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3D Poisson equation problem coupled with Nernst-Planck equation

November 20, 2012, 8:30 pm
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I used coupled Electrostatics physics and Transport of Diluted Species physics to simulate this problem.
The source term in Poisson equation is the difference between cations and anions.
Usually, the difference should be zero due to the electroneutrality. In some conditions, it would be not.

My problem is that, when I used 2D model, the simulation converged even with a coarse mesh. When it comes to 3D situation, it no longer converged, no matter how fine the mesh is.(as far as I think, the mesh is fine enough)

The error is as below:

Failed to find consistent initial values.
Segregated group 1

Undefined value found.
Undefined value found in the stiffness matrix..
For mesh-case 1 there are 111061 equations giving NaN/Inf in the matrix rows for the variable mod1.H.
and similarly for the degrees of freedom, NaN/Inf in the matrix columns.
Last time step is not converged.

I'm not clear about the numerical methods. It seems that the solving processes of 2D and 3D are quite different. 3D used segregated groups, which is not shown in 2D process.

I need your help.
Thanks in advance.
Mingjie Jia
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Subsonic Ramjet Model

November 26, 2012, 1:41 pm
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I am attempting to optimize the design of a ramjet to be created for rocket experimentation. I have been working on the model for the fall semester attempting to refine it. I am now using a 2d axis symmetric turbulent flow and heat transfer models. The heat transfer is being used as a rough model for combustion. I could get the model to converge before I realized the heat transfer model and turbulent flow model were not coupled. But once I coupled them the model will only converge sporadically for very coarse meshes. Could you help me figure out how to get the model to converge? Any tuning that could be done to make the model more accurate would be very helpful as well.

The model beginning with "test coupling" has the two models coupled (at least I think) but will not converge.
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Low Reynolds Turbulent Flow

November 30, 2012, 5:45 am
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Hi folks,

does anyone know what the "spf.wall_spf" variable means in the AKN Low Reynolds k-ep model.

I can see it in the equation view, but other than that, there is no more info...

I am really trying to understand this one, because I am trying to implment my own Low Reynolds model (coupled with some other physics) but I am having a hard time defining the Epsilon at the wall.

And I believe it may be due to the variable I mentioned above.

Hope some can help.


Best Regards,

Rui Silva
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bubbly flow: udrift in laminar flow...

December 4, 2012, 12:35 am
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Hi,

I am currently working with the bubbly flow application mode in a laminar flow.

The gaz velocity, ug, is computed thanks:
ug=ug+uslip+udrift

In the help, is it said that the drift velocity is computed only in a turbulent flow. However, in the laminar flow, we can see that this dift velocity is non equal to zero. Does anyone know if, from a physical point of view, if this velocity should be equal to zero in a laminar flow?

thanks a lot!

Patrick

PS: in version 3.5, the drift velocity was equal to zero in a laminar flow
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Assigning Materials

January 25, 2013, 12:58 am
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While assigning materials to some solids in my model, I am getting a red cross below the material name in Model Builder. What does that mean? What changes should I do to use those materials for the analysis??
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Simulationtime for mapped meshes in 2D and 3D

January 25, 2013, 1:11 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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Manipulating data from tables

January 25, 2013, 3:05 am
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Hi all,
I'm working on a model using piezodevice physics in the frequency domain, and I'm trying to plot admittance in dB (rather than siemen). I've found the admittance by taking the surface integral of the current density (pzd.JZ) over one of the electrodes, which results in a table with admittance values for each frequency. Is there any way to apply additional mathematical manipulation to the values in that table? To convert to dB, I need to take 10*log10(x) for each admittance value. I've tried doing the 10*log10(abs(pzd.JZ)) as the expression in the surface integral, but that doesn't work... though admittedly my math is a little fuzzy as to why not.

It seems like there must be an easy way to manipulate data in a table, but I can't seem to find it. Any advice would be greatly appreciated!

Thanks,
Jillian
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ALE question

January 25, 2013, 3:17 am
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Hi,

I'm interested in the transient effects of a moving charged object on a grounded one (see image).

I have first modelled this using a parametric sweep on the object's x position. But of course, this gives a series of steady state solutions and gives no information on transient induced currents/voltages etc. If I use a time dependant solver I'm not allowed to enter time dependant positions of geometry objects. As I understand, this is because the time dependant solver does not loop through the geometry and re-mesh for each time point, it can only use the original mesh.

I have since tried to use ALE to achieve this motion, by applying the "free-deformation" BC to the air domain and the ellipse and then the "prescribed mesh velocity" BC to the ellipse only (with a velocity in the x direction). The motion would go from one side of the grounded object to the other.

Unfortunately, I think this is too much motion for the mesh deformation to handle. Do I understand correctly that the ALE is unsuitable for this amount of motion (from one side of the picture to the other)? When I solve the model, I get a weird, unrealistic deforming of the electric field around the ellipse.

Either way, do you have any insight on the correct way to implement this amount of motion into a comsol model? ALE or something else?

Thanks and kind regards,
Dominic.
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Straw Poll: Who would like to see the model gallery documentation overhauled???

January 25, 2013, 5:26 am
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Hello all,

I thought I'd post this here to get the community opinions before suggesting it to support.

If I want to learn how to use an aspect of Comsol I work through example models and I visit the forums, and often the advice from the forum is to work through a particular example model. This is, in my opinion, the best way of learning how to use any technical software.

I believe that this process is quite seriously hampered in Comsol compared to other products like Labview/Matlab or even things like Adobe Creative Suite. I think it is hampered because of the structure of the model documentation.

Basically, I think it is near enough useless to give users a step-by-step instruction sheet for how to build an example model without telling them WHY they are doing what they are doing. In almost all cases, when I try to adapt what I have learned from working through the example models to my actual applications, there comes a point where something fails and I have no way of figuring out which aspects of the example models are crucially important and which aspects should be changed because I often don't know what they were doing in the example model or why.

One clear example is in the application of boundary conditions to particular physics (which BCs are necessary for the physics to solve, which are problem-dependant).

Another example of the type of problem is that the built-in material AIR cannot be used with the electric currents interface without changing its conductivity from 0 to some small positive value. I realise the physical reasoning behind this, but this must be such a common problem that I can't believe there isn't some up-front info in the model library examples that explicit state this. It's the kind of problem that can hold new users up for days/weeks and it's not the only one: a user might get past this only to run into another similar problem in the very next step of his/her model.

The way the documentation is written, you might come across a line that says something like "change the value of conductivity from 0 to 1 S/m in the material properties". But there is no explanation of why you are doing this, or whether this is generally important or only important to the specific problem in the example. This means that you could be searching around the documentation for an answer to your problem only to read right over it again and again.

Basically I think the barrier for entry into COMSOL is needlessly high because of the way the documentation is handled. With a relatively tiny amount of work on the part of Comsol (hell, it could even be done by the community) the documentation could be updated and thousands of new users would be able to answer their own questions instead of raising hundreds and hundreds of forum and support tickets. All that is needed is the inclusion of one or two explanatory sentences after each section of the step-by-step instructions.

This is not a rage post, it doesn't relate to a specific problem I am having right now, I genuinely think that Comsol are wasting an opportunity to grow their community and broaden their user-base.

I'm interested to hear your thoughts.
Regards,
Dominic.
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Using step function in kinetic reaction simulation

January 25, 2013, 5:26 am
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Hi

I am trying to simulate the kinetic reaction of biomolecule interaction in fludic channel expecting to have association and dissociation phase (concentration of complex versus time) in the same curve. I knew that it is possible to use step function or smoothed Heaviside step function from COMSOL in order to change the concentration of analyte species at the inlet from C=x mol/m3 to C=0 mol/m3 at a certain time. I have no idea how to find this function and what kind of variables I need to defind. Anyone has any experience on this?

Regards,

Pattamon
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Save the Date - COMSOL Conference 2013 Boston, Oct 9-11

January 25, 2013, 6:23 am
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Hello,

We are excited to announce that the COMSOL Conference 2013 Boston will be held on October 9-11th.

With over 50 events to choose from, including training and presentations, this is the premier event for multiphysics simulation. Express your interest in the conference and we'll keep you updated as more information becomes
available: www.comsol.com/conference2013/usa/

Be sure to visit our 2012 Highlights page to see photos, videos and more from the COMSOL Conferences in Boston, Milan and Bangalore last year.

Hope to see you there!

Kristen O'Connor
Conference Registrar
COMSOL, Inc.
kristeno@comsol.com
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just start with a known solution for first time step

January 25, 2013, 6:45 am
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Hi,
there is a simple solution, or?
I perform a time-dependent study. But the input variable for the time is not long enough, so I want to repeat this pattern several times. Unfortunately it does not work as I imagine it. How to start with the known solution from the previous study only for the first step of the new study?
Thanks
Markus
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Embedding a Circle on the Curved Face of a Cylinder.

January 25, 2013, 6:57 am
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Hello,

I am a new user. I am trying to create a model where there is a flux coming into the curved side of a Cylinder. The flux enters from a circular area connected to the curved edge of the cylinder, so I thought a circle on the side and boundary conditions setting an inward flux would be the easiest way to go.

I have not been able to do this. I have tried embedding a circle from 2D, but it only seems to work if I am embedding to a 2D surface.

If anyone knows a simple way to do this, or can suggest another method it would be greatly appreciated.

Thank you
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Error-Automatic remeshing

January 25, 2013, 8:09 am
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Hi,

I encountered a problem about "Automatic remeshing" with "deformed geomerty" and Time-Dependent Solver

When I ran the program, Comsol always displayed a Error:
Unsupported operation on this type of object.
-Feature: Time-Dependent Solver 1 (sol1/t1);
-Error: Unsupported operation on this type of object.

i try all possible mesh quality expressions and resonable threshold vaules, it seems when the program reaches this value,

the above error happens immediately.

Who can tell me what's wrong?

Your information and help is deeply appreciated!

Luyao






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Electrostatics and Electric Circuit

January 25, 2013, 3:54 pm
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I am relatively new to COMSOL. I am trying to simulate a two part problem.
1. An electric pole is placed underwater
2. A circuit is built around it to measure the voltage drop across a resistor

The setup can be better understood from my schematic that I have attached.

I have done the first part in which I ran the Electrostatics Physics and got the potential plot. From different posts in this forum I was able to make a little headway into the second part, but it does not work. I assigned two terminals 1,2. Added a voltage source between 1 and 2. Added a External I vs U between 2 and 3 and then a resistor of 10000 ohm between 3 and 0. When I try to solve this, I get the following error

Errors during multiphysics.
The DAE is structurally inconsistent.
- Variables: 2
- Equations: 0

I have attached my files.

The reason may be that the two terminals are not connected. But the water should technically close the circuit. And there should be a reading on the voltmeter. I have verified this experimentally to be of the order of 300 mV. But COMSOL does not solve my problem. Is there something that I am doing wrong or COMSOL is not designed to solve such a problem.

Any help will be highly appreciated. Thanks.
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DG method with periodic boundary conditions.

January 25, 2013, 11:22 pm
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I am solving the incompressible Euler equations in 2d in vorticity-streamfunction formulation. The vorticity equation is
w_t + grad.(Uw)=0 where U is the velocity and w is the vorticity. To solve the vorticity equation, I am using DG method and the flux is the upwind flux. For periodic boundary condition, how do I modify the flux?
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maximum (or minimum) value in a time domain

April 25, 2012, 1:48 am
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Hi,

I have a problem I cannot solve and you might help me.

In my time-dependent analysis I would like to know the maximum of value of a variable, which has been reached at previous time steps. But I can't figure it out how to that.

If my variable is v(t), then I would like to know max(v(t=t0), (t=t1), ..., v(t=ti)) at each time step.

Do you think it is possible to do that?

Thanks,
Daniel

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How to export data in excel format?

May 31, 2012, 10:35 am
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Hey Ivar,
I want to export my figure into excel format. and replot it in matlab. Is it possible ?
Thank you very much!!

Baofeng
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electromagntic propagation through three layers

January 18, 2013, 1:54 am
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hello
I am student on electronic and I am trying to simulate with comsol on rf module , in summarize my work is to simulate the sun light propagation through three layers bonded to each other and I had used the floquet periodic condition and this witch include this equations :

Parameter

lambda0= 0.5e-6[m]
k0_emwh = 2*pi/lambda0
alpha1 =30[deg]
n1 1.97235
k1x= k0_emwh*real(n1)*cos(alpha1)
k1y =k0_emwh*real(n1)*sin(alpha1)
freq c/lambda0
c= 3*10^8[m/s]
n2 0.125
alpha2= asin(sin(alpha1)*real(n1/n2))
k2x= k0_emwh*real(n2)*cos(alpha2)
K2y= k0_emwh*real(n2)*sin(alpha2)
n3 1.97235
alpha3 =asin(sin(alpha3)*n2/real(n3))
E0z= exp(-i*k1x*x)
Ez1= E0z[V/m]
k3x= k0_emwh*real(n3)*cos(alpha3)
k3y= k0_emwh*real(n3)*sin(alpha3)

and I put the boundary condition above and below the layers
Periodic condition 1
K vector for folquet periodicity :
k1x
k1y
Port 1
In the front of the layer
Put the wave excitation in this port (on)
Pin=1
Téta(in) = 0
E0= Ez1
Beta =k1y
Port 2
Put the wave excitation in this port (off)
E0= Ez1
Beta =k1y

in this simulation i would like to study the propagation of the electromagnetic wave through three micrometer thickness of material, and each material have property , and my problem when reduce the thickness the electromagnetic wave propagate with difficulty and when i increase the thickness the wave propagate easily, i don't know if my simulation is true.

Thank you for your help

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Structural Mechanics Equation System

January 19, 2013, 4:46 pm
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I am trying to understand how my structural mechanics model is computing its solution. The model solves and gives me a reasonable solution. However, I don't understand how it is arriving at its answer. I am looking at the Subdomain Settings - Equation System panel where it shows the PDE which it is solving. I am viewing it in coefficient form, and all of the coefficients are set to zero. How am I arriving at a solution when the PDE which is being solved is zero?

Ryan
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