Problem when performing modal analysis in PyMAPDL

[arun-maniam]

I am trying to convert my APDL code into PyMAPDL code (Modal Analysis). My APDL code for the modal analysis is as follows:
! ***************** Modal analysis ***************************
/SOLU
ANTYPE,MODAL
MODOPT,lanb,10,1,f1
SOLVE
! **************************************************************

And the corresponding PyMAPDL code is as follows:
############Modal Analysis ######################
mapdl.allsel()
mapdl.slashsolu()
mapdl.antype('MODAL')
mapdl.modopt(method='lanb', nmode=10, freqb=1, freqe=1000)
mapdl.solve()

I am getting the following error:
CRITICAL - pymapdl_global - logging - handle_exception - Uncaught exception
Traceback (most recent call last):
File ".......", line 321, in
mapdl.solve()
File "C:\Users....\AppData\Local\Programs\Python\Python311\Lib\site-packages\ansys\mapdl\core_commands\solution\analysis_options.py", line 3450, in solve
return self.run(command, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users....\AppData\Local\Programs\Python\Python311\Lib\site-packages\ansys\mapdl\core\mapdl.py", line 2990, in run
text = self._run(command, verbose=verbose, mute=mute)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users....\AppData\Local\Programs\Python\Python311\Lib\site-packages\ansys\mapdl\core\mapdl_grpc.py", line 906, in _run
response = self._send_command(cmd, mute=mute)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users....\AppData\Local\Programs\Python\Python311\Lib\site-packages\ansys\mapdl\core\errors.py", line 161, in wrapper
raise MapdlExitedError("MAPDL server connection terminated") from None
ansys.mapdl.core.errors.MapdlExitedError: MAPDL server connection terminated

Could anyone guide me on how to solve this issue? The problem is actually with the mapdl.solve() command. Thanks in advance!

[mikerife]

Hi @arun-maniam
It helps to know the PyMAPDL and MAPDL version being used. And enough of the script to run and troubleshoot. So please use this as an exmaple; run it and let us know what happens:

import os
from ansys.mapdl.core import launch_mapdl
path = os.getcwd()
mapdl = launch_mapdl(run_location = path, additional_switches = '-smp')
print(mapdl)
mapdl.prep7()

mapdl.et(1,"BEAM188")
mapdl.keyopt(1,3,3)
mapdl.mp("EX",1,200E9)
mapdl.mp("NUXY",1,0.3)
mapdl.mp("DENS",1,8000)
mapdl.sectype(1,"BEAM","RECT")
mapdl.secdata(2,2)
for i in range(0,11):
    mapdl.n(i+1,i)
for i in range(1,11):
    mapdl.e(i,i+1)

mapdl.finish()

mapdl.slashsolu()
mapdl.antype("MODAL")
mapdl.mxpand(10,'','',"YES")
mapdl.modopt(method='lanb', nmode=10, freqb=1, freqe=1000)
mapdl.d(1,'UX',0,0,1,1,'UY','UZ','ROTX') 
mapdl.d(11,'UY',0,0,11,1,'UZ')

mapdl.solve()

mapdl.exit()

mike

[arun-maniam]

Hi @mikerife,
thanks for the reply. I tried running your code and it works fine. I get this output in the terminal (btw I'm using V.Studio):

Product: Ansys Mechanical Enterprise Academic Research
MAPDL Version: 23.1
ansys.mapdl Version: 0.65.2

[arun-maniam]

I have added the first part of your code into mine ( from import os ... until mapdl.prep7()).
Now it somehow worked but I'm still getting some errors.

` S O L U T I O N O P T I O N S

PROBLEM DIMENSIONALITY. . . . . . . . . . . . .3-D
DEGREES OF FREEDOM. . . . . . UX UY UZ ROTX ROTY ROTZ
ANALYSIS TYPE . . . . . . . . . . . . . . . . .MODAL
EXTRACTION METHOD. . . . . . . . . . . . . .BLOCK LANCZOS
NUMBER OF MODES TO EXTRACT. . . . . . . . . . . 10
MODAL EXTRACTION RANGE. . . . . . . . . . . . . 1.0000 TO 200.00
GLOBALLY ASSEMBLED MATRIX . . . . . . . . . . .SYMMETRIC
NUMBER OF MODES TO EXPAND . . . . . . . . . . . 10
ELEMENT RESULTS CALCULATION . . . . . . . . . .ON

                  L O A D   S T E P   O P T I O N S

LOAD STEP NUMBER. . . . . . . . . . . . . . . . 1
THERMAL STRAINS INCLUDED IN THE LOAD VECTOR . . YES
PRINT OUTPUT CONTROLS . . . . . . . . . . . . .NO PRINTOUT
DATABASE OUTPUT CONTROLS. . . . . . . . . . . .ALL DATA WRITTEN

                     ***********  PRECISE MASS SUMMARY  ***********

TOTAL RIGID BODY MASS MATRIX ABOUT ORIGIN
Translational mass | Coupled translational/rotational mass
0.63898E-27 0.0000 0.0000 | 0.0000 0.19722E-30 -0.63898E-28
0.0000 0.63898E-27 0.0000 | -0.19722E-30 0.0000 0.15974E-27
0.0000 0.0000 0.63898E-27 | 0.63898E-28 -0.15974E-27 0.0000
------------------------------------------ | ------------------------------------------
| Rotational mass (inertia)
| 0.85190E-29 -0.15974E-28 -0.49304E-31
| -0.15974E-28 0.52885E-28 -0.19722E-31
| -0.49304E-31 -0.19722E-31 0.61400E-28

TOTAL MASS = 0.63898E-27
The mass principal axes coincide with the global Cartesian axes

CENTER OF MASS (X,Y,Z)= 0.25000 0.10000 0.30864E-03

TOTAL INERTIA ABOUT CENTER OF MASS
0.21292E-29 0.13393E-42 -0.23973E-44
0.13393E-42 0.12949E-28 -0.93782E-45
-0.23973E-44 -0.93782E-45 0.15074E-28
The inertia principal axes coincide with the global Cartesian axes

*** MASS SUMMARY BY ELEMENT TYPE ***

TYPE MASS
1 0.619256E-27

Range of element maximum matrix coefficients in global coordinates
Maximum = 3.384531323E-19 at element 82.
Minimum = 7.888609052E-31 at element 1050.

*** WARNING *** CP = 6.062 TIME= 08:47:35
Coefficient ratio exceeds 1.0e8 - Check results.

*** ELEMENT MATRIX FORMULATION TIMES
TYPE NUMBER ENAME TOTAL CP AVE CP

    1      1000  SHELL281      0.312   0.000313
    2        25  COMBIN14      0.000   0.000000
    3        25  MASS21        0.000   0.000000

Time at end of element matrix formulation CP = 6.0625.

BLOCK LANCZOS CALCULATION OF UP TO 10 EIGENVECTORS.
NUMBER OF EQUATIONS = 18675
MAXIMUM WAVEFRONT = 49
MAXIMUM MODES STORED = 10
MINIMUM EIGENVALUE = 0.10000E+01
MAXIMUM EIGENVALUE = 0.20000E+03

Memory allocated for solver = 38.834 MB
Memory required for in-core solution = 37.065 MB
Memory required for out-of-core solution = 16.949 MB

*** NOTE *** CP = 6.609 TIME= 08:47:36
The Sparse Matrix Solver used by the Block Lanczos eigensolver is
currently running in the in-core memory mode. This memory mode uses
the most amount of memory in order to avoid using the hard drive as
much as possible, which most often results in the fastest solution
time. This mode is recommended if enough physical memory is present
to accommodate all of the solver data.

Total memory available for LANCZOS = 13.325508 GB

Process memory required for in-core LANCZOS Workspace = 72.292397 MB
Process memory required for out-of-core LANCZOS Workspace = 2.192757 MB
Lanczos Memory Mode : INCORE

*** WARNING *** CP = 7.172 TIME= 08:47:36
Fewer modes than the requested number of modes ( 10 ) were computed.
The range specified (FREQB= 1, FREQE= 200) contains only 0 modes.

*** NOTE *** CP = 7.172 TIME= 08:47:36
There are no modes in the specified range.

*** NOTE *** CP = 7.203 TIME= 08:47:36
There are no modes in the specified range.

*** MAPDL BINARY FILE STATISTICS
BUFFER SIZE USED= 16384
0.125 MB WRITTEN ON ELEMENT MATRIX FILE: file.emat
1.000 MB WRITTEN ON ELEMENT SAVED DATA FILE: file.esav
7.625 MB WRITTEN ON ASSEMBLED MATRIX FILE: file.full
0.062 MB WRITTEN ON MODAL MATRIX FILE: file.mode
0.750 MB WRITTEN ON RESULTS FILE: file.rst`

What could be the reason that there are no modes in the specified range, even though there actually exist a few modes?

[mikerife]

Hi @arun-maniam
The numbers for the model are very, very small. A total mass of ~1E-27 !!! The largest stiffness matrix term is ~1E-13 !!! Can you rescale the geometry and material properties so that the values are more reasonable? Basically chose a different unit system so that the stiffness and mass values are closer to unit values.

Also see the LANBOPTION command. You could activate the Sturm check to see how many modes are in the range. And then try the alternative LANB solver as well.