[hossein65]

I am trying to simulate light distribution in an open channel with water, which is exposed to light (top boundary condition of Xw/m2). I am using DO model, non-gray (number of bands=1, 400-700nm), and the Angular Discretization is set on default. As far as the pond is exposed to light intensity, I set the top boundary condition as a was with the follwoing setup at Radiation tab:
BC type: Semi-transparent
Direct Irradiation: 50w/m2
apply direct irradition parallel to the beam is checked.
Beam direction: x=0, y=0 and z=-1 (because I want it to be exposed from the top)
diffuse irradiation = 0

My problem is that:
1-when the physical properties of the liquid in the channel regarding radiation is set to 0, when I set the direct irradiation to X W/m2, I get a 2X W/m2 on the top, after several time steps.
2-when I change the physical properties of water (like scattering and absorptio coefficients), I get a random number on the top, although I have set it to X W/m2 in boundary conditions tab.
How can I set all these to specific values and baiscally, how it works?