Symmetric boundary conditions

Vikas_22008
Vikas_22008 Altair Community Member
edited June 4 in Community Q&A

Hi,

I have an assembly model which is completely symmetric with respect to one of the planes even the loads are symmetric. The model is not in circular shape it is rectangular in shape.  I would like to use only half of the model for the simulation to reduce the computation time.  Is there a way to implement symmetric boundary conditions in the hypermesh?

Thanks 
Vikas 

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Answers

  • Adriano A. Koga
    Adriano A. Koga
    Altair Employee
    edited May 31

    you need to create the adequate boundary conditions in order to setup the symmetry conditions.

    Use the constraints to fix the DOFs accordingly.

    An example from a google search: https://www.clear.rice.edu/mech403/HelpFiles/CW_sym_anti-symmetry_BC.pdf

  • Vikas_22008
    Vikas_22008 Altair Community Member
    edited June 2

    Hi Adriano,

    thank you very much for the suggestion. 
    I looked into the pdf and implemented the symmetric BCs for a simple rectangular slab. I am uploading the results here as a screen shot. 

    I know that the results in itself make no sense because of the high stresses but the objective was to get the similar results using symmetric BCs and I achieved it, thanks to you :-)

    a) Full model with the required BCs as usual. 500N of force on either side of the center node. 

    image

    b) Half model with only 500N. As the model is symmetric about YZ plane, the displacement perpendicular to that plane must be restricted (Ux=0) . The rotations parallel to that plane should be constrained based on the requirement (Ry, Rz = 0).

    image

    c) Results with as error of 0.6%. 
    image

    Thanks
    Vikas M B

  • Vikas_22008
    Vikas_22008 Altair Community Member
    edited June 3

    Hi @Adriano A. Koga,

    I tried to implement the symmetric BCs for a simple assembly of 2 plates. I have attached 3 FEM solverdeck files. 1. symmetric BCs (half of the assmebly), 2. Full assembly with 1 RBE for the load, and 3. Full assembly with 2 RBEs for the load. 
    I want to know the following things:
    1. Are the symmetric BCs correct?
    2. Are the results correct?


    1. Results when i used 1 RBE(right side) to apply the load  image

     

    2. Results when i used 2 RBEs(right side) to apply the same load.
    image

    3. Why there is a difference between the results when i apply the load using 2 different RBEs (please check full_2_RBE.fem) and when i apply the same load with 1 RBE (full_1_RBE.fem)? 

     

    Thanks 

    Vikas M B

  • Vikas_22008
    Vikas_22008 Altair Community Member
    edited June 4

    Hallo Adriano,

    I figured it out what the problem was. In both the above examples and in the beam analysis, the BCs are correct but according to this example, OS-T: 1070 Analysis of an Axi-symmetric Structure (altair.com), one should assign the set of nodes on the symmetric plane to a local coordinate system (refer the link) then and only then the symmetric effect can be seen. 
    image

    even the stresses are almost similar
    image

    Thanks
    Vikas M B

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