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Geometry variation within Time-Dependent solving.

April 18, 2011, 9:41 am
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Hello,

I was wondering if it's possible to change geometry according to time ( via internal or external function(MatLab)) ?

ie: Making a block depth function of time.


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Electromagnetic Periodic Boundary Condition

August 9, 2012, 7:00 am
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Hi all,

I am trying to simulate a 2D periodic electromagnetic structure using periodic boundary condition in comsol multiphysics. the whole structure is 3D but it has 2D periodicity in x and y directions.
There are a lot of options in periodic conditions Tab including "Type of periodicity", "Constraint Type".
I can just use one of those options because the rest makes error.
What I am using is "continuity" in "Type of Periodicity" and "Unidirectional" in "Constraint Type".
At the rest options, simulation runs up to some percentage and then it stops and makes error after a while.
Although I can simulate the structure with above mentioned periodic conditions but when I am calculating reflection and absorption of that structure it gives wrong results.
I am calculating reflection and transmission using Poynting theory. So I am calculating the power transmitted through different transmission and reflection planes.
When I am using this method by PEC an PMC boundary conditions, It gives correct results which the results correspond even to calculated results with scattering parameters, but this calculation does not work for periodic BC.
The structure should have resonance at 0.87 THz which it has resonance at the same frequency but the amount of absorption (A=1-R-T) gets higher than 100% or even it changes when I am changing the reflection plane of my calculations.
I am not sure now if my method of using periodic boundary condition is wrong or my calculation is wrong, but I think that I may do wrong periodic boundary conditions.
The surprising thing is that I tried aslo to use exactly same mesh on symmetric boundaries but the results are totally different from what I got with nonsymmetric mesh.
When I want to use periodic BC, I am using two periodic boundary Tabs and I am putting pairs of symmetric boundaries in each of them. I am selecting boundaries in x direction in one of the Tabs and boundaries in y direction in another Tab. I am using scattering BC for z direction boundaries.

I appreciate if someone can help me.
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use modulo for periodic step function

September 6, 2012, 4:31 am
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Hi!

I want to create a periodic step function to control the input concentration of a contaminant in an aquifer. I want the concentration to be 1 for 3 months and 0 for the rest 9 months.

I have created a rect function with upper limit=0, lower limit=3*24*30*3600 (months in sec), and a rise time=0.5*24*30*3600 (1/2 month for smoothing).

Then I use as a dirichlet boundary condition that C=1 * rect1(mod(t,12*24*30*3600)).

This should work, no? Is there a way to plot the input function before runing the model? (the model is very big with many meshes and it takes a long time to solve).

When I try to run it I get an error at time=1 year (12*24*30*3600). It says that solver can not converge to a solution. Any idea why I get this? I thought that it could be the sharp step increase.. however it worked ok at t=3months where there was a step decrease... I have also used a 1/2 month smoothing which should be ok.. I have set the solver to take "free" time steps while solving it, so I was hoping that it will take smaller time steps around the concentration changes.

thank you so much!
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Discretization schemes, micromixer tutorial

September 6, 2012, 7:33 am
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Dear Comsol users,

I kindly ask you to help me with the following task. I have to specify the discretization schemes employed in my model. Since I used an existing tutorial model 'micromixer.mphbin' and did not change any of the tutorial's solver procedures could you please help me specify/identify the discretization schemes employed?

I tried to get this information from the tutorial's pdf (under 'Study 1' section) though unsuccessfully.
Thank you very much for your help,
best regrads!
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Moving ferromagnetic objects inside magnetic field

September 6, 2012, 10:33 pm
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Dear COMSOL experts,

for our research we would need to simulate moving metallic leaves of finite extent inside a magnetic field generated by multiple coils carrying a DC current (i.e. no changing fields). We are interested in the influence of the movement on the magnetic field as well as in the forces on the metallic leaves, which includes eddy-currents induced in the leaves and probably in the coils. The velocity of the leaves would be rather slow, so a quasi-static calculation would hopefully be accurate enough.

While it is certainly possible to model this scenario in the static case (where the leaves are not moving) using the COMSOL AC/DC Modul, I am not sure if simulating the moving metallic leaves would be possible.

Through reading online COMSOL documentation, I got the impression that including the velocity into the Lorentz / Maxwell equations is only possible in COMSOL under fairly restrictive conditions, for example it appears to be possible if the object moving is infinite in length alongside the direction of movement (infinite rail) or if you are looking at axis-symmetric object rotating around its axis of symmetry.

However, I have found no possibility so far of modelling a scenario where you have a moving conductive projectile inside a magnetic field.

So my questions are:

a) Can above described scenario be modelled in COMSOL with sufficient accuracy?
b) If yes, is it possible to feed COMSOL with a trajectory for the single metallic leaves and simulate the magnetic fields and forces over time?

Any help is gladly appreciated,

Tristan
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Average Nusset number and Standard Deviation Calculation

September 7, 2012, 10:10 am
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Please help me How to calculate Average Nusselt number over the heated wall and standard deviation of the temperature over the whole surface at different Ra values in COMSOL?
(I am cosidering a square cavity and trying to validate benchmark buoyancy flow posed by de Vahl Davis).

Thanks
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Heat Generation From Impact

September 8, 2012, 9:17 am
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I am attempting to model the heat generated from the impact of two metallic spheres. I was able to solve the structural mechanics portion of the problem, but I am having trouble coupling the heat transfer portion. COMSOL will solve the problem, but it gives me the following warning: Dangerous use of time-derivatives in pointwise constraints. And there is no temperature increase shown. Any help would be much appreciated!

Ryan
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Electric Currents Boundary Conditions Question

September 9, 2012, 2:17 pm
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Hello,

I am a relatively new user of Comsol, and I am trying to learn to use the electric currents Physics module. I want to model two copper cylinders with a contact resistance, and 1V applied across the whole structure. I also want to see what the electric potential around this structure would look like in air.

If I do not have a box of air around the cylinders, Comsol solves the potential drop along the cylinders with no problem, but when the air box is introduced, a "Singular Matrix" error shows up**, which I understand is usually due to poorly formed boundary conditions.

How can I form boundary conditions to have Comsol complete this simulation? Also, in general, what do I need to be careful with in the electric currents module?

Thank you so much,
Robert



**The error is:
Failed to find a solution.
Singular matrix.

There are 21056 void equations (empty rows in matrix) for the variable mod1.V
at coordinates: (-3.5355,-3.53557,-5), (-3.17193,-3.86508,-5), (-2.77782,-4.15737,-5), (-4.15732,-2.7779,-5), (-3.86502,-3.17201,-5), ...
and similarly for the degrees of freedom (empty columns in matrix)
Returned solution is not converged.
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Extremely thin geometries

September 9, 2012, 11:13 pm
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I created a cylinder with radius 1e-6 m and thickness 9e-10 m in 3D to make a continuum simulation of graphene. I have initial radial stress, an uniform body load along the thickness of the cylinder and use fully clamped boundary conditions on the radial edge. When I try to mesh it the process never finishes, and earlier when I had a mesh on a similar geometry the simulation didn't converge.

Question: What do I need to consider while trying to simulate extremely thin geometries?
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What's Axial symmetry 1?

September 9, 2012, 11:24 pm
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Hi, everyone

I think people using 2D axisymmetry mode know the default Axial symmetry 1 condition. I feel this condition strange, as most of time there's no variable, no weak expression or no constraint. Then what's imposed on this boundary? Does anyone know this?

Thanks a lot!
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Today is the *Hotel Savings Deadline* for the COMSOL Conference

September 10, 2012, 5:20 am
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Hello,

The COMSOL Conference is right around the corner. The Boston Marriott Newton is providing guests of the Conference a special discounted room rate for the event. The deadline is today, Tuesday, *September 11th*.

The block of rooms is filling up fast. To book your room at the special Conference rate of $179 per night, visit: http://comsol.com/c/fol

Note: This rate is available only to Conference attendees, so if you have not already registered and would like to, please do so at http://www.comsol.com/conference2012/usa/register/.

Best regards,
Lauren

--
Marketing Manager
lauren.sansone@comsol.com
781-273-3322

P.S: Want to learn more about the COMSOL Conference? Watch this short highlight video at:
http://www.comsol.com/video/399/
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Cumulative sum ???

September 10, 2012, 9:02 am
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Is it possible to achieve (within comsol) a cumulative sum of a time varying data. Here I'm interested in the calculation of the cumulate mass of water going through a porous media. It is quite easy to get the transient evolution of mass flow rate, but I do not manage to get the cumulative mass flow unless exporting data to Matlab.

Any sugestions are welcome.

Best regards,

D. L.
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Integral Constraint

September 10, 2012, 9:16 am
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Hey there,

I want to impliment an integral constraint: the subdomain integral of one variable (say A) shall be constant. In the COMSOL guide on page 274 it is told to define the integration coupling variable (I call it int_A), with a random point outside my geometry as destination, and then to set its value at "equation system->point settings". There I choose "constr" and type in the field for example "int_A-1" if I want the integral to be 1. (Actually, I define a constant A0, make an integration coupling varialbe "int_A0" and then type in that constr. field "int_A-int_A0", but this should not make the difference I think.) This does not give me an error, but the solution does not fulfill the constraint! What is my mistake? I also tried the recommendations in this thread (http://www.comsol.com/community/forums/general/thread/11/), but also without succes.

This constraint is surely not a rare thing to want, so if you know how to do it please let me know. :)
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Visualization problems

September 10, 2012, 2:49 pm
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The java heap space problem is solved, but now the simulation reaches 100% of the linear solver and nothing happens, Comsol uses a core of four available cores, I give same time that Comsol takes to reach 100% complete linear solver and nothing happens, still in the same state.
In other attempt for to solve this problem, I remove the check in "Generate default plot" box, and them Comsol finish the simulation without problems or errors, but when I tried to plot the "particle trayectory", happens that, when Comsol attempts to load time data of "Particle" do not finish this task, at least for one day of elapse time.

The features of simulation are:
>>1 600 000 elements (magnetic field solution)
>>The elements for particle tracing should be more low, because is solved for part of entire geometry.
>>The time of simulation is for one second and steps of 1e-6 seconds (1 million of steps)
>>The size of Solution file is 8 Gb aprox. in .mph file, and the .mph file size is 4 GB aprox. (ccompressed file).

Thanks for your help.....
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Different results with parametric sweep in induction heating model

September 10, 2012, 3:02 pm
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Hi all,

I'm using COMSOL 4.0a to model a 2D axisymmetric model of an induction heater. The real device runs at 62kHz, and when I run a Frequency-Transient study at that frequency for 60 seconds, I get a peak temperature of 426K. However, when I run a Frequency-Transient study with a parameter sweep between 0 and 100kHz, the peak temperatures I get are MUCH lower than without the parameter sweep.

However, when I look at the solution for 60 seconds at 60kHz, I get a peak temperature of 312K. Why is this? I would love to attach the input file, but its 20Mb and it won't let me. Where should I look? What should I check?

Thanks!
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compressible flow with heat transfer

September 10, 2012, 8:29 pm
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I am attempting to model hydrogen flowing through a heat exchanger at high speeds and temps (250m/s and 200K). In order to do this, the change in density of the hydrogen must be taken into account as it travels down the exchanger and heats up. I have been using the conjugate heat transfer model to do this, but it does not seem to be taking into account the density change. Rather, it seems to be modeling it as an incompressible flow. I know that choosing incompressable or compressible flow is an option for the fluid transfer modules, but I have not been able to find it for the conjugate heat transfer module. Does anyone know how enable this function?
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using function

September 10, 2012, 8:58 pm
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Hi,
I have some data like bellow and I want to use the fitting curve through this data as a function. I have tried the interpolation function but this error appeared

>> interpolation grid axis values must be sorted in ascending order in each dimension

11 0.35
13 0.35
14 0.355
16 0.35
23 0.341
28 0.3365
33 0.33
36 0.3225
37 0.31
38 0.32
40 0.312
42 0.2908
44 0.284
45 0.282
47 0.2608
48 0.25
49 0.242
50 0.235
51 0.2288
52 0.205
53 0.206
54 0.1996
55 0.2
56 0.1819
59 0.164
60 0.17
61 0.178
62 0.1549
64 0.155
65 0.145
66 0.16
67 0.14
68 0.134
69 0.146
70 0.138
74 0.134
77 0.126
79 0.124
84 0.119
89 0.114
94 0.1
104 0.1
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Solid Vibration

September 11, 2012, 3:29 am
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Hi,

I'm a bit new on this and I really don`t know if what I'm trying to do is possible.

I have a solid in wich there are two membranes in the middle, the objective of this part is that by a vibration of this two membranes the top of the piece moves up and down, with also a vibration.

For simulate this I'm trying to use the Acoustic-solid and solid mechanic solvers, but i'm having some issues.

Am I doing properly or should try another way?

Thanks a lot.

P.D.: if someone knows about any tutorial of Accoustic solid it would be very helpfull.

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How to calculate a value between two points?

February 12, 2010, 9:14 am
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Hello,

I have modelled a simple 2D electric potential plot of a dielectric between two plates with a void in the middle. I want to know the voltage between across the void. I can get the plots on individual points on teh void but am unclear how the points are specified?

Does anyone know how to specify the function: voltage_diff = voltage at point 1 - voltage at point 2?

Thanks, David.
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Thermoelectric Model

July 12, 2010, 12:23 pm
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Dear All,

I am looking to find and study example models relating to thermoelectric effect modeling using COMSOL.

Does anyone have any reference source/model on thermoelectrics which could be used ?

Thanks
Piyush
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