The dimensionless ratio Epsilon /d (where Epsilon is the average height of the surface irregularities and d is the diameter of the pipe) is termed the relative roughness. The physical interpretation of this functional equation is that the friction factors of pipes are the same if their flow patterns in every detail are geometrically and dynamically similar. 2598 The term Epsilon indicates the height of the irregularity above the boundary surface only; hence it is apparent that, dependent upon the thickness of the boundary layer adjacent to the surface, the projection can either lie submerged within the boundary layer or else project outside it.