Controlling the quality of simulations

When analyzing results from a simulation, it is crucial to ensure the quality of the simulation. There are a number of ways to do this, and to locate the error if there is something wrong, explained below.

Look at the animation and see if it looks physically realistic
- Elements distortions
- Penetrating parts
Plot the time history of the energy to study energy conservation
- Mode 1, ASCII, GLSTAT
- The energy ratio should for example be close to 1
Vector plot of nodal velocities
- Can reveal location of model problems
Stress plots and Rcforc plot can also reveal contact issues or other unrealistic phenomena

If something seems to be wrong and/or there is an error termination, there are some common errors that can be the cause, listed below.

Wrong Units
Bad contact definitions
Unrealistic Material properties
Load, boundary conditions, initial conditions
Element aspect ratios, angles and warpangle
Too large time step

Search for “time step size should not exceed” in the d3hsp file and note the time step.
Search for “T i m i n g i n f o r m a t i o n”, close above there should be a list with the time steps for every iteration formatted as below. Make sure the time step does not exceed the maximum allowed.

Cycle

time

Size of time step

1965000 t 3.4859E+02 dt 1.55E-04 flush i/o buffers 10/06/13 20:38:48

1967596 t 3.4900E+02 dt 1.55E-04 write d3plot file 10/06/13 20:43:16

1970000 t 3.4938E+02 dt 1.64E-04 flush i/o buffers 10/06/13 20:47:23

The time step can be scaled in the *CONTROL_TIMESTEP card with tssfac, if needed.

Error messages

If the simulations is of bad quality, there might me a premature error termination. In that case it will produce a so called error termination, with some information about the cause for terminate. The error messages can normally be found in the d3hsp file or message file. Open these in a text editor and search for error, read the message and try to understand what they mean.

The simulation might not start at all. In this case you will get an error termination and information about the cause. Generally, it is due to errors in writing the k-file. Some examples:

Empty lines
To long lines
The use of “TAB”
Badly formatted “cards”
Duplicate nodes and elements
Wrong ID referencing, e.g. material, part etc.

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