Problem

How does HAMMER compute the Darcy-Weisbach friction factor (f) for each pipe in the model?

Background

HAMMER always uses the Darcy-Weisbach friction method during a transient simulation, regardless of the initial conditions friction method. It computes the "f" friction factor based on the initial conditions headloss (total headloss across the pipe including both friction and minor losses). The friction factor 'f' is shown in the "PIPE INFORMATION" section of the Transient Analysis Output Log. See more here: Reporting the Darcy-Weisbach Friction Factor (f)

There are a few different variations of the calculation depending on the situation:

Solution - non-zero initial flow

For non-zero flow pipes (initial conditions flow above "flow tolerance" transient calculation option) it uses this equation:

f = hl / [ (L/D)(V^2/2g) ]

Where:

hl = headloss across the pipe (ft, m)
L = pipe length (ft, m)
D = Diameter (ft, m)
V = Velocity (ft/s, m/s)

Solution - Zero initial flow

If the initial flow through the pipe is less than the "flow tolerance" transient calculation option, the user-entered friction coefficient will be directly converted to a Darcy-Weisbach 'f'. 

In particular if the initial conditions friction method is set to Darcy-Weisbach (with a roughness height 'e' entered), it uses the Von Karman equation to compute the friction factor:

DW f = (1.0 / (2.0 * Log10(Diameter / roughness height) + 1.14)^2

See Also

Unexpected results seen in pipes with zero initial flow

Error computing HAMMER model: "The Darcy-Weisbach friction coefficient xxx is too large.