Flow Friction – Coatings; Flow drag reduction techniques and friction factor measurement

Site last update April 29th_2021

Keywords: Flow Friction pipe internal coatings surface characteristics Flow drag measurement structured surfaces

Just as solid friction, resulting from the relative movement between two solid elements in contact, fluid friction, resulting from the relative movement between a solid element and a fluid, is of great importance. It concerns a large number of applications and is the origin of numerous losses of energy transformed into heat and contributing to global warming. This degraded energy is unrecoverable (increase in entropy). It is found in turbines generating electricity or in pipelines transporting natural gas requiring the use of large compression units. These two examples fall into the category of internal flows. It is also found in all means of transport, planes, trains, cars, trucks or boats. These latter examples fall into the category of external flows.

This chapter deals with different forms of fluid friction, viscous and turbulent, on an element in displacement relatively to a liquid or gaseous fluid bringing some information concerning friction on smooth and rough surfaces, the calculation of aerodynamic drag for an external flow as well as pressure losses in a conduct for an internal flow.

Surface characteristics

Different forms of surface roughness and surface deformation are presented, in particular, surface undulations (or ripples) sometimes encountered with certain types of internal pipe coatings. These coatings, although having a locally smooth surface (suggesting a low coefficient of friction), can generate significant pressure losses due to surface deformations with relatively large amplitude and long wavelength.

Coating aerodynamic testing

Some internal pipe linings are described (epoxy with or without solvent, epoxy in aqueous phase and polyamide), as well as an aerodynamic test apparatus for the measurement of the friction factor of internal coatings under conditions of high pressure (up to 100 bar) and very high velocity (up to 40 m / s), that is to say in conditions of very thin boundary layer thickness. Transcribed in terms of Reynolds number, these results are representative of the gas transport under a pressure condition “X” times higher and a gas velocity “X” times lower (for example, 500 bar and 8 m / s or 1000 bar and 4 m / s).

This chapter then presents several techniques aimed at reducing the friction of a flow on the surface of internal coatings (aerodynamic or hydraulic drag).

Turbulent drag reduction: structured surfaces

The drag reduction technique more particularly developed in this chapter concerns structured surfaces, three forms of which of different generations (two-dimensional, three-dimensional type 1 and three-dimensional type 2) are described. This section discusses the numerical techniques used for the geometric definition of these structured surfaces, a test apparatus for the measurement of aerodynamic performance, manufacturing methods as well as potential industrial applications. Structured surfaces produced on different types of internal pipe coatings have also been subject to mechanical tests, in particular, to analyse the behaviour of a structured profile under the force of a rolling load (smart pig simulation).

Other flow drag reduction techniques

In this chapter are also discussed other techniques for reducing aerodynamic or hydraulic drag such as compliant surfaces, porous coatings, walls inducing fluid slow down or acceleration (parietal sliding) of flow as well as the very specific case of two cylindrical walls in relative motion.