I have just started learning COMSOL and have a problem now.
I derived the surface temperature distribution of a sphere using COMSOL.
After that, I calculated the net radiation force of the sphere, i.e. the surface integrations of 
, where 
is the temperature and 
is the unit normal vector.
However, the result of surface integrations given by COMSOL seems to be incorrect.
Then I did another test with a uniform surface temperature distribution. The result of surface integrations should be zero in theory due to the symmetry, but it is in fact not. I am confused now. Is there something wrong about the surface integration function in COMSOL?
I would appreciate it if someone offers some help.