SOLIDWORKS Simulation: The mystery of symmetry – Part 1

SOLIDWORKS Simulation offers you a great tool to iterate a lot of different idea’s in a short time. But, sometimes an idea is becoming quite large and you might run out of time. A helpful solution is to make use of the symmetry of your design, so you can significantly decrease the calculation time. In the first of a two-part tech blog, I want to clarify the symmetry fixture type and its application. During the second part I will provide more in-depth information about the cyclic symmetry fixture.

The main idea of a symmetry fixture is to run a study on a 1/2, 1/4, or 1/8 portion of your model, while Simulation still thinks that you are running the study on a full model. This can save you a lot of elements and this way it will influence the calculation time. Also, the results of a symmetry study are equal to the results of a full model study. ­So let’s take a look on how to accomplish this.

Model preparation
First of all, you need to know if your model is suitable for applying a symmetry fixture. Your model and setup need to meet the following requirements:

  1. The model must be fully symmetric.
  2. The fixtures must be fully symmetric.
  3. The loads (e.g. force, pressure) must be fully symmetric.
  4. In case of an assembly, verify that your contact conditions (e.g. bonded, no penetration) are fully symmetric.

See the image below for an example where all the requirements are met. Note that a small difference in symmetry can greatly impact your results.

Symmetry1

If your model meets the mentioned requirements, you can simply cut it in half. This can be done with an Extruded Cut feature or Cut with Surface feature. For an assembly you can use the Extruded Cut Assembly Feature, the advantage of this feature is that you can cut your assembly in half with just one feature. Also, the cut will only be made at assembly level, so the parts will not change. Additional tip: Use configurations to create a symmetry configuration, which you can use for your simulation study.

Configuration

Study setup
After your model is correctly split in half, you can create your simulation study. To apply a symmetry fixture, right click Fixtures in the study tree and select Advanced Fixtures. Then you select all the faces which were created by the Symmetry Cut feature. The image below shows an example. Be sure to select all the necessary faces. If you forget one, this means that this face can move in its normal direction and this will badly influence your results.

Symmetry_fixture1

Other things to keep in mind when using a symmetry fixture:
1. If you apply a force on a face and this face is split in half due to the symmetry, then you will need the half of the force. See the image below for an example.

Symmetry2

 

2. When a hole for a bolt connector is split in half due to the symmetry, don’t forget to use the symmetry option, listed under Advanced Option in the bolt connector definition. In case of 1/2 symmetry, you need to select the symmetry plane. Also, for the bolt preload you can use the total value, but for the bolt mass you will need to define 1/2 or 1/4 according to the selected symmetry type. Finally, when you list bolt forces after running a study, the results equal 1/2 or 1/4 of the total force.

Symmetry_bolt

3. In case of using a 45-degree slice of your model, instead of using a 1/2 or 1/4, the following message appears when applying a symmetry fixture:

Symmetry_error

The intended use of the symmetry fixture is only for cases of true orthogonal symmetry. This means that the model needs to be cut by using one, two, or three orthogonal (perpendicular) planes, resulting in 1/2, 1/4 or 1/8 of the whole model. To still use symmetry at a 45-degree slice, you can use a Roller/Slider fixture instead. This fixture type also restraints the normal direction of a face, so it will produce the exact same behaviour as a Symmetry fixture.

Study results
After you run the study, you can start analyzing the results. At first the results will be projected at the half of the model, as seen below.

Symmetry_result1

But luckily there is a nice trick to show the full model for the report. To do this, go to Edit Definition of the plot. Under Advanced Options, select Display symmetric results. This will cause the plot to be mirrored, so you can still show the full model.

SOLIDWORKS Simulation

I also want to point out that you have to be careful when analyzing result forces. The resultant force on a face which is split in two by symmetry will be 1/2 of the total force on that face. So don’t forget to multiply these results!

Conclusion
I started this tech blog by saying that you can significantly decrease the calculation time. So let’s see how that worked out for the model from the study results section.

Result_comparison

As you can see the number of degrees of freedom decreased with 50% for the symmetry model. And the total solution time even decreased with 60%. That are big numbers! So you can even analyse two symmetry models in the time of one full model.

This is bringing me to the end of this tech blog. As said before, this was part 1 of 2, so next time I will tell you more about cyclic symmetry fixtures.

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  • Blake

    Is the main benefit of the symmetry fixture (as opposed to the roller/slider fixture) the post-processing option to display symmetric results? Other than that I’m not sure what distinguishes the two fixture types.

    • Martijn Visser

      That’s correct! In fact roller/slider and symmetry fixture both fix the normal direction on a face.
      So the post-processing is the big difference.

      • Mostafa Ayman Mohammed Nageeb

        If I may add something, sometimes symmetry must be used to give more convenient results. I was testing all different methods to do a simple plate with a hole tension simulation. When I subjected the object to a force from one side and fixed the other side once with fixed fixture and the other with slider fixture, both resulted in inaccurate results; namely, neither the stress distribution nor deformation was symmetrical.