Learning Forum

[nirvana]

Hi!

I would like to get your opinion about a doubt regarding the symmetry region. I have done a simulation using the symmetry region by activating beta options in order to get a full body graphical visualization in the stress output. However I was wondering if the releted stress value refers to half of the model or to a full model. Each half is the mirror copy of the other so they share the same contour and values, but should I double the values of the stress?
Thank you for the attention.

 

[peteroznewman]

You don't double the stress on a half symmetry model.

If you applied a force to a face that was cut in half by symmetry, then you would apply half the full model force because the other half of the force is on the missing half of the geometry.

If you applied a non-zero displacement to a face that was cut in half by symmetry, you don't apply half the displacement, but when you request Reaction Force, you double that to find the full model Reaction Force because the other half of the force is coming from the missing half.

 

[Miguel Sanchez]

Hi, I understand the explanation but have a doubt about the particular case of the force being applied through a joint-force scoped to a traslational joint on a body cut in half by the symmetry plane (meaning that the joint free DoF and, therefore, the force line direction are aligned with/contained in the XZ symmetry plane), does the same reasoning still apply? Should the applied joint-force be also half of that in the full model?: 

Thanks.

[nirvana]

Hi Peter, thanks for your ansewar.

Regarding your first point "If you applied a force to a face that was cut in half by symmetry, then you would apply half the full model force because the other half of the force is on the missing half of the geometry" ok this is clear, and in this case the stress values refers to the full model, right?

Instead if I need to apply a force in a region far away from the symmetry one as in the following picture I should apply of course the whole value, right? In that way I will get by symmetry the other half of the model with the same force applied on the opposite side.

One last point. Tha Ansys guide states that only flexible body scoping is supported for symmetry region. Thus if I cut in half a rigid body what can I do? I need to apply on a face of a rigid body (the face is split in half) a force load. In this case do I need to apply half of the force of the full force? How can I include the symmetry region on a rigd body? 

 

[peteroznewman]

Say you had a beam that was symmetric, 100 mm long and a cross section of 20 x 6 mm and you wanted to apply a tension load along the length. Let's say the tension load was 64 N, so in the full model you have 64 N at each end (which won't solve, by the way).

If the first symmetry plane was to cut it in half at the center so it was only 50 mm long, you still have 64 N at the end you kept.

If the second symmetry plane was to cut it in half in the 20 mm direction, now there is only half the cross section, so that would have an end load of 32 N.

If the third symmetry plane was to cut it in half in the 6 mm direction, now there is only a quarter of the cross section, so that would have an end load of 16 N.

[nirvana]

Hi,

probably you replied me before I had posted my edited reply so I'm going to repeat the last edited parte of my post.

The Ansys guide states that only flexible body scoping is supported for symmetry region. Thus if I cut in half a rigid body how can I scope half of the cut face as symmetric region?  I need to apply on a face of a rigid body (the face is split in half) a force load. In this case do I need to apply half of the force of the full force? How can I include the symmetry region on a rigid body?

Moreover when you said that by probing the reaction forces in case of applied displacement these reaction values must be doubled, is that true only for a displacement since I apply the entire value? If I apply half of the total force to a cut face and I want to probe reaction forces I shouldn't double those values, right?

[peteroznewman]

Don't use Rigid bodies if you need to use symmetry.

 If you apply half the total force to a cut face due to symmetry, then you would double the reaction force to recover the full model reaction force.

[nirvana]

Hi, 

so if the rigid body is cut in half and I cannot apply a symmetry condition can I apply a remote displacement to fix at least the degree of freedom in the direction that should be normal to the symmetry region? This because I apply a force on the half face of that rigid body

Thank you very much ! I really appreciate your support for this and all the previous questions I asked! 

[peteroznewman]

Have you tried to apply a force to a Rigid body?  I find Mechanical does not allow me to assign a force to a rigid body.

[nirvana]

If I use a remote force I am allowed to scope to the face of the rigid body, but then in the results the missing half of that rigid body is not displayed

 

[peteroznewman]

Okay, so if you apply a remote displacement on the symmetry plane, you can extract the reaction force from that remote displacement, is that correct?

[nirvana]

tha's not my situation. I apply a remote force to the half face of the rigid body and to the half face of all the bodies which are cut by the symmetry plane I used to slice my model. the scoped regions become red meaning that they have been correctly selected thus it seems to work. But when I analyse the stress output of the model I note that all the bodies are mirrored except the rigid body so I'm not sure it worked properly and the stress output is not reliable.

Thus, I'm going to try two ways:

the first one is to switch the behaviour of the rigid body to flexible.

the second one is to keep it rigid, apply the remote force again on its face, use the symmetry region on the faces of the flexible bodies and constrin on dof of the rigid body usign a remote displacement since there I can manipulate all the degrees of freedom.

what do you think?

 

[peteroznewman]

You will have more confidence if you use flexible bodies. The trade off is longer wait times for the solution.