Applies To 

Product(s): 
HAMMER 
Version(s): 
10.00.xx.xx, 08.11.xx.xx 
Area: 
Modeling 
Original Author: 
Jesse Dringoli, Bentley Technical Support Group 
Problem
How does HAMMER compute the DarcyWeisbach friction factor (f) for each pipe in the model?
Background
HAMMER always uses the DarcyWeisbach 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. The friction factor 'f' is shown in the "PIPE INFORMATION" section of the Transient Analysis Output Log.
There are a few different variations of the calculation depending on the situation:
Solution  nonzero initial flow
For nonzero flow pipes (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 userentered friction coefficient will be directly converted to a DarcyWeisbach 'f'.
In particular if the initial conditions friction method is set to DarcyWeisbach (with a roughness height 'e' entered), it uses the Von Karman equation:
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 DarcyWeisbach friction coefficient xxx is too large.