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Natural frequency analysis

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Samviens95

Samviens95

Aerospace
Joined
Nov 17, 2020
Messages
8
Location
CA
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

FEMAP_Analysis_ajc3bd.png
 
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what output have you asked for ? frequencies and displaced shape (ie nodal displacements) ??

another day in paradise, or is paradise one day closer ?
 
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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.
 
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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 ?
 
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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
 
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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 ?
 
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Here is a screenshot of the analysis output request:

analysis_ej31tp.png


analysis2_pq9eul.png
 
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This is the kind of plot I am looking for:

Plot_I_want_xnxiaw.png
 
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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 ?
 
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When I but my excitation of 0.25G in the Y axis I get this for the accelaration vs frequencies:
AY_zhxw2x.png


When I put the excitation in the X axis I get this for acceleration and displacement:
Ax_oz93cu.png

T1_xlctxb.png


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?
 
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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:

Natural_freq_e1t5jt.png


Does it make sens or not at all?

By the way thanks for your answers rb1957
 
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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 ?
 
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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.
 
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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 ?
 
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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:
iberisa.wordpress.com

• Análisis Dinámico Avanzado con FEMAP y Simcenter Nastran-I: Modal Frequency Response (SOL111)

La intención de este POST es enseñar porqué a menudo un simple Análisis Estático Lineal (SOL101) con FEMAP y Simcenter Nastran no basta para validar un diseño, siendo necesario realizar Análisis Di…
iberisa.wordpress.com iberisa.wordpress.com

BRACKET-SETUP-ANIMATED_sruchx.gif


Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48004 BILBAO (SPAIN)
WEB: Blog de FEMAP & NX Nastran:
 
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