Welcome to the forums. Please post in English or French.

You are not logged in.

#1 2017-11-14 16:05:37

From: Ingenieurbüro für Mechanik, DE
Registered: 2009-11-04
Posts: 643

Bug in modal analysis with gyroscopy


there is a bug in modal analysis with gyroscopy when using SEG2-elements and discret point elements.

In very short: the results change if another coord syst is choosen to define the parameters of the discrete elements. But the results should not depend on this.

CODE-ASTER-courses at Ingenieurbüro für Mechanik, Germany

     30 Nov - 01 Dec 2017

     05 Feb - 09 Feb 2018

(1) is the main problem but I point also some small deficiencies in the doc

(1)    -----------------------------------------------------------------------------------

In  testcase sdll124a  the moments of inertia of the 3 mass disks are all defined in a local coordinate system.
When defining them in the global coordinate system, the results are completely wrong, see attachment.

I suppose this is a general problem because I discovered it with another rotor model and arrived to the testcase sdll124a only
because I searched for "my" error

(2)    -----------------------------------------------------------------------------------

in the doc of sdll124a  the moments of inertia are specified from the thicknesses and inner and outer radius of the disks.
With a thickness>0  it follows (from know formula) that  Iaxial>Ipolar/2. The input file   sdll124a  contains moments of
inertia which fulfill  Iaxial=Ipolar/2. This is in contradiction to the doc of the testcase, V2.02.124, where thicknesses>0
are specified

(3)    ----------------   remarks to U2.06.32    --------------------------------------------------

chap 2.1 Modélisation des parties tournantes
Code_Aster doit donc permettre, pour chaque élément de rotor, un traitement dissocié des termes de
masse et de rigidité.

and later: this is done with
deux structures de type cara_elem , pour la rigidité et pour la masse

Question: Can a reference to a testcase be added, or a piece of code shown? I can't see how 2 concepts of caracteristics can be
introduced into ASSEMBLAGE

About the coordinate direction of the rotor axis (of beam elements):

Remarque sur l'orientation de la ligne d'arbres et des éléments discrets :
Quelque que soit le système étudié, l'axe de rotation de la ligne d'arbres est fixé dans Code_Aster : il
s'agit de l'axe z ,

this is not true, because in sdll125c  the y-axis is the axis of rotation. Could it be explained better?

Les éléments discrets doivent être définis dans le repère local, par exemple..

also not true, for example the discret stiffnesses in sdll124a are defined in the global coord system

chap 3.2.4 bilan:
there seems to be some rubbish in AFFE_CARA_ELEM(VALE=(KXX1 etc
and some rubbish here (virgule): VALE=(0.0383,3.E-6,1.8E-6,1.8E, 6,0.,0.,0.,0.,0.,0.), ),)

Can be explained what is the

"""règle empirique des 17°"""

(4)    ----------------   remarks to U2.06.31    --------------------------------------------------

Dans Code_Aster, il est possible, moyennant
quelques précautions dans la mise en donnée, de modéliser un rotor avec n'importe quel axe de

in contradiction to U2.06.32. Or holds this only for 3D models?

chap 2 Example
"""V6.02.126"""  does not point to a rotordynamics case. It should be V2.02.126

(5)    ----------------   doc V2.02.126    --------------------------------------------------

chap 1.1
mass input of disque D1:
Iyy=Izz=0.6463858 is wrong. It must be   0.06463858

chap 3.1, 4.1, 5.1, 6.1  Caractéristiques de la modélisation
13 elements and 14 nodes,  (!! not 130 elm and 131 nodes !!)

testcase_sdll124a_ToForum.zip, Size: 223.88 KiB, Downloads: 2

Ingenieurbüro für Mechanik
D 91717 Wassertrüdingen / Germany

www.code-aster.de                                                Training & Support for NASTRAN and CODE-ASTER


Board footer