Learning Forum

[jeremy leong]

Hi, I have a problem when I solve the residual vectors in modal analysis. Could anyone give me some help, please? Thanks in advance.

I want to solve the residual vectors of a large-scale 3D FEM and output them together with the modeshapes in MMF format. I know the residual vectors can be calculated in ANTYPE,MODAL and RESVEC should be activated. And actually I successfully obtained a residual vector under one concentrated force. But according to the theory of residual vectors method presented in Section 14.8.5 of Theory Reference, they are calculated by a matrix of multiple concentrated forces. So my question is how to define this forces matrix to obtain all residual vectors under multiple concentrated forces. The attempt I have made is shown below:

1)my APDL of computed a residual vector under one concentrated force

/SOL
ANTYPE,2
MODOPT,LANB,NUM_FREQ,0,0,,OFF
SOLVE
FINISH

/SOL
ANTYPE,2,RESTART
MXPAND,NUM_FREQ, , ,0 
RESVEC,ON                                  !RESIDUAL VECTOR METHOD FOR HIGHER FREQUENCY MODES
D,21653,ALL,SUPPORT                !SPECIFY PSEUDO-CONSTRAINTS TO PREVENT RIGID-BODY MOTION
D,21965,ALL,SUPPORT                !SPECIFY PSEUDO-CONSTRAINTS TO PREVENT RIGID-BODY MOTION
num_NODE_=node_num(1)
F_DOF_INDEX_=F_DOF_INDEX(1)
F,num_NODE_,F_DOF_INDEX_,1     !Generate load vector
SOLVE

2) use the RESTART of MODAL ANALYSIS

I try to define every concentrated force in every step of RESTART to copmpted the residual vectors respectively. But in the second RESTART, an error occurs: 

 "Residual vectors (RESVEC command) were calculated during the first      
 modal analysis.  The modal analysis cannot be restarted.  If you need   
 to restart a modal analysis, make sure you only do the mode extraction  
 (and possibly expansion) during the first analysis without generating   
 residual vectors."

I guess this is due to the second residual vector's calculation need to call the Jobname.mode to get the modeshapes without  any resiudal vectors, however,  Jobname.mode have one residual vector after first RESTART. My APDL is as follows:

/SOL
ANTYPE,2
MODOPT,LANB,NUM_FREQ,0,0,,OFF
SOLVE
FINISH

/SOL
ANTYPE,2,RESTART
MXPAND,NUM_FREQ, , ,0 
RESVEC,ON                                  !RESIDUAL VECTOR METHOD FOR HIGHER FREQUENCY MODES
D,21653,ALL,SUPPORT                !SPECIFY PSEUDO-CONSTRAINTS TO PREVENT RIGID-BODY MOTION
D,21965,ALL,SUPPORT                !SPECIFY PSEUDO-CONSTRAINTS TO PREVENT RIGID-BODY MOTION
num_NODE_=node_num(1)
F_DOF_INDEX_=F_DOF_INDEX(1)
F,num_NODE_,F_DOF_INDEX_,1     !Generate the first load vector
SOLVE

FDELE,ALL,ALL 

num_NODE_=node_num(2)

F_DOF_INDEX_=F_DOF_INDEX(2)

F,num_NODE_,F_DOF_INDEX_,1   !Generate the second load vector

SOLVE

 

Another method I guess is that a restarted modal analysis to compute normal modes is needed before solving every residual vectors but this procedure is time-consuming.

[dlooman]

It's not necessary to do restarts.  You can issue modcont,on and then do multiple solves in the modal.  Only the first solve will do the mode exctraction.  The remaining solves will create additional load vectors.

resvec,on

modcont,on

F,num_NODE_,F_DOF_INDEX_,1     !Generate the first load vector

solve    ! modal extraction and first load vector

F,num_NODE_,F_DOF_INDEX_,1   !Generate the second load vector

solve   ! create second load vector

F,num_Node...

solve

etc...