## Friction factor. Skin-friction coefficient

A dimensionless number characterizing the frictional force at the boundary between fluid and a wall; it is defined by the identity:

a) in the case of flow in pipes

b) in the case of external flow past a body

where

*σ*

_{w}is the shear stress at the surface of the body (wall);

*ρ*is the density of the fluid;

*w*

_{0}is the characteristic velocity of the flow (in the case of flow in pipes, it is the average value over the cross section; in the case of external flow past a body, it is the value in the external flow).

*Comments.*1. A distinction is made between the "instantaneous local friction factor," the "average instantaneous friction factor over the surface," the "time-average local friction factor," and the "time- and surface-average friction factor."

2. In the case of a steady-state flow, the friction factor also characterizes the energy losses due to friction; it should not be confused with the "coefficient of resistance," which includes not only energy losses due to friction, but also energy losses of a different nature.