Tagged: damper, psd, responsespectrum


March 24, 2022 at 3:17 pmMichelebSubscriber
Hi everyone,
I'm working on a project which involves a body suspended on a series of springs (COMBIN14 with k and c given through Real constants) and I need to perform a Spectrum Analysis. They gave me the PSD levels [G^2/Hz] and I successfully performed the base eccitation. In order to use Miles' eq. I extracted the RPSD value of RELATIVE acceleration @CoG in /post26: is this the transmissibility I need?
I also took a look at the "RPSD values of fixed nodes in terms of ABSOLUTE acceleration" MINUS "RPSD values @CoG in terms of ABSOLUTE acceleration" but I don't find a correlation with relative acceleration; what does this mean?
Thank you for helping!
Michele :)

March 28, 2022 at 11:03 pmBill BulatAnsys EmployeeHello Michele This may not answer all of your questions, but for what it's worth, the following MAPDL input posits a springmass system with a damping ratio (DMPRAT command) subjected to a flat unit acceleration PSD. The one sigma UY displacement of node #2 is determined three ways:
1) In POST1 from the SET,3,1 results set
2) In POST26 by taking the square root of the integral of the RPSD of the UY response of node 2 over frequency (a trick I learned in college)
3) Miles' equation (a trick I learned from you)
Results from all three agree well. Notice that I defined the input PSD values in units of acceleration rather than G^2/Hz because of the note in the source I found while researching Miles' equation:
https://femci.gsfc.nasa.gov/random/MilesEqn.html
"NOTE: in this form of the equation, since we want units of length, the ASDinput must be in units of [length/sec2]2/[Hz].You will most likely have to convert your ASD data from g2/Hz to something else."
Best Bill
fini
/cle
/sys,del file*.png
C**********************************************
C*** PARAMETERS
C**********************************************
pi=acos(1)
k=1000 ! STIFFNESS (N/m)
m=1 ! MASS (kg)
zeta=0.05 ! DAMPING RATIO
wn=sqrt(k/m) ! UNDAMPED NATURAL FREQUENCY, DO NOT EDIT
fn=(1/(2*pi))*wn
Q=1/(2*zeta) ! Q FACTOR, DO NOT EDIT
/title,m = %m% kg, k = %k% N/m, zeta = %zeta%
C**********************************************
C*** MODEL
C**********************************************
/prep7
n,1
n,2,,1
et,1,14,,2
r,1,k !,c
et,2,21,,,4
r,2,m
type,1 $real,1 $e,1,2
type,2 $real,2 $e,2
d,1,ux
d,1,uy
d,2,ux
fini
C**********************************************
C*** MODAL ANALYSIS
C**********************************************
/solu
anty,modal
modo,lanb,1
mxpa,all,,,yes
alls
save
solve
fini
C**********************************************
C*** PSD ANALYSIS
C**********************************************
/solu
anty,spec
spopt,psd,1,yes
psdu,1,acel
psdfreq,1,,0.75*fn,1.25*fn ! FLAT SPECTRUM IN Y DIRECTION
psdv,1,1,1
sed,,1
d,1,uy,1 ! BASE EXCITATION
pfact,1,base
psdres,disp,abs
psdres,velo,abs
psdres,acel,abs
dmprat,zeta
solve
fini
/solu
anty,spec
psdcom,,,,total ! USE PSD COMBINATION METHOD
solv
fini
C**********************************************
C*** POST PROCESSING
C**********************************************
/post1
set,3,1 ! READ 1 SIGMA DISPLACEMENT RESULTS INTO DATABASE
uy1=uy(2) ! ONE SIGMA UY DISPLACEMENT OF NODE 2
pldi
fini
/post26 ! ALTERNATE "MANUAL" CALCULATION OF ONE SIGMA UY DISPLACEMENT OF NODE 2
store,psd
nsol,2,2,u,y
rpsd,3,2,,1,1,UY2
int1,4,3,1,,INT_UY2
*get,uy2,vari,4,extrem,vmax
uy2=sqrt(uy2)
C************************************************************
C*** MILES EQUATION
C*** https://femci.gsfc.nasa.gov/random/MilesEqn.html
C***
C*** "NOTE: in this form of the equation, since we want units of length C*** the ASDinput must be in units of [length/sec2]2/[Hz].
C*** You will most likely have to convert your ASD data from g2/Hz to something else."
C*************************************************************
uy_miles=sqrt((q*1)/(32*pi**3*fn**3))
/ann,dele
/tla,0.50,0.90,UY1 = %UY1% (POST1 NODAL RESULTS FROM SET,3,1)
/tla,0.50,0.85,UY2 = %UY2% (SQRT OF INTEGRAL OF RPSD IN POST26)
/tla,0.50,0.80,UY_MILES = %UY_MILES% (RMS VALUE FROM MILES' EQUATION)
/axl,y,UY of NODE #2
plva,3
/sho,png $plva,3 $/sho,close
/eof

April 12, 2022 at 1:27 pmMichelebSubscriberThank you we have been very helpful! I really appreciated the script part along with the comments. I have done some testing and I think I can say that this methods converge only if there's just one natural mode and if the input is a step input, otherwise Miles' one gives strange results.
Linked to this, how do you think I could calculate GRMS of the second or third peak of acceleration? The problem with method n.1 is that SET,3,1 gives me the maximum, whereas methods n.2 involves the integral of RPSD of U, so it changes if I change the bandwidth (extremes of integration).
Thank you again for your time
Michele :)

 You must be logged in to reply to this topic.
Simulation World 2022
Check out more than 70 different sessions now available on demand. Get inspired as you hear from visionary companies, leading researchers and educators from around the globe on a variety of topics from lifesaving improvements in healthcare, to bold new realities of space travel. Take a leap of certainty and check out a session today here.
Earth Rescue – An Ansys Online Series
The climate crisis is here. But so is the human ingenuity to fight it. Earth Rescue reveals what visionary companies are doing today to engineer radical new ideas in the fight against climate change. Click here to watch the first episode.
Ansys Blog
Subscribe to the Ansys Blog to get great new content about the power of simulation delivered right to your email on a weekly basis. With content from Ansys experts, partners and customers you will learn about product development advances, thought leadership and trends and tips to better use Ansys tools. Sign up here.
 How to work with STL file?
 drawing a geometry by importing a table of points
 Parameters not imported into Workbench 18.2 from Solidworks/Inventor
 material properties
 section plane
 Cant import spaceclaim model into fluent mesher
 Ansys Mechanical Unable to get model
 Missing faces on solid bodies
 how to fill solid between two surfaces?
 Importing drawing file to Ansys Design Modeler

1141

1078

495

412

204
© 2022 Copyright ANSYS, Inc. All rights reserved.