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 σw is the shear stress at the surface of the body (wall); ρ is the density of the fluid; w0 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.