Friction loss

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Friction loss is the loss of energy or “head” that occurs in pipe flow due to viscous effects generated by the surface of the pipe.[1] Friction Loss is considered as a "major loss" and it is not to be confused with “minor loss”, which includes energy lost due to obstructions. In mechanical systems such as internal combustion engines, it refers to the power lost overcoming the friction between two moving surfaces.

This energy drop is dependent on the wall shear stress (τ) between the fluid and pipe surface. The shear stress of a flow is also dependent on whether the flow is turbulent or laminar. For turbulent flow, the pressure drop is dependent on the roughness of the surface. In laminar flow, the roughness effects of the wall are negligible because, in turbulent flow, a thin viscous layer is formed near the pipe surface that causes a loss in energy, while in laminar flow, this viscous layer is non-existent.[2]


Friction loss has several causes, including:

  • Frictional losses depend on the conditions of flow and the physical properties of the system.
  • Movement of fluid molecules against each other
  • Movement of fluid molecules against the inside surface of a pipe or the like, particularly if the inside surface is rough, textured, or otherwise not smooth
  • Bends, kinks, and other sharp turns in hose or piping

In pipe flows the losses due to friction are of two kinds: skin-friction and form-friction. The former is due to the roughness of the inner part of the pipe where the fluid comes in contact with the pipe material, while the latter is due to obstructions present in the line of flow--perhaps a bend, control valve, or anything that changes the course of motion of the flowing fluid.

Calculating friction loss[edit]

One of the accepted methods to calculate friction losses resulting from fluid motion in pipes is by using the Darcy–Weisbach equation. For a circular pipe:[3]

h_l = f_D \left ( \frac{L}{D} \right ) \left ( \frac{V^2}{2g} \right )


hl = Head loss due to friction, given in units of length
fD = Darcy friction factor (see Confusion with the Fanning friction factor )
L = Pipe length
D = Pipe diameter
V = Flow velocity
g = Gravitational acceleration


  1. ^ Munson, B.R. (2006). Fundamentals of Fluid Mechanics 5th Edition. Hoboken, NJ: Wiley & Sons. 
  2. ^ Munson, B.R. (2006). Fundamentals of Fluid Mechanics 5th Edition. Hoboken, NJ: Wiley & Sons. 
  3. ^ Brown, G.O. (2003). "The History of the Darcy-Weisbach Equation for Pipe Flow Resistance". 

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