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frequency domain analysis

Posted Apr 11, 2012, 4:20 p.m. EDT Structural Mechanics Version 4.2a 7 Replies

Arjan de Wildt
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Dear all,

I'm trying to simulate a cantilever-like hinge, as the first simple step of a large project. For that I'm trying to run a frequency domain analysis but somewhere I'm doing something wrong cause even this VERY simple model is driving me crazy. I have already spent many hours searching through the Comsol support and have read every post on cantilever and frequency domain but it does not yet answer my question.

The results I'm getting are strange, I namely plot frequency (x) over y-displacement of a point on the right tip of the 2D cantilever (y). However, I'm just getting a flat line instead of some spikes here and there for the resonance frequencies. It doesn't matter what frequency domain I'm sweeping it with, with the added model it is between 1 and 1000 in steps of 10. The 2D cantilever is fixed on the left and I applied a point load on the top right node. After solving I make a 1D plot, point graph but it gives me nothing just a flat line. What am I doing wrong / forgetting? Could someone PLEASE help me it's driving me mad, must be so simple!

Thanks in advance!
Henk de Winter
7 Replies Last Post Jul 19, 2012, 5:18 p.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Apr 11, 2012, 4:49 p.m. EDT
Hi

which variable are you plotting (in solid the displacements are u,v,w and not x,y,z) could that be the point ?

The next item could be the excitation, do you have an excitation defined, with a non-zero amplitude ? (check the equations and your BCs)

Just a few guesses, not easy to say more ;)

--
Good luck
Ivar
Hi which variable are you plotting (in solid the displacements are u,v,w and not x,y,z) could that be the point ? The next item could be the excitation, do you have an excitation defined, with a non-zero amplitude ? (check the equations and your BCs) Just a few guesses, not easy to say more ;) -- Good luck Ivar
Arjan de Wildt
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Posted: 1 decade ago Apr 11, 2012, 5:15 p.m. EDT
Dear Ivar,

Thanks a lot for your quick reply! What you suggested regarding what I'm plotting could be the problem, although I thought of it already and tried my best to plot the right variables, but to no alas. What I've done:

1) indeed in solid mech. the displacements are in u, v, w, (i believe the problem to be here) but wherever I look I cannot find a possibility to plot u, v, or w on the y-axis. I've tried to plot everything from displacement field, displacement amplitude, material coordinate - all in Y direction - but all give weird results. The best sounding choice is "displacement field Y, u component" but also no real results are obtained. Is this sufficient or would you have an idea what I could try?

2) as regards the excitation with 0 amplitude : I have rigidly attached the left side of the beam and applied a downward force (-10 N) in the Y direction. (also tried higher forces) Then, for the frequency domain analysis I define a frequency range from (tried all possible combinations) 1:10:1000 Hz. With 'no function applied'. Have also tried applying a exp^10 from range(2,0.1,3) but nothing works. Should I apply a sin? What range should be used then (again: tried many combinations already).

Would you have any idea where I'm going wrong? All I want for this simple model (so I can move on with the hard part) is a graph showing resonance peaks. I manually calculated the first eigenfrequency to be around 18.000 Hz but don't see anything happening around the higher frequencies either.

Thanks so much for your help.
I have attached my model. THE ATTACH FILE BUTTON FAILS, SORRY

Best regards,
Henk de Winter
Dear Ivar, Thanks a lot for your quick reply! What you suggested regarding what I'm plotting could be the problem, although I thought of it already and tried my best to plot the right variables, but to no alas. What I've done: 1) indeed in solid mech. the displacements are in u, v, w, (i believe the problem to be here) but wherever I look I cannot find a possibility to plot u, v, or w on the y-axis. I've tried to plot everything from displacement field, displacement amplitude, material coordinate - all in Y direction - but all give weird results. The best sounding choice is "displacement field Y, u component" but also no real results are obtained. Is this sufficient or would you have an idea what I could try? 2) as regards the excitation with 0 amplitude : I have rigidly attached the left side of the beam and applied a downward force (-10 N) in the Y direction. (also tried higher forces) Then, for the frequency domain analysis I define a frequency range from (tried all possible combinations) 1:10:1000 Hz. With 'no function applied'. Have also tried applying a exp^10 from range(2,0.1,3) but nothing works. Should I apply a sin? What range should be used then (again: tried many combinations already). Would you have any idea where I'm going wrong? All I want for this simple model (so I can move on with the hard part) is a graph showing resonance peaks. I manually calculated the first eigenfrequency to be around 18.000 Hz but don't see anything happening around the higher frequencies either. Thanks so much for your help. I have attached my model. THE ATTACH FILE BUTTON FAILS, SORRY Best regards, Henk de Winter
Arjan de Wildt
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Posted: 1 decade ago Apr 11, 2012, 5:18 p.m. EDT

Dear Ivar,

Thanks a lot for your quick reply! What you suggested regarding what I'm plotting could be the problem, although I thought of it already and tried my best to plot the right variables, but to no alas. What I've done:

1) indeed in solid mech. the displacements are in u, v, w, (i believe the problem to be here) but wherever I look I cannot find a possibility to plot u, v, or w on the y-axis. I've tried to plot everything from displacement field, displacement amplitude, material coordinate - all in Y direction - but all give weird results. The best sounding choice is "displacement field Y, u component" but also no real results are obtained. Is this sufficient or would you have an idea what I could try?

2) as regards the excitation with 0 amplitude : I have rigidly attached the left side of the beam and applied a downward force (-10 N) in the Y direction. (also tried higher forces) Then, for the frequency domain analysis I define a frequency range from (tried all possible combinations) 1:10:1000 Hz. With 'no function applied'. Have also tried applying a exp^10 from range(2,0.1,3) but nothing works. Should I apply a sin? What range should be used then (again: tried many combinations already).

Would you have any idea where I'm going wrong? All I want for this simple model (so I can move on with the hard part) is a graph showing resonance peaks. I manually calculated the first eigenfrequency to be around 18.000 Hz but don't see anything happening around the higher frequencies either.

Thanks so much for your help.
I have attached my model. ; I tried to attach it but the button fails

Best regards,
Henk de Winter


[QUOTE] Dear Ivar, Thanks a lot for your quick reply! What you suggested regarding what I'm plotting could be the problem, although I thought of it already and tried my best to plot the right variables, but to no alas. What I've done: 1) indeed in solid mech. the displacements are in u, v, w, (i believe the problem to be here) but wherever I look I cannot find a possibility to plot u, v, or w on the y-axis. I've tried to plot everything from displacement field, displacement amplitude, material coordinate - all in Y direction - but all give weird results. The best sounding choice is "displacement field Y, u component" but also no real results are obtained. Is this sufficient or would you have an idea what I could try? 2) as regards the excitation with 0 amplitude : I have rigidly attached the left side of the beam and applied a downward force (-10 N) in the Y direction. (also tried higher forces) Then, for the frequency domain analysis I define a frequency range from (tried all possible combinations) 1:10:1000 Hz. With 'no function applied'. Have also tried applying a exp^10 from range(2,0.1,3) but nothing works. Should I apply a sin? What range should be used then (again: tried many combinations already). Would you have any idea where I'm going wrong? All I want for this simple model (so I can move on with the hard part) is a graph showing resonance peaks. I manually calculated the first eigenfrequency to be around 18.000 Hz but don't see anything happening around the higher frequencies either. Thanks so much for your help. I have attached my model. ; I tried to attach it but the button fails Best regards, Henk de Winter [/QUOTE]
Megha Agrawal
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Posted: 1 decade ago Jul 17, 2012, 9:24 a.m. EDT
hey have u been able to solve ur problem?
hey have u been able to solve ur problem?
Arjan de Wildt
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Posted: 1 decade ago Jul 18, 2012, 4:50 a.m. EDT
Hey, no unfortunately, I have not received any response and am still stuck.
Regards, Henk
Hey, no unfortunately, I have not received any response and am still stuck. Regards, Henk
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jul 19, 2012, 1:49 a.m. EDT
Hi

have you really tried a search on the FORUM, the model library and the model exchange ? I'm sure this has been discussed a few times and that there are models around here

--
Good luck
Ivar
Hi have you really tried a search on the FORUM, the model library and the model exchange ? I'm sure this has been discussed a few times and that there are models around here -- Good luck Ivar
Sankha Mukherjee
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Posted: 1 decade ago Jul 19, 2012, 5:18 p.m. EDT
Hi,

Can you share your model, may be then we can look at it and get a better understanding as to why you are having this problem.

-Sankha
Hi, Can you share your model, may be then we can look at it and get a better understanding as to why you are having this problem. -Sankha

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