Darcy Friction Factor

The pressure drop equation developed on the preceding page is the starting point for the development of the Darcy friction factor that is used to compute the head loss in the pipe. The Darcy friction factor can be determined analytically for laminar flows but is based on experimental measurements for turbulent flows. Another friction factor encountered in fluid dynamics is the Fanning friction factor. The two differ by a factor of four.

It has been observed that the pressure drop is proportional to the pipe length to diameter ratio.  This allows the equation for the pressure drop from the previous page to be rewritten as

 


The relationship, F, between the Reynolds number and the relative roughness is defined by a parameter, f, called the Darcy friction factor


The Darcy friction factor for laminar flow can be determined analytically and give by the equation 


Experimental measurements were conducted by Colebrook to determine the Darcy friction factor for turbulent flows.  One form of the Colebrook equation is


 

Learning Objectives

Learn the definition of the Darcy friction factor.