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Lumped Capacitor in the RF Module, eigenfrequency

August 3, 2015, 10:43 pm
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Hello all,

I'm using 4.4, and I realize it is impossible to add Lumped Capacitor and Lumped Inductor in RF Module Eigenfrequency study.

(but lumped elements with Z value is possible, and Lumped Capacitor or Lumped Inductor is possible for frequency study)

I think it is because Comsol is looking for a frequency, so if you only give capacitance, how it can calculate impedance lol. My current solution is hand calculate a Z value with approximated frequency and input this Z value, then look for eigenfrequency.

Then my question is, how to use lumped element with capacitor type in Comsol? Is 5.1 or 5.0 can fix this problem? Or any other brilliant solutions?

Regards,
Jared
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MEMS Piezoelectric (Voltage vs. Eigenfreq)

August 19, 2010, 5:58 pm
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Hi,

I am changing the voltage across the piezoelectric beam actuator I've created. I expect to see a different set of eigenfrequency numbers when I change the voltage. However, the numbers remain the same.

Under solve parameters
Analysis Types: Piezo Solid (smpz3d) Eigenfrequency with Eigenfrequency Solver.

I'm reading the eigenfrequency numbers from
Point evaluation > Solution to use > eigenfrequency

Any ideas??

-Albert
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FSI and Damped Eigenfrequency Analysis (Comsol v4.0a)

September 3, 2010, 11:20 am
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Hi to all,
I'm trying to solve some FSI models which correctly ran with version 3.5a.
The models calculated the eigenfrequencies of a solid structure vibrating in a fluid environment. Thanks to the "Damped Eigenfrequency analysis", that was possible, and results were correct.
Now, unfortunately with version 4.0a, without changing any BC, the eigensolver always fails with the error "singular matrix".
Clearly such a study is not a "default" one (as usual, the Fluid Flow module does not have any defaul analysis for the frequency domain) and thus I don't know if it is already possible with v4.0a.
I'm wondering if it is a problem regarding the Fluid Flow equations (they cannot be converted in their frequency counterpart) or regarding the eigensolver.
Is this last already able to solve Damped Eigenfrequency analyses too?
Is there someone which has already tried (also with other physical couplings) this kind of analysis?
Thanks in advance for the answer.

Alessandro
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problem with eigenfrequency

February 23, 2012, 9:04 am
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Hi all,

My project consists in calculating the eigenfrequency of a piezoelectric beam. For the experience, I apply 10V on the top, the ground is on the bottom. After, I mesh the beam, I solve and I obtain the eigenfrequency value.

But my problem is that when I change the mesh (I increase the number of elements), my eigenfrequency value change. And this until the solve stop because I'm out of computer memory. I think it isn't normal.
I can't obtain a correct eigenfrequency and I don't know why! If someone have a solution, please write me.

I work in 2D with MEMS module/Structural mechanics/Piezo plane stress

Thank you

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Eigenfrequency Analysis

May 31, 2012, 2:22 pm
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Hi everyone,

I've been trying to do vibration analysis but keep running into an error that says:

The following feature has encountered a problem:
-Feature: Compile Equations: Eigenfrequency (sol1/st1)
-Error: Out of memory in mesh processing.

From what I've read it sounds like I have too many mesh elements, which I've tried to reduce by choosing extremely coarse on the Size Settings of the Mesh submenu. What else can I do to reduce the number of elements? Any help at all is greatly appreciated.

If there's a tutorial of something similar online, a link to that tutorial would suffice.

Thanks!!
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Eigenfrequency analysis on shells

July 27, 2012, 12:18 pm
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I have been working in 3.5a for a few years now, but have recently started a new project and decided now was the time I should upgrade to 4.2a. In 3.5a, when I had a model of a coupled system of a shell and a solid. I could set the solver to solve for eigenfrequencies on the shell and a frequency response solver on the solid. This would allow me to see purely see the eigenfrequencies generated by the shell structures and how these have effected the solid structure by plotting the displacement on the subdomains. Can this be done in 4.2a or 4.3 (I am about to upgrade again)? At present I am getting many more eigenfrequencies generated than I am interested in.

Hope that makes sense.
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Relation Eigenvalue/Eigenfrequency problem

October 4, 2012, 9:07 am
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I am currently investigating two-dimensional dielectric cavities and their electromagnetic TM/TE eigenmodes. For these studies I use the "Electromagnetic Waves" module while only considering the out-of-plane vector (corresponding to TM modes) and a perfectly matched layer in order to restrict myself to outgoing boundary conditions (Sommerfeld radiation condition). I have a few questions regarding such simulations in COMSOL:

1) How are the eigenvalue and the eigenfrequency studies related to each other?
Typically, in two dimensions the three dimensional EM equations without any sources boil down to

(∇² + ε_r(x,y) k²) E(x,y) = 0

such that one would assume that the eigenvalue λ should be related to the eigenfrequency k simply by λ = k². However, when I simply switch from an eigenvalue study to an eigenfrequency study. I get completely different solutions. The solutions from the eigenfrequency study are not found in the eigenvalue study and vice versa.

2) Comparing the eigenfrequencies from the calculations for a simple circular dielectric cavity to the actual analytic results (which can easily be obtained for such a system), the calculated eigenfrequencies don't match, although convergence regarding the mesh discretization has been reached. Although the real parts of the eigenfrequencies roughly match, the imaginary parts are off by 5 orders of magnitude.

I would be happy for any suggestions!

Best regards,
Tommy
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CONNECTION between 3-D eigenfrequency analysis & 2-D mode analysis

October 9, 2012, 10:12 pm
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I use 3-D Eigenfrequency Analysis vs 2-D Mode Analysis to solve the eigenmodes in conventional ridge waveguides. Even though I have successfully got the expected results, there are still some problems or concepts needed to be clarified, and I seperate them into two main parts:
------------------------------------------------------------------------------------------------------------------------------------------------
Firstly,

Please check the attached PNG file:

I respectively get “Eigenfrequency” in 3-D Eigenfrequency Analysis and “Effective Index” in 2-D Mode Analysis, BUT:

1. What’s the connection between this two calcuation modes, 3-D Eigenfrequency Analysis and 2-D Mode Analysis?

2. How to get the “Effective Index” from 3-D Eigenfrequency Analysis, and “Eigenfrequency” from 2-D Mode Analysis?

I believe these two distinct tools must be able to be linked together!!

------------------------------------------------------------------------------------------------------------------------------------------------
Secondly,

I found there is a package tool contained in the RF Module, called "Eigenvalue mode", which are also shown in my attached file.

I wanna set this tool to be "3-D" mode analysis, but I'm curious about the definition of eigenvalue in the EM wave cases. To my knowledge, the eigenvalues in the EM wave cases described by Maxwell equations should be k0, which is a wavenumber of light propagating in the structure. But as I designate one initial eigenvalue for the solver, it can't derive the same electric field distribution as that in the 3-D Eigenfrequency analysis.

So, here, I just need some "instruction" for how to use the Eigenvalue Mode to achieve the same results as 2-D Mode Analysis and 3-D Eigenfrequency Analysis.
------------------------------------------------------------------------------------------------------------------------------------------------

Your kind reply would be highly appreciated. Thank you for your patience,
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Eigenfrequency Problem

March 12, 2013, 9:49 am
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Good evening,


I'd like to know if there's a way to find the highest, maximum , eigerfrequency of a structure using COMSOL.
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Eigenfrequency study with FSI interface in 4.3a

April 18, 2013, 1:54 pm
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I attempted to use the FSI interface with the eigenfrequency study step using its default configuration under 'custom studies' in the model wizard. The study step indicates a warning, however, so I suspect the eigenfrequency solver settings are not compatible with the physics settings. I included a simple model to illustrate the fault. All eigenfrequency solutions appears at 'NaN.'

Has anyone had any luck with this interface using an eigenfrequency study?
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Cantilever Strain at Resonance Frequencies

June 4, 2013, 1:50 pm
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Hello,

I am new to Comsol and am trying to simulate strain in various structures at their eigenfrequencies. I have no problem finding the eigenfrequencies, but the amplitude in the eigenfrequency study is taken to be around a meter when the cantilever or structure is on the micrometer scale. Because of this amplitude I can't use the eigenfrequency study to find strain in different resonating structures. If I could fix the total energy of the system to limit amplitudes that would be ideal.

Is there a way to limit the energy in the eigenfrequency study so that I can limit the amplitude of oscillation and have the same energys for higher order resonant frequencies? If not, how can I simulate the max strain in a cantilever or any structure vibrating at its resonant frequencies?

Thanks,
Dustin Bigham
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Relative Energy from Eigenfrequency Analysis

October 9, 2013, 2:59 pm
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Hello All,

I have been working with COMSOL 4.0 (mostly heat transfer modules) but now need to analyze a structure with an eigenfrequency analysis.

I can generate the model and the eigenfrequency(s), but I would like to end up with the results presented as a plot of frequency vs energy.

Anybody know the steps I need to take using 4.0a?

Thanks Everyone
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Rigid connector as rotation sensor

January 31, 2014, 8:23 am
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Hello,

I'm trying to use the rigid connector as an angle sensor, but I'm having troubles interpreting the values.

In my case I have a 3D solid mechanics simulation of an object, and I apply a rigid connector constraint to one surface. The "rigidization" of the surface is not a problem for me, since that face is indeed mostly rigid. Additionally, I constraint the rotations around 2 axes (x and z), so that the surface is now only allowed to rotate around y axis. Since my case only allows rotation around Y axis, what I'm hoping to obtain is a number representing the angle I'm rotating around that axis (I know that rotations around more axes become quite complex to understand, but my case should be simpler since is only around Y). And my problem is that I'm unable to extract that number from the simulation.

I run an eigenfrequency analysis, and I can see in the global evaluation that it is possible to access both the "rotation vector" and the "rotation matrix". I also notice that the displacements for each mode are quite large relative to the size of the structure (I'm using mass normalization and it is a light structure), and therefore the angle that I'm looking for is near 90 degrees (I can see that in a deformed shape with scale factor 1).
In the rotation vector, I only have non-zero values on direction Y, but they are complex numbers. for instance:
Eigenfrequency Rotation vector, y component (rad)
11.26198 -6.28319+44.13022i

I also tried looking into the terms of the rotation matrix. According to wikipedia, the rotation matrix of a elementary rotation around Y should look like: upload.wikimedia.org/math/2/8/...c2031127e6dacfb84b96446d8.png. I do see the inner cross of 1 and zeros, but the corner positions are not clear. The (x,x) and (z,z) positions are always 1, which is strange since that position is supposed to be cos(tetha), and tetha is almost 90 (I evaluated those with solid.rotzZ_rig1 and equivalents for other DOFs). Also, I checked (x,z) and (z,x) positions and they have large values (like 1.2e5), which is estrange since those are supposed to be sines of an angle.

So, all in all, I don't know how to extract the angle out of the global evaluations.
Any idea?
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Rigid-body Μodes

February 14, 2014, 9:56 am
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Hello guys.

I am using Comsol more and more these days, so you will be hearing from me regularly from now on. I hope we'll have a nice chat. To the point now:

In Eigenfrequency Analysis of a system, is there somewhere an option not to calculate the Rigid-body modes, or at least not to present them as results?

Thanks a lot,
Nicholas
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Question about complex frequencies

April 2, 2014, 9:54 pm
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Hi!

I'm simulating a MEMS resonator using Thermoelasticity and an eigenfrequency study. As might be expected, I'm obtaining complex frequencies due to the damping.

I looked all over the forum and the answers about the meaning of the real and imaginary parts of the frequency are very confusing to me since nobody makes clear for what stands the real part of the frequency.
So, my question is, what means the real part? Is the undamped resonance frequency or the damped resonance frequency?
Is the imaginary part the damping ratio 'zeta', or the damping coefficient 'c' that is used with the critical damping to obtain the damping ratio?

Thanks for the help!
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Eigenfrequency Error "Cannot evaluate expression"

May 22, 2014, 7:21 pm
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I am trying to compute the Eigenfrequency physics for a mesh I created and imported into COMSOL. The mesh is 3D, complex, and asymmetrical but I have had no problems computing with it in other studies. However, when I run the study I receive the following error:

"Cannot evaluate expression.
Failed to evaluate variable.
-Variable: comp1.acpr.p_t
-Geometry: 1
-Boundary: 1
Failed to evalulate expression.
-Expression: comp1.acpr.p_t
-Plot: surf1 (Surface)"

I suspect the error has to do with the fact I cannot select my mesh as the Sound Hard Boundary (Wall) 1. It says (not applicable) next to the mesh selection. Can I somehow create a boundary of the mesh to use for this? Is this why I am receiving the above error?

Thank you
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Velocity in Eigenfrequency Study

July 11, 2014, 4:34 pm
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I trying to compute the eigenfrequecies and eigenmodes of a model. I have computed the Eigenfrequency study correctly. However, when I look at the generated values for the velocity (local RMS velocity, instantaneous, etc.) they are not what I am looking for and are all close to zero throughout the model. Is this because I am using the Sound Hard Boundary condition? Is the study even computing the velocity? Can I use another study or add another dependent variable that will actually calculate the velocity?

Thanks for any help
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Velocity in Eigenfrequency Study

July 11, 2014, 4:34 pm
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I trying to compute the eigenfrequecies and eigenmodes of a model (material is water) in the pressure acoustics and frequency domain. I have computed the Eigenfrequency study correctly. However, when I look at the generated values for the velocity (local RMS velocity, instantaneous, etc.) they are not what I am looking for and are all close to zero throughout the model. Is this because I am using the Sound Hard Boundary condition? Is the study even computing the velocity? Can I use another study or add another dependent variable that will actually calculate the velocity?

Thanks for any help
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Eigenfrequency with change in temperature

September 8, 2014, 6:41 am
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I have a gold beam fixed at both the ends. I would like to know the eigenfrequency(resonance frequency) of the beam with change in temperature(cooling the surroundings), please help me how to do that, how do I change the material properties(Young's modulus, density, Possion ratio) as function of temperature.
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Symmetric meshing

April 29, 2015, 11:30 am
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Dear all,

I'd like to learn if it is possible to have symmetry axis in mesh. I'm working on a 3D model, and the geometry is symmetric in both x and y axis. I'm studying on eigenfrequency analysis and I'd like the mesh to be symmetric in x-axis only.

To be more clear, I don't expect to see symmetric results for all mode shapes of course but when the mode shape is known to be symmetric by analytical means I want to see a more realistic illustration of it.

Thanks,
Utku
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