Plot Velocity-Mobility FRF

[ansysnewbee]

Hello,

Iam trying to get a mobility plot (velocity/frequency) from my ansys model. I ran a full harmonic analysis and I have the displacements.

I would like to convert these displacements to velocities, to plot mobility FRF's.

Could anyone tell me how to do it in ANSYS?

Sorry if its a basic 101 question, new to dynamics..and ansys..

Emily

 

[Drej]

You can't do this directly, and certainly not in ANSYS. Your harmonic analysis is based on calculations done in the frequency domain, hence you can't obtain velocities since the derivative of displacement wrt frequency is some unknown quantity (and is certanly NOT velocity). Sorry to be the bearer of bad news...


Cheers,

-- drej --

 

[ansysnewbee]

I strongly feel you are wrong. There are tons of papers where researchers have calculated the velocity from response obtained from a harmonic analysis.

Velocity is simply {v}=j*omega*{u}
where {u} is nodal displacement and {v} the nodal velocity.

Emily

 

[Drej]

Wrong how? Maybe there are tons of papers but (1) you don't say where these tons of papers are or (2) which code was used to produce these velocities or (3) whether you've bothered to look in the ANSYS help file regarding velocities from harmonic analyses. So let me quote you the help file:

"The results data for a harmonic analysis are the same as the data for a basic structural analysis with the following additions: If you defined damping in the structure, the response will be out-of-phase with the loads. All results are then complex in nature and are stored in terms of real and imaginary parts. Complex results will also be produced if out-of-phase loads were applied. See Review the Results in Structural Static Analysis"

"Command(s):
NSOL for primary data (nodal displacements)
ESOL for derived data (element solution data, such as stresses)
RFORCE for reaction force data"

Your query isn't very clear, but it appears to be that you're asking two very simple questions:

(1) how do you calculate velocities from displacements?
(2) how do you do this in ANSYS?

The output from a harmonic analysis in ANSYS is like I said to you before: primary nodal ouput (displacements) and secondary (stresses) based on harmonic frequencies. If you want to calculate velocity BASED ON MANUAL CALCULATIONS of your displacements, then of course that's very easy within ANSYS.

 

[ansysnewbee]

Again, everything you said is correct, but I dispute your 'fact' that velocities/accelerence cannot be obtained from a FE displacement result set.

Look at this thread..I had posted the same question:

http://www.eng-tips.com/viewthread.cfm?qid=115189&page=1

See, others do understand me..

Emily.

PS: When I say I have read tons of papers in which ther have done this, I mean it. As a matter of fact, in acoustics, it is common practise (see ANSYS/LMS SYSNOISE, ANSYS/Comet, Nastran/CDH) to convert displacements to velocities, and then use these velocities as B.C's in acoustic models.

 

[Drej]

Sigh. If you read my response *carefully* I'm not disputing the fact you can obtain velocities from displacements; I know this, I've been working with ANSYS for a long time.

I gave a response based on what you asked. What YOU were appearing to ask was: does *ANSYS* give you velocities from harmonic analyses? No. Can you calculate these from harmonic displacements? Yes. Simple.

PS. > See, others do understand me..
So why aren't there more responses to your thread in here? Everyone appreciates a clear, unambiguous and detailed description of the problem, hence hopefully in the future everyone can benefit from this thread (from both the content and any *other* lessons learned).

 

[ansysnewbee]

Ok Drej, Iam sorry..didn't read your post properly.

Yes, its ANSYS implementation was what I was after..Sorry if my post wasnt clear.

So, what I did was(working in Hz.):

created variable 100 (-1*Pi)
created variable 101 (100*freq)
created variable 102 (101*disp)

Variable 102 is mobility..

Am I right?

Many thanks..

Emily.

 

[ansysnewbee]

BTW: My orginal post does carry the 3 magic letters.. FRF

-Emily.