[MBDyn-users] question about eigenanalysis

Louis Gagnon louis.gagnon at polimi.it
Fri Nov 17 16:10:29 CET 2017


Dear Iris,
I am not familiar with eigenanalysis, but this is indeed a peculiar issue.
I however noticed that if you use a position with z not equal to 1, the 
issue is resolved.
Try
 > reference: arm,
 >   reference, global, 0., 0., 1.01,
Alternatively, you can change the way lapack handles the problem and 
change to the following line:
use lapack,balance,permute;
which also solves your issue
Best Regards,
-Louis


On 14-Nov-2017 08:12 AM, lengjun wrote:
> Dear all,
> I have a problem about the eigenanalysis of a cantilever. I set a 
> reference called arm:
> reference: arm,
>   reference, global, 0., 0., 1.,
>   reference, global, eye,
>   reference, global, null,
>   reference, global, 0., 0., omega;
> and all nodes and elements are in the 'arm' reference.
> the result of frequency are incorrect:
> ##############################################################################
> # Key for lines starting with "Step":
> # Step Time TStep NIter ResErr SolErr SolConv Out
> Step 0 0 0 0 0 0 0 1
> Step 1 0.001 0.001 0 0 0 0 1
> # Derivatives solution step at time 0 performed in 0 iterations with 0 error
> Info: 0, success BALANC="B" ILO=7 IHI=226 ABNRM=4.62438e+16 
> BBNRM=4.62438e+16
> Mode n.       Real           Imag             Damp %     Freq Hz
> # Key for lines starting with "Step":
> # Step Time TStep NIter ResErr SolErr SolConv Out
> Step 0 0 0 0 0 0 0 1
> Step 1 0.001 0.001 0 0 0 0 1
> ##############################################################################
> However ,when I changed the reference 'arm' into:
> reference: arm,
>   reference, global, 0., 0., 0.,
>   reference, global, eye,
>   reference, global, null,
>   reference, global, 0., 0., omega;
> 
> it goes well. the results are:
> # Derivatives solution step at time 0 performed in 0 iterations with 0 error
> Info: 0, success BALANC="B" ILO=7 IHI=226 ABNRM=1.24038e+10 
> BBNRM=1.24038e+10
> Mode n.       Real           Imag             Damp %     Freq Hz
>         7: -3.07437e-08 +       38.281 j     8.03108e-08          6.0926
>         9:  1.11572e-09 +       38.281 j    -2.91457e-09          6.0926
>        11:   1.4287e-08 +      234.684 j    -6.08777e-09         37.3511
>        13:  1.41642e-08 +      234.684 j    -6.03545e-09         37.3511
>        15:  7.43393e-08 +      635.257 j    -1.17022e-08         101.104
>        17:  1.41226e-07 +      635.257 j    -2.22314e-08         101.104
>        19:  1.84635e-09 +      697.431 j    -2.64735e-10             111
>        21:  1.25765e-07 +       1189.2 j    -1.05756e-08         189.267
>        23: -1.27215e-07 +       1189.2 j     1.06975e-08         189.267
>        25:  1.05445e-07 +      1863.02 j     -5.6599e-09         296.508
>        27: -1.74265e-09 +      1863.02 j     9.35393e-11         296.508
>        29:  6.88544e-08 +      2622.68 j    -2.62534e-09         417.413
>        31: -4.36833e-08 +      2089.71 j      2.0904e-09         332.588
>        33: -7.66471e-08 +      3438.49 j     2.22909e-09         547.253
>        35:  9.22549e-08 +      2622.68 j    -3.51757e-09         417.413
>        37: -1.77564e-08 +      4283.13 j     4.14566e-10         681.682
>        39: -6.07606e-08 +      3438.49 j     1.76707e-09         547.253
>        41:  7.44935e-09 +      3474.15 j    -2.14422e-10         552.928
>        43:  -6.6307e-08 +      4283.13 j      1.5481e-09         681.682
>        45: -3.87825e-08 +      4845.22 j     8.00427e-10         771.141
>        47:  1.29202e-07 +      5124.28 j    -2.52136e-09         815.554
>        49:  1.03002e-06 +      5881.29 j    -1.75136e-08         936.036
>        51:  4.21351e-07 +      5124.28 j    -8.22264e-09         815.554
>        53:  2.84468e-07 +      5881.29 j    -4.83682e-09         936.036
>        55:  2.56009e-07 +      6196.96 j    -4.13121e-09         986.277
> I am wodering if the results are related to the position of the 
> reference. The attachments are the input files.
> Best wishes.
>   Iris
> 
> 
> 
> 
> 
> 
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-- 
Louis Gagnon, Ph.D.
Postdoctoral fellow
Dipartimento di Scienze e Tecnologie Aerospaziali
Politecnico di Milano
+39 02 2399 8642
http://louisgagnon.com/research/


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