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termination technique in eigenvalue solver

April 20, 2016, 5:52 pm
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Hello,

I would like to ask is there any way to stop the eigenvalue solver after some iterations, like it can be done with segregated solver?
If there is no such way, is there another solver which returns eigenvalues and also allows to stop the running on a precise number of iterations?

All the Best
Ashot
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EMW Analysis Methodology

April 20, 2016, 9:56 pm
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Can anyone offer any specific details on the the Robust, Intermediate, and Fast settings and how they relate to the Mesh and Solver? There isn't much coverage in the reference manuals and guides and various blog posts - I've only ever seen it mentioned in passing. Thanks.

Nick
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Heat Source in Turbulent High mach number flow !

April 22, 2016, 3:57 am
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Dear COMSOLERS ;

I am trying to simulate an electric arc in a flowing gas through a nozzle. as you can see in the attached model, the arc heat is been added as a heat source in the hmnf interface. also i created a ramp parameter to sweep the heat source value in the auxiliary sweep.

Now , as the ramp value goes higher , the solver gives the following error :

Failed to find a solution for all parameters,
even when using the minimum parameter step.
In Segregated Step 2:
Segregated iteration leads to undefined function value.
There was an error message from the linear solver.
The relative residual (1.2e+003) is greater than the relative tolerance.
Returned solution is not converged.

I tried different type of meshes , ramping the materiel properties and it didn't work out.
it would be great if any could help me solve this problem.

Best Regards ;
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Current conservation issue when integrating charge density

April 4, 2016, 2:30 pm
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I am working with an AC/DC, 3D, Stationary, Electric Currents model.

I am currently studying charge density between deep brain stimulation electrodes of different surface areas and geometries. To represent an oversimplified electrode, we created a cylinder with high conductivity (1e10 S/m) and a point current source at it's center. The cylinder was in a conductive medium similar to the brain, 0.2 S/m.

When evaluating this simplified problem, I encountered some error that seems violate current conservation. Specifically, I am finding 47% error between the point current source (0.001 A) and the integral of current density (ec.normJ) over the surface area of the cylinder.

Note: I did conduct a mesh convergence study - below the current mesh size (0.00002), the error does not notably improve.

I then explored this issue with different geometries (spheres, ellipsoids, cubes, rectangular boxes, etc). See page 1 of the attached TroubleshootingComsol.pdf for the results. To quickly summarize, symmetric geometries - spheres and cubes - integrate to a current within 2% error. However, non symmetric geometries, ellipsoids, cylinders, rectangular boxes - have unacceptable integration error (11-63%).

*****Question: Any ideas of the possible sources of error in the simple problem? Any suggestions to further troubleshoot?

An an alternative method, I also explored representing the electrode as a cylindrical shell with floating potential of 0.001 A. In this case, the integral of normJ is 0.0004- less than half than expected. The integral of nJ was closer - 0.0009. To troubleshoot this, I went back to a sphere, with similar results - integral of normJ was 0.00048 and integral of nJ was 0.00097. See page 2 of the TroubleshootingComsol.pdf for more details.

*****Question: I originally believed that normJ, the total current density norm, was the output of interest when studying charge density on a surface, but I am confused by the fact that it is substantially lower than nJ in the floating potential situation.

I greatly appreciate in advance any tips, explanations or corrections.
Sincerely,

Ashley
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Phase change not working, warning inverted element

April 24, 2016, 5:49 pm
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I am trying to simulate the heating and phase change of a low melting point metal. In the mph file that I've uploaded to Google drive (drive.google.com/file/d/0B3QaD21DDcpNeFdlYjJwNWE4Rzg), you can see the innermost helical heating coil, surrounded by a helical silicone tube. A small air gap exists between these two domains. The silicone tube is immersed in the low-melting point metal which is in turn contained by two concentric cylinders of silicone.

When solving, I get a warning (Inverted element near ~0,0,0). This cannot be, there is no material near the origin.

The solution shows that the model is being heated but I cannot find any evidence of phase change in the Field’s metal (you can see this by plotting out Table 1 in results section). As far as I can see, I have set the Physics options correctly. Can you please give me a clue as to what I might be doing wrong?

When I improved mesh quality, warning disappeared, but solver hung as soon as it was time for phase change (at melting point) and seemed to stay there forever. I tried to improve convergence by changing fully-coupled settings (max no.of iterations- 25 and tolerance - 1e-3), but this did not work either.

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Assembly Convergence Problem

April 24, 2016, 6:22 pm
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I have a model in which I would like to model contact between two domains. The physics of the model (boundary conditions etc) works just fine if I don't include the contact. Using "form union" at the end of the geometry sequence, the model behaves as I would expect. But if I wish to model the contact, I have changed this to "form assembly".

Having looked at a few tutorials and documentation, I know that I also need to set "create pairs" to true and set "pair type" to contact pair, then include the contact pair in the physics sequence. But this does not seem to be enough, as with these changes made, but with no other changes the model suddenly does not converge.

The converging model is attached as "g_p_reflow", while the broken model is attached as "g_p_reflow_ass".

Any suggestions?
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TIME to FREQUENCY FFT

March 20, 2016, 9:36 pm
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Dear all,

I am simulating pulse echo technique using ultrasound. I have my results in time domain. Now I need frequency spectrum of my time dependent study result. In study 1 "time dependent " time range is = range( 0, 5.0e-8, 9.0e-5 )s and generated ultrasound frequency is 1 MHz.

So I add study 2 "Time to frequency FFT" in my COMSOL model. Now I dont know what to do. I tried once and get 0 Hz frequency result.

Thank you
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inverted mesh element near coordinate

January 15, 2015, 4:56 pm
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Hi dear all

I want to see the mechanical changes due to flow of current through two parallel wires in COMSOL. To do this, first I used two "magnetic field" and "solid mechanic" modules. After getting the results I noticed there is no filed change resulted by mechanical changes which happens due to magnetic forces. After reading previous discussion, I used "moving mesh" module to execute the changes. But then I faced this error:
"inverted mesh element near coordinate"
What I have to do in order to solve the error?

I used this terms in my modeling:
1)free deformed(for all domain, 2-wire and air)
2)prescribe mesh displacement(just for boundary of wire )

With best thanks and regards
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Energy balance with variable density

January 22, 2016, 6:04 am
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Hello everyone,

I am interested in finding the temperature in a system that changes the density in time and I would like to formulate the energy balance using this variable density.

What I have in mind is to use

Cp * d( rho T)/dt = Q

but I find it pretty tricky to implement it in COMSOL, because of the derivative. Does anyone have any experience with this?

Nomenclature: Cp is the specific heat, rho is the density (rho = f(t)), T is the temperature and Q is the heat source in W/m3, which can be any number (just for reference)]
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How do I compare the voltage difference of 2 boundaries with respect to the position along it?

April 26, 2016, 10:03 pm
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Hello COMSOL users,

Im am trying to replicate the study attached that was performed at a COMSOL conference but i have ran into complications. (The file is attached below)
I don't understand how to get the voltage difference between 2 points on a boundary. These 2 points must be a part of the boundary (but not specifically defined) and the voltage differential I want to get has to lie directly across from it.
So say I'm working the 2D axial symmetrical plane and have a boundary at r .5 and another at .505. These lie directly across from each other and have the same dimension in z. Using an electrostatics module (or electric current) I apply a voltage to my structure and wish to know the difference in voltage in relation to the z component.

Please let me know if you have any ideas on how I should go about solving this. The paper says that I should use extrusion but I am not clear on how. I have read many different threads and COMSOL examples and still don't fully understand how to use it. I tried using the boundary probe but this did not seem helpful.

Note: I believe this paper has some mistakes in the writing.

Regards,
Stephen
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Linking to PSPICE Software

April 27, 2016, 2:13 am
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Hi,

I am new to COMSOL and would like to model a piezoelectric energy harvester in structural mechanics module. Gradually, I would want to model the mechanical part in COMSOL and link it to PSPICE where the electrical circuit part will be done. So please provide me with the proper steps and guidance so that I can try it and study the performace analysis.



Thanks in Advance,
Jain Jose
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Combine two solutions

February 5, 2016, 8:53 am
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Hey all,

I have two consecutive time dependent studies and two solutions sol1 and sol2. In postprocessing I would now like to "stitch" these two solutions together, so that instead of having to deal with two data sets sol1 and sol2 I just have one data set solComb that contains the combination of these consecutive two solutions.

I want to be able to access the time(1) which lies in sol1 and time(6) which lies in sol2 with just one data set.

Thanks in advance for your help

best

Paul-Remo Wagner
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How to sweep through different boundary conditions using parametric solver

April 27, 2016, 6:52 pm
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Hi,

I am investigating the current distribution in water, it requires me to send a voltage into a medium (water) through an electrode and measure through another electrode.
My problem is how to change the boundary condition (i.e. Electric Potential) using parametric sweep. I have tried using using the tag ({pot1}) to reference the parameter names in parametric sweep and using the selections within the parameter value list (grouped the boundary condition using Explicit (Definitions)).

Many thanks,
A.Onilude
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Defining a variable limited only for the boundary

April 28, 2016, 7:46 am
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Can we define a dependent variable for a physics which is only valid for particular boundary and not for the whole domain?
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maxm no. of newton's iterations reached & parametric solver

January 2, 2011, 1:35 am
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Question about newton's iterations:

I keep getting the following error

Failed to find a solution.
Maximum number of Newton iterations reached.
Returned solution has not converged.

I am unable to locate where the solver is pulling up these Newton's iterations from. Can someone please guide me where to find the number for Newton's iteration? Also, any insight about why I am getting this error will also be very helpful.
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Question about parametric solver:
I am wondering if there are any tutorials/examples that show me how to use the parametric solver. I am trying to use it along with a stationary solver because I am having convergence issues but I am not sure how to use it. For my problem when I choose to use the default solver, I get the iterative solver. So I am a little confused about how the parametric solver works. Should I be using it along with the iterative solver?

Thanks for the help.

Sirisha

PS: Happy New year to all!
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Could you please tell me the name of the variable for time steps in comsol?

April 26, 2012, 6:21 pm
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Hi

I am working in comsol 4.2a version for hydrogen storage system modelling. Could you please tell me the name of the variable for time steps in comsol?
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max timestep

April 29, 2016, 11:35 am
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Hi guys,

is it possible to connect (max) timestep of the solver with a variable x (which is changing with time)?
if x gets big the next timestep should be smaller.

max timestep on auto does work but does not give the best result
and if max timestep is set to something small the results are much better until a point where the solver fails ...
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Warning: SOR line did not find any lines.

April 30, 2016, 5:30 pm
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Hi everyone,
I am running a laminar two-phase flow (level set) simulation with a 3D geometry. I had previously simulated the same geometry in 2D with the same initial values and boundary conditions and have no problem reaching a solution. However, when I try to run the simulation with a 3D geometry, the warning "SOR line did not find any lines" keeps coming up and eventually I get error saying
Nonlinear solver did not converge.
Divergence of the linear iterations.
Time : 0
Last time step is not converged.

Has anyone encountered this warning before and know what to do with it? Thanks a lot in advance!
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COMSOL Shut Down

May 2, 2016, 2:58 am
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When I am done making a model, I click on "Compute" in "Study 1" to start computing. But for some reason, before the computation is complete, the program automatically shuts down. I was told a separate window for progress should open up, but it does not. When I look at the progress at the bottom right of the COMSOL window, I notice that when the program was just about to run PARADISO, it automatically shuts down. What could be the reason? I just recently installed this program on my laptop. (My computer runs on Windows 10.)
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Weak form for 2-D elasticity

April 15, 2011, 2:03 pm
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In an effort to learn COMSOL, I decided to model the two-dimensional elasticity equations using the weak form. I was surprised looking through the documentation that there are very few example problems, so I didn't have much to go on. I think I'm deriving these correctly, but I could use another pair of eyes:

-(lamb+2*mu)*test(ux)*ux-mu*test(uy)*uy-(lamb+mu)*test(ux)*vy

-(lamb+2*mu)*test(vy)*vy-mu*test(vx)*vx-(lamb+mu)*test(vy)*ux

lamb and mu are the lame coefficients. u=u(x,y) and v=v(x,y) are the displacement fields.

One question that I can't figure out is how to apply stress boundary conditions. Can anyone help with that?
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