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Straw Poll: Who would like to see the model gallery documentation overhauled???

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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.

Using step function in kinetic reaction simulation

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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

Electrostatics and Electric Circuit

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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.

Meshing Internal Volume of CAD import

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All,

I am trying to model fluid flow inside of a CAD object.

How do I create a domain inside the object that I can mesh?

I have capped all the openings.

Thanks,
John

calculating scattering and absorption cross section of gold nanosphere in comsol4.3a

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Hello,

I'm working on Comsol4.3a these days, and this is my first time using this software. Actually, I started with building the " Optical Scattering Off of a Gold Nanosphere" model, as it is described in the Pdf file. And I would like to calculate the scattering and absorption cross sections, so could someone please help me to figure out the steps for doing that or is there any guide that I can use?

Thank you,

Integration Coupling Variables

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I am trying to find the volume of a spherical shell using Comsol 4.3a in a 2D axisymmetric coordinate system. I attempted to do this by creating a integration coupling variable along the boundary of the shell with the expression pi*r^2, but it returns a value that is significantly different from the expected 4/3*pi*r^3. Can anybody help me find the volume contained within the boundary of a spherical shell?

Incidentally, integrating along the boundary with the expression 2*pi*r for the surface area returns the expected value, as does integrating the contained subdomain with the expression 2*pi*r for the volume, but I would like to be able to calculate the volume using boundary integration.

Thanks in advance,
Sean MacGahan

Problem with meshing

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Sir,

While meshing my model, I am getting an error message as follows "Out of memory for mesh processing". What does that error mean and what are the correction requirements??

RAM requirement

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What is the probable RAM requirement for mesh processing??

Please send me tutorial for ultrasonic wave propogation through liquid

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I need tutorial on Ultrasonic wave propogation through liquid.

Eigenfrequency and Temperature in a Cantilever

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Hello,

I'm using COMSOL for some months. I'm simulating the eigenfrequency of a clamped-free beam (cantilever MEMS).
The value of the eigenfrequency at room temperautre is correct (about 6.4MHz) but the increase in temperature produces an increase in the frequency.
Theorically and experimentally, if the temperature rises on the structure, the eigenfrequency decreases.

I tried to solve it including a HT module but there was no change. What is wrong?


Thanks. Kind Regards.
-Francisco

PS I simulated a clamped-clamped beam (bridge) and it worked correctly.

Dubinin Sorption Model on Dual Porosity System

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Hope this message finds you all well. I have a little technical question. I am currently trying to build a dual porosity model using the earth science module of comsol multiphysics 3.5a. However, I have hit a little bit of a roadblock and I do not know whom else to ask. As well as dual porosity I am trying to model coalbed methane sorption by using the dubinin sorption model. Using this model the matrix storage term turns out to be

Matrix Storage=c1*exp(-(c2*ln(c3/p))^2)

where c1, c2, and c3 are constants and p is the pressure in the system. and the second term of the material balance equation turns out to be the derivative of this matrix storage. The problem is that comsol's default equation has the second term of the material balance equation expressed as da*p' where da is the damping mass coefficient and p' is the derivative of pressure with time. However, using the dubinin sorption model it is not possible to simplify the equation in such a way that it ends up with a coefficient multiplied by a time derivative of pressure. I have tried fiddling around with the equation system but it has yielded no results.

Any ideas on how to modify the equation system so as to fit my dubinin model? Any help on the matter will be very gratefully appreciated. Attached is a word document with the numerical model, perhaps it makes the question clearer. Appologies if the question is not very clear, or if it is very long winded, it is my first time in this forum.

Kind Regards,

Andres Bracho.

Questions of the modelling of a light bulb using Heat Transfer Module

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Dear all:

Now I am trying to carry out some simulations using heat transfer module. To start with, I downloaded the "free convection in a light bulb" model, which can be obtained from:

www.uk.comsol.com/showroom/documentation/model/303/

After investigated this model, I still have some questions and I will appreciate if can get your help.

1, for the tungsten filament, which is set as the heat source, its surface emissivity is set to be 1 (can be found in surface-to-surface radiation 2). Does that mean it is assumed to be an ideal blackbody radiation?

2. the tungsten is able to emit heat using surface-to-surface radiation, what is the relationship between surface-to-surface radiation and blackbody (or greybody) radiation?

3. From my understanding, the heat emitted by the heat source (tungsten) heats up the inner wall of glass envelope by both radiation and convection. The inner part of the wall will also radiate heat and that's why it is set to surface-to-surface radiation as well (its surface emissivity is set to be 0.8). Is this understanding correct?

4. The glass is set to opaque, does that mean there is no heat radiation can go through the glass from its inner wall to its outer wall? If so, how could the temperature of the outer wall increase, e.g. by conduction?

5. After this process, the outer wall of the glass envelope is heated up. The final temperature of the outer wall can be obtained by considering surface-to-ambient radiation as well as convective cooling. Is this correct?

Sorry for many questions which may stupid to you. But as a newbie of COMSOL, any help from you is highly appreciated. Many thanks in advance.

Regards,

Fangjing

Propagation loss in SAW devices

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Hello!
I use Comsol for SAW device simulations and design some time domain models for SAW delay line. Results (impulse responses) are rather good. Now I want to introduce propagation losses in my models. Standart definition of it is loss per unit length in dB / 1e-6s (dB to microseconds). How can I introduce it in my simulation?

import a bmp image into COMSOL geometry

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Hey,

as stated in this webpage: www.comsol.com/products/formats/ Comsol can both read and write few types of images. However, when I tried to import a bmp image as my structure for flow simulation, it was not possible, because the options given include only mphtxt, dxf, etc. Does any one know how I can get a bmp or jpeg image imported? Thanks!

Chunyan

Coupled magnetic and solid-mechanics simulation

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I have a question about coupling a magnetic study to a solid mechanics study.

I attach to this email a simple model to explain my question. In the model you can find a permanent magnet with a round soft-iron path to guide the magnetic flux, and an air gap (and an air volume surrounding it). I added electromagnetic physics and solid mechanics physics, and I would like to perform a fully coupled study. That is, the magnet generates magnetic flux, which in the air-gap generates a force. That force deforms the soft-iron, therefore reducing the air-gap and increasing the flux. Thus, there is 2 couplings: the magnetic force deforms the solid object, and the deformations change the magnetic path.

What I see when running the simulation (study 3) is that I can indeed deform the structure thanks to the magnetic force. Nevertheless, I don’t think the magnetic flux is being recalculated after the deformation. To verify this I added a boundary load to the solid mechanics physics which pushes the air-gap closed. When performing the simulation, I do see a difference in the deformation, but I see no difference in the flux (you can check it with the surface average node in the derived values). The same happens if I chance the young modulus of the soft –iron, even to un-realistic values: no matter the final size of the air-gap, the flux doesn’t change. I tried a fully coupled study (instead of segregated) and get the same result.

Do you know what am I doing wrong?

A second question is that when I run the simulation with a force of 0 or 200N (you can change it in the parameters) it runs fast and easy, but if I try a parametric study with those 2 values, it gets stuck. You can see it in the study 2.

Cluster job that shows speed-up when using multiple nodes

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Hi,

I would be highly interested if someone could post an mph model file that they have shown to see speed-up when distributed across multiple nodes. I am having difficulty seeing any speed up for a number of models and understand from previous threads that speed-up is highly model dependent.

If someone could post an mph file that they know to show speed-up it would be greatly appreciated.

Best regards,
Josh Thomas

Import geometry from Matlab into Comsol

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Hello,


First of all, I want to emphasize that I am a beginner with Comsol.

I am trying to import a geometry generated with Matlab into Comsol in order to study the wave propagation in this geometry.

My geometry is simply a n*n matrix, with 2 values : 0 and 1. The value 1 is the material and the value 0 is the air inside the material.

How can I import this matrix into Comsol to work with ?

Thank you for helping out.

Mie scattering in 4.3

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Hello,
I have already seen some discussion about mid scattering of a gold nanosphere. But I the discussions I have consulted, no one has yet implemented a mie scattering program in Comsol 4.2 or 4.3 that works as the one in 3.5. Has this problem been solved yet? Can anyone post the correct program here?
Thank you very much for the help

particle size and geometric constraint of channel

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Hi,

I designed a microfluidic channel with gaps at 10 um. Of course, particles with diameter at 1 um can go through this gap, but particles with diameter of 15 um still can go through this gap, which is not possible. How can I add geometric constraint besides the different forces for different sized particles?

Thank you.

MAKING STRIATIONS IN A SOLID CYLINDER

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how do i make striations in a solid cylinder? I know it should be possible but some how cannot figure out how?
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