Transient maximum result is different from the highest value seen in a time history graph

  Applies To 
  Product(s): HAMMER
  Version(s): V8i, CONNECT Edition
  Area:  Output and Reporting
  Original Author: Scott Kampa, Bentley Technical Support Group

Problem

The maximum transient pressure value in the Properties display, the Transient Analysis Detailed Report and the Transient Results Viewer profile are different from what is
reported in the Time History graph at the same location. Meaning, the graph peaks at a value less than the reported maximum.

Solution

This is likely occurring because of the report period set in the Transient Calculation Options. If you have a fast-moving transient occurring (such as an air pocket collapse in an air valve) and your report period is set too high, then the peak could be occurring between the report timesteps, in which case it would not show in the time history or profile animation. For example if your report period is set to "10" and your maximum HGL occurs at timestep 105 and only lasts one timestep, then it will be missed since the results will report at timestep 100 and 110.

Conversely, the maximum value plotted in the profile (red line) and shown in the properties of an element is always based on all timesteps, so it will always show the "true" maximum.

To resolve this, you will need finer results granularity. Go to Analysis > Calculation Options and double click the active transient calculation option to view the information in the properties. Find the field "Report Times". Change this to "At All Times" and compute the model again. When you graph the time history at the element in question, the results will coincide with the Transient Analysis Output Log and the properties.

Note: check your user notifications after computing the model and make sure you do not see one that mentions the report period was increased by a certain factor. This can happen if there are too many report points in the model. In this case, reduce the number of Report Points, choose to report "only if on path" and/or consider skeletonizing the model. Note that this notification may only apply to the granularity of profile animations.

To see the true / calculated max/min transient results, see the transient statistical results section of the properties, or in flextables. To see the maximum/minimum at each point along the length of a pipe (since the Method of Characteristics calculates at intermediate points), use the “extreme pressures and heads” table. See more in the second article below. Note: the maximum and minimum results reported in the Properties grid and FlexTables, as well as the maximum and minimum results in the profiles, are the actual maximum results. These are calculated prior to the reporting period being used.

Note also that the transient profile in the Transient Results Viewer will include intermediate points along the pipe, whereas the standard profile plot of min/max will only plot between the nodes. So, if a peak occurs between the nodes, the standard profile will not pick up on it since it is only reporting at the nodes.

See Also

Recommended HAMMER Calculation Option Reporting Settings

"The period between path histories has been increased by a factor of X to limit the number of path records to Y. "

How can you find the minimum and maximum transient pressure or hydraulic grade results in a HAMMER model?

Results from the Extreme Pressure and Heads table are different from results found in the transient profiles

Recommended
Related