Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

Oasys LS-DYNA Environment: User Guide (Version 8.1)
Singular Behaviour
Some events that are analysed, such as crash simulations, can exhibit singular behaviour as the
panels buckle and contact each other, this leads to the spread of results that is often observed.
When such an event is analysed using LS-DYNA, on a single CPU, the order in which the
calculations are performed ensures that the program will always predict that each panel will
buckle in the same way, leading to consistent results.
If a complex model is analysed using the parallel version of LS-DYNA the roundoff errors can
influence the local buckling modes of some panels. This leads to a spread of results being
obtained for different numbers of CPU’s. Although all of these results may be valid it can be
very confusing for users who are use to a single set of consistent results.
OBTAINING CONSISTENT RESULTS
To eliminate the roundoff issue for users LS-DYNA has significant work done to force the
operation sequence for multiple CPU's to be the same as for a single CPU. Users are now offered
a choice in the form of an 'accuracy' flag to control whether these options are used as they can
have an overhead of between 5% an 20% depending on the analysis. This option is activated by
setting ACCU to 1 on *CONTROL_PARALLEL, this is the default option in LS-DYNA.
RESTARTING
If a LS-DYNA analysis that was run using the parallel option has to be restarted, from a dump
file, then it is advisable to restart the analysis using the same number of CPU’s that the first
analysis was run with. Under NO circumstances should more CPU’s be used but less may be
used with caution.
The above section is based on the following paper :
Dennis Lam, “Parallel Processing of LS-DYNA”, 3rd International LS-DYNA3D
Conference, 1995.
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