The geometric roughness height and sand-grain roughness height parameters have different effects and need to be specified separately. The geometric roughness height modifies the transition location and the sand-grain roughness modifies the wall shear stress and heat transfer once the flow is turbulent. The geometrical roughness is required inside the transition correlation – meaning that it applies to the entire solution domain instead of only being active in the near-wall region. The GUI specified value will therefore be used everywhere unless restricted by UDF.