Natural convection and periodic boundary layer
 

Hai everyone, I am a beginer in Fluent.Now iam doing project on the heat and flow in airside radiator ducts.Here the ducts will be rib roughened.This is a free convection case.Here the ducts are on constant heat flux and air is allowed to move around it.Here the rib roughness for over the duct is symetrical.So can i take a model a part of it and give periodic boundary condition?Or,since the temperature of the air changes as it moves away,would the periodic boundary condition be a bad choice in this natural convection case. Thanks, George.