In modal analysis, the requested frequencies have been calculated which can be seen in the solution data. but the solver continues on to another “Lanczos cycle” without converging further. How to stop this?
June 6, 2022 at 9:58 amFAQParticipant
This behavior is most usually due to a very ill-conditioned system. Try changing the solver from Block Lanczos (direct) to subspace. Subspace better handles indefinite matrices than the Block Lanczos (BL) method. This can be done in Analysis Settings -> Solver Type -> Subspace. Usually what is happening is that there is a disagreement between the number of modes to be found between what the BL found and what the Sturm sequence check finds. So a new BL cycle is done with a new shift, but then the same disagreement is found but in the opposite direction. the subspace does not perform a Sturm check by default, so it can bypass the issue altogether and ‘fix’ what is causing the ill conditioned model. But at the expensive of a possible less accurate model. There is also an option to turn off the Sturm checking for the BL method via the new LANBOPT command.
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