| Product(s): | HAMMER |
| Version(s): | 08.11.XX.XX and higher |
Problem
In the transient calculation options in HAMMER, when should the property "Is User Defined Time Step?" be set to "True", and what are some guidelines for selecting a reasonable custom timestep? ("Time Step Interval" property)
Solution
The default setting for "Is User Defined Time Step?" is "false", which means HAMMER will attempt to set the time step for you, based on a reasonable balance between performance and wave speed or length adjustment. See: Understanding length/wave speed adjustments and their impact on results
To view HAMMER's suggested time step, go to the Home or Analysis ribbon > Time Step Options. In this dialog you will see the suggested timestep on the left, and you can check the "Use custom time step" box on the right, enter a value and click "Update" to see the resulting max/mean/rms adjustment.
In most modeling situations you will likely want to set a custom time step. Generally speaking a value of 0.01 seconds is very typical for most transient simulations, and in some cases 0.005 seconds is also acceptable.
For most models, when time steps are smaller, calculated results are more accurate. For some sensitive models, changing model data, including calculation time step, can change calculation results. Engineering judgement is needed to determine if the selected timestep is producing more accurate results.
Note that although the results are calculated at the selected time step interval, the results seen in the Transient Results Viewer may not be reporting at every time step. In the transient calculation options, the "Report Times" option is used to control how often the results are saved. For example if it is set to "Periodically" and the "Report Period" is set to 10, the results in time histories (graphs) will be shown at every 10 time steps. See more in the "Improving performance..." article below.
What if a very small timestep produces inaccurate results?
It is not necessary to use too small calculation time steps (such as 0.0001 seconds). For some models, calculated results are not perfectly accurate at each time step and so using a very small time step can cause larger accumulated errors due to having too many time steps.
If reducing the calculation time step does not result in smoother results, do not reduce the time step.
See Also
Understanding length/wave speed adjustments and their impact on results
Improving performance of transient simulations (recommended reporting calculation options)