# Friction Loss (laminar flow)

## Description

In fluid flow, friction loss (or skin friction) is the loss of pressure or “head” that occurs in pipe or duct flow due to the effect of the fluid’s viscosity near the surface of the pipe or duct. In mechanical systems such as internal combustion engines, the term refers to the power lost in overcoming the friction between two moving surfaces, a different phenomenon.

Laminar flow is encountered in practice with very viscous fluids, such as motor oil, flowing through small-diameter tubes, at low velocity. Friction loss under conditions of laminar flow follow the Hagen–Poiseuille equation, which is an exact solution to the Navier-Stokes equations. For a circular pipe with a fluid of density ρ and viscosity μ, the hydraulic slope S can be expressed as shown here.

Related formulas## Variables

S | friction loss or hydraulic slope (dimensionless) |

ν | kinematic viscosity (m^{2}/s) |

g | Standard gravity |

V | flow velocity (m/s) |

D | pipe diameter (m) |