I am trying to model flow through one channel of a heat exchanger fin using conjugate heat transfer. Air is flowing through the channel. The top wall has a constant heat flux, and the left, right, and bottom are completely insulated. I tried to model the surrounding walls and the fluid as separate bodies but am getting confused with assigning boundary conditions to all generated walls/shadows. For example, how do i know which boundaries to couple? Essentially I am trying to get the heat transfer rate of the fin and the pressure drop of the fluid. Also I am looking to change the roughness of the interior wall surface touching the fluid. How would i go about doing that? Can someone give me advice and a potential example to help me with this process?