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rigid body modes in OP2

Experimenter
Experimenter

I am running a normal modes analysis of an unconstrained model.  The modal has 800K DOF.  The f06 file lists 361 roots found with 6 rigid body modes (frequencies << 1) but the op2 and pch files contain only 359 modes with 4 RBMs.  Here is an excerpt from the f06 file:

 

                          E I G E N V A L U E  A N A L Y S I S   S U M M A R Y   (READ MODULE)


                                     BLOCK SIZE USED ......................    7

                                     NUMBER OF DECOMPOSITIONS .............    5

                                     NUMBER OF ROOTS FOUND ................  361

                                     NUMBER OF SOLVES REQUIRED ............  108

1    NORMAL MODES                                                           JANUARY   2, 2017  NX NASTRAN  5/ 5/15   PAGE    26
                                                                                                                                    
0                                                                                                            SUBCASE 1              
 
                                              R E A L   E I G E N V A L U E S
   MODE    EXTRACTION      EIGENVALUE            RADIANS             CYCLES            GENERALIZED         GENERALIZED
    NO.       ORDER                                                                       MASS              STIFFNESS
        1         1       -9.803721E-08        3.131089E-04        4.983284E-05        1.000000E+00       -9.803721E-08
        2         2       -3.928240E-08        1.981979E-04        3.154417E-05        1.000000E+00       -3.928240E-08
        3         3        9.201642E-09        9.592519E-05        1.526697E-05        1.000000E+00        9.201642E-09
        4         4        1.158430E-07        3.403572E-04        5.416953E-05        1.000000E+00        1.158430E-07
        5         5        1.661520E-07        4.076175E-04        6.487434E-05        1.000000E+00        1.661520E-07
        6         6        2.147217E-07        4.633807E-04        7.374933E-05        1.000000E+00        2.147217E-07
        7         7        4.318702E+03        6.571684E+01        1.045916E+01        1.000000E+00        4.318702E+03
        8         8        2.199661E+04        1.483125E+02        2.360467E+01        1.000000E+00        2.199661E+04
        9         9        3.198790E+04        1.788516E+02        2.846512E+01        1.000000E+00        3.198790E+04
       10        10        3.519191E+04        1.875951E+02        2.985668E+01        1.000000E+00        3.519191E+04
       11        11        5.078170E+04        2.253480E+02        3.586524E+01        1.000000E+00        5.078170E+04

What could cause this anomoly?

 

3 REPLIES

Re: rigid body modes in OP2

Phenom
Phenom

I am counting 6 freq. below 10.45Hz. (value of order 1e-5)

Production: NX9.0.3.4, NX10.0.2.6
Development: VB.NET (amateur level !)

Re: rigid body modes in OP2

Experimenter
Experimenter

Yes, the f06 file shows 6 rigid body modes.  But the op2 and pch files contain only 4 RBMs.  When I read the op2 file into FEMAP, the first mode listed in the results has a mode number of 3 and has the same frequency as the third mode listed in the f06 file.  So the first 2 modes are omitted for some reason. The same thing occurs when I read the pch file into Matlab.  Only 4 RBMs are read and the first 2 listed in the f06 are not there.

Re: rigid body modes in OP2

Experimenter
Experimenter

I figured out the problem.  The model file I was running was created from a SOL 111 modal frequency response file which I manually editted to run a SOL 103 to compute normal modes.  The original file had multiple subcases for applied loads which I neglected to delete.  When I removed all the SUBCASE case control commands the modal solution yielded 6 RBMs in the op2 and pch files as it should.  Interestingly, when I left a single subcase in the model it also correctly computed 6 RBMs.  But when I included 2 subcases, only 5 RBMs were computed.  And including 3 subcases resulted in 4 RBMs.  So the moral of this story is be careful when you edit a model file to perform a different analysis.