Frequency Dependent Damping

[ryan.bolin]

Hello,

I have noticed when running random vibration (and sine frequency response) that if you use the SPCD method of base excitation with PARAM,RESVEC,ON, the responses are not scaled by the TABDMP1 frequency dependent damping card properly.  They seem to look at only the first value of damping and apply it to all frequencies called for in the FREQi cards.  Why is this the case?

I made a simple 2-DOF system with base excitation to demonstrate the issue.  The picture shows the response of node 2 of the 2DOF system, with 4 different damping tables (TABDMP1).  The input deck is below, and I would just change the TABDMP1 9999 card to match the damping I want. 

In the attached .PNG file, I would expect Case 1 (red line) to have a response that matches the Case 3 (green lin) at 26 Hz, as it has 1% critical damping specified above 15 Hz.  Similarly, I would expect Case 4 (blue dots) to match the Case 2 (yellow line) at the 26 Hz mode, as it has 7.5% critical damping specified above 15 Hz.  Why is this?

Thanks,

Ryan Bolin

NASTRAN SYSTEM(319)=1
ID F44940,Femap
SOL SEMFREQ
CEND
  TITLE = 2DOF system to investigate frequency dependent damping
  ECHO = NONE

  set 1 = 101,2,3
 
  ACCELERATION(plot,phase,psdf) = 1
 
  SPC = 2
  FREQUENCY = 1  
  SDAMPING = 9999  
  METHOD = 1

 
SUBCASE 111        
  $ X-excitation
  subtitle= Random Vibe Lateral (X)
  DLOAD = 104
  xydatagenerate,1,123,1,grid   
       

$********************************************************************************
BEGIN BULK
$********************************************************************************


PARAM,PRGPST,NO
PARAM,OGEOM,NO
PARAM,AUTOSPC,NO
PARAM,GRDPNT,0             
param,resvec,yes   
param,maxratio,1.+8    

$--------------------------------------------------------------------------------
$             VIBRATION PARAMETERS
$--------------------------------------------------------------------------------
FREQ1          1      1.      1.     199
FREQ4          1      1.    200.      .1       3
EIGRL          1            200.                                    MASS
$--------------------------------------------------------------------------------
TABDMP1     9999    CRIT                                                +       
+             1.    .075     15.    .075    15.1    .075   2000.    .075+
+        ENDT  
$--------------------------------------------------------------------------------
$             SINE VIBRATION PARAMETERS
$--------------------------------------------------------------------------------
$ X-direction = 1g
TABLED1  11      LOG     LOG                                                       
         1.0          1.   2000.      1. ENDT
$--------------------------------------------------------------------------------
$             CONSTRAINTS AND LOADS
$--------------------------------------------------------------------------------    
SPC1           2  123456     101
SPC1           2   23456       2       3           
$-------2-------3-------4-------5-------6-------7-------8-------9-------10------
SPCD         102     101       1   386.4
RLOAD1       101     102                      11            ACCE
DLOAD        104      1.      1.     101   
$--------------------------------------------------------------------------------
$                            FEM BULK MODEL DATA
$--------------------------------------------------------------------------------
$--  Stiffnesses and masses produce F1 = 9.8 Hz and F2 = 25.8 Hz theoretically
$-------2-------3-------4-------5-------6-------7-------8-------9-------10------
pbush         99       K    1.+5
$-------2-------3-------4-------5-------6-------7-------8-------9-------10------
grid         101       0      0.      0.      0.       0
grid           2       0      0.      0.      0.       0
grid           3       0      0.      0.      0.       0
$-------2-------3-------4-------5-------6-------7-------8-------9-------10------
cbush          1      99     101       2                               0
cbush          2      99       2       3                               0
$-------2-------3-------4-------5-------6-------7-------8-------9-------10------
conm2          3       2             10.
conm2          4       3             10.


ENDDATA