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Quantitative dimensionless measure  f of the friction forces between fluid pipe flow and inner pipe walls based on Darcy–Weisbach equation.

It depends on Reynolds number  \rm Re and inner pipe walls roughness  \epsilon  (see Fig. 1): f =f({\rm Re}, \, \epsilon).

Fig. 1. Schematic chart showing how Darcy friction factor depends on Reynolds number  \rm Re and roughness  \epsilon (following [1])

In engineering practice one can use either Moody Chart or Darcy friction factor @model to estimate the actual value of Darcy friction factor.

Darcy friction factor  f  takes only positive values  f > 0 but has singularity at zero flow velocity:  {\rm Re} \rightarrow 0 \Rightarrow f \rightarrow \infty which may cause computational challenges.

Using Reduced Friction Factor  \Phi = f \cdot {\rm Re} / 64  instead can help in computations as it stays finite for all finite values of Reynolds number  \rm Re

See also

Physics / Fluid Dynamics / Pipe Flow Dynamics / Darcy–Weisbach equation 

Darcy friction factor Single-phase @model ] [ Reynolds Number for Multiphase Flow @model ]

[ Moody Chart ] [ Reduced Friction Factor (Φ) ]

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