Hello Juan and Victor and thank you very much for your comments and clarifications. They have been very helpful.
This is so far what I get and my conclusions. Sorry if the post is too large.
1-Ccx orders negative eigenvalues starting from biggest absolute value (confusing). If the eigenvalues extraction is done with the same criteria, the result will miss the lowest negative eigenvalues and large negative Eigenvalues may show on the list.
2-Ccx v2.17 from Mecway, v2.15MT and v2.16 Spools from Prepomax compilation have preserved the sign of all the eigenvalues in this particular model.
It helps to identify the load direction of each mode.
-Ccx v2.17 from Mecway match well when both configurations are compared. (Seems the right one to me). If Nastran standard is applied, I think this could be the golden test for the buckling solver. Both configurations ( positive and negative loads should provide the same results ¿isn’t it?) . We probably need to increase the accuracy to compare properly. It seems that the last modes on the list are less accurate. I have noticed some missing or shifted modes when comparing results.
- ccx v2.15MT and ccx v2.16 Spools provides a new set of modes with low values, but Mode shapes shows noise, are not symmetric and some gives varying results between several runs.
-All later solvers tested in my machine gives large negative values under internal pressure. Some of them do not respond or gives varying results between several runs.
Ccx 2.17.1 SPOOLS - (- 0.6/0.6 Mpa)
Ccx 2.18 Dynamic Website - (- 0.6/0.6 Mpa)
Ccx 2.18 Estatic Website - (- 0.6/0.6 Mpa)
Ccx 2.18 Website - (- 0.6/0.6 Mpa)
Ccx 2.19 Dynamic Website- (- 0.6/0.6 Mpa)
Ccx 2.19 Static Website- (- 0.6/0.6 Mpa)
From my point of view, it would be perfect to sort all the modes according to the Nastran standard but pointing to the user which modes belong to the requested load configuration and which to the opposite loading case (as it may be impossible and without interest to the user).
Victor, would be nice to be able to adjust the accuracy. ¿Is it possible with “modify Keyword”?
My calculation abstract is attached.
I have one question. ¿Is the gravity with inverted sign also considered into the possible set of buckling modes?.
Thanks in advance