abaqus; radiation; boundary conditions

Dear all,

I have a problem assigning surface radiation to my model. Lets say the model consists out of two same size cubes. I want to simulate one cube sliding on top of the other. So lets say in the beginning the lower cube is covered 50 % by the upper cube and at the end it is covered 70% by the upper cube. Hence, at all times a part of the surface of the lower cube is free to the environment and another part is covered by the upper cube. However, due to the sliding process the free surface is changing in time.