Friction Loss (hydraulic slope) - related to pressure change


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.

When the pressure is expressed in terms of the equivalent height of a column of that fluid, as is common with water, the friction loss is expressed as S, the “head loss” per length of pipe, a dimensionless quantity also known as the hydraulic slope, as shown here.

Related formulas


Sfriction loss or hydraulic slope (dimensionless)
ρfluid density (kg/m3)
gStandard gravity
Δpchange in pressure (pascal)
Lunit length of pipe (m)