7.10.       MASS_WEIGHTED_EFFIND Alter

 

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

$  Rigid Format 103 - Normal modes analysis

$  MSC/NASTRAN Version 2001 or NX Nastran Version 1.0

$

$

$     ******************************************************

$     *****            COPYRIGHT  (C)  2003            *****

$     *****          BY ATA ENGINEERING INC.           *****

$     *****             ALL RIGHTS RESERVED            *****

$     ******************************************************

$

$     09-03- ATA/Paul Blelloch

$

$  Description:

$

$  This alter performs an mass weighted effective independence scheme to\

$  optimally select A-set DOF.  It starts from the A-set defined by the user and

$  iteratively removes DOF until it reaches a user specified number.  At

$  each iteration it eliminates the DOF with the highest ratio of diagonal

$  stiffness to mass.  This will tend to do the best job at predicting

$  low frequency modes.

$

$  There are two ways to use this alter.  The first is to not define any

$  ASET DOF in the model.  In this case the alter starts from the F-set.

$  The alter always forces Nastran to eliminate massless DOF from the

$  problem before moving to the eigensolution.  If the F-set is large

$  (> 5,000 DOF), run times and scratch space can be extensive.  The

$  alternative is to define a large ASET.  In this case the run time is

$  a function of the size of the starting ASET.  This is the recommended

$  approach, with a starting ASET size < 5,000.  A number of methods can

$  be used to generate this initial ASET including grid point kinetic

$  energies or residual kinetic energies.

$

$  There are two variations on the alter.  The default multiplies the mode

$  shapes by the full mass matrix at each step, while the alternative

$  uses the square root of the diagonal of the mass matrix.  Preliminary

$  experience doesn't show any clear preference between these methods, but

$  they will give different results.

$

$  The number of DOF eliminated at each step of the iterative process is

$  controlled by the parameter EFFILT.  The default value of 1.0 will

$  eliminate one DOF at each step.  Setting EFFILT to a value less than

$  1.0 may result in a larger number of DOF eliminated at each iterative

$  step.  All DOF with a stiffness/mass ratio greater than EFFILT times

$  the largest ratio are eliminated at each step.

$

$  If the value of EFFILT results in deleting enough DOF that the

$  remaining number is less than NEFYAN at any step it is internally

$  reset to 1.0.  From this point on one DOF will be eliminated at all

$  following steps.

$

$  The alter will optionally calculate two error norms at each step in the

$  iteration.  The error norm is based on the following orthogonality

$  error matrix:

$

$  EORT = [I] - PHIX'*MXX*PHIX

$

$  The first norm (NORT) is the RSS of all the terms in this matrix.  The

$  second norm (MORT) is the maximum absolute value of all terms in

$  this matrix.  The user must supply the modes to be used for this

$  calculation, and can optionally supply a range of modes (N1 to N2)

$  to be used for the calculation.

$

$  Currently this alter will calculate the eigenvalues for the reduced

$  mass and stiffness matrices, but will then fail.  It will write the

$  selected DOF list to the PCH file.  This can be used to create ASET

$  cards to perform a Guyan reduction.

$

$  Special instructions to use this alter:

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

$  FILE MANAGEMENT SECTION (FMS)

$

$  If no ASET is defined, or the ASET is large (>1000 DOF) a considerable

$  amount of scratch space may be required.  This may require the addition

$  of a card of the following form

$

$  INIT SCRATCH LOGICAL=(SCR1(20GB)),

$               SCR300=(SCR300(20GB))

$

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

$  EXECUTIVE CONTROL DECK

$

$    SOL 103

$    Include this alter immediately before the CEND statement

$

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

$  CASE CONTROL DECK

$

$    No special input is required.

$

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

$  BULK DATA DECK

$

$    Optional parameters:

$

$    PARAM,ENORM,CHAR   'YES' : Calculate error norm

$                       'NO'  : Do not calculate error norm (default)

$                

$    PARAM,WRTMAT,CHAR  'YES' : Write reduced matrices at end of run

$                       'NO'  : Do not write reduced matrices at end of run (default)

$                

$    PARAM,DMASS,CHAR   'YES' : Multiply modes by square root of diagonal of mass

$                       'NO'  : Multiply modes by full mass matrix (default)

$                

$    PARAM,NDOF,I             : Number of DOF in final set (default = 1000)

$

$    PARAM,EFFILT,RS          : Filter to remove DOF at each step (default = 1.0)

$

$    PARAM,OMODES,I        <0 : Read FEM modes from OUTPUT2 file UNIT = |OMODES|

$                           0 : Read FEM modes from DMIG cards

$                          >0 : Read FEM modes from OUTPUT4 file UNIT = OMODES

$

$    PARAM,N1,I               : 1st FEM mode to use in error norm calculation (default = 1)

$

$    PARAM,N2,I               : Last FEM mode to use in error norm calculation (default = all)

$

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

$  EXAMPLE NASTRAN DECKS

$

$

$    ASSIGN  MASTER='gpsc_aset.MASTER'

$    ASSIGN  DBALL ='gpsc_aset.DBALL'

$    $

$    SOL     103

$    INCLUDE iter_guyan.v2001

$    CEND

$    $

$    TITLE    = GENERAL PURPOSE SPACECRAFT

$    SUBTITLE = SELECT DOF

$    LABEL    = MASS WEIGHTED EFFECTIVE INDEPENDANCE METHOD

$    $

$    SPC = 1

$    METHOD = 50                 $ Modes to 50 Hz

$    $

$    BEGIN BULK

$    $

$    PARAM,NDOF,50             $ Select best 50 DOF

$    PARAM,EFFILT,0.9            $ Eliminate DOF with > 90% of max ratio

$    $

$    EIGR,50,AHOU,1.0,50.0

$    $

$    INLCUDE 'gpsc.blk'

$    $

$    ENDDATA

$

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