Natural frequency analysis

Question

Natural frequency analysis

Samviens95 (Aerospace)

(OP)

17 Nov 20 19:19

Hi,

I got my natural frequencies from Siemens NX and went in FEMAP to obtain the plot of my peaks at those frequencies. The problem is that I am new to this and I don't know how to get a good plot. I watched a lot of tutorial but nothing is working. Here's a description of my analysis:

- A CubeSat fixed on one end (4 tips)
- Frequency range: 20 to 2000 Hz
- First natural freqeuncy: 211.218 Hz

Note: I am only analysing the structure with no added mass

rb1957 (Aerospace) · Answer 1 · 2020-11-17 21:15:08

what output have you asked for ? frequencies and displaced shape (ie nodal displacements) ??

another day in paradise, or is paradise one day closer ?

Samviens95 (Aerospace) · Answer 2 · 2020-11-18 15:31:57

I ask for the displacement in output request from «Manage» of the study. And in my plot I asked for T2 translation since my excitation is in this axis.

rb1957 (Aerospace) · Answer 3 · 2020-11-18 16:09:08

yes, but did you ask NASTRAN to output these values ? (so FeMap can display them)

another day in paradise, or is paradise one day closer ?

Samviens95 (Aerospace) · Answer 4 · 2020-11-18 16:12:30

I am not sure what you are asking. Where do I have to ask NASTRAN to do it? Because I am using FEMAP for my study. Is it in the Manage window that we ask this?

Thanks

rb1957 (Aerospace) · Answer 5 · 2020-11-18 17:05:02

FeMap is only the front and back end of the FEA (the pre-and post-processor). NASTRAN is the FE engine under the hood; when you select model\analysis.

another day in paradise, or is paradise one day closer ?

Samviens95 (Aerospace) · Answer 6 · 2020-11-18 17:13:24

Here is a screenshot of the analysis output request:

Samviens95 (Aerospace) · Answer 7 · 2020-11-18 17:19:00

This is the kind of plot I am looking for:

rb1957 (Aerospace) · Answer 8 · 2020-11-18 18:38:34

so you see you are using NASTRAN ... "Simcenter Nastran"

you have asked for displacement at each node, but can't plot the deformed shape ?

another day in paradise, or is paradise one day closer ?

Samviens95 (Aerospace) · Answer 9 · 2020-11-18 19:08:05

When I but my excitation of 0.25G in the Y axis I get this for the accelaration vs frequencies:

When I put the excitation in the X axis I get this for acceleration and displacement:

The peak is at a frequency of 190 Hz instead of 211 Hz as I was expecting with the X excitation and I dont understand why it doesn't start at 0 displacement?

Samviens95 (Aerospace) · Answer 10 · 2020-11-18 21:03:00

I think I have something better now. I did my 3 axis back to back on the complete frequency range and it gives me this:

My natural frequencies are suppose to be:

Does it make sens or not at all?

By the way thanks for your answers rb1957

https://files.engineering.com/getfile.aspx?folder=088bcede-ecfc-4d8d-948c-9

rb1957 (Aerospace) · Answer 11 · 2020-11-19 13:47:19

when you say "are supposed to be" ... is this school work ?
or are you rerunning someone else's model ?

this thing is 350mm tall ?

your tables are confusing to me ... is ordinate "350" the top or the bottom of the model ? (the restrained end or the unrestrained end)

Not sure why none of your results look like the plot you want (17:19 on the 18th Nov, showing the maximum response of a input (ay = 0.25g) ?

another day in paradise, or is paradise one day closer ?

Samviens95 (Aerospace) · Answer 12 · 2020-11-19 14:47:36

In fact I use the same excitation of 0.25G, but the graph I sent at 17:19 on the 18th Nov was an example I followed. My dimensions are:
(100 x 100 x 200) mm
I fixed one end and put my excitation at the same end. I look at 1 node at the other end. In my graphs, x axis is the frequency and y axis is the acceleration. My frequency range is 20 to 2000 Hz. In my last post I put a word file if you didn't see it. But I got somethnigs to change still.

rb1957 (Aerospace) · Answer 13 · 2020-11-19 15:56:20

when you "fix one end" ... is that constraining 6 dof at a bunch of nodes ? That may not be correct. You should model exactly as your example. Maybe they said "fixed" but maybe they constrained 3 dof (translation) at each node. Even this is not "good" practice ... the constraint should avoid infinite stiffnesses.

another day in paradise, or is paradise one day closer ?

BlasMolero (Mechanical) · Answer 14 · 2020-11-19 22:44:46

Hello!,
Take a look to this post and you will learn how to perform a modal frequency response dynamic analysis (SOL111) using FEMAP & Simcenter Nastran:
https://iberisa.wordpress.com/2020/01/07/analisis-...

Best regards,
Blas.

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Ingeniero Industrial
Director

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