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total DOF > 3*(# total nodes) for a linear tetrahedral mesh

Posted 24.10.2011, 08:21 GMT-4 Mesh, Modeling Tools & Definitions, Parameters, Variables, & Functions, Structural Mechanics Version 4.2 4 Replies

kodanda
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Hi,

I find it strange and puzzling! I have a simple unit cube meshed with tetrahedral elements. I chose "Linear" for the interpolation order, linear elastic material and static study. Used displacement BCs, no other type of constraints. See the attached model file.

When I export mesh, I see 33 linear tets and total number of nodes is 15. Since, each node has 3 degrees of freedom (viz. {u v w}, I checked in the equation view as well for the field variables in the solver node), I must have 3*15 = 45 DOF.

However, while solving in the log file it reports 222 DOF! What are these extra DOF, which are nearly 5 times the "actual" DOF?

In a 3D model with 20k nodes, this difference in DOF is a big blow on the computation time and memory for me. So I really need to understand what is this and how to get rid of these excess DOF.

Best regards,
Kodanda

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4 Replies Last Post 25.10.2011, 07:17 GMT-4
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 24.10.2011, 11:23 GMT-4
You are using a quadratic discretization of the displacement field
You are using a quadratic discretization of the displacement field
kodanda
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Posted: 1 decade ago 24.10.2011, 12:02 GMT-4
Hi Nagi,


You are using a quadratic discretization of the displacement field


It fits with the explanation of a quadratic interpolation for the field.

But, I am using Linear interpolation. As to my understanding, the only place where the interpolation order is chosen is on the "Mode Node". I manually had selected the setting on the Model node from "Automatic". i.e. Quadratic to Linear.

In addition, the # DOF does not change for what ever interpolation I select from the Model Node. This means the manual selection is not effective. Where else I can enforce it? I have always been re-building each goemetry and mesh nodes after changing the interpolation.


Best regards,
Kodanda
Hi Nagi, [QUOTE] You are using a quadratic discretization of the displacement field [/QUOTE] It fits with the explanation of a quadratic interpolation for the field. But, I am using Linear interpolation. As to my understanding, the only place where the interpolation order is chosen is on the "Mode Node". I manually had selected the setting on the Model node from "Automatic". i.e. Quadratic to Linear. In addition, the # DOF does not change for what ever interpolation I select from the Model Node. This means the manual selection is not effective. Where else I can enforce it? I have always been re-building each goemetry and mesh nodes after changing the interpolation. Best regards, Kodanda
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 24.10.2011, 13:04 GMT-4
That one deals with the geometry and it’s not the one you want. The other one is under the “Solid Mechanics” node. There should be a section titled “Discretization”. If you don’t see it, click on the “Show” button in the Model Builder window and toggle ON “Discretization”.

Nagi Elabbasi
Veryst Engineering
That one deals with the geometry and it’s not the one you want. The other one is under the “Solid Mechanics” node. There should be a section titled “Discretization”. If you don’t see it, click on the “Show” button in the Model Builder window and toggle ON “Discretization”. Nagi Elabbasi Veryst Engineering
kodanda
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Posted: 1 decade ago 25.10.2011, 07:17 GMT-4
Hi Nagi,


That one deals with the geometry and it’s not the one you want. The other one is under the “Solid Mechanics” node. There should be a section titled “Discretization”. If you don’t see it, click on the “Show” button in the Model Builder window and toggle ON “Discretization”.

Nagi Elabbasi
Veryst Engineering


Thanks for pointing it out.

Best,
Kodanda
Hi Nagi, [QUOTE] That one deals with the geometry and it’s not the one you want. The other one is under the “Solid Mechanics” node. There should be a section titled “Discretization”. If you don’t see it, click on the “Show” button in the Model Builder window and toggle ON “Discretization”. Nagi Elabbasi Veryst Engineering [/QUOTE] Thanks for pointing it out. Best, Kodanda

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