Behavior of Collimated Beam Irradiation in DO model

How does an incident collimated beam irradiated on a surface behave in the DO model?

To explain this phenomenon, a simple square domain is taken for illustrative purposes in the PDF file shown below. A collimated beam is incident on a small portion of this wall. A structured grid is employed for computation. Various parameters such as number of phi divisions, diffuse fraction, and beam direction are varied to study the impact of this incident radiation.