What is a Friction Factor
Also known as the Moody Friction Factor or Darcy Weibach friction factor it is a dimensionless number used in internal flow calculations with the Darcy-Weisbach equation. Depending on the Reynolds_Number, the friction factor may be calculated one of several ways.
In laminar flow, the friction factor is independent of the surface roughness, . This is because the fluid flow profile contains a boundary layer where the flow at the surface through the height of the roughness is zero.
For Re<2100, the friction factor may be calculated by:
For (transitional flow regime), the friction factor may be estimated from the Moody Diagram.
Methods for finding the friction factor f are to use a diagram, such as the Moody Diagram, the Colebrook-White Equation, or the Swamee-Jain Equation.
Using the diagram or Colebrook-White equation requires iteration. Where the Swamee-Jain equation allows f to be found directly for full flow in a circular pipe.
The Colebrook-White equation is used to iteratively solve for the Darcy Weisbach Friction Factor f.
- For Free Surface Flow:
- For Full Flow (Closed Conduit):
Where f is a function of:
- roughness height, e (m, ft)
- hydraulic radius, R (m, ft)
- Reynolds Number Re (unitless)
Because the iterative search for the correct f value can be quite time-consuming, the Swamee-Jain equation can be used to solve directly for f.
The Swamee-Jain Equation is accurate to 1.0% of the Colebrook-White Equation for and .
- roughness height, Œµ ( ft)
- pipe diameter, D ( ft)
- Reynolds Number, Re (unitless).