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## #1 2021-10-24 13:41:37

filjan
Member
From: Poland, Cracow
Registered: 2016-07-11
Posts: 85

### Unexpected influence of boundary conditions on buckling modes

Hi the Code_Aster Community!

I obtained unexpected results from simple linear buckling analysis - I've noticed 2 points for discussion:

1. Depending on imposing rotation DOF on boundary edges I got a lot of artificial, unexpected buckling first multipliers (and modes). At last 8th buckling modes with restricted rotations corresponds to the first with free rotations (see attached pictures and snippets from .mess file below)! How to explain this first 7 unreal buckling modes?

2. Whole model is symmetrical to the XY plane, and the first real buckling mode should also be symmetric, but on picture you can find the bigger positive DZ displacement than in negative direction. What can be source of this discrepancy?

In the attachment you can find pictures, .comm file and  .rmed mesh and .mess file

A)

First minimum buckling multipliers with free rotations (with COQUE_NCOU=5)
------------------------------------------------------------------------
Calcul modal : Méthode d'itération simultanée
Méthode de Sorensen

numéro    charge critique    norme d'erreur

1       6.07851E+00        3.42566E-10

2       7.54621E+00        2.42043E-10

3       7.65019E+00        1.62089E-10

4       1.00785E+01        1.03446E-10

5       1.04750E+01        1.15403E-10

6       1.13024E+01        6.11925E-11

7       1.25364E+01        8.58257E-11

8       1.36543E+01        5.32246E-11

9       1.42141E+01        4.96179E-11

10       1.47606E+01        3.89146E-11

Norme d'erreur moyenne   :  1.25432E-10

------------------------------------------------------------------------

B)
First minimum buckling multipliers with imposed zero rotations on boundary edges (with COQUE_NCOU=5)

------------------------------------------------------------------------
Calcul modal : Méthode d'itération simultanée
Méthode de Sorensen

numéro    charge critique    norme d'erreur

1      -6.09552E+00        2.48475E-13

2      -5.81936E+00        3.22682E-14

3      -4.53943E+00        2.11344E-14

4      -3.49609E+00        2.73799E-14

5       3.48710E+00        2.97868E-14

6       4.53262E+00        2.53528E-14

7       5.81460E+00        4.86050E-14

8       6.03323E+00        1.60771E-11

9       6.07375E+00        4.76493E-13

10       6.82361E+00        1.92509E-13

Norme d'erreur moyenne   :  1.71791E-12

------------------------------------------------------------------------

Best regards
filjan

Last edited by filjan (2021-10-24 13:43:23)

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## #2 2021-10-24 16:10:35

jeanpierreaubry
Guru
From: nantes (france)
Registered: 2009-03-12
Posts: 3,776

### Re: Unexpected influence of boundary conditions on buckling modes

hello

in the vicinity of group "force" the boundary conditions do not represent a symmetry in any one of the two conditions

jean pierre aubry

freely available here https://framabook.org/beginning-with-code_aster/

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## #3 2021-10-24 17:22:49

filjan
Member
From: Poland, Cracow
Registered: 2016-07-11
Posts: 85

### Re: Unexpected influence of boundary conditions on buckling modes

jeanpierreaubry wrote:

hello

in the vicinity of group "force" the boundary conditions do not represent a symmetry in any one of the two conditions

jean pierre aubry

Of course - symmetry boundaries wasn't purpose here - I mean discrepancy in results in symmetrical areas mesh (see picture).
Boundary conditions shouldn't introduce any eccentricity in buckling modes, but mode occured uneven.

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## #4 2021-10-25 07:55:16

jeanpierreaubry
Guru
From: nantes (france)
Registered: 2009-03-12
Posts: 3,776

### Re: Unexpected influence of boundary conditions on buckling modes

please post her a med 4.0 file so i can run the problem on 15.2 version of code_aster

freely available here https://framabook.org/beginning-with-code_aster/

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## #5 2021-10-25 15:55:49

filjan
Member
From: Poland, Cracow
Registered: 2016-07-11
Posts: 85

### Re: Unexpected influence of boundary conditions on buckling modes

jeanpierreaubry wrote:

please post her a med 4.0 file so i can run the problem on 15.2 version of code_aster

Here you are. Attached mesh was converted into MED 4.0.0 from 4.1.0 in salome_meca, because I did in gmsh and used C_A 15.4 then.

Best regards
Filjan

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## #6 2021-10-25 15:59:04

filjan
Member
From: Poland, Cracow
Registered: 2016-07-11
Posts: 85

### Re: Unexpected influence of boundary conditions on buckling modes

and unrolled .geo from gmsh in case

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## #7 2021-10-26 13:56:59

jeanpierreaubry
Guru
From: nantes (france)
Registered: 2009-03-12
Posts: 3,776

### Re: Unexpected influence of boundary conditions on buckling modes

hello

with this

qu9_mesh = DEFI_GROUP(
reuse = qu9_mesh,
MAILLAGE = qu9_mesh,
CREA_GROUP_NO = (
_F(TOUT_GROUP_MA = 'OUI',),
_F(
NOM='symtop',
NOEUD=(
'N13','N427','N428','N429','N14','N430','N431','N432','N15',
'N29','N804','N803','N802','N20','N869','N868','N867','N30',
),
),
),
);

restr[0]  = AFFE_CHAR_MECA(
MODELE = model,
DDL_IMPO = (
_F(
GROUP_NO = 'restraint',
DX = 0.0, DRX = 0.0,
DY = 0.0, DRY = 0.0,
DZ = 0.0, DRZ = 0.0,
),
_F(
GROUP_NO = 'force',
DX = 0.0, #DRX = 0.0,
DRY = 0.0,DRZ = 0.0,
#DZ = 0.0, #DRZ = 0.0,
),
_F(
GROUP_NO = 'symtop',
DX = 0.0, #DRX = 0.0,
DRY = 0.0,DRZ = 0.0,
#DZ = 0.0, #DRZ = 0.0,
),
),
);

these boundary on 'force' and 'symptop' depict a true symmetry about yOz plane
i get good result for the quadratic mesh on results_0

by the way i think a linear mesh with DKT is good enough for your problem

jean pierre aubry