Darcy-Weisbach Equation
Because of non-empirical origins, the Darcy-Weisbach equation is viewed by many engineers as the most accurate method for modeling friction losses. It most commonly takes the following form:
| h L | = | Headloss (m, ft.) | |
| f | = | Darcy-Weisbach friction factor (unitless) | |
| D | = | Pipe diameter (m, ft.) | |
| L | = | Pipe length (m, ft.) | |
| V | = | Flow velocity (m/s, ft./sec.) | |
| g | = | Gravitational acceleration constant (m/s 2 , ft./sec. 2 ) |
For section geometries that are not circular, this equation is adapted by relating a circular section’s full-flow hydraulic radius to its diameter:
D = 4R
This can then be rearranged to the form:
| Q | = | Discharge (m 3 /s, cfs) | |
| A | = | Flow area (m 2 , ft. 2 ) | |
| R | = | Hydraulic radius (m, ft.) | |
| S | = | Friction slope (m/m, ft./ft.) | |
| f | = | Darcy-Weisbach friction factor (unitless) | |
| g | = | Gravitational acceleration constant (m/s 2 , ft./sec. 2 ) |
The Swamee and Jain equation can then be used to calculate the friction factor.