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  7. [SOLVED] Mesh dependency in Modal Analysis

[SOLVED] Mesh dependency in Modal Analysis

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9 years 2 months ago - 9 years 1 month ago #8438 by men
[SOLVED] Mesh dependency in Modal Analysis was created by men
Hey everyone,

I came across another Issue today. I tried to run a modal analysis and got unrealistic results. I am using Code_Aster version 12.4 and working on a volumetric model. The code uses the command ASSEMBLAGE to build the Stiffness and Mass matrices and the command CALC_MODES to calculate the Eigen-frequencies. I thought that my BC were not well defined in a first place. Then I found out this morning by building a volumetric model of the first example of the document V2.08.011 that I have a dependency of the results on the mesh-size. This sounds reasonable, however the differences were huge, up to double of the reference value. The code as well as my mesh refinement study are attached. I am really wondering if I use the code in an inappropriate manner? Is there anyone who came across similar problems or could help me out with this?

Thanks a lot
Menb
#---------------------------------------------------------------------------------------
# Author: 	menb
# Date: 18.11.15
#
# UNITS: 	MASS	LENTH	TIME	FORCE	STRESS	ENERGY
# 			ton 	mm 		s 		N 		MPa 	N-mm
#---------------------------------------------------------------------------------------



DEBUT(LANG='en',);

#---------------------------------------------------
# MESH
#---------------------------------------------------

mesh=LIRE_MAILLAGE(	FORMAT='MED',									# read in string						
						UNITE=21,
						INFO=1,);
   

#---------------------------------------------------
# MODEL
#---------------------------------------------------

model1=AFFE_MODELE(MAILLAGE=mesh,										# define the model
                   AFFE=_F(TOUT='OUI',
                           PHENOMENE='MECANIQUE',
                           MODELISATION='3D',),);
						

#---------------------------------------------------
# LOAD AND BC
#---------------------------------------------------

BC=AFFE_CHAR_MECA(MODELE=model1,										
                  DDL_IMPO=(_F(GROUP_MA=('fix',),						
                               DX=0.0,
                               DY=0.0,
                               DZ=0.0,),),);
                               
CuBe2A=DEFI_MATERIAU(ELAS=_F(E=210000.0,								
                             NU=0.3,	
                             RHO=7.8e-09,
                             ALPHA=0.01,),);



matfield=AFFE_MATERIAU(MAILLAGE=mesh,
                       AFFE=_F(TOUT='OUI',
                                MATER=CuBe2A,),
	);
                                    
                                    
ASSEMBLAGE(MODELE=model1,
           CHAM_MATER=matfield,
           CHARGE=BC,
           NUME_DDL=CO('NUM'),
           MATR_ASSE=(_F(MATRICE=CO('Stiffn'),
                         OPTION='RIGI_MECA',),
                      _F(MATRICE=CO('Mass'),
                         OPTION='MASS_MECA',),
                      _F(MATRICE=CO('Damping'),
                         OPTION='AMOR_MECA',),),);


#---------------------------------------------------
# MODAL SOLVER 
#---------------------------------------------------

reig0000=CALC_MODES(OPTION='BANDE',
                    SOLVEUR_MODAL=_F(METHODE='SORENSEN',),
                    MATR_RIGI=Stiffn,
                    MATR_MASS=Mass,
                    CALC_FREQ=_F(FREQ=(0.0,30000.0,),),
                    VERI_MODE=_F(STOP_ERREUR='NON',),
                    AMELIORATION='NON',);



IMPR_RESU(FORMAT='MED',
          UNITE=80,
          RESU=_F(RESULTAT=reig0000,),);



FIN();

Mesh Refinment Study
# Vol. Elem1, Mode [Hz]
107500168.4
16100171.6
2871182.3
631205.1
120296.9
Last edit: 9 years 1 month ago by men.
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9 years 2 months ago #8440 by men
Replied by men on topic Re: Mesh dependency in Modal Analysis
Hi there,

I came across the solution to my problem and found that I was using a linear mesh which caused the unprecise results. When the mesh was changed to quadratic elements, the solver CALC_MODES worked just fine. :) I was not aware that a linear mesh would have such a big influence on the results. I hope this may be helpful for others.

Cheers
Menb
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9 years 1 month ago #8441 by kwou
Replied by kwou on topic Re: Mesh dependency in Modal Analysis
hoi menb

the solver probably also works just fine with linear meshes; but the linear mesh behaves stiffer indeed.
You can have a look here:
caelinux.org/wiki/index.php/Contrib:KeesWouters/staticshell
Interest: structural mechanics, solar energy (picture at 'my location' shows too little pv panels)

--
kind regards - kees
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9 years 1 month ago #8446 by men
Replied by men on topic Re: Mesh dependency in Modal Analysis
Hey keeswouters,

Thanks for that link, I think that the grid-refinment study for the different mesh types is quite useful to get a feeling about the accuracy a certain mesh sould achive. It describes exactly the problem I faced..
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