Pipe friction due to different materials and surfaces

In practice, the surfaces of pipe walls are always associated with a certain roughness. This surface roughness is created partly as a result of the production process and partly due to deposits or corrosion caused by operation. The pipe’s material also has a decisive infl uence on the roughness. In laminar flow the roughness of the pipe has a very small influence on the pressure loss, because the fl uids in the region of the boundary layer have very low flow velocities or in some cases do not move at all.

In turbulent flow, however, what is decisive is whether the thickness δ of the laminar boundary layer extends over the unevenness of the pipe surface k and conceals it. In this case they are referred to as hydraulically smooth pipes and the roughness does not affect the pressure loss. If the surface roughness of the pipe extends far beyond the laminar boundary layer, the sliding effect of the boundary layer is lost. In this case they are hydraulically rough pipes and the roughness has a considerable effect on the pressure loss.

Hydraulically smooth pipes

The laminar boundary layer is sufficiently pronounced to cover the unevenness in the pipe surface. The turbulent pipe fl ow can flow freely.

Pipes in the transition region

In practice, hybrid forms occur depending on the flow condition and the nature of the pipe. If the laminar boundary layer is considerably pronounced but does not entirely cover the unevenness, this is referred to as pipes in the transition region.

Hydraulically rough pipes

The laminar boundary layer is not sufficiently pronounced to cover the unevenness in the pipe surface.