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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. 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.
Roberts, 1
Source:
Dictionary of Mining, Mineral, and Related Terms
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