[MBDyn-users] question about eigenanalysis

lengjun lengjun1007 at 163.com
Sat Nov 18 09:21:31 CET 2017


Dear Louis,
Thank you for your advice and I think it is a good way to deal with the problem. I also found that
the problem can be well solved if I change the final_time and time step.
Maybe the key to the problem is choosing suitable initial sets and methods to solve matrix equations.
Best wishes,
-Iris








At 2017-11-17 23:10:29, "Louis Gagnon" <louis.gagnon at polimi.it> wrote:
>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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