@wikipedia

Quantitative dimensionless measure  $//<![CDATA[ \begin{array}{l}f\end{array} //]]>$ of the friction forces between fluid pipe flow and inner pipe walls based on Darcy–Weisbach equation.

It depends on Reynolds number  $//<![CDATA[ \begin{array}{l}\rm Re\end{array} //]]>$ and inner pipe walls roughness  $//<![CDATA[ \begin{array}{l}\epsilon\end{array} //]]>$  (see Fig. 1): $//<![CDATA[ \begin{array}{l}f =f({\rm Re}, \, \epsilon)\end{array} //]]>$.

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  $//<![CDATA[ \begin{array}{l}f\end{array} //]]>$  takes only positive values  $//<![CDATA[ \begin{array}{l}f > 0\end{array} //]]>$ but has singularity at zero flow velocity:  $//<![CDATA[ \begin{array}{l}{\rm Re} \rightarrow 0 \Rightarrow f \rightarrow \infty\end{array} //]]>$ which may cause computational challenges.

Using Reduced Friction Factor  $//<![CDATA[ \begin{array}{l}\Phi = f \cdot {\rm Re} / 64\end{array} //]]>$  instead can help in computations as it stays finite for all finite values of Reynolds number  $//<![CDATA[ \begin{array}{l}\rm Re\end{array} //]]>$. 

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) ]