The CDS super element should be used when it is anticipated that a large number of residual runs will be made on a very large model at the higher end of the frequency range of study. For example, this approach should be used when studying a variety of inputs on an automobile model in the frequency range of 400 to 800 Hz. For the residual analysis to run as fast as possible, all components, except very small ones, should be converted into CDS super elements. The major limitation of this approach is that it takes longer to generate the CDS super elements than with the other super element creation methods. Also, the analysis must be performed at the fixed set of frequencies specified when the CDS super element is formed. The major benefit of the CDS super element is that the residual run will be much faster than with super elements created by other methods.

For an example of the body CDS super element generation, see the input data for a body-in-white below. The special data for this input are the case control data: CDSMETH = 1; the FREQ card which restricts the residual analyses to just those frequencies; the CDSMETH data (see the CDSMETH card definition); and the BNDFREE data which defines the exterior points on the component.

CDS Body Super Element Example

MODEL = NONE

TITLE = BODY-IN-WHITE CDS MODEL

$-------------------------------------------------------------------------------

SUBCASE 1

FREQUENCY = 1

METHOD = 2

CDSMETH   = 1

BEGIN BULK

$==01==><==02==><==03==><==04==><==05==><==06==><==07==><==08==><==09==><==10==>

FREQ1          1     1.0     0.2      94

FREQ1          1    20.0     0.5     159

FREQ1          1   100.0     1.0      99

FREQ1          1   200.0     2.0     200

EIGRA          2          1000.0

CDSMETH        1

 

BNDFREE1 123456  1000000    through 1006001

INCLUDE '/MODELS/BODY/BODYNEW.dat'

ENDDATA

 

CDS super element is saved in the file: XXX_CDS.h3d

The interface points (exterior points) are where the component is attached to another super element directly or to the residual structure. These interface points must be independent degrees of freedom. If they are the dependent point of an RBE3, they must be made independent by transferring the dependencies to one of the independent grids referenced by the RBE3 element using UM data on the RBE3 definition, or use PARAM,AUTOMSET,YES. In the Bulk Data section on the RBE3 element, the UM parameter shows how to redefine the dependency. The RBE3 can also be changed to an RBE2, but this could stiffen up the local area as a result.

In order to be formed into a super element, a component FEA model has to be complete. All grids referenced in the super element must be in the component model file. This includes local coordinate systems grids. All properties and material referenced in the components must also be included in the component. The component model must be able to be run successfully by itself in a modal analysis run.

The OSET field on the CDSMETH entry can be used to recover responses from the interior grids of the CDS superelements.

Residual Run Using the CDS Super Elements

The residual run on the full model must be run with the direct analysis approach. Also, the same or a subset of the frequencies specified in the CDS super element generation run must be used in the residual run.

The reduced matrices from CDSMETH will be used in residual run thru ASSIGN,H3DCDS. For CDSMETH, no DMIG selector entry such as K2GG is applicable but all the matrices in H3D file are used in residual run.

$ FOLLOWING CARD RETRIEVES THE H3D FILES FOR THE CDS SUPER ELEMENTS

ASSIGN,H3DCDS,AX,’myfile_CDS.h3d’

$

TITLE = P/T FULL VEHICLE ANALYSIS

SUBTITLE = FVM DIR H3D

SPC = 1

MPC =   406

SUBCASE 1

LABEL = UNIT LOAD INPUT INTO CDS MODEL

DLOAD = 110

FREQUENCY=1

SET 2 = 1006001,9106012

ACCELERATION (PUNCH,SORT2,PHASE) = 2

$

BEGIN BULK

$

$==01==><==02==><==03==><==04==><==05==><==06==><==07==><==08==><==09==><==10==>

FREQ1          1    1.0      0.2      94

FREQ1          1   20.0      0.5     159

FREQ1          1  100.0      1.0      99

FREQ1          1  200.0      2.0     200

EIGRL          1                   800.0  

$==01==><==02==><==03==><==04==><==05==><==06==><==07==><==08==><==09==><==10==>

$

INCLUDE  '/LOADS.dat'

$

INCLUDE  '/CONNECTIONS_BETWEEN_COMPONENTS.dat'

$  

INCLUDE  '/NON_CDS_COMPONENTS.dat'

ENDDATA