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Laser soft/hard tissue ablation

November 1, 2015, 6:25 pm
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Greetings!

I am quite new to COMSOL, but i have uneasy task to do as far as i understand. I have to model a moving pulsed heat source (laser) and simulate ablation of human tissue. So far i have already implemented pulse heat source using explicit events. From what i have read in forums, adding movement to heat source means defining a function of motion and ablation means usage of deformed mesh. The problem is that i do not completely understand how it can be done and i would be very grateful for any tips and advices anyone can give or share.

Thanks in advance!
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Mesh problem in a magnetic core and coil model

November 2, 2015, 3:07 am
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I built a structure contains a magnetic core and multi-turns of coils wounding it,
when it comes to mesh, it works perfectly at first,
but after I add an air(medium) block covering it, the mesh always failed.
I tried to mesh step by step, mesh the coil first, magnetic core second, it turns out successful.
When it comes to the air, I got an error 'Intersecting face elements', I don't know why.
If anyone knows how to solve it, that will be great.
Thanks

PS:The geometry was created by SolidWorks, and I imported into Comsol.
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Symmetric Meshing Error

November 2, 2015, 7:11 am
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Hi all,

I have a yz-axis symmetrical 3D model and I want to copy the mesh from one side to the other. In other words, I want to mirror the Mesh.

I've tried what Henrik Sönnerlind suggested in this Thread:
www.comsol.com/community/forums/general/thread/73291
but this only works if I have just ONE domain.
My model consists of several domains mirrored at the yz-axis. I fail to copy the mesh of several domains at once. I always got the error message "Different number of edge elements".
I tried it with two different Mesh operations, with "Copy Domain" and "Copy", but both gave the same error as stated above.
For clarity I attached the model with the error message.

Does anybody have experience with symmetric meshing?

Thanks in advance
Christian

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Mesh : resolving a part much thinner than the rest of the volumes

November 3, 2015, 7:49 pm
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Hi,

I don't know much about the different options for meshes and their mechanism of creation. I am studying a geometry where the is a layer much thinner than the rest of the geometry. 1cm becomes 1 micrometer on this layer, where the temperature will vary very fast ( time dependent study of a few microsecondes).

I try different options without really knowing their impact. Sometimes the mesh can't be created because the smallest element size is too large and if I bring it down enough, I end up with meshes with far too many points and it slows the calculation.

Do you have any advice for me ?
For example, how could I get elements much larger in two directions than in the third one ?

Thank you very much
Simon
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Mean operator requires an adjacent entity of higher dimension

November 4, 2015, 5:22 am
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Hi there.

I am trying to simulate my model as a stationary turbulent flow model. I cant get it to run since it keeps comming with that problem warning box: "Mean operator requires an adjacent entity of higher dimension". I have tried to alter the mesh to some crazy fine ones and the other way around. I need som help here to get this model running I think, because i really dont know what is wrong and how I can fix it!. Is it really a mesh problem or is it a boundary problem?
Hope someone can help me out.
I cant attach the file since it is to big. Here is a link to a folder where the file can be found: www.dropbox.com/s/mpgpvwqqjt5n5dd/Model.mph?dl=0

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Issues with Multiple Types of Meshing in Model- Faces Locked by Adjacent Domain

November 5, 2015, 3:03 pm
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Hello all!
I've been having issues with my COMSOL project. I figured out how to add Swept Mesh (Triangular) to where I needed it, but I could not add Free Tetrahedral Mesh to the remaining geometry because some of the domains and faces/boundaries need Triangular Mesh. I was able to add Free Triangular Mesh to the two domains I needed, but when I try to do the same with the faces, I kept getting an error that claimed "The face is locked by adjacent mesh and cannot be remeshed." I even got the error message when I tried to use Boundary Layers to mesh the faces. These faces are adjacent to my swept mesh domains but I cannot get rid of the swept mesh nor can I just leave parts of my model without meshed.
Can anyone help me figure out how to completely mesh my model without compromising my swept mesh? Please let me know if you need more information on my project.
Thank you,
Devin
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Meshing Error

July 17, 2012, 8:01 pm
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I am new to COMSOL and am trying to model an electrostatically actuated cantilever where the beam is thicker near the tip than at the anchor. When I try to build the swept mesh, I get an error that says "Unsupported topology of domain: domain must be bounded by one connected face component only". What does this error mean? What is the best way to get around it?

I attached my model file where block 1 and 2 form the beam, block 3 is a grounded terminal, and block 4 is the surrounding air.

Thanks,
Scott
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3D Symmetry of Model

July 1, 2013, 5:41 pm
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Good afternoon all,

I am modelling two electrodes (tips facing each other) and the electric field between them. There are two Z-axis layers to each of the electrodes, measured in nanometers, whereas the width of the electrodes are measured in the hundreds of micrometers. Further, the electrode tips are half-cylinders. As you can imagine, this leads to some mesh-compiling problems (where it only compiles when the electrode width is set to a few dozen microns, instead of the desired hundreds), and I'm looking for solutions to be able to create and render a larger mesh.

The best hope to a solution (unless there are any you can recommend) seems to be symmetry, as both electrodes and the environment around them are identical.


I don't think 2D axial symmetry, extruded into 3D, would be able to suit my needs.

How would I do 3D symmetry along an axis? I've scoured the COMSOL forums, and while they are normally very helpful (thank you to everyone who takes the time to respond, you aren't just helping the OP) all the advice on symmetry is very vague and esoteric to a beginner like myself.

Thank you for your help,
Daniel
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Simple heat exchanger model error

November 11, 2015, 7:49 am
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Hi everyone,

I'm working on a heat transfer problem and could use some help. It concerns a large horizontal pipe containing an elongated u-shaped pipe (the 'loop'). The pipe contains air and the loop contains water. The water inside the u-shaped pipe is flowing and is laminar. The air in the larger pipe is initially at rest. There is heat being produced inside the u-shape. So heat is being transported from the loop through the pipe by convection and out of the model (using the external natural convection option inside the heat flux node). I'm interested in the steady state temperature in both pipe and u-shaped pipe.

Initially I started working on a simpler model which only contained 2 cylinders (ie. I used a cylinder instead of the loop). This model worked perfectly, but when I switched the smaller cylinder for the loop I started getting errors.

1. The direct solver gave the error of a singular matrix
2. the iterative solver returned a error message saying it ran into an undefined value.

I was wondering if these errors can be caused by poor meshing, since my boundary conditions are exactly the same for the working model and the non working model? Any help on solving my problem would be greatly appreciated!
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Different Solution when "Non-Relevant" Parameter is Changed in 3D (but not in 2D)

November 18, 2015, 9:38 am
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Hi all

First of, thanks in advance for reading and hopefully reply to this thread.
The main problem is, that when I change a parameter in a 2D simulation, the results do not alter and that's how it's suppose to be. However, when I change the same parameter in a 3D model, all of a sudden the results jump randomly.

If you're interested in the problem, read the subsequent section. If not, you can just simulate the attached COMSOL file with different "margin_side" values.

I try to simulate the reflection and transmission of a mirror.
Here is how the Simulation works:
- An input and an output port are defined.
- The input port is excited with an electromagnetic wave
- The wave travels within a waveguide towards the mirror
- At the mirror the wave is then partially reflected and partially transmitted
- Thus, some of the incident wave reaches again the input and some the output

Here is how the Reflection and Transmission is calculated:
- The scattering parameters at the input and output port (S11 (reflection) & S21 (transmission)) are a measure of how much of the incident wave reaches the input and the output port
- In order to calculate the reflection of the mirror, the power reflected at the mirror is divided by the power reaching the mirror. This can be expressed in a formula like this:
P_in=P0*exp(2*imag(emw.beta_1)*z0)
P_ref=10^(S11dB/10)*exp(-2*imag(emw.beta_1)*z0)
P_ref / P_in = 10^(S11dB/10)*exp(-2*imag(emw.beta_1)*2*z0)
where P0 is the excitation power which was chosen to be 1W
z0 is the distance between the input port and the mirror. In the simulation z0 = "margin_side"
-The transmission is calculated likewise.

So now let's come to the problem:
In theory, when I change the distance between input port and mirror, neither the reflection nor the transmission should change. When I simulate the problem in 2D, the simulation works like expected (R & T do not alter).
In 3D however, changing z0 gives a drastic change in reflection as well as transmission. Additionally, the change is not monotonic but the R and T jump from value to value in a random manner (see attached pdf file).

Does anybody have a clue where this problem might come from? Has anybody had similar issues?
I don't think that meshing is the problem, because the problem still exists if the mesh is made finer. Furthermore, finding the wrong excitation mode at the input port can also be omitted, as the effective refractive index stays the same for all different distances (see attached pdf file).

Best Regards
Chris


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Request for basic help

November 19, 2015, 11:08 pm
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Hello everyone,

I'm very new to comsol and am looking for some help with a capacitor simulation.

Right now I have 2 parallel plates and a charged sphere simulated. I need the plates to not move and to calculate the force on the sphere.

It seems there are many different "fixed mesh" kind of things. What do each of these do? What's the best way to accomplish what I'm looking for?
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Fault Modeling

November 24, 2015, 12:34 pm
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Hello everyone,

I was just wondering can I model a fault, or generally a discontinuity in COMSOL? If yes, how ? Can ((deformed geometry)) be used ?


Thanks a lot,
Faraz
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Coupling poisson equation and level set method

November 25, 2015, 9:33 am
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Hi all,

I have been trying this for months now, couldn't figure it out.

Is it possible to couple poisson equation with level set method,

I have a simple pressure equation in the form of poisson equation and the interface moves because of the pressure difference of inside the domain and outside. And I want to track the moving interface using level set method.

Can I achieve this in comsol?

Best regards
Ashan
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Electric field for concentric spheres with a current source

November 26, 2015, 7:19 am
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i am trying to compute electric field and potential for a four concentric spheres with different connectivity. inject a constant 1 [A] current from one cylinder (source) and eject from other one(sink).
for selection and symmetric purposes i had created a number of cylinders in different positions but i am using only two of them at once.
i have done some refinement for mesh purposes.

i am trying to impose the following boundary conditions:
1. Continuity of the normal component of the current density in all the interior boundaries.
2. Make the potential uniform on top of all the cylinders surfaces .
3. Electric insulation in the external boundaries.

initial values:
1. 1[A] source
in the process of building the geometry with layers i made the radius a bit larger? is it necessary?
do i need used Dirichlet Boundary Condition ?
do i need Laplace or just use the Electric Currents is sufficient?
how can i set a source of 1 A from one of the cylinders?
if i using a 1 V as initial value from one cylinder and disable the current source the solution converges, why?
i think my mesh is fine but again i am not sure.

i am know i am doing something wrong but i am not sure what.
how can i know if the solution is reasonable?
any suggestions will be very appreciated.
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Meshing helix geometry

November 27, 2015, 5:33 pm
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Dear Ma'am/Sir,

I am trying to mesh helix geometry since last 1 month, but everyday i encounter new errors. So, till date i have seen 15-20 type of meshing errors.

I tried different meshing techniques but all the techniques have without fail generated error.

Can anyone plz help me in chalking out a method or sequence or manner by which i can mesh this helix.

The file size is big so if anyone wants to have a look plz mail me(cdesai@mtu.edu) or post on this thread.

I am attaching image of it for primary reference.

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Simulationtime for mapped meshes in 2D and 3D

January 25, 2013, 6:11 am
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Hi,

Normally I used triangular meshes to simulate thin films on cantilever beams. Because of the very high aspect ratios I 've tried to use a mapped mesh for the rectangular geometry. The amount of mesh elements decreases drastically while the result is nearly similar.

Now I've got two questions about the simulation-time:

a) If i compare the simulation-time between triangular and mapped meshes with the same geometry and amount of elements the simulation with a mapped mesh needs nearly twice as long as the other one. Why is that? I assume Comsol calculates internally with triangular meshes and therefore with the double amount of elements for a mapped mesh. Is my assumption correct?

b) The same problem in 3D needs much much more callculation-time for a mapped mesh than for a tetrahedral mesh. Why is that?

Thank you
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Fluid-Structure Interaction Particle blockage

November 30, 2015, 2:46 am
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Hello! Have a nice day!
I am solving one problem in COMSOL Multiphysics Fluid-Structure Interaction model (FSI) at the moment. I need to take results in which particles must blockage porous media. I solved this task until halfway after then appeared errors: Condition for remeshing still fulfilled after remeshing. When one particle close porous throat this particle automatically remeshes until minimum quality after that mesh quality reaches minimum and run time stops. And thus another particles could not reach other porous throats. I put below some movies about particle tracing.
My email address: dansho.91@mail.ru

My questions:
1) How can I stop meshing a particle which is blocked pore throat and for other particles let continue meshing?
2) Is it possible to change domain property when this domain collides with another domain (For example: When particle collides with wall change particle's domain to fixed constraint)?
3) Can I release geometry by time from FSI model and automatically give material property (like Particle
tracing model there this parameter is available)?
4) Can I give boundary condition in FSI as Particle tracing model (bounce, stick, freeze etc)?

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Meshing

November 30, 2015, 1:01 pm
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Dear Comsol Users,

how I can make meshing for tow parts, they are cylinders, but one of both is too big In comparison with other one.
the first Cylinder R: 16 mm Hight: 10000 mm the other one R: 30000 mm, Hight: 60000 mm.
If any one know how I can make this meching, I well have pleasure if anyone could to help me.

I face always as this messages:
1-Edge is much shorter than the specified minimum element size
2-Face is (or has a narrow region that is) much smaller than the specified minimum element size.

Regards
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Meshing without high mesh density and with high minimum mesh quality

October 28, 2014, 7:47 pm
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I'm trying to mesh the interface between what is essentially a cylinder and its base. The model is imported and I would prefer not creating a chamfer or fillet unless I can edit an imported model without much hassle.

I tired using smaller elements but the resultant minimum mesh quality remains poor. To the order of e-5.
I would really appreciate any inputs. Kindly find a model attached with this post.

Cheers!!

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Eccentricity in Rotating Electrical Machines

December 4, 2015, 2:44 pm
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Dear all COMSOL experts,

I am a rotating electrical machine designer and I would like to study a machine under eccentricity fault. I aim to study a 2D geometry with the AC-DC package and in particular rotating machinery package. Is it possible to do such a study in comsol? My concerns are the following:

- usually for rotating machines the center of rotation is considered to be centered in the origin (0,0). As far as I know I always done like that in order to get reliable values of electromagnetic quantities, in particular about the electromagnetic torque. For eccentricity is it possible to prescribe rotation also around another axis getting fair values or is it better to change the stator position accordingly?

- in order to set up the mesh for time stepping simulation, how can I do? For healthy rotating machines (no accentricity) the mesh geometrical constraint is set by a pair formation along the airgap. However in this situation it does not work. Is there any other kind of method for dealing with an air-gap which is changing thickness at each time step?

- is it possible to simulate dynamic eccentricity? In this case the rotor will rotate around his axis which is displaced by the healthy one, and at the same time it will be whirling with a certain vrotational velocity around the healthy axis. In this case the axis position along which the rotor is really rotating is varying his coordinates at each time step. Is it possible by any chance to set up a simulation like this one?

Do you have any kind of advice?
If you have any advice or suggestion I will be really grateful.


Sincerely,
Nicola
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