Darcy Equation Fluids Flow Equation

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Darcys Equation Fluids Flow Equation - also called Darcy–Weisbach equation.

In fluid dynamics , the Darcy–Weisbach equation is a phenomenological equation, which relates the head loss — or pressure loss — due to friction along a given length of pipe to the average velocity of the fluid flow.

The Darcy–Weisbach equation contains a dimensionless friction factor, known as the Darcy friction factor . This is also called the Darcy–Weisbach friction factor or Moody friction factor . The Darcy friction factor is four times the Fanning friction factor , with which it should not be confused.

The frictional head loss can be calculated using a mathematical relationship that is known as Darcys equation for head loss. The equation takes two distinct forms. The first form of Darcys equation determines the losses in the system associated with the length of the pipe.

Darcy–Weisbach equation


f = friction factor (unitless)
L = length of pipe (ft)
D = diameter of pipe (ft)
v = fluid velocity (ft/sec)
g = gravitational acceleration (ft/sec2)

Example: Darcys Head Loss Equation

A pipe 100 feet long and 20 inches in diameter contains water at 200F flowing at a mass flow rate of 700 lbm/sec. The water has a density of 60 lbm/ft3 and a viscosity of 1.978 x 10-7 lbf-sec/ft2. The relative roughness of the pipe is 0.00008. Calculate the head loss for the pipe.


The sequence of steps necessary to solve this problem is first to determine the flow velocity. Second, using the flow velocity and the fluid properties given, calculate the Reynolds number. Third, determine the friction factor from the Reynolds number and the relative roughness. Finally, use Darcys equation to determine the head loss.

Darcy–Weisbach equation

Use the Moody Chart for a Reynolds number of 8.4 x 107 and a relative roughness of 0.00008.

Darcy–Weisbach equation