Heat flux at the boundary
The gradient of temperature is not available at the boundary faces. But the
In my test case I had a wall surface with id =6 and defined a heat flux bc there. The following UDF implemented over that case yielded the same value of heat flux as was specified in the BC.
int id=6; /* for my case the wall surface id was 6 and I checked heat flux at that surface by providing a heat flux boundary condition */