Short/Long wave radiation Red Brick

[Michael Hurd]

I'm seeing some odd results when selecting red brick for Short wave radiation absorptivity. I can select another color/product with similar numbers (red tiles for instance) that differs by only .01 in absorptivity, yet will produce a completely different result. This leads me to believe that the short wave absorptivity is affected by not just the color but by the material properties of say the brick versus concrete. Looking in the help section I don't see an explanation of this behavior. Can you confirm, what I assume, that short wave absorptivity is affected by properties beyond just the color of the material?

[Manfred Kehrer]

No I cannot confirm your hypothesis, the color (or to be more specific the brightness) should be a good indicator for the solar absorptivity. A change of the solar absorptivity by 0.01 should not have a big impact on the results. In case it does there might be some numerical problems, i.e. at least one of your cases did not produce physical correct results.

[Michael Hurd]

So, If I select two different materials with two nearly identical "brightness" numbers, I should expect to get two nearly identical results - which did not happen in my case. This makes me wonder if something is incorrect with the red brick selection within the program itself. I say this because, I ran multiple other scenarios with very close "brightness" numbers and they did not produce the odd results I get with the red brick, not sure where to go from here other than perhaps to throw out the red brick....

[Manfred Kehrer]

Michael, I am not sure what you mean with "material with nearly identical brightness"

The material properties in WUFI do not have a brightness, because solar absorption happen only at exterior surfaces, therefore the solar absorptivity is to be found at the surface transfer dialog.

There is nothing incorrect with the red brick, as I have used it multi dozen times with proper results. It is a combination out of many things which end up in convergence issues. I would check the balances in the result/summary first.

[Michael Hurd]

Manfred, I only used the term "brightness" because you did. You are correct the balances are thrown off on the south and west elevations, but not on the North and South - I am just floored that a .01 difference in absorptivity can create results that are so varied. If I select red brick absorptivity my south and west elevations fail miserably, if I select Red Tile (.01 different absorptivity) the north, south, east and west all pass with flying colors - I'm using the same inputs on all the elevations, is this really possible?

I enabled the time steps on the two elevations in question and the calc's grind so slow I'll need a shave by the time it finishes, if it finishes. When I switch to the old air, it sails through the calcs and all cases perform well. Thoughts?

[Manfred Kehrer]

Yes this is really possible by our experience. I am also pretty sure no one in our support team can explain exactly under which circumstances it happens.

Regarding the "old air", yes we know this too, The old air is more forgiving due to higher moisture buffering (numerically spoken the higher buffering makes the main diagonal in the final matrix more dominant, which is a good indicator for successful solving).
You did the correct thing, use the "new air" and switch back to the "old air" in case numerical stability cannot be reached.

[Michael Hurd]

Manfred, thanks much, I appreciate being able to bounce these situations off you guys!