Cancel
Showing results for 
Search instead for 
Did you mean: 

Meaning of "Number of Generalized DOF" parameter - SOL103 Superelements

Genius
Genius

Hi,

I am using NX Nastran 12, for model substructing using Craig Bampton method (i.e. SOL 103

 Superlement).

  1. What does it mean "Number of Generalized DOF" in a SOL103 Superelements? 
  2. As default, this parameters is equal to 500. Is there a rule of thumb to evalutate this parameter for each FE model?

 

Immagine.png

 

 

I found somethink in this or this old NX Help , but I do not understand.

Regars,

A

5 REPLIES 5

Re: Meaning of "Number of Generalized DOF" parameter - SOL103 Superelements

Valued Contributor
Valued Contributor

Hi @AlbertoM ,

 

I also use component-modal synthesis (CMS) and I also asked myself the meaning of this parameter.
I think it is concern to the memory allocated for the simulation, but I am not sure.
Maybe someone more experienced can give you more information (and me too).

 

Regards,

 

Ivan

Re: Meaning of "Number of Generalized DOF" parameter - SOL103 Superelements

Siemens Phenom Siemens Phenom
Siemens Phenom

It is the number of CB modes you will be saving. Nastran uses an SPOINT to create a placeholder for each mode, you need to create 10 SPOINTS for 10 modes to be saved. This number needs to be large enough to save all of the CB modes that you request, or they will be truncated, not saved.

 

Regards,

Re: Meaning of "Number of Generalized DOF" parameter - SOL103 Superelements

Genius
Genius

I am pretty confused @fembrackin .

 

To simplify the SuperElement (SE) analysis I directly check the matrices of SE exported by a "SOL103 - Flexible body".

I specified 10 constraint modes (i.e. I put a "Fixed Boundary Degrees of Freedom" on two dinstict nodes and I constraint in total 10 DoF). I ask to the solver to look for normal modes of my model until 40 kHz (i.e. "Lanczos data"->"Frequency range - upper limit"=40000).

 

Then, I tried to adjust the "Number of Generalized DOF" and see what happen and those are the results:

  •  "Number of Generalized DOF" = 2500, I obtain a superelement matrices with size = 30.
  • "Number of Generalized DOF" = 500, I obtain a superelement matrices with size = 30.
  •  "Number of Generalized DOF" = 3, I obtain a superelement matrices with size = 13.
  •  "Number of Generalized DOF" = 1, I obtain a superelement matrices with size = 11.

I do not really understand these results. Could you explain me exactly what happen?

Regards

Re: Meaning of "Number of Generalized DOF" parameter - SOL103 Superelements

Siemens Phenom Siemens Phenom
Siemens Phenom

As previously stated, Nastran internally stores component modes on QSET dof. You need to have at least as many QSET DOF as you have component modes. If you have fewer QSET DOF than component modes, then only some of the modes will be stored (the rest will be discarded).If you have more QSET DOF than component modes, the extra QSET dof are unused.

 

For the data you show, it looks like you have 20 component modes. If you specify 19 or fewer QSET dof, some will not be retained and your matrix size changes. If you specify 20 or more QSET DOF, all component modes are retained and the size of your matrix is unchanged.

 

The number of component modes is typically on the order of tens. The default for number of generalized DOF in Simcenter was chosen to be 500. This is generally many more than are required, so essentially users do not need to worry about how many QSET DOF are requested. It would be very rare for a model to have more than 500 component modes.

Re: Meaning of "Number of Generalized DOF" parameter - SOL103 Superelements

Phenom
Phenom

On a side note, it would be nice to implement MSC Nastran's PARAM,AUTOQSET,YES to not have to guess or use a overly large QSET number (although NX Nastran offers PARAM,QSETREM,YES)