one more quastion. Do you have an experiance with solar radiation model in Fluent? In fluent solar radiation panel is an option to establish a grid orientation (north and east). I have a boxs (cube) with gravity X=0, Y=0, Z=-9,81 and I`d like to have only a top exhibited on solar rays.

I'm not sure that I'm right but I think that a good option could be imposing mixed thermal condition at the top wall.

I hope it helps but please correct me if I am wrong

thank you Albert for reply. Yes I`m using mixed conditions. My problem is I don`t know haw to orientated a grid in Fluent (norh and west vectors). I thought that one vector (north for example ) will orientate my grid. I don`t understand it.

thanks one more time