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Internal stress does not affect mechanical eigenvalues.

December 11, 2013, 10:55 am
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Hello,

I am trying to use "Initial Stress and Strain" in Solid Mechanics to tune the mechanical frequency of a (doubly-clamped) micro-beam.

I am trying to use "Pre-stressed Eigenfrequency Analysis" which lets the beam relax then should use this as the linearization point for calculating the eigenfrequencies.

This technique works okay in 2D but not for a 3D model. No matter what I apply to the "initial stress and strain" tensor, I cannot change the eigenvalues. I do know the stationary solver is working; adding stress does change the displacement in a sensible way.

Is there a solution to this? Is it a bug? Enclosed is what I'm looking at.

Thanks in advance,
Jack
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Webinar - New Features of COMSOL Multiphysics 4.4 - December 12

December 11, 2013, 11:10 am
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Hello!
Join us for a free webinar tomorrow, December 12th, to learn about what's new in COMSOL Multiphysics 4.4.

2:00pm CET (Paris)
www.comsol.com/events/1585/int...on-to-comsol-multiphysics-4-4/

2:00pm EST (Boston)
www.comsol.com/events/1583/int...on-to-comsol-multiphysics-4-4/

Ask your questions about COMSOL 4.4 during the webinar, Bjorn Sjodin, VP of Product Management, will answer them during the Q&A session.

See you at the webinar,
Valerio

--
Valerio Marra, Ph.D.
Technical Marketing Manager
COMSOL, Inc.
+1-781-273-3322
valerio@comsol.com
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COMSOL 4.4 Installation-Error

December 11, 2013, 1:50 pm
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Hi,

I am trying to install COMSOL 4.4 in my machine. I have already installed COMSOL 4.3b and its license manager. I would like to keep 4.3b version in my machine too. When I setup a new license manager for 4.4, and open the COMSOL 4.4, I am getting the following error.

License error: -25
License server system does not support this version of this product.

Could anyone help on this?.

Thanks,
Krish
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Electrified fluid...

June 27, 2011, 2:06 am
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Hello,

I am just getting started with Comsol (4.1) and am attempting to model the shape of a fluid droplet as it is deformed by an externally applied electric field (i.e: electrospray).

So far I have managed to calculated the electric field for a chosen geometry (stationary solution) using the Electrostatics physics module. I have also used the laminar two-phase flow, level set physics module to calculate the dynamic shape of a fluid meniscus at the end of a capillary with pressure applied.

The problem I am having is with combining these two types of physics so that I can correctly model my situation.

What is the appropriate method to use in modelling this experiment?

Is it possible to add the force from the electric field into the level set calculation?

I have read that the ALE method may be more appropriate than the level set one because there is always a clearly defined boundary for the electric field to act upon. The problem is that the fluid meniscus is expected to deform into a thin jet. This might be too large a deformation for the ALE method to handle.

What about an iterative method in which the geometry is re-meshed between steps? Is this possible to achieve in Comsol?

Any advice that you can offer will be gratefully received.

Thanks,

Matt
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Creating solution objects and assigning values from MATLAB in COMSOL 4 (similar to femsol)

September 9, 2011, 3:24 am
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Hello,

I am trying to adapt from COMSOL 3.5a to COMSOL 4, and so far, the biggest functionality that I miss while using the MATLAB connection is the following:

I want to be able to create solution objects and assign these solution objects to fem's, so that I can use COMSOL solvers to process the dataset I want, or more importantly, to be able to plot those datasets.

I was able to do this by using the following command in COMSOL 3.5:

fem.sol = femsol(u,'tlist',tlist)

where the solution data set I want to have in fem is present in u. In other words, I was able to manipulate the doffs by manipulating u. Is there a corresponding functionality in COMSOL 4?

Thanks in advance.
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parameters names must not be empty

September 13, 2012, 1:20 pm
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Hey,

again, this is an error I ha with comsol 4.3: parameters names must not be empty

I need a quick help

attached is my comsol file, you just need to run it and see the error.
and also i attached a printed screen of my parameters table and the error window in case some one is not having comsol to run my file.

Naema
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How to Export the material data to .mphtxt file?

December 4, 2013, 5:57 am
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I am trying to export the data from Comsol. I could get the mesh data if I click the 'mesh export'.
But I also need the material data for each mesh cell (tetrahedron). Since I cannot link the Comsol with Matlab, I was wondering if I could export the material data to .mphtxt file or something I could read it by some codes in Matlab.
By the way, my Comsol is 4.3a.
Thank you in advance.

haozhan
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Need help in output acoustic pressure in air domain

December 11, 2013, 6:40 pm
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Hello,everyone
I used Pressure acoustic, frequency domain and electromechanics Physics, 2D-axisymmetric to simulate a vibration membrane to get the output pressure in the water domain. The result is good. But when I changed the water domain into air domain, the shape of the output pressure line is very strange, the line is very flat at the beginning, but change very sharp unexpected.
The x-coordinate is frequency. I don't know why.could anyone can give me some advises please? Thank you in advanced!
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How to calculate the normal heat flux on a boundary

December 12, 2013, 1:59 am
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Hi everyone. I´m a beginner with COMSOL and I don’t understand how it calculates the normal heat flux on a boundary.

In order to evaluate how to derive values in the Results section, I have followed the Heat sink tutorial included in COMSOL 4.2a and once I get the results of the simulation, I calculate the Surface Integration of the Total normal heat flux (nitf.ntflux) on surfaces 2, 3, 4 and 5. These surfaces have Thermal Insulation boundary conditions by default; hence, I expected to obtain 0 W on each surface. However, the values calculated by COMSOL are nitf.ntflux = 0.01353 W (surface 2), nitf.ntflux = -0.03251 W (surface 3), nitf.ntflux = 0.03545 W (surface 4), nitf.ntflux = 0.01363 W (surface 5) and nitf.ntflux = 0.0301 W (surfaces 2, 3, 4, and 5). In this model, there is only a heat source of 1 W on surface 8; therefore, the calculated Total normal heat flux on the isolated surfaces is higher than 3% of the supplied power.

Are these results misunderstood? Or there is something wrong with the model? Or…

Could anyone help me to understand these results?

Thank you and regards.
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Disk error while writing to to temporary file

December 12, 2013, 5:33 am
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When I try to open up larger COMSOL files I get this error. What does it mean and how can I avoid it?

I have tried in two different versions of COMSOL (4.3b and 4.4) and get the same error no matter where I save the file.
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recover matrix

December 12, 2013, 5:55 am
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hello everyone

I made a resolution a PDE on COMSOL and I would like to to recover the resolution matrix using Matlab.

is that possible?

thank you for your help
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Plotting ratio of displacement

December 12, 2013, 6:10 am
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Hi,

I am using COMSOL 4.2.1.166, and am simulating a circular membrane as a Structural mechanics 'plate'. I simulate to get the mode shape of the fundamental mode for a uniform load in the normal direction and intial stress in the x-direction. I now would like to plot the ratio of the vertical defelction along the x and y axis as a function of the radius, i.e. something like
w(x=0,y,z=0)/w(x,y=0,z=0)
with vertical displacement w of the fundamental mode vibration. I can't find how to access the vertical displacement along a line (e.g. x-axis w(x,y=0,z=0) ) to be able to plot it. Does anyone know how this can be done?

Best
Martin
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Best way to find the average velocity over a cross section of an axisymmetric flow

December 12, 2013, 7:00 am
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I'm attempting to find the average velocity over a cross section (of an 2d axisymmetric flow profile) vs distance along the flow channel (say z direction) in COMSOL. The only way I can find to do this is to create cut lines at intervals then create a 1-d plot with the data and export this to a spreadsheet for further processing. Is there a better way? I was trying to calculate an average using the cut-lines but what ends up happening is that COMSOL takes all of the velocities in the cut lines and averages them, so I end up getting the average velocity in the channel.

Thanks in advance for any advice!
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Meshing problem

December 12, 2013, 8:01 am
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Dear colleagues,

I'm modelling a thin walled element (solid geometry) and I want to mesh it with an hexahedral mesh with only one element along the thickness. I attached an image of the geometry.
The main problem is the complex geometry with circular holes and curves. I try to mesh a surface with quadrilateral element, but I can't do it on surfaces with holes.
Is it possible to mesh it with quadrilateral elements? If it is possible I can extrude them and obtain what I want.
If it is a problem I'll work with a triangular mesh.

I hope I was clear enough

Thanks for any suggestions
Best regards

Simone
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UMFPACK

December 12, 2013, 3:23 pm
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I am using COMSOL 4, I want to solve a nonlinear equation. I can not find UMFPACK in solver list!!! Could you tell me where it is?!

Thanks.
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Piezoelectric Simulation (pzd) in comsol 4p2 for FBAR filter

January 20, 2012, 7:50 am
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Hi,

There is a parameter in COMSOL 4.2, in piezoelectric simulation, pzd.Y11 which gives you input impedance if you simulate a one port device. I think this correspnds to Y11 parameter of Y matrix. I am simulating a 2 port device (filter) and need to get Y21 and Y11 parameter to be able to calculate S parameters (Y12 and Y11 is not necessary since it is symmetric and reciprocal). I couldnt see pzd.Y21 parameter in the tutorial or help, but when I defined it as variable, it looks it is working if I set the voltage of the terminal 2 as 0 V. When I set the voltage of the second terminal different from 0 V comsol cannot calculate pzd.Y21 parameter, which makes sense to me. I believe that is correct but would like to verify with someone else if anyone tried using such a paramater.

Thanks for the help!

Seyit

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Geometric Nonlinearity problem

February 2, 2012, 10:49 am
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I have an odd problem. I am trying to recreate the Large Deformation Beam model (www.comsol.com/showroom/documentation/model/204/) before I start my own project to investigate buckling. I was following along with the model, but when I go to Study 1 > Step 1: Stationary and open Study Settings, I do not see any buttons that say "Include Geometric Nonlinearity". Indeed, I do not see any buttons at all.

When I download the support file for the Large Deformation Beam model, I do see this button.

However, if I delete the stationary solver and add a new one, I do not see the button any more.

It seems like I must be doing something dumb here, but I can't seem to figure out what. Any help would be appreciated.

Thanks,
-jfb
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Recovering desired eigenfrequencies

December 20, 2012, 7:38 am
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Hi,

I'm performing an eigenfrequency analysis of a model in the Comsol 4.3a RF module. My problem is that, though I ask for only 1 eigenfrequency, Comsol ALWAYS gives me 2 eigenfrequencies: one purely real, and one complex. This is frustrating me because when I try to probe the eigenfrequency, it always probes the wrong eigenfrequecy (the complex one), so the only way I can extract the data for a 50 point sweep is to manually go through and extract the proper eigenfreq. The correct eigenfreq is always at the bottom of the dropdown menu, which in previous versions of Comsol always corresponded to the first solution.

I figure there are two solutions to my problem:

1. Figure out how to make Comsol only give me 1 eigenfrequency (should work if the bottom solution does still correspond to the first one it found).

2. Tell Comsol to probe eigenfrequency 1.

I can't seem to figure out how to execute either of these solutions. I'm attaching a couple screen shots of my model builder to help explain.

Any advice would be greatly appreciated.

Thanks, Chris
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Mesh Problem

December 6, 2013, 9:16 am
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Hello,

I have problem with mesh generation for my model. I made a 3D glass beads with lots of grains.
When I want to mesh that I faced some error in meshing. sometimes it meshed but then during study meshing makes a problem: (Failed to generate mesh for domains :...)
I spent lots of times to figure out the problem, but no success,
I would appreciate if any body could help me.
Thanks,
Saeed
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Hollow fiber membrane

December 12, 2013, 5:02 pm
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I want to model hollow fiber membrane, I have a problem with reaction term in tube side. this term changes the continuity equation into nonlinear type. this model has three section; tube, membrane and shell. unfortunately, NO CONVERGE! do you have any idea? or experience about this type of model.

Thank you.
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