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solidDisplacementFoam - problem with weakly nonsymmetric result on symmetric geometry

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Old   October 3, 2012, 13:20
Default solidDisplacementFoam - problem with weakly nonsymmetric result on symmetric geometry
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Hello.

I have this problem. I'd like to compute stress in basic school case - beam of prism shape witch square cross-section strained in pulling. Symbolic list of vertices in my blockMeshDict file follows:

(-L 0 0)
(L 0 0)
(L T 0)
(-L T 0)
(-L 0 T)
(L 0 T)
(L T T)
(-L T T),

where L is length of the beam and T is thichness of beam and both are positive numbers. I use tractionDisplacement boundary condition type. Nonzero stress as boundary condition is only put on sides

(-L 0 0)
(-L T 0)
(-L T T)
(-L 0 T)

and

(L 0 0)
(L T 0)
(L T T)
(L 0 T).

On these sides the stress is put in direction outward from body of beam and perpendicular to corresponding side and constant on entire corresponding side.

On rest of sides are stresses put equal zero.

I derived this case from tutorial case placed in tutorials/stressAnalysis/solidDisplacementFoam/plateHole. In file constant/mechanicalProperties I changed Poisson ratio from 0.3 to 0 and Young's modulus of elasticity from 2e+11 to 1e+10.

When I run solidDisplacementFoam solver, the result of displacement D is not symmetric according to plane YZ, it means plane with normal (1 0 0). Near the side with x-coordinate -L has a little bit another size than near the side L, although in these areas the displacement should be of same size.

What is the reason of that??

Result of stress is good - except for numerical error of constant size on entire body of beam.

One solution could be to use symmetryPlane boundary condition type. In this case only half of beam is considered. More precisely: Instead of -L in x-cocrdinates of vertices the 0 is used and symmetryPlane condition is put on side (0 0 0)(0 T 0)(0 T T)(0 0 T) and only on side with L in x-coordinate of vertices the nonzero stress as boundary condition is used. In this case the results are even exact, without numeric errors which in previous case probably are.

So is using symmetryPlane bc type the good solution for above described weak nonsymmetry? What do you think?
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