# Energy error differences

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Hi

I am running an analysis on a brake assembly and I am trying to learn use of 3d elements in this case.

When I use hex elements, I see only -3% energy error at the end of simulation whereas for tetra it is -43%. Why is there so much difference?

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Hi,

this is because of the different contact interface definition:

pohan likes this

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Hi,

Thank you.

So after making Ifric = 2 and Iform = 0 on both models, I am getting energy errors -37.6 % for tetra and -36.4% for bricks. I think that's a fair match. However, why is there so much negative energy percentage in both cases? Recommended limits are -15% to +5%, isn't it?

Also, from T01 files, if I compare energy plots for Internal, Kinetic, Contact, External forces work and (Total - Global External Work), all of them are different except for the last one (total - external). Why this difference?

Then which element type is better in this case? Since energy errors are same and (total energy- external) is also maintained.

Regards

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Hi Skill

I think if they have the same energy errors, which one gives you less elements is better than the other one.

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Contacts with friction and hourglass energy are usually a source of negative error (energy dissipation) since they are not counted in the energy balance. The guidelines you mentioned are valid, but can not be generalized to all models (although the source of discrepancy has to be identified). From Radioss>Frequently Asked Questions>Results Checking:

Quote

Large contact energies relative to total energy can cause large negative Energy Errors because contact energy is not part of the Energy Error equation. If the simulation has friction and a lot of sliding contact, then the large contact energy and resulting energy error can be considered acceptable.

All else being equal, there should not be much difference in the results between brick or tetra mesh. Brick meshing is usually harder, especially if structured mesh of good quality is required, but it can be more computationally efficient on the other hand.

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