eigenfrequency - Latest Discussions - COMSOL Forums

problem with eigenfrequency

Thu, 23 Feb 2012 12:04:43 +0000

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

difference between eigenfrequency and frequency .

Fri, 27 May 2011 08:01:53 +0000

hi

can anyone explain what is the difference between eigenfrequency and frequency .

let us consider an eigenfrequency : 2.34567e5i or 4.87965e7 + 2.567e8i

so what will be frequency in both case .?

Eigenfrequencies of a Thin Rectangular Plate

Fri, 11 Feb 2011 15:54:22 +0000

Hello,

I am trying to run an eigenfrequency analysis on a thin rectangular plate. I selected a random material, Steel AISI 4340, and dimensions of 12 in. x 16 in. x 0.04 in. The boundaries along the thin part of the plate are set to fixed. The mode shapes seem to be correct, but I am getting many errors of the following type:

Error: The relative residual is greater than the relative tolerance.

Error: The relative error is greater than the relative tolerance.

I have tried two different mesh types, but I receive the same type of errors with both. The first mesh that I tired was a free tetrahedral mesh. The second type was a swept mesh that swept from surface to surface through the thinnest part of the plate. With the swept mesh, I have set the distribution of elements to various settings from 3 elements up to 15 elements.

I have attempted to check my answer using plate theory, but so far, the answers do not match. I have attached my file. Also, as a note, I do not have the structural mechanics module, and I am using the base COMSOL Multiphysics.

Does anyone know what the problem is?

Problem with the eigenfrequency solver using matlab

Wed, 09 Feb 2011 10:09:52 +0000

Hi,

I use Comsol Versoin 3.5. I'm searching for eigenfrequencies of a vibrating membrane under stress.

After I solved the eigenfrequency problem, I saved the m-file and reopened it with Comsol. Unfortunately, it now calculates eigenvalues instead of eigenfequencies.

To reproduce the problem, create a new file (MEMS Module -> Structural Mechanics -> Solid, Stress-Strain -> Eigenfrequency analysis), define a random block, initialize mesh and solve the problem. Go to Plot Parameters -> General -> Solution to use, and note, that it shows some eigenfrequencies there. Then go to File-> Save as.... and save it as a Model M-file. Reopen the just saved m-file and again, go to Plot Parameters and there you will see, that it calculated eigenvalues instead of eigenfrequencies.

Checking the solver parameters after loading the m-file, I noticed that Comsol is no longer able to auto select the solver. It also doesn't show the eigenfrequency solver, just the eigenvalue solver.

Is there a way to calculate eigenfrequencies using matlab?

Thanks a lot

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

Fri, 03 Sep 2010 12:20:30 +0000

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

MEMS Piezoelectric (Voltage vs. Eigenfreq)

Thu, 19 Aug 2010 18:58:04 +0000

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

Eigenfrequency Analysis in Fluid-Structure Interaction

Fri, 25 Jun 2010 09:39:31 +0000

I'm trying to calculate the resonance frequencies of a fluid filled hollow shell and the results I get are NaN for each frequency, but it works when I use the Pressure-Acoustics (acpr) physics mode (I get frequencies and realistic mode shapes) on some arbitrary shape.

Here is my setup process (using Acoustics Module in version 4.0)<check attached file for further clarification>:

Load physics and the liquid water material. Load the Linear Elastic Material Model (from the physics tree "branch")

Draw two concentric spheres, set the outer one as solid elastics model, leave the inner as acoustics pressure.

From the Union.

Acoustic-Structure boundary is automatically assigned to their interface (8 surfaces)

Set material properties (Young's Modulus, Poisson Ratio, Density and get three green checks) manually in the solid outer sphere.

Do a normal Free mesh and then compute 18 or so eigenfrequencies.

Everything else was left to default

This results in all NaNs.

From my understanding, the computer now knows everything it needs to calculate the frequencies: forces and accelerations from material properties; geometrical relationships in the functions; and the mesh. It knows what part is fluid, what part is solid so I don't understand why its not working. It might be something in the solver

Matlab script for effective modal mass extraction

Fri, 15 Jan 2010 12:15:27 +0000

Hi everbody,

as posted frequently in this forum, COMSOL does not provide effective modal masses in eigenfrequency analysis for structural analysis. But since effective masses are a very convenient way to determine critical modes from less critical ones, and since this information is often required for documentation, I wrote a Matlab script that extracts the effective masses from an eigenfrequency solution and exports them to an Excel file.

This is how it works:
1. Copy the attached matlab script to your disk (preferably to a directory that is already in the Matlab include path, e.g. C:\Users\...\Documents\MATLAB on windows).
2. Run COMSOL and do the eigenfrequency analysis there.
3. Save your COMSOL eigenfrequency analysis to an .mph file of your choice.
4. Connect COMSOL to Matlab (you don't need to export the fem object to Matlab though).
5. In Matlab, execute the attached effective_mass_analysis.m script (open/run or if located in the Matlab workspace simply type 'effective_mass_analysis' in the command window).
6. The script asks you which .mph file to open, calculates the effective masses and saves them to a Microsoft Excel spreadsheet (you will be prompted where to save it).

In the Excel file there are four different sheets: 'Total Masses', 'Effective Masses', 'Mass Fraction' and 'Cumulative Mass Fraction'. These results resemble the standard output of other FEM software, such as Ansys that provide effective mass output by default.

Unfortunately, this script does not work for large models since the matrix size that Matlab can handle is limited and it may also fail when more than one application mode is used. I have cross-checked the results with only one reference case from Ansys, so there is no guarantee that the script works correctly with all models. Any comments and suggestions are greatly appreciated.

Cheers,
Lars

2D plane strain mode vs. 2D axial symmetry stress-strain mode

Wed, 23 Sep 2009 13:37:16 +0000

I am relatively new to structural mechanics and am trying to explore the various application modes of the structural mechanics module. I have used the 2D plane strain mode to obtain the eigen frequencies of the radial and torsional modes of a circular 2D geometry. Is there a way to obatin the same results through the 2D axial symmetry stress-strain mode?

I am also trying to obtain the longitudinal vibration modes of a very long cylinder. Can COMSOL solve for these modes? What is the best suited module to solve for these modes and if possible, what would be the constraints/symmetry conditions to selectively find these modes, without the radial/torsional/flexural modes coming up in the solution set as well?

Thanks!

eigenfrequencies analysis

Wed, 16 Sep 2009 11:27:11 +0000

Hello
I have built 3D geometry with structural module to find eigenfrequencies, but the results are different from fem model realized with another tool. In fact the first three frequencies obtained with comsol are 2.6,53,73 Hz but they should be 3.6, 68, 125 Hz. Both models have no damping, the same weight, the same materials. I try to change apps mode properties(large deformation on) but I have found the same results. I don't understand where i could make a mistake.
Thank you
Luca