Study Impact of Time Step Selection


  1. Provide a simple example of the impact of time step size on accuracy of results.
  2. Illustrate the run time advantages of more implicit methods.


Make a subdirectory for this exercise and download this input file into it.

Problem Definition

This is a hypothetical variation on a real test series named Marviken. A vessel is filled with heated water (>500K) at about 5MPa. The transient begins by opening a rupture disk a the bottom of a pipe exiting the bottom of the vessel. The sample deck has a maximum time step size of 1.0 seconds. Run it with NOSETS at both the default and 1. For the remaining three runs, restore the default value of NOSETS (actually 0) by unchecking it in the Namelist dialog. Make runs with the maximum time step set to 0.1 and 0.01. Make a final run that sets the maximum time step to 0.1s until 300s in the transient, drops it to 0.01 until 350s, then raises it to 0.2s until the transient ends at 500s. Plot pressure and void fraction in Cell 4 of Pipe 3 (near bottom of the vessel. Plot mass flow rate at the last cell face in Pipe 3 and plot CPU time for all runs. Each of these four plots should contain curves from all 5 runs.


Make all of your runs before plotting results. Each plot will have 5 curves. From the AcGrace "Plot" menu, select the "Set Appearance" item, and use it to set legends for each curve that describe the run (e.g. "dt=1.0s", "Semi-Implicit",...).

If you want to get a feel for Choked Flow, try increasing the pressure in the break by a factor of 2, and compare exit mass flow rates.

What is missing from this input model?

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Created and Maintained by John Mahaffy : 2003