How to export mass and stiffness matrices of a constrained model?

I'd like to export mass and stiffness matrices of a constrained model (e.g. MPC applied) and retain information on the ASET, i.e. the independent degrees of freedom. Is that possible somehow?

Up to now I've used to obtain the relevant matrices only under free-free conditon by using PARAM,EXTOUT,DMIGPCH

Best regards,

Johannes

0 Vivek Bhardwaj over 5 years ago

You can use the same method, MSC Nastran takes care of SPC grids and MPC's. Only thing you need to care is to define ASET's not on dependent grids and SPC grids.

Other option you can use is : EXTSEOUT (STIFFNESS MASS ASMBULK EXTID = 10 DMIGPCH)
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0 Don.Graff over 5 years ago

You can also do this using an Alter (if you have DMAP in your licensing)… the alter below will print and punch the MAA and KAA matrices (if you do not want it to print, just remove the MATGPR lines). Insert this alter just before the CEND line of the Nastran input file:

MALTER 'MALTER.*KAA.*MAA.*' $

MATGPR GPLS,USET0,SILS,MAA//'A' $

MATGPR GPLS,USET0,SILS,KAA//'A' $

MATMOD KAA,EQEXINS,USET,,,/,/16/1/1/s,n,dum1/////////'KAAEXT'/'A' $

MATMOD MAA,EQEXINS,USET,,,/,/16/1/1/s,n,dum2/////////'MAAEXT'/'A' $

Test it out with the attached sample.

Attached Files (1)

punch-dmig_kaa-maa.bdf.zip
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0 JohLut over 5 years ago

Thanks for your quick replies, Vivek and Don!

However, I'm still wondering how boundary conditions are considered in the MAA and KAA matrices...

By the way, what exactly is the difference between "PARAM,EXTOUT,DMIGPCH" and "EXTSEOUT (STIFFNESS MASS ASMBULK EXTID = 10 DMIGPCH)".

Why are there these two commands existing?
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0 Don.Graff over 5 years ago

The ASET is what is left over after SPC (constraints), AUTOSPC, MPC dependencies (rbe1, etc), and any explicit ASET (or OSET) entries are taken care of.

The PARAM method was an earlier means of getting the reduced matrices out of Nastran in a DMIG format. The EXTSEOUT command is a newer means of reducing and exporting a Nastran External Superelement… this has many additional option over the older PARAM method.
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0 JohLut over 5 years ago

"The ASET is what is left over after SPC (constraints), AUTOSPC, MPC dependencies (rbe1, etc), and any explicit ASET (or OSET) entries are taken care of."

That's what I was assuming, too.

I asked about the boundary conditions because the matrices' dimensions are somewhat inconsistent.

Although the dimensions of my matrices are reduced properly when I apply only MPC, this is not the case when I consider static displacement BC. More specifically, the resulting matrix dimensions in the latter case are equal to those I get under free-free conditions.
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0 Don.Graff over 5 years ago

How are you defining the displacement boundary condition? If using SPC/SPC1 entries, they are turned on via an SPC case control command. Maybe that is missing?
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0 JohLut over 5 years ago

Oh no... You're right. That was too obvious.

Thank you very much for your patient support!
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