Product(s): |
Bentley Hammer |

Version(s): |
08.11.XX.XX, 10.01.XX.XX |

Area: |
Layout and Data Input |

# Problem

How can I model a frictionless pipe during my transient analysis in HAMMER?

# Solution

To model a frictionless pipe, simply use a very large Hazen-Williams friction coefficient (such as 99,999,999,999), so that the computed headloss during the initial conditions is zero or close to zero. (per the "Headloss" field in the "Results" section of the pipe properties. If using the Darcy-Weisbach friction method instead of Hazen-Williams, you would use a very small friction factor.

To ensure that the pipe is indeed frictionless for the transient simulation, you would open the Transient Analysis Output Log (located under Reports > Transient Analysis Reports) after computing the transient simulation, then scroll down to the section beginning with "PIPE INFORMATION". You will see a table showing each pipe and the Darcy-Weisbach friction factor that HAMMER used for the simulation (HAMMER always converts pipe head loss to a Darcy Weisbach friction factor for the transient simulation calculations). Your frictionless pipe should have a zero for the friction factor.

If it still has a positive friction factor listed, then the computed headloss for the initial conditions is probably still slightly greater than zero. In this case, you can use the pipe rounding calculation option. In your transient calculation options, you will see a field called "Round Pipe Heads?". Set this to "true", then enter the number of decimal places to round to. Basically you would set it so that the head loss (for the initial conditions) for the frictionless pipe would round to zero. Check the computed headloss in the pipe and make sure it's zero with one decimal place.

An alternate, but advanced way to model a frictionless pipe would be to use the "Specify Initial Conditions" transient calculation option. With this set to true, the user must manually enter the initial conditions, instead of them being computed using the steady state/EPS solver (Analysis > Compute Initial Conditions). In this case, you would manually enter the head at each end of the pipe along with the flow. For a frictionless pipe, you would simply set the start and stop head to the same value.

Note that this is demonstrated in the included "Sample1" model, located in the Samples subfolder within the installation folder.

# Effect

In the absence of friction, transient waves will not dampen. So in a system like the "Sample1" model, the waves will continue to propagate and reflect, and a final steady state will never be reached.

# See Also

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