Question-and-Answer Resource for the Building Energy Modeling Community
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Maybe you could cheat. The reflected part of the radiation on the flood will be distributed area weighted on the remaining surfaces in the zone. So if you make your floor really reflective, you could throw some of that energy at the other surfaces. That's far from exact science, but it's an idea.
From what I've seen from daylighting simulations of atriums, the direct beam falling on vertical surfaces moves quite fast from position to position, never heating any one spot for very long. The longest time is still on the floor surface.
