problem with the frequency of ansys calculated and scipy calculated are different

Hello, everyone,

I have a problem with the frequency calculated. when I use ansys calculated I can get [2871,6168,8689,11468] When I try to get the mass and stiffness matrix from a .full file with the scipy.sparse.linalg.eigsh module to calculate, Python give me [15.811 15.812 15.813 15.814]. I think they should be the same.

Here is the code I use for testing:

import pyansys
import os
import scipy
from scipy.sparse import linalg
import numpy as np

fullfile = os.path.join('G:\STIFF', 'CURVE.full')
full = pyansys.read_binary(fullfile)
dof_ref, k, m = full.load_km()   # returns upper triangle only

k += scipy.sparse.triu(k, 1).T
m += scipy.sparse.triu(m, 1).T

k += scipy.sparse.diags(np.random.random(k.shape[0])/1E20, shape=k.shape)

w, v = linalg.eigsh(k, k=20, M=m, sigma=10000)

f = np.real(w)**0.5/(2*np.pi)
print('First four natural frequencies')
for i in range(4):
    print('{:.3f} Hz'.format(f[i]))

[FredAns]

Hi, For what I see, this mathematical problem seems to be a bit hard to solve, and I see the Mapdl Lanczos algorithm making a lot of iterations to converge. My feeling is that scipy func gives wrong results, because of the numerical difficulties.

I’ve checked using an alternative Modal Algorithm for Symmetric Matrices ( MODOPT,SUBSPACE,4,1), an I get very close solutions to Lanczos, but SUBSPACE converge faster because it’s more robust in this numerical situation :

FREQUENCY RANGE REQUESTED= 4 MODES ABOVE 1.00000 HERTZ

1     8.528801109020
2     12.49936641695
3     14.83630288045
4     17.04379504882

[akaszynski]

I’m running into the same issue as you in regards to the python solve. However, when solving this within MAPDL using MAPDL Math, I’m ending up with a different set of eigenvalues than you:

! read in matrices
*SMAT, stiff, D, import, full, file.full, STIFF
*SMAT, mass, D, import, full, file.full, mass

! solve
/SOLU
ANTYPE,MODAL
MODOPT,LANB,4,1
*VEC, MYVEC, D, ALLOC, 0
*eig,STIFF,MASS,,MYVEC 

Which returns:

  FREQUENCIES AT CURRENT LANCZOS CYCLE
     1  0.85280110E+01     2  0.12485649E+02     3  0.14800782E+02
     4  0.17033644E+02     5  0.21613667E+02     6  0.22647486E+02
     7  0.26119110E+02     8  0.29797630E+02     9  0.30706920E+02
    10  0.31378644E+02    11  0.33062955E+02    12  0.34077783E+02
    13  0.36827381E+02    14  0.42039170E+02

     number of steps      :      2
     eigenvalues found    :     14
     total no. eigenvalues:     14

  LANCZOS CYCLE NUMBER =   2

     new shift:  1.4948D+04     modes still needed:        0
  Allocate a [4] Vector : MYVEC

 *** ANSYS - ENGINEERING ANALYSIS SYSTEM  RELEASE Release 18.2.2   18.2.2   ***
 ANSYS Mechanical Enterprise                       
 88888888  VERSION=LINUX x64     12:22:41  OCT 15, 2020 CP=      0.752

                                                                               



 *** FREQUENCIES FROM BLOCK LANCZOS ITERATION ***

  MODE    FREQUENCY (HERTZ)      


   FREQUENCY RANGE REQUESTED=    4 MODES ABOVE   1.00000     HERTZ

    1     8.528010965666    
    2     12.48564886186    
    3     14.80078189537    
    4     17.03364386520    

Please note that I’m running this on ANSYS v18.2, and there might be a fundamental difference in full files between v17 and v18.2. Can you try running the above on v17?

Pinging @FredAns. Is there something we’re potentially missing here?