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Boundary Variable not being Recognized

September 26, 2014, 10:17 am
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I am having a bit of a problem with defining a boundary variable that has a counterpart defined on the model domain but with a different value.

For a simple spherical particle, I am modeling the classical diffusion equation dc/dt + div(J) = 0 Where J is the flux defined as J = -D*grad(c) using a general form PDE. For the domain I am defining a domain variable D as a function of concentration c (i.e. D(Domain) = f(c)) and for the boundary that completely encloses the domain I define a boundary variable D as a constant value.

For additional information, I define a boundary flux "Jo" on the boundary as well.

When I solve this problem and plot intermediate "D" values, I find that D changes on the boundary which I did not expect to happen.

Can someone please give me some insight on what might be going on or what I am possibly doing wrong?
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Too Much Stress & Strain Cause Damages

September 27, 2014, 12:07 pm
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The release of Windows, rumoured to be called Windows 9 or possibly Windows 8.2, is just a few days away. Here, we'll track the latest leaks and rumours about the update, letting you know what to expect from the next version of Windows, which Microsoft has confirmed will be announced on 30 September. Alain Crozier, president of Microsoft France, accidentally let slip the "Windows 9" name during a press event held ahead of the 30 September launch, which was live streamed on the Microsoft France website. His slip of the tongue has been taken as confirmation the next version of operating system will indeed be called Windows 9, although the head of marketing and communications in France has attempted put a stop to the rumour. He told ZDNet the event at the end of the month is about "the future of Windows, which doesn't currently have a name". The gaffe comes shortly after a Microsoft post on Chinese blogging site Weibo appeared to suggested the next version of its OS will indeed be called Windows 9. The post, which seemed to be of the operating system's logo, has since been taken down, and may have been no more than a slip-up by local staff, but adds weight to reports that Microsoft is keen to drop the tarnished Windows 8 brand in favour of a fresh start with Windows 9. According to recent rumours, Microsoft is set to show off Windows 9, codenamed Threshold, to the press at a dedicated event on 30 September. This rumour stems from an article posted on The Verge that claims it received information about the event from “sources familiar with Microsoft’s plans". Details about what exactly the Windows 9 launch event will show off is thin on the ground. However, it is tipped that the new hybrid Start Menu will be unveiled along with the removal on the Charms bar.

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Meshing Tiny Thermal Vias

September 18, 2014, 3:53 am
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Hi everyone,

By using the Joule Heating Modul, I am trying to get the max. temperature around an electronic component. I have studied similar models before; but this time addionally placed several tiny (in radial direction) and long thermal vias across the PCB.

You can imagine those as really narrow and relatively long cylindrical tunnels connecting both sides of the PCB.

I figured that the "build all" takes forever and meshing gives memory errors. Even if I use an extra coarse mesh the simulation seems like it would take ages.

What would be an ideal (meshing or some other type of) approach to those structures?

Thanks,
Bartu
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Piezoelectric Transducers

August 25, 2014, 7:03 am
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Hi
I am analyzing the signals in piezoelectric transducers. Can somebody help me in finding some of the difficulties I am facing.

1) How could I measure electric potential of the container on which an piezoelectric is placed.

2) How could I calculate the vibrations being observed by the piezoelectric when one piezoelectric is used to emit the vibrations and other is used to receive the vibrations.

Kindly suggest me an answer to the above questions, which would be a great help to my project.
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error message

September 28, 2014, 7:31 pm
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I got a message in equation complaint like the picture show, what 's the problem it is?
how can I solve it?
"Interpolation grid axis values must be sorted in ascending order in each dimension.
Error in multiphysics compilation"
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Two Phase Fluid Flow Convergence Error half-way into solution

September 29, 2014, 7:59 am
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Hi All,

Time dependent solver for Laminar Two-Phase Flow, Level Set crashes half-way through the solution with error message "last time step didn't converge". Do you know what could cause this? The geometry is a simple y-junction. The mesh is user controlled and done by creating distributions (rectangular) and sweeping them.

Thanks
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Solving Self - Consistent Schrodinger and Poisson with MATLAB and COMSOL LiveLink.

September 6, 2012, 12:44 am
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I'm a COMSOL 4.2a user. I need to solve 2D self-consistent Schrodinger and Poisson equation. Thus, I need to feed the potential values and the wavefunctions to the Schrodinger and Poisson equations as argument and solve iteratively.

The thing I need to know, how can I feed the updated the wavefunction and potential values as argument iteratively in my model? I have livelink with MATLAB and I intend to solve the problem inside a loop (i.e. Calling one models while passing the result of another as argument). I have solved the models with PDE solver but cant do it iteratively with LiveLink.

One more thing, is it possible to access the results directly from MATLAB and thus use them as variable ( though a class of course) from MATLAB for post-processing, like we get the solution matrix in MATLAB?

--
Thanks
Mehedi
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Convergence problem in RF module

April 1, 2013, 8:16 am
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I am doing some plasmonics simulation using silver nano particles in comsol. For silver particle convergence is either very very slow or there is no convergence in most cases for some specific wavelengths. The same model works fine for gold nano particles. Kindly help.
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To demonstrate a "sieve" at the particle tracing module

February 27, 2014, 1:14 am
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Hi all, now I'm trying to demonstrate a particle sieve using the particle tracing module.
I expected that the particles bigger than the pore size of the sieve could be filtered and stuck on the sieve surface.
But it seems that the particle tracing module cannot demonstrate such effect because every particle can pass through the pores even which are much smaller than those particles!

Do you think that I have to set up a sort of force equation or "if-condition" which can describe the sieve effect by the size difference??

Thanks!!

Dukwoo
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Piezoelectric Transducers

August 25, 2014, 1:03 pm
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Hi
I am analyzing the signals in piezoelectric transducers. Can somebody help me in finding some of the difficulties I am facing.

1) How could I measure electric potential of the container on which an piezoelectric is placed.

2) How could I calculate the vibrations being observed by the piezoelectric when one piezoelectric is used to emit the vibrations and other is used to receive the vibrations.

Kindly suggest me an answer to the above questions, which would be a great help to my project.
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Convergence versus meshing granularity

September 12, 2014, 6:47 pm
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I have noticed interesting behaviour in my models.

If the model will solve using a coarser mesh, it may not solve well using a finer mesh.

And although use of the boundary layer tool can reduce the wall lift-off, it does not necessarily reduce convergence time, and often can lead to convergence instabilities. Sometimes I can only add 2 boundary layers.

Also, if I use an initial field from a coarsely meshed solution, the convergence behaviour for the finer mesh will only begin to act erratically earlier.

I have noticed these effects using the k-epsilon and k-omega turbulence models.
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Filling of a capillary channel

September 23, 2014, 3:02 am
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Remco, hi!

Have you ever tried to simulate filling of a capillary channel in 3D? I've been trying to do that for some time, but still with no success. Maybe you know how to do that correctly?

Regards,
Kirill
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Catching errors in parametric sweeps

September 23, 2014, 3:12 am
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Dear all

Is there an elegant way to catch errors in large parametric sweeps?
E.g.: When varying values of parameters one might well reach a point when convergence (or other) problems are encountered.
It would be a very nice feature if COMSOL would log this error and continue the sweep with the next parameter value(s) instead of stopping the whole sweep.
Is this possible even when not running COMSOL via the Matlab interface?
Or would something like the "try catch" method in Matlab be the only way to achieve this?

How do you treat this issues?

Thanks for interesting hints on this and best regards,
Juergen

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"Update scaled absolute tolerance"

September 23, 2014, 3:56 am
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About the "Update scaled absolute tolerance" check box.
This feature of the Time-dependent solver has been on by default in the last few versions of Comsol.

In some models in the Model Library, the instruction is to uncheck it, but it's given without comment. For example, in these models:
• Capacity Fade in a Lithium-Ion Battery
• Peristaltic Pump
• Nonlinear Acoustics: Modeling of the 1D Westervelt Equation

I can't find a single theme in them (but I haven't looked in-depth).

From the documentation, when the check box is checked, the behaviour is described essentially as an interpolation in time, between the absolute tolerance value as entered into the "Absolute tolerance" field, and the same thing multiplied by the maximum value of the dependent variable taken over the entire field, where it is smaller than 1. (Plus, the L2 norm of it is added, which will always be smaller than the L-infinity norm, so that must be for the case where L-infinity norm is greater than 1.)

So it seems reasonable (and, in fact, better) to adaptively adjust the absolute tolerance to be relative to the max value or the L2 norm of the dep. variable, like that.

My questions are:
• When should this *not* be used?
• In particular for the above sample models, why has this feature been disabled?
• Is there a way to access the value of the absolute tolerance as it's being adapted throughout the time-dependent simulation?
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problems with linear projection

September 23, 2014, 9:07 am
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Hi all,

I developed a model for calculation of AC losses and I am using Linear projection operator to ensure sufficient current flow (Point-wise constraint later). The problem is that my linear projection operator fluctuates a lot. You can see that in attached graph. What could be the reasons?
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Simulating capacitive device operating at collapse voltage

September 24, 2014, 2:29 am
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Hi,

I'm trying to simulate an capacitive ultrasound transducer operating at collapse voltage. The model, made of two parallel capacitor plates, are expected to collapse into each other during simulation, then its eigenfrequencies are to be calculated. Is this possible with COMSOL? If not, would it work if I start the simulation using an already collapsed model?

Thanks,

Sergei
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Simulation in Finned Tubes

September 24, 2014, 6:02 am
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How to do heat transfer and flow simulation in Finned Tubes

I tried a lot ; but i could not able to give correct boundary conditions.

Please help me to solve this problem.
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Difference between optimization and parametric sweep?!

September 24, 2014, 9:28 am
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Hi Comsolers,

Could you kindly tell me that what is the difference between optimization module and parametric sweep in comsol, please?
I know that maybe it is simple question, But I want to need when a user choose optimization or parametric sweep for his/her model?

Thanks in advance for your help.
Sepideh
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Smoothing battery current changes

September 24, 2014, 6:28 pm
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I've been having issues with the capacity fade model and I think I may have narrowed down the problem.

See the model in question here: www.comsol.com/model/capacity-fade-of-a-li-ion-battery-12667


I am trying to increase the number of cycles, and I usually encounter errors in the transition between constant voltage discharge and constant current charge. The abrupt change in current density from ~+0.1 A/m^2 to 10 A/m^2 usually results in a lot of error and a large voltage spike which completely interrupts the charging-discharging cycle determined using the events interface.

As I understand it, the discontinuity of the current input is not something COMSOL likes.

I see that there are smoothing functions available, but I am not sure how to use them in concert with the events system used to determine the current load in this model. Any help would be appreciated.
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resistance measurement

September 25, 2014, 1:18 am
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Hi,
I am working on joule heating and thermal expansion module.
I want to measure resistance between two terminals. No voltage is applied to the terminals, no current flow initially. Later through induction or channel formation current will flow and it will change the resistance.
How do i measure the resistance initially and after current flows.
Thanks
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