I am simulating 2 phase flow in a pipeline, how I going to define the material and the roughness of the wall?

From what I know you only need the wall roughness in turbulent flows. Therefore if you turn on the turbulent flow model you can enter the wall roughness as boundary condition, but not if you use the 2 layer zonal model as near wall treatment because it use a one equation turbulence model to solve the turbulent flow in the near wall region. If you use the wall function approach to describe the influence of the walls on your flow the wall roughness should appear in the boundary condition pannel for walls.

Regards Phil

Is the wall conducting/insulating heat? You can try planar conduction, which is only available for 3D problems in Fluent 5.4 with material property and thickness of wall. Roughness in the wall, may be possible.This planar conduction model works with only segregated solver.

Ashutosh