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How to mesh touched thin busbars for magnetic field calculation with AC/DC module

November 18, 2014, 6:54 am
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Hi all,

I'm calculating the magnetic field distribution of a system, which mainly consists of bus bars as shown in the attachment. (The model is imported into COMSOL via the LiveLink interface.) Since the bus bars are relatively thin, we want to use a swept mesh in order to reduce the degrees of freedom. However, the swept mesh doesn't work well because some parts of bus bars touch. Then, how can I generate an efficient mesh? Note that, there are some holes on the bus bars, and the busbars are surrounded by an air box. Thanks in advance.

Regards,
Tony
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Creating meshes in Matlab with given set of vertices, tetrahedra and faces

September 30, 2011, 6:51 am
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Hello everyone!

I have a procedure which generates a tetrahedral mesh externally. So now I have a set of vertices, tetrahedra and faces. Can anyone tell me what would be the Matlab syntax to create a Comsol 4.1 compatible model containing this mesh? I have done this previously with Comsol 3.4, but have not yet been able to find an equivalent in COOMSOL 4.1 Livelink with Matlab. I was planning to have every tetrahedra as a single domain...


I hope this makes sense.
Thanks!
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Why the solution only saves result from the last study step?

February 27, 2014, 2:29 pm
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Hi, I have a time dependent problem to solve. Because at the start and ending stage of the simulation variables change more significantly, I decide to use several time dependent study steps, for the first and last steps the time step is smaller while in the middle I use a larger dt. In the solvers steps taken by solver is set to be strict. However, I find that at the export, only the time steps in final study step show up in the stored output times, why? In the solvers, I've set Times to Store to be specified values in Output tab. Thanks!
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Dependent variables for Transient Mode ? Equation (in Wave Optic Module)

November 18, 2014, 5:20 pm
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In Wave optic module, there are 4 types of mode.
For the transient mode, however, the dependent variable is magnetic vector potential ('A' expressed in COMSOL equation configuration). The other mode has the electric field or its relevant variable as a dependent variable.

I would like to know why 'A' have to be dependent variable for transient mode.

[Four mode in Wave Optic Module]
- Frequency domain (Dependent variable : Electric field)
- Beam envelop (Dependent variable : Envelop of Electric field)
- Time explicit (Dependent variable : Electric field, Magnetic field)
- Transient (Dependent variable : Magnetic Vector Potential)
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Parallel plate capacitor 2D vs 3D issue

November 19, 2014, 7:55 am
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I'm a beginner COMSOL user. I came across this problem while i'm doing some test simulations. I've simulated a parallel plate capacitor in both 3D and 2D. I wanted to see if I can extract the same results from the 3D if I use a cut plane. So I created a cut plane so the resultant would be similar to a 2D situation. However when I extract the electric field directions, they do not look even remotely close to the results from 2D simualtion. I've attached two images from two simulations. I'm using "Electrostatic (es)" in both cases and all the materials are same in both cases. I would be very grateful if anyone can point out what I'm doing wrong.

Thank You.
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Add LiveLink for MATLAB after installation of COMSOL 4.4

November 19, 2014, 10:34 am
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Hi!

I have both COMSOL 4.4 and Matlab R2014b installed, however in the "wrong order", hence I was never asked to install LiveLink at the time of my COMSOL installation, since I did not have Matlab installed at that point. Now I need the LiveLink for Matlab feature.

So, how can I add it at this point without having to reinstall COMSOL?

Thanks in advance,
Carsten
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Thermocapillary recirculation

September 18, 2014, 9:37 pm
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EDITED
Hello,

I am a beginning user of COMSOL.
Currently, I am trying to create a preliminary model of Marangoni convection, like this one available on the COMSOL site: www.comsol.com/model/marangoni-convection-110

The main difference is that mine is axisymmetric, rather than two-dimenional, and instead of a temperature difference, the fluid is heated with a point heat source on the upper, free surface.
I was planning on doing both a stationary and time dependent study, but both are running into errors.

EDIT: The problem with the strange temperature distribution has been solved. The heat conductivity was improperly defined. However, now it reports errors in user defined functions.

The error message is as follows:

Error in user-defined function.
- Function: rho
Failed to evaluate variable.
- Variable: comp1.ht.rho
- Defined as: ((unit_kg_cf*rho((comp1.T/unit_K_cf)))/(unit_m_cf^3))
Failed to evaluate expression.
- Expression: d((-2*comp1.ht.rho*comp1.ht.Cp*(comp1.ht.ur*comp1.Tr+comp1.ht.uz*comp1.Tz)*test(comp1.T)*pi*r)*(dvol),{test@15})
- Feature: Stationary Solver 1 (sol2/s1)


Could someone help me get this right?
I've attached my model to this post.

With regards,
Aaron Ayal
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Problem of the boundary condition at bubble surface

November 19, 2014, 5:55 pm
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Hi，
I'm now trying to use N-S and continuity equations to calculate the drag cofficient of spherical bubbles.
I use the PDE and 2D axisymmetric model module to simulate. when setting the boundary condition of zero shear stress at bubble surface, I encounter a problem that the boundary condition is "t.T.n=0"as shown in the image.
I wonder if someone knows how to transform this condition so that it can be used in comsol.
I've shown my idea in the image and I'm very pleased if someone gives me any idea about this topic.
Thank you very much.
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How to calculate photonic band gap

November 20, 2014, 4:07 am
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Hi Jens,
Here you asked about photonic band gap calculation www.comsol.com/community/forums/general/thread/36237/
and you uploaded two pictures with result obtained. I want to understand how to lead photonic band gap calculation in COMSOL and have tried to reproduce your results but did not obtained good agreement with yours. Could you share your file?
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Pulse input precision problem

November 20, 2014, 5:48 pm
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Dear all,

I try to define a pulse input having a period of 0.01s, and the pulse itself is just 8e-9s.

but when I plot the function I defined, the figure turns out to be strange. It seems that only 1000 points are used to describe the function I defined.

So when I decrease the period to 1e-6s, I can get the right pulse signal as I want.

Could anyone tell me how to increase the precision of the function in COMSOL?

Or is there any other way that I can use to define such a pulse input?

Thanks in advance.
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Problems in assigning inlet-outlet boundary condition for a two phase flow

November 21, 2014, 1:25 am
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Dear all,

I am trying to simulate a laminar two phase flow passing over a cylinder with a phase-field model where the cylinder is submerged into the water (heavier fluid), as a part of my project. I have tried a couple of candidates for the boundary condition at the inlet and the outlet. Initially, I gave a uniform velocity at the inlet for both phases plus a zero pressure at the outlet, then I assigned a mass flow rate for both phases and mass flow rate at the exit and a few other failed attempts. Also I have tried to give a step function at the inlet for the volume fraction at the level of my free surface and give a uniform velocity which I could not even implement.

Receiving a piece of advice, I have recently tried to give a zero pressure gradient at both sides and a body force or a small acceleration in the x-direction (as my driving force), but it seems that after sometime (4% of run-time in my case) the "reciprocal of the step size" increases and causes a failure.

I would be glad if there is a solution for the problem. I also attached the basic model here.

Amin,
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weak constraint in" benchmark model of ccp"

November 21, 2014, 1:58 am
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Hi ,guys. I am confused by the use of weak constraint in model library ”Benchmark model of a capacitively coupled plasma" .more specific I don't understand why "J_total= intop1(-d(V_lm,t))" ,ie the total current density equals the time derivative of the Lagrange multiplier of electric potential.
Could you guys tell me what indeed "V_lm" is and how it relates to the total current density at the driven electrode? Thanks a lot!
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Different materials in Mixer

November 21, 2014, 5:44 am
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I have questions about how can I define two different materials in the mixer. For example, half water and half air in the mixer. Can anyone explain how to draw streamlines like the Mixer Module website showed? The attach file is what I using now.

Thanks.

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Simulating bipolar electrodes, problems with coupling transport and electric currents

November 21, 2014, 6:38 am
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Hey,

I'm trying to simulate a system in 1D where a slab of metal surrounded from both sides by a liquid and subjected to electric field. I use Transport of Dillute Species in liquid phases, and tried both Electric Currents and Coefficient form PDE for the current.

The idea is that I apply an external electric field on the metal. Two electrode reactions take place at the metal-liquid boundary. The reaction rates are defined by Butler-Volmer kinetics, where the overpotential term is defined as V(metal)-V(liquid)-E0(standard potential of the redox couple). Similar model has been described here: www.electrochemsci.org/papers/vol6/6072609.pdf

So basically I define the dependence of current on potential on both metal boundaries, and potential should go to a value where the current is conserved (current in = current out). The model is solved and the solution looks ok, but relative error is high, ca. 0.2, so I get the warning message from the solver.

I've tried the system both with Electric currents, and Coefficient form PDE, where I basically just defined same governing equations as in Electric Currents, and have the same situation with both systems. Is this because the potential of the metal is not well defined? Or any other suggestions?

Best regards,

Pekka Peljo
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DBD plasma actuator model

November 21, 2014, 11:23 am
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Hello everyone,
I am trying to study gas discharge at atmospheric pressure using a dielectric barrier discharge plasma actuator (2D geometry) consisting of two electrodes, one exposed to air gas at atmospheric pressure and the other covered by a dielectric material. Air plasma kinetic model was modelled by air cross-sections from file, with N2, O2, N2_1p, O2_1p as species. Volumetric and surface reactions were also considered. N2 species has the largest mass fraction and it was chosen to fulfil the mass constraint. Initial mass fraction of O2 species is equal to 0.22. Initial number density of O2 ions is zero. Electric potential BC is applied on the exposed electrode (Vrf = 5e3[V]*sin(2*pi*1e3*t[1/s])), Ground BC on the other electrode.
I set a time-dependent study step from 0 to 10 ms, but I get the error:

Repeated error test failures. May have reached a singularity.
Time : 1.985019325484047e-005
Undefined value found.
Undefined value found in the stiffness matrix.
There are 2 equations giving NaN/Inf in the matrix rows for the variable mod1.wN2_1p.
at coordinates: (0.00100937,0.00120891), (0.00102875,0.0012092), ...
and similarly for the degrees of freedom, NaN/Inf in the matrix columns.
Last time step is not converged.

The same model was successful solved by applying an electric potential with same freq but lower amplitude (1000V), so I think that the problem is the too much high amplitude of the electric potential Vrf.
I tried to increase the Vrf’s amplitude gradually by using previous solutions (obtained with lower amplitude) as initial values of dependent variables (both variables solved for and variables not solved for), but I tried the same error with amplitudes > 1000 V.

Can someone help me?

Chiara D.P.
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meshing error

March 7, 2011, 2:08 am
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hi,
i got following error several times with different models while meshing.
'failed to respect boundary element on geometry edge'.
may i know the reason behind this?
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Internal fluid-fluid boundary issue

November 22, 2014, 4:00 am
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Hello all,
Inexperienced comsol user here. So I have a 3-d model of a cylinder. In the lower half of the cylinder there is water while in the upper we have air. Air is rotating around the cylinder due to an inlet and an outlet. My problem is that I want to have an interior boundary on the air-water surface that is as follows: ux_air=ux_water and uy_air=uy_water and w_z=0 so that there is no mixing between air and water.
Do you have any suggestions on how this could be applied?

Thanks in advance,
Lefteris
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Modeling the Lithium Ion Battery

November 23, 2014, 8:28 am
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Hello,

I am working on modeling a lithium-ion battery based on the Kandler paper attached here. I have successfully inputted the governing equations into COMSOL with one exception: the boundary conditions for the charge, solid phase equation on page 3 of the Kandler paper. The model runs when a Dirichlet boundary condition is used for the solid phase equations, which sets the potential of our modeled electrodes to zero at their respective boundaries. This is incorrect since I am trying to set the derivative of these boundaries equal to zero (the zero flux boundary condition). However, the only way I have been able to eliminate the error messages is with these Dirichlet boundary conditions. How can I satisfy this condition with our model?

We have labeled the corresponding partial differential equations as "psn" and "psp" in our COMSOL model. As you can clearly see, I have implemented a Dirichlet boundary condition for both equations. The equation was taken from Table 1 on page 3 of the attached Kandler pdf.

Any help would be greatly appreciated.
Thank you for your time.

Regards,

Adib Samin
The Ohio State University
samin.2@osu.edu
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Help needed on permanent magnet simulation

November 9, 2014, 9:37 pm
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Respected sir,
I am a masters' student working on a thesis on on-chip magnetic aggregation of cells.
I would appreciate if you can help me with the simulation of magnetic flux density inside microfluidic channel in PDMS, I would be grateful for your support.
I am a beginner in COMSOL and do not have a good idea about it. I have attached a schematic of the design having different layers as glass, NdFeB magnets, PDMS and then PDMS cover.
If you can provide me your email ID I can send you the file which I am unable to upload here.

Thanking you.

Yours sincerely
Sohail Faizan
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Which BC needed?

November 23, 2014, 10:28 pm
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Hey there,

I am quite new to comsol so maybe there is an easy way to do it. I have a pipe with an internal helix structure which is repeated after a 180° twist. I want to use this symmetry to save computational power. Therefore I need to find boundary conditions, where the flow field at the outlet is equal to the flow field at the inlet.

I already tried periodic flow conditions but it resolves in a flow speed of zero since the inlet/outlet pressure difference is equal.

The model is attached. Do you have any suggestions for me?

Best regards
Jonas

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