Learning Forum

[simse]

Hi everyone,

Does anybody know how nodal difference/nodal fraction can be used to document if a stress in a hotspot has converged? I was wondering if you can say that if the difference between the averaged nodal stress and the nodal difference of the same node is below XX % then the stress for that node has converged?

[Sahil Sura]

Nodal Difference, Nodal Fraction do help in determining the mesh quality. If there is a significant difference in the values of the elements with shared nodes, a mesh refinement will be needed for the same.
This can be the case of checking the convergence manually.
Please find the generic guidelines for accuracy acceptance used-
Low levels of accuracy are acceptable for demonstrations, training, and test runs. Allow for a significant level of uncertainty in interpreting answers. Very low accuracy is never recommended for use in the final validation of any critical design.
Moderate levels of accuracy are acceptable for many noncritical design applications. Moderate levels of accuracy should not be used in a final validation of any critical part.
High levels of accuracy are appropriate for solutions contributing to critical design decisions.
At maximum accuracy, when convergence isnotsought, studies of problems with known answers yield the following behaviors and approximated errors:
Less than 20% error for peak stresses and strains, and minimum margins and factors of safety.
Between 5% and 10% error for average (nominal) stresses and elastic strains, and average heat flows.
Between 1% and 5% error for average stress-related displacements and average calculated temperatures.
5% or less error for mode frequencies for a wide range of parts.
When seeking highly accurate, converged Results, more computer time and resources will be required than manual control, except in some cases where the manual preference approaches highest accuracy.
You can also use the convergence tool in Ansys to avoid manual iterations.
References-
Convergence (ansys.com)
Using Adaptive Convergence (ansys.com)
Thanks Sahil
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[simse]

Thank you very much for the answer. For peak stresses and strains a 20 % error or less is acceptable. Does that mean that I can have a nodal fraction in a hotspot of 20 % and the averaged stress result will be relatively close to an accurate solution? Seems a bit high :)