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Changing the predefined equations in COMSOL /adding more variable in the equation

July 6, 2012, 1:03 am
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Is there any feature to change the predefined equations present in any module (in my case the battery module) to edit the predefined equations to add extra variables in the predefined equation ?

If not how else u can do the same task of adding an extra variable to the predefined equations ?

In my case I want to add an extra stress variable to the concentration equation for the active particle in porous electrode.
Any one can help me out for the same ?

Best Regards,
Utsav
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Mixing two different fluids in fluid flow

July 12, 2012, 1:46 am
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Hi

I have been working on a flow analyzer that has two inlet and an outlet. I have two fluids, one is very dense and the other is a very dilute substance, both of them are completely different and they do not react with each other. I have to inject them separately to two inlets at different velocities. There is only one domain for the whole device.
I used laminar flow and transport of dilute species physics but my problem is that I cannot use two distinct liquids to inject. I made two different material but one of them always overrides the other and finally either one of them is only used in the process and the density plot of the device show that the density is uniform throughout. How can this be solved in comsol ?
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heat transfer simulation

July 13, 2012, 4:15 am
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Hi,
I am trying to simulate heat transfer of a 3D closed box with a fluid(Nitrogen) inside it and some solid electronic components that are heat sources.
Actually I am a little confused with different modules in COMSOL. What is the best module for this purpose? Is it the Conjugate Heat Transfer(heat transfer module) or Non-isothermal Flow in Fluid Flow module?
When I run both of them on a simple example I got different results!

I really appreciate your time and kindness.
Thank you,
Razieh
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Fluid-Porous Slip boundary conditon

August 19, 2012, 11:29 pm
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Hello,

I focus now on the flow and solute transportation simulation in the free flow and porous media. There are traditionally one singe formulation, typically Brinkman and two-domain coupling with Navier-Stokes/Stokes and Darcy's or Brinkman's. For the former unified one I guess as exerted in fp (free flow and porous media), there is no need to specify the boundary conditon along the interface between the two medias, which provides continuous velocity profile across the transition zone. However, I would also like to try the coupling with specification of a slip jump velocity across the interface. There is some literature in comsol conference proceedings. Could anyone explain to me in detail how to impose this jump boundary condition? What is the difference between the choices of 'slip conditon' and ''slip velocity'', could ''slip velocity'' be prescribed as experssion like the Beaver&Joseph boundary dU/dy=α/√k(Uf-Ud), or necessarily assign a value? What is the difference with the maxwell boundary conditon? Could anyone explain to me more in detail or find me some reference?

Best

Yuexia
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Failed to call interpolation function in material

August 20, 2012, 10:51 pm
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Hi everyone,

I want to insert a function in a material.
Function is frequency and relative permittivity relationship.
I make function from a text file and then call the function by typing the function name in relative permittivity expression.
The function name is DC.
Like attach picture.
When I try to compute the study ( I used freq. domain in electric current module), it gives error message
Segregated group 2
Failed to evaluate variable.
- Variable: DC
- Geometry: 1
- Domain: 28 37 45 54

What is the error mean?
Did I do the correct step?

Thank you
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Level Set Method: Unable to Re-compute

August 21, 2012, 4:32 am
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Hello,

I'm using two-phase flow, level set to simulate mixing of two fluids (gases) and got my model to run (is not correct in current state but did run), however, when I make a small change, I cannot get it to compute. I tried clearing all meshes, all solutions and resetting, but it still doesn't work.
Any thoughts?
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Same BC for 2Axis Symmetric and 3D, yet different Delta_P

August 21, 2012, 6:03 am
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Please Delete, I made a silly geometry error!! UGH!
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Flux Calculation

August 21, 2012, 8:21 am
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Hi all,

I am doing simulation for a problem that involves coupling between electrostatics and transport of diluted species.

In the problem I used "No Flux" condition for all species at one wall. I was able to obtain results. However, when I plot the total normal flux along that wall (with "chds.ntflux_c1" expression), the flux is NOT ZERO, not even close.

I refined the meshes near the wall but got the same problem.

Does someone know the reason and how to fix it? I am using 4.2a.

Thanks.
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Export 3D simulation data to reuse it in a 2D simulation

August 21, 2012, 10:49 am
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Hi,

I simulated the laminar flow in a tube-like structure in 3D. Then I want to use the transport of diluted species in the centerline cut plane. I recreated the 2D centerline cut plane geometry in a separate model. Is there a way to export the velocity profile, which is time dependent, to a data file and use the velocity profile in the transport of diluted species?
I tried export V_x including all the time steps from the 3D model and import as an interpolation function in the 2D model. But COMSOL cannot recognize the time steps. I think COMSOL takes the time as the third dimension z.
Any suggestion is appreciated. Thank you!
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point source in RF model

August 21, 2012, 11:44 am
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Hello,

I met with a problem that when I add a point source, for example,electric dipole or magnetic dipole, in 2d structure, the point source can only excited in a limited small range, although the material is air. So I wonder how can I excite the wave in a large area. Thanks you very much.

Thank you very much
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How to consider groundwater table in soil

August 21, 2012, 6:04 pm
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Hi I am a beginner of COMSOL, I am trying to use COMSOL to model soil behaviour. As you know the soil always water in it, which make a difference of stress state, can be total stress and effective stress.

When I use Geomechanics model, there is no option for groundwater, do I need the "subsurface flow module" to make for it?

I tried to use "porous material module", but it can not show some specific geomechanics feature, such as soil plasticity, friction angle, cohesion, etc.

thank you
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define frequency value in global definitions -> variables

August 21, 2012, 7:57 pm
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Hi,

I need to use electric current interface and frequency domain study.
I want to give applied input voltage in terminal 1 as Vo=V0*sin(2*pi*f*t)

So in the Global definitions -> variables

I set V0=0.05 V, and t=5 s

I didnt set the "f" because I thought i didnt have to do that since I am working in frequency domain.

However, in the terminal1 node I got the Vo=V0*sin(2*pi*f*t) was in yellow colour which means something wrong happened.

What did I miss?????

PS: Actually I need the frequency is in a range of value which I can set in frequency domain range.
But how can I define the sinusoidal input voltage like I mentioned above?

Please somebody help me
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Genearal question: Mesh density

August 21, 2012, 8:23 pm
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Hi all,
I have a general question, whether the high mesh density will give a good convergence, or not ? As i simulate a nano-particle optical scattering, i found that use a smaller mesh, the resonance frequency will slightly shift upwards.



Bruno Berg
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integration points

August 21, 2012, 9:47 pm
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Hi
I need to extract the value of a parameter with in an element. I studied in one of the forum that I can do that by using gauss integration points while exporting the data. But How and where I can find that option????......
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help

August 21, 2012, 11:02 pm
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Anybody know what the command "mf.murinvz"does?
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Customizing node numbering of elements

August 22, 2012, 2:07 am
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Hi Everyone

Is it possible to customize how Comsol assign node-numbers to the mesh nodes?

For example if we consider a 4 node quad element, Comsol starts at the lower left corner as node 1, and then goes counter clockwise as shown below:

4 3
1 2

Is it possible to user define this process so that the nodes are numbered as below?

3 4
1 2

I have already tried to study the scripts in MatLAB using livelink with no luck.

Thank you in advance

With regards

Robin
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Using deformation results as initial conditions for the next study

August 22, 2012, 2:55 am
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Hi all,

I have a linear elastic model with a stationary study whereby I apply a boundary load and deform the model domain - this is simple and works fine.

Next, I would like to use the deformed results from the first solution as the initial condition for a second solution where I re-apply the same boundary load. I hope to be able to see that the deformation builds up with each successive solution/load (I plan to repeat this 10 times eventually).

**However, I would like to know how to take the deformed results from the first solution and set them as the initial condition for the next solution?** and **Is it possible to alter the geometry as part of this, as the boundary with the boundary load is bound to deform too?**

Any help would be greatly appreciated!

Cheers, James.

"Using deformation results as initial conditions for the next study"
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Injecting Fluid into another fluid, problems solving with various Inlet BC's

August 22, 2012, 5:04 am
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Hello Everyone,

I currently have a pipe with water flowing through it. I have verified/solved this model to as accurately as I can. I then have insert a small nozzel at one end of the pipe (1/20th the length of the pipe). This is injecting fluid with Exactly The Same Properties, but the Flow Rate will be different.

Using Pousille I have calculated for my flow rates an expected inlet velocity for the pipe and the nozzle.
I then set my inlet BC's to "Mass Flow Rate" using the desired inlet's. It solves wonderfully, my Delta_P is what I expected. My exit flow rate is the combination of both mass flow rates, and *My inlet velocity on the pipe and nozzle are what I calculated*

Question - However, when entering any other inlet conditions I'm unable to solve. If I enter my BC's as "Velocity" it will not solve. The same goes for setting laminar inflow "Average velocity" and "Flow Rate". Though I suspect the laminar inflow case is because Comsol is having trouble with two assumed full developed flows intersecting, thus no longer being fully developed.

The Reynolds number is around ~ 400. The flows should exhibit no turbulence, though I know their is natural turbulence when flows will mix, is this causing the problem?

Thanks for any tips understanding the various inlet conditions. I assume if I can get it working with the correct physics for mass flow rate, I should be able to calculate velocity and pressures and have it solve as well.
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Importing dxf files created in autoCAD into Comsol 4.0a

June 6, 2011, 9:16 am
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Hi

I am trying to import dxf files created in autoCAD into Comsol as to create a 2d model. I am having problems when it comes to anything circular. Square or rectangular objects are fine, but when the objects are circular or elliptical the error message "The imported file did not contain any geometries" comes up.

If I were to import everything I needed at the same time - which seems to work - how do I then seperate them out?

Any help would be grealty apprecaited.

Kind Regards

Gareth
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transient electromagnetic waves study

August 6, 2011, 7:24 pm
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Hi,
I want to study the propagation of electromagnetic waves with time in a coaxial waveguide. What I want is at the beginning the wave is incident form the bottom of the waveguide and then propagating in the waveguide. I use the transient electromagnetic waves study and choose time dependent. But there is always error when computing. I can not figure out the reason. Attached is the model I made. Could any one please tell me where is wrong and how to fix it? Thank you!

Best,
Xinyu
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