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2.4  NASTRAN interfacing

• Non-linear springs
• Superelement generation (Craig-Bampton type)
• Superelement generation (Free mode variants)
• Storing advanced SDT options in bulk format
• Data export

For generalities about superelements see section 1.6.1. This section discusses the specifics of superelement generation using NASTRAN.

2.4.1  Non-linear springs

For spring representations of volumes or surfaces, a first common approach is to use so called rigid elements. NASTRAN supports

• RBE2 : rigid connections where the spring is connected to a master node with 6 DOF which enforce motion of a number of slave DOFs.
• RBE3 : flexible connection where the spring is is connected to a slave node with 3 or DOF which depend from a set of master nodes. Note that in RBE3, the normal operation is to define a single slave node. UM cards in the RBE3 element can be used to change this behavior by selecting which DOF get eliminated.

Laws without internal states are similar to PGAP and import will be implemented in the future.

2.4.2  Superelement generation (Craig-Bampton type)

The superelement generation by NASTRAN is saved to an .op2 file that is automatically transformed to the SDT superelement format by FEMLink. A sample file is given in ubeamse.dat.

ASSIGN OUTPUT2='./ubeam_se.op2',UNIT=30
$
ID DFR
SOL 101
GEOMCHECK NONE
TIME 100
$
CEND
TITLE=Generic computation of mode shapes
METHOD=1
DISP(PLOT) = ALL
SPCFORCES(PLOT)=ALL
MPCFORCES(PLOT)=ALL
$ Now extract stresses on base
SET 101=1 THRU 16
STRESS(SORT)=101
$
MPC=1
SPC=1
$ RESVEC(NOINRL)= YES
EXTSEOUT(ASMBULK,EXTBULK,EXTID=100,DMIGOP2=30)
PARAM,POST,-2
PARAM,BAILOUT,-1
$
BEGIN BULK   
$EIGRL,SID,V1,V2,ND,MSGLV,MAXSET,SHFSCL,NORM
EIGRL,1,,,20
$ DOF and nodes to support modal DOF
QSET1,0,1000001,THRU,1000050 
SPOINT,1000001,THRU,1000050
$ Master DOF 4 base corners
BSET1,123,1,5,8,12
$ 
$ Residual on 3 DOF of input node 104
USET,U6,104,123
$
$ Residual associated with CAMP1 vector
CDAMP1  161     2       114     1       244     1
PDAMP*  2               1.              
$
include 'ubeam_include.bdf'
ENDDATA

The resulting basis has the following form

The op2 file contains nodes and superelement definition. It is advised to read the bulk file to obtain a model containing elements and material properties

• The interface DOF are defined in NASTRAN usingBset cards. These are stored in SDT as a DofSet entry to the model.
• QSET correspond to modal/generalized DOFs. These require the definition of a QSET card (to declare existing DOFs), SPOINT grids (to have node numbers to support these QSET DOFs). Note also that the SPOINT numbers should be distinct from other NodeId. The number of modes defined in the EIRGL card should be lower than the the number of SPOINT and the QSET card.
• Fixed interface modes φ_c are computed by specifying the EIRGL card.
• Residual loads b_res are defined as follows and lead to additional shapes using the residual vector procedures of NASTRAN
  • point loads simply declared using the USET,U6 card
  • relative loads simply obtained by declaring a CDAMP element that generates a relative viscous load between its two nodes.

[b] is defined by an DAREA real loading and possibly DPHASE definition. It should be noted that in SDT, it is strongly advised to define the phase using the input, since a complex input shape matrix has no sense in the time domain. The input is defined using a RLOAD2 B(f)e^{iφ(f)+θ−2π f τ} or RLOAD1 (C(f)+iD(f))e^{iθ −2π f τ}

2.4.3  Superelement generation (Free mode variants)

When using a free mode computation, NASTRAN provides mechanisms to compute residual vectors, you should just insert the RESVEC=YES card. An example is given in the ubeamfr.dat file. The resulting basis has the following form

The main mechanisms to generate residual vectors are

• Free interface modes φ_c are computed by specifying the EIRGL card.
• Residual loads b_res are defined as follows and lead to additional shapes using the residual vector procedures of NASTRAN
  • point loads simply declared using the USET,U6,NodeId,DofList card. Alternatively RVDOF (MSC but possibly not NX-NASTRAN) can given a list of up to four NodeId,Dof per card.
  • relative loads simply obtained by declaring a CDAMP element that generates a relative viscous load between its two nodes.

2.4.4  Storing advanced SDT options in bulk format

For upcom parameters, export is done using design variables.

SDT-NLSIM provides an harmonic definition mechanism (see hdof). Storage in NASTRAN bulk format is as follows

$   1  $$   2  $$   3  $$   4  $$   5  $$   6  $$   7  $$   8  $$   9  $
$ All DOFs with sin(omega t) and cos(omega t) 
DTI     HDOF    1       ALL     123456  CS1     ENDREC
$ Gradual building of full list of DOFs 
DTI     HDOF    1       N1      THRU    N2      123     S1      N3
        THRU    N4      1       C1      N5      123456  S1      ENDREC

Node numbers are first specified using ALL all (independent) nodes, N1 THRU N2 a list of consecutive node numbers, N5 a single node number. Associated DOFs are then written using the CM field of RBE2 (Component numbers of the dependent degrees-of-freedom integers 1 through 6 with no embedded blanks). A third field then specifies the harmonics. cs1 is a short cut for both cos(1 ω t) and sin(1 ω t).

The specification of target frequencies follows the normal NASTRAN format using FREQ or FREQ1 cards. Provision for a single call generating responses at multiple amplitudes (hbm_solve AFMap .Freq and .Amp fields) is specified as a DTI HBMAmp entry with all target amplitudes given.

$   1  $$   2  $$   3  $$   4  $$   5  $$   6  $$   7  $$   8  $$   9  $
EIGRL,10,,,1
$FREQ,SID,F2,F2,F3
$FREQ1,SID,F1,DF,NDF
FREQ          10   0.318      1.     3.0     4.0
RLOAD1        10       1                       1

$   1  $$   2  $$   3  $$   4  $$   5  $$   6  $$   7  $$   8  $$   9  $
DTI     HBMAmp  1       1.0     2.0     3.0     ENDREC

To specify loads, a number of formats are defined.

$   1  $$   2  $$   3  $$   4  $$   5  $$   6  $$   7  $$   8  $$   9  $
DTI     Name    1       SID     101     FORM    Amp     UN1     UN2    
        Harmi   ACi     ASi     ENDREC

• Name is an arbitrary string (at most 8 characters) but should be unique and differ from internal NASTRAN tables. By default it is proposed to use strings of the form P101 where 101 is the property number.
• IREC (field 3 of the DTI) is only used when considering multiple entries with the same name and should be set to 1.
• SID : first the string SID in field 4 then, in field 5, the property identifier (integer) which should correspond to the set identification number SID for which this amplitude dependence is defined.
• Form (selected with the string on field 7) is the form name with the following formats defined

  Form
  Amp	amplitudes	{u(t)} = C0+∑k∈ H Sk sin(kω t) + Ck cos(kω t)
  AmpT	Amp table	{u(t)} = C0(ω)+∑k∈ HSk(ω) sin(kω t) + Ck(ω) cos(kω t)

• Harmi number of retained harmonic. 1 for cos(1 ω t) and sin(1 ω t).
• ACi, ASi amplitudes associated with the cosine and sine harmonic contributions. In the AmpT form integer numbers referring to table entries in the bulk.

2.4.5  Data export

nas2up supports writing of field at DOF results op2 format.