Calibration simulations
InfoWorks WS provides an Automatic Calibration facility that significantly speeds up work on large all mains models where friction factors have a predominant role in calibrating real-life networks. The auto-calibration method is based on repetitive peak demand snapshot analysis during which friction factors are adjusted until a minimum sum of differences between model and field data is reached at pressure and flow observation points.
The software repeats the calculations for the snapshot timestep over and over again, adjusting all friction factors within a selected range by a varying factor, until it finds a factor that gives the least difference between model and field data. It then writes out the friction parameters adjusted by this factor as part of the simulation results.
The user can adjust the friction factors in a new copy of the network using these results.
In models that may include hundreds of thousands of pipes this is a valuable, and may be the only method, for providing a better estimate of friction factors for hydraulic analysis.
Method
The loss due to pipe friction can be represented by the equation:
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where Dh - headloss k - headloss coefficient V - velocity g - acceleration due to gravity (9.81 m/s2) |
The software takes the Minimum Friction Factor and Maximum Friction Factor values entered in the Calibration Options dialog and determines equivalent k values for them. During the calibration process it multiplies the pipe roughness by a scaling factor and determines the equivalent k value for the new pipe roughness. If the new k value is within the limits the software will use it to solve the network at the snapshot time, otherwise the software will use the original roughness value.
Roughness values will be adjusted if they fall within the calculated headloss coefficient (k) limits. This adjustment will take place for all roughness models used in the network.
If you mix roughness models then you may find roughness values adjusted for pipes that do not use the roughness model you specified in the Run Calibration Options dialog for the calibration limits. In general, it is a good idea to stick to one roughness model in a network.
Scheduling a Calibration Simulation
To set up a calibration run and use the results to update your network:
- Set up the run as normal. See Creating
and Scheduling a Run for full details.Please Note
A Live Data Configuration is required for a calibration run.
- Choose Calibration in the Run
Type box and click the Options button
to display the Calibration
Options dialog.
Show image.
Use this dialog to set up the parameters for the calibration run. Click
the OK button to save your chosen parameters.The software converts the Minimum Friction Factor and Maximum Friction Factor values entered to equivalent minimum and maximum headloss coefficients. It makes no alteration to values that are less than the minimum or greater than the maximum calculated headloss coefficient. The values are first multiplied by the Initial Scaling Factor. The scaling factor then varies as the software iterates towards optimal values.
The Snapshot Time is the timestep for which the friction values are optimised. This timestep would normally be an important point in the simulation, such as the time of maximum demand in the system.
The Live Data Configuration contains the field data to which the network is being calibrated.
- Run the simulations by clicking the Run button as described in Creating and Scheduling a Run. Fix any problems with simulations that have failed, and reschedule until the simulations complete successfully.
