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How to use a superelement with gravity loading?

Stumpy

I have a part delivered from a supplier as a dynamic superelement.  When I run a solution 101 and perform a gravity checkout on the assembled system model, the mass is reported from the J set and is missing exactly the mass of the superelement.  The OLOAD prints out the applied load confirming the missing mass is not present.  The QRG for the EQUILIBRIUM output request contains this statement: "The EQUILIBRIUM Case Control command is applicable to Linear Static analysis (SOL101)
only, and does not produce output if any superelements are present."  This leads me to believe MSC is aware of something affecting the force balance with superelements.

I am able to read the superelement mass matrix and can construct force and moment cards to apply gravity "manually".  How does Nastran want this to be done?  I imagine items like an aircraft wing with an engine can be modeled with differential stiffness due to gravity with a reduced engine model.  Or am I wrong in that assumption?  In other words, if I run a normal modes on a wing with a reduced engine using STATSUB, will I get the engine weight deflecting the wing?

Basically the question is how to apply gravity loading to a super element for a system level checkout?

  • 0

    The mass matrix is not generated in static analysis. As a consequence, inertia loads (GRAV and RFORCE) are not applied to external superelements during the
    assembly run. 

    In static analysis, inertia Loads (GRAV, RFORCE) are calculated as load vectors for each superelement and brought down to the residual (the mass matrix is not reduced in static
    analysis).

  • 0 in reply to Vivek Bhardwaj

    Thank you for the explanation.  Does this mean I need to include the inertia load when reducing the model within the PA matrix?  If that's true, does it then become impossible to apply inertia loading if the vendor doesn't include inertia in the reduction?  Alternatively, is it be possible to load the superelement mass matrix and use M2GG to add it back in?

  • 0 in reply to Stumpy

    In the original method for superelements in the assembly run if there was a reduced load, mass matrix and an inertial
    load then the inertial load could end up being applied twice.
    The method was changed so that loads would not be generated from the reduced mass matrix and only the reduced loads applied.

    Therefore if you have the inertial load in the reduction run then these will be applied in the first subcase of the assembly run.
    If you have multiple subcases then the same subcase structure should be used in the reduction run to the assembly run to get the loads in to the correct subcases.

    If you want to apply inertial loads through the reduced mass matrix(and not through the inertial loads in reduction). This would need some DMAP in the assembly run.

  • +1 in reply to Vivek Bhardwaj

    You can try this dmap in assembly job, I tested it till 2014 not sure if it works in latest versions:

    assign inputt2='tip4-a.op2', unit=30
    SOL 101
    compile extin $
    alter 'EQUIVX.*EXTDB.*MXX' $
    type db zuzr02 $
    equivx extdb/zuzr02/-1 $
    compile phase1dr $
    alter 'call extin'(4)
    type db zuzr02 $
    CALL SELG SLT ,BGPDTS ,CSTMS ,SILS ,EST ,
    MPTS ,ETT ,EDTS ,ZUZR02 ,CASEW ,
    DIT ,MATPOOL ,EQEXINS ,CASES ,DEQATN ,
    DEQIND ,USET0 ,MAA ,GPSNTS ,SCSTM ,
    MEDGE ,DRG ,GEOM4S ,ESTL ,DYNAMICS,
    MGG ,KGG ,HARMELPT,ROT3ELPT/
    PJ ,PTELEM ,NEWSLT ,PJGRV ,PMMOFL /
    STATICS /SEID /LUSETS /(1.,0.) /(1.,0.) /
    GRDPNT /ERROR /PARDCMP /IRES /PRPJ /
    NLHEAT /IMAGE /NPL /NASOUT /0 /
    PEXIST /SETYPE /RSONLY /S400LIN /S400NLS /
    S400KEP $
    add5 pj,pa,,,/xxx $
    equivx xxx/pa/-1 $

    $ ***********************************************************************
    CEND
    $ === CASE CONTROL SECTION ====

  • 0 in reply to Vivek Bhardwaj

    Again thank you for the detailed explanation, this is very helpful.  Forgive my DMAP ignorance, I was unable to find the documentation for subDMAP SELG.  But I found in the 2024 reference guide page 440 that SELG is SSG1.  When I looked up SSG1 in the 2024 DMAP guide I see a different number of inputs, so I don't believe SELG is SSG1, or maybe it changed somewhere.  Can you provide any guidance as to the documentation of your alter?  I am using versions 2017 and 2024.  When I used it as is, I didn't get additional inertia loading.  In the 2017 run, I am using SOL 101 with one subcase and GRAV of 9.81.  The residual mass is 461 kg and applied load is 4522 N.  The SE mass is 22 kg and should add another 215N.  When I compare the OLOAD and SPCFORCE for runs with and without the DMAP I get the same results.

  • 0 in reply to Stumpy

    You need not require to learn the DMAP

    Ensure that you do not have grav loading in the reduction run. And then in the assembly job insert the highlighted portion in the executive section(after SOL101 and before CEND) and see if it does not print any fatal error, further you can also compare the results, I think it should work in v2017. Try with simple model first

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