112.5 135.0 157.5 ! View angles - ALWAYS (-180:step:180-step) 00.000 ! Obstruction height for opening 1, view angle 1 (i.e. between -180 and -157.5 degrees or N and NNE for Northern Hemisphere) 00.000 ! Obstruction height for opening 1, view angle 2 (i.e. between -157.5 and -135 degrees or NNE and NE for Northern Hemisphere) 00.000 ! Obstruction height for opening 1, view angle 3 (i.e. between -135 and -112.5 degrees or NE and ENE for Northern Hemisphere) 00.000 ! Obstruction height for opening 1, view angle 4 (i.e. between -112.5 and -90 degrees or ENE and E for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 5 (i.e. between -90 and -67.5 degrees or E and ESE for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 6 (i.e. between -67.5 and -45 degrees or ESE and SE for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 7 (i.e. between -45 and -22.5 degrees or SE and SSE for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 8 (i.e. between -22.5 and 0 degrees or SSE and S for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 9 (i.e. between 0 and 22.5 degrees or S and SSW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 10 (i.e. between 22.5 and 45 degrees or SSW and SW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 11 (i.e. between 45 and 67.5 degrees or SW and WSW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 12 (i.e. between 67.5 and 90 degrees or WSW and W for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 13 (i.e. between 90 and 112.5 degrees or W and WNW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 14 (i.e. between 112.5 and 135 degrees or WNW and NW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 15 (i.e. between 135 and 157.5 degrees or NW and NNW for Northern Hemisphere) 90.000 ! Obstruction height for opening 1, view angle 16 (i.e. between 157.5 and 180 degrees or NNW and N for Northern Hemisphere)" 

I get a diffuse fraction of 0.6111 instead of 0.5 or 0.25.
Where is my reasoning mistake ? And do you have any tips that could make our lives somewhat easier ? 

Best Regards, and have a good weekend,
Jeroen

--
Ir.Jeroen Van der Veken
Afdeling Bouwfysica
Katholieke Universiteit Leuven
Kasteelpark Arenberg 40
3001 Heverlee
T: +32 16 32 13 47
F: +32 16 32 19 80
@: jeroen.vanderveken@bwk.kuleuven.be



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