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Where are weak contributions added in the governing equations?

January 29, 2013, 6:35 pm
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When adding weak contributions to a particular physics, where are the weak contributions added in terms of the governing equations? Is it always on the left or right hand sides, or is it added somewhere else?

For example, in the marangoni_convection file from the model library, where is test(u)*(gamma*Tx) added with regards to the boundary conditions?

Thanks in advance for any help!
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Try converting to triangular mesh on these faces

January 29, 2013, 10:35 pm
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Dear sir,

While meshing my model, only one domain didn't get meshed.

And the error shown for a face which is not meshed is "Try converting to triangular mesh on these faces". And, I was not able to convert it to triangular mesh as it is constrained by the mesh on adjacent faces etc..

Help me out about how to solve this problem, please?
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Undefined Value Found

January 29, 2013, 10:46 pm
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I was runing a 2D model and came up with this error..
anyone can help me??


Undefined value found.
- Detail: Undefined value found in the equation residual vector.
.There are 912 degrees of freedom giving NaN/Inf in the vector for the variable mod1.E10
. at coordinates: (-2.83333e-007,-8.5e-007), (-2.91667e-007,-8.5e-007), (-3e-007,-8.41126e-007), (-3e-007,-8.32252e-007), (-2.94182e-007,-8.26412e-007), ...
There are 284 degrees of freedom giving NaN/Inf in the vector for the variable mod1.E20
. at coordinates: (-2.94182e-007,-8.35286e-007), (-2.85849e-007,-8.4416e-007), (-2.62218e-007,-8.43432e-007), (-2.73067e-007,-8.37592e-007), (-2.92567e-007,-8.08802e-007), ...
There are 284 degrees of freedom giving NaN/Inf in the vector for the variable mod1.E21
. at coordinates: (-2.94182e-007,-8.35286e-007), (-2.85849e-007,-8.4416e-007), (-2.62218e-007,-8.43432e-007), (-2.73067e-007,-8.37592e-007), (-2.92567e-007,-8.08802e-007), ...
There are 630 degrees of freedom giving NaN/Inf in the vector for the variable mod1.Ez
. at coordinates: (-3e-007,-8.5e-007), (-2.75e-007,-8.5e-007), (-2.875e-007,-8.5e-007), (-3e-007,-8.36689e-007), (-3e-007,-8.23379e-007), ...
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Problem with periodic boundary condtion.

January 29, 2013, 11:27 pm
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I am trying to implement the advection equation with periodic boundary conditions using DG. I have specified homogenous Neumann flux on all the boundaries and then I have used the periodic condition on the opposite pair of boundaries. But the periodic movement does not occur. Is it a problem with the entering of the boundary flux?
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Random nucleation

January 30, 2013, 2:22 am
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Dear all,

I am currently involved in a phase change simulation.


I have not yet found the way to deal with random nucleation occurring for instance during the quenching : during the transient heat transfer calculation, I would like to randomly generate at each time step crystal nuclei at a spesific rate of nucleation ( dependant of : temperature, number of nucleation, the activatin energy, many parametres...)

I am aware of the random function but cannot figure out how to use it in this specific context.


Any help would be much appreciated !
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Problem in direction of magnetic field

January 30, 2013, 4:10 am
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Dear all ,
I simulated the Two coil system with ferrite core , with primary coil and secondary coil and both are separated by distance , i have problem that , if i am exciting primary with voltage source and my field of lines are not perpendicular towards secondary , please let me know what could be the reason
and second thing how can i add infinite elements to particular domain
i am attching my model
--
kishore Naik Mude
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Can't save file after deleting one model

January 30, 2013, 6:44 am
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I started with one model, then created a second and afterwards deleted the first one completely. Now I am unable to save the file and I also can't run any study. I think there are some variables left from the first model I deleted. Here is the error message when I try to save the file:

The object 'Type: Geometry
Tag: geom1
Dimension: 2
Current child: r3 is not contained in a resource.


In general, how can I access variables and delete them?
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Heaviside function simulate Acoustic emission signal

January 30, 2013, 7:02 am
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Hello,

I need to apply a Heaviside function difference to simulate a Acoustic emission burst signal. Can help, how can I do this? What model should I use? I'm new working with COMSOL.

Thanks,

Nuno Gueiral
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building a multicomponent gas mixture in comsol

July 11, 2011, 9:49 am
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Hello All,

I was wondering if someone could help me with building a multi-component gas mixture in COMSOL 4.2. I have my biogas composition of 66% CH4, 33% CO2 and 2% H2S and I need to define parameters in comsol for these proportions for modelling gas flowing through water in a bubble column reactor. If anyone could help me with this it would be much appreciated. As of right now, I have CO2 flowing through water with no problem but I need to build a gas mixture and since the bubbly flow interphase only allows for one gas and one liquid I assume I need to build my own material with the proportions listed above. Unfortunately, I can't figure out how to define it in the material builder.


Thanks!!

Nick
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wave guide

October 25, 2011, 10:27 pm
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Hi,
I am starting study using Femlab simulation. I have problem. Actually, i want to simulate my metamaterial design but before simulate my design, i try simulate simple design. I just make rectangular wave guide and define air inside of wave guide. The dimension of base is 100 micro meter x 80 micro meter. I think the result is in all frequency, the wave will propagate till the end of wave guide. But i get graph like high pass filter, and freq cut off is around 1.6 e12 Hz. When freq was under 1.6e12 Hz, the wave could not propagate till the end of wave guide. Any body can explain about this phenomena?

Another question, I want to simulate the movement of nano particle under electric field. But i could not successful to get result. Any body have experience to simulate particle movement using Femlab?

Thank you very much
Hadi Teguh Yudistira
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Problem Modeling Capacitive Cantilever Sensor

March 16, 2012, 6:57 am
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Hi,

I want to find out the pull-in voltage of a capacitive cantilever sensor. To do so I did the following:

I chose to do the 2D model first to get an idea, then I added Moving Mesh (ale), solid mechanics, and electrostatics (in order) for my physics and chose a stationary solver.

In geometry I made 3 rectangles (300um * 2 um). I chose the top and bottom rectangles to be gold; the top one is going to be the cantilever and the bottom one is going to be a fixed ground plane.

I chose only the top and bottom rectangles for the solid mechanics module. I chose the middle rectangle, representing the gap to be air. Then is solid mechanics module I fixed the left, right, and the bottom boundaries of the bottom rectangle (the ground plane) and fixed the left boundary of the top one, using the fixed boundary constraint.

I chose air for the electrostatics module. In the electrostatics module, I chose the bottom face of the middle rectangle to be ground and the top face to have a fixed potential.

At the end, I chose the middle rectangle only for the ALE, chose it to have free deformation (free deformation domain), and added a prescribed mesh displacement (after the free deformation) overriding the default mesh displacement. I chose the top and bottom face of the rectangle, and chose u and v for dx and dy.

I used a physics-controlled mesh.

At the beginning when I tried to solve the problem it gave me a Newtonian n iteration number error so I increased the number of iterations; however, the y displacement result that I'm interested in, still does not make physical sense (it is ~3E-308, which is zero).

Would someone be kind to tell me where I am making the mistake? Eventually I am going to need to model the #D version; I tried the 3D version using a similar approach and keep getting 0 displacement as answer. I don't get any errors or warnings from the solver. I have also tried playing with the fixed boundary conditions, and the voltage level to no avail.

Thanks.
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How to set a radial polarized illumination ?

September 21, 2012, 3:13 pm
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Hello,

I am simulating a model and the source of the model is radial polarized illumination. However I do not how to set this incident light. I hope anyone who knows the answer can tell me. I use the RF module. Thank you for all of you.
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Parallel computing MatLab Comsol

December 5, 2012, 11:24 pm
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Hi,

I am using Comsol 4.3 with Livelink for Matlab. I run an optimization code in Matlab where i call Comsol to perform frequency response simulations. Now i would like to start two parallel frequency response simulations that are independent of each other from my Matlab code.
My approach so far has been to open another Matlab worker by using matlabpool and parfor. However, only one of the Matlab workers are then connected to Comsol. Is it possible to perform parallel computing in Comsol when using the Matlab livelink?

//Anders

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Modeling a Sliding Constraint

January 30, 2013, 11:10 am
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I am trying to model a sliding constraint, basically a pin in a hole.

To try to understand how best to apply the contraints I created a simple model of a tube and rod. The tube and rode are both 1 m long, the rod is inserted .1m into the tube the Tube ID and Rod OD are the same. I fix the free ends of the tube and rod. Then I run an Eigenfrequency study on the assembly.

If the geometry is a union (or assembly with continuity at the sliding interface), the first mode of the two parts is as expected a single wave bow of the system.
If the geometry is an assembly with a roller pair at the sliding interface, the model behaves as if the roller is fixed in space. i.e. the area where the rod is in the tube does not deflect. Rather the first couple modes are of the rod bowing then the tube bowing.

So how does one model a sliding contraint that can move with the surrounding geometry?

I have attached a version 4.3a model that I have been querying. Switch between the continuity 3 and Roller 1 constraint to see the differences as descriped above.
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Modeling a Sliding Constraint

January 30, 2013, 11:10 am
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I am trying to model a sliding constraint, basically a pin in a hole.

To try to understand how best to apply the contraints I created a simple model of a tube and rod. The tube and rode are both 1 m long, the rod is inserted .1m into the tube the Tube ID and Rod OD are the same. I fix the free ends of the tube and rod. Then I run an Eigenfrequency study on the assembly.

If the geometry is a union (or assembly with continuity at the sliding interface), the first mode of the two parts is as expected a single wave bow of the system.
If the geometry is an assembly with a roller pair at the sliding interface, the model behaves as if the roller is fixed in space. i.e. the area where the rod is in the tube does not deflect. Rather the first couple modes are of the rod bowing then the tube bowing.

So how does one model a sliding contraint that can move with the surrounding geometry?

I have attached a version 4.3a model that I have been querying. Switch between the continuity 3 and Roller 1 constraint to see the differences as descriped above.
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Stiffness in bending mode

January 30, 2013, 12:43 pm
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Hello Everyone,

I have been COMSOL for quite sometime and have come across a problem that I can not solve very recently.

My objective is to find out the bending stiffness of a metallic nanoplate. What I have been doing is provide COMSOL with Young's modulus, Poissons ratio and density of the metal and got correct values of stiffness for longitudinal mode. However when I go for bending mode the numbers do not match with atomistic simulations. I think this can be solved by introducing anisotropic elastic constants of the metal (C11.C12,C44). Can anyone advise me as to how can I plugin these numbers in the solid mechanics module? Or if I were to plugin these values in the piezoelectric module what should I use for Coupling matrix, relative permittivity, loss factors?

Thanks

Sankha
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parasitic capacitance

January 30, 2013, 10:28 pm
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hi,
i m designing a capacitive comb drive accelerometer in this i want to take parasitic capacitance in to account in order to find capacitance but i cant find where it is...could you please help me out
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difference between "optimization study node" and "optimization module" for an optimization problem

January 31, 2013, 3:37 am
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Hi!

Seems an optimization problem can be performed by either "optimization study node" or "optimization module".

If use "optimization study node", the available optimization solvers include SNOPT and Levernberg-Marquardt

while if use "optimization module", then the available optimization solvers are Nelder Mead solver, Coordinate Search, Monte Caro.

What is the difference of above two approach? And how the two different set of solvers are related?
Thanks a lot in advance!

Xiaogai
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mesh error

January 31, 2013, 7:25 am
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Hey,

I was trying to follow the PDF on comsol website ( Model ID 15) to build a model for Helmholtz coil. I followed almost all the steps except for the dimensions, so the geometry should have no problem. But when I was trying to compute the results, it gives me error saying that "not all domains are meshed, no mesh on domains 1,2", which are the domain for air. I have already set default domains to air and I did not see anything mentioned in the PDF about creating a mesh for air domains as well. Can you help me solve this problem so it won't give me a error? Thanks.
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Gain Medium with Circularly polarized light in COMSOL

January 31, 2013, 8:19 am
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Dear All
COMSOL follows exp(-iwt) modeling in the and hence the absorption of the material is given by imag(permittivity) >0.
If I want to simulate a gain medium, is it trivial to replace the imag(permittivity) < 0 ? or am I missing something.

The other question is regarding : how to set the circularly polarized light
Assume wave propagates in +x direction
E =(0 0 1) implies linear polarization
if I set E = ( 0 i 1) does this mean its a circularly polarized light since there is phase difference between Ex and Ey?


Any help will be appreciated.
Thank you.


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