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How does HAMMER compute the Darcy-Weisbach friction factor (f) for each pipe in the model?
a pipe with zero initial flow and the Darcy Weisbach friction method (or when the initial flow below the "Flow Tolerance" calculation option).
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. There are a few different variations depending on the situation:
For non-zero flow pipes (flow above flow tolerance 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)
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:DW f = (1.0 / (2.0 * Log10(Diameter / roughness height) + 1.14)^2
Unexpected results seen in pipes with zero initial flow