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Particle-Particle Interaction Cutoff

March 21, 2017, 10:10 am
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Hello!

I am trying to use the mathematical particle tracing module to accumulate some particles in a material.
Ultimately I would love to be able to spawn particles, have them travel in some direction then stick to a wall. However, I also do not want them to touch each other.

I figured I could make this happen by using the particle-particle interaction node and applying something like damped force when the particles get close to each other to reach an equilibrium distance.

Anyway, I am running into a problem with the cutoff length I think. I've attached a sample file where I release one particle, let it drift, then release another one some time later. I am using the cutoff length setting, so the first particle shouldn't "feel" any interaction force since the second particle hasn't been released yet. However, it does! I'm not too familiar with this physics set so I don't know exactly how it works, but something doesn't feel right.

Any help would be greatly appreciated.
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COMSOL 5.2 Linux version installation

March 21, 2017, 1:17 pm
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Hi,
I am new to COMSOL. Please help me to get the installer file. It was asking for trial passcode. Can you please help me to get it.

Thanks!
-Dipanjan.
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Undefined value found

October 19, 2009, 6:22 am
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Hi,

i can solve my semiconductor model with a relatively coarse mesh.
However the found solution is not good enough. It shows some
differences to a reference (PC1D) semiconductor model.

That is why i want to refine the mesh.


But if i refine the mesh i get following error:

---------------------------------------------------------------------
Error: 7248
Undefined value found.

Details:
Exception:
com.femlab.jni.FlNativeException: Undefined value found Solvermodel
Messages:
Undefined value found Solvermodel

Stack trace:
at solvermodel.cpp, row 1837, (Undefined value found in the equation residual vector.
There are 85 degrees of freedom giving NaN/Inf in the vector for the variable phii
at coordinates: (0.000493076,-5.8675e-005), (0.00049406,-5.80051e-005), (0.000495044,-5.73351e-005), (0.000494026,-5.58557e-005), (0.000493041,-5.65256e-005), ...
)
at com.femlab.solver.FlSolver.femStatic(Native Method)
at com.femlab.solver.FemStatic.run(Unknown Source)
at com.femlab.server.FlRunner.run(Unknown Source)
at com.femlab.util.i.run(Unknown Source)
at com.femlab.util.aa.run(Unknown Source)
----------------------------------------------------------------------------

The model is quite complex and has 5 coupled application modes.
I use the Parametric segregated Solver.

I think if there would be a problem with my initialisation values or boundary contitions,
i would not be able to solve the model even with a coarse mesh?

I found out, that the problem especially occurs, if i refine the mesh in a region with large gradients.

The Comsol solution is represented by green circles and the reference solution is shown as line in the attached figure.

Has anyone an idea how the finer mesh causes "undefined value"-problems and how to overcome them?

Thank you,

Stefan







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global equations (ODE) and global variables

December 9, 2010, 6:27 am
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Hi, I'm trying to define an equation in global equations and it is corresponded with the coordinates x and y (or certain global variable), e.g.:

States
Name (u) Equation f(u,ut,utt,t) Init (u)
u u-rho 0

where rho is defined as sqrt(x^2+y^2) in Global Expressions. However, I get an error as follows:

Error: 6170
Failed to evaluate variable.
- Variable: x
- Global scope
Failed to evaluate variable.
- Variable: rho
- Defined as: sqrt(((x)^2+(y)^2))
Failed to evaluate expression.
- Expression: u-rho

I would like to ask if there is a way to define the global equation such that it accepts a global variable like rho? If not, how can I use coupling variable or some other ways to get the coordinates in the whole domain. By the way, how can I obtain the Jacobian numerically in Comsol? Thank you very much for your advice and help!

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PCF design

January 9, 2012, 3:21 am
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How can I get imaginary part of effective mode index using comsol4.0 ? Help me.
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Parameter estimation

April 24, 2012, 5:26 pm
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Hello,

I want to determine one parameter k1, which is used over the entire domain. However, objective function values should be compared only at a boundary. According to the Optimization user manual, I think I need to use “least square objective" at a boundary and either “control variable field over the entire domain” or “global control variable”. When I try these, I get an error message of “Incorrect number of coordinates in least-squares objective”. Can I define an objective function at a boundary and use a control variable applied to the entire domain so that the control variable can be evaluated over the entire domain in COMSOL? In the attached model, the Optimization module is used in Study 2.

I contacted technical support and heard that they would post a model example in the future (unscheduled for now). If you know how to do it, please advise. I'd greatly appreciate your help!

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Input Port Excitation Power for 2D RF Simulation

January 15, 2013, 8:50 am
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Hello, I'm having difficulty understanding exactly what the input power is for the port in the RF module in COMSOL 4.3. The unit is declared as watt, but unless there's some predefined thickness, the unit doesn't make sense (I would expect watt/m).

Just as an example, I tried simulating a silicon waveguide and set a line integration variable on the input where I measured the emw.Poavx (perpendicular to the line) at different lines on the waveguide and multiplied this value (which is in watt/m) by my expected thickness and got values that were much smaller than what I expected for what I put in as my input power.

Does this mean that the unit for input power in the port excitation is in fact watt/m? If so, that would explain some discrepancies in my other results I've been simulating.
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rigid particle carried by a channel flow

April 15, 2013, 4:16 am
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Hi

I have to simulate the displacement and rotation of a rigid prticule in a fluid flow and I am not a professional on comsol. So, is there anyone who could help me? or if you have any examples of this models.

Thanks
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Damped Eigen frequency Analysis

March 5, 2015, 12:57 pm
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Hei,

I am working on calculating Q factor from eigenfrequecny analysis using structural mechanics. I know that Q factor= abs(imag)/2*abs(real) but my problem is when I add damping in the form of isotropic loss factor, it would increase the real part too (which is frequency of the system) that should decrease with the increase in damping. Since both the real and imaginary part are increasing simultaneously, there is a little difference in the values of Q factor with or without damping.
I know that adding damping to eigenfrequency gives imaginary frequencies, but I am not sure how to deal with it.
Please give me some suggestions on how to proceed.
Any help would be appreciated.
Regards,
Saleha
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Far field of dipole antenna on substrate

December 8, 2016, 6:46 am
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Dear COMSOL community

I try to model a nanodipole antenna on glass substrate , first I create a model for a nanodipole in air and the far field seems good to me ( the tours shape typical for dipole antenna ) , where the far field calculation is made by a surface surround the dipole and placed lamb/2 far away from the dipole.

Now I placed the dipole on the top of a glass substrate , while the far field domain this time will be half air and half substrate , unfortunately the shape of the far field seems to me incorrect , I read on the thread that the far field domain has to be homogeneous, but how to calculate the far field in the case of antenna placed on substrate

Thanks in advance

M.H.Elshorbagy

UCM , Spain
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A Swept Quad Mesh Within Free Tet - Quality Concerns

March 17, 2017, 10:00 pm
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Hi everyone,

I'm looking for some advice on a meshing problem that I have - I have a heat load imposed on a 3D rectangular beam embedded within a larger beam of the same material; the two beams form a union. The area where the heat load is applied is very finely meshed with a mapped distribution so that I can control the exact number of elements in my area of interest.

This is great but my problem is that in order to cut down on computation time/memory cost - I am converting the boundaries of this swept block (or another swept block around it) to tri's and then putting in free tetrahedral mesh for the large remainder of the geometry. This has the unfortunate effect of creating a region of poor mesh element quality around the swept block where I need my mesh to be the most accurate.

I have attached an image of what the mesh element quality looks like in one iteration of this model. It seems that every time I try a slightly different meshing scheme - my results are significantly different and I have tried many variations. If anyone can suggest a better alternative to my current meshing scheme, or point me in the right direction of increasing the mesh quality, I would greatly appreciate it.

Finally, my heat loads are asymmetric and so unfortunately I cannot exploit any symmetry to cut down on the computational cost.
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The transition zone is too large. Two transition zones overlap

March 21, 2017, 3:22 pm
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Hello everyone,

When I use the Rectangle function in Comsol5.2, I set the Lower limit and Upper limit as 0.03029 and 0.
05818, respectively. But it gets an error saying that "The transition zone is too large. Two transition zones overlap". I indeed believe that the accuracy of Comsol is enough, so how can I solve it.

Thanks!

Jiaqing
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Couple heat and mass transfer of porous media

March 21, 2017, 9:16 pm
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I am new in using COMSOL in solving couple heat and mass transfer process. Using transport of dilute species in porous media (tds) and heat transfer in porous media (ht), and finer mesh, i wasn't able to solve the couple multiphysics problem. After running the simulation i got an error message "The following feature has encountered a problem: Invalid variable for lower limit. Feature: Time-Dependent Solver 1 (sol1/t1)", as shown in the attached image. Please, i need someone to help me figure out how to solve the problem.
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Graphis Display Overlap

March 21, 2017, 10:21 pm
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Unsteady incompressible N-S solver

March 22, 2017, 12:10 am
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Hello all
I have question about the method used by COMSOL for solving the unsteady incompressible laminar flow. I know it has an option for pressure-correction approach (projection method), but I have no idea about the default method used to deal with pressure.
I would appreciate if you could share some information with me.

Thanks,
Farzad
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Magnetic fields of moving objects

March 22, 2017, 4:20 am
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Hello everyone,

I am pretty new to COMSOL so this might be a simple question to answer.

I want to 2D simulate magnetic field distribution between a moving magnet and an iron bar. Stationary problem is solved easily. But when I want it to be time dependent, the fields don't move even though there are deformation indeed. So, the magnet moves but in the plot we have the distribution of the fields as in the solution of the stationary problem. I have tried to include Moving Mesh before the Solid Mechanics module but without success.

So my question is, how can I force the system to take into account spatial movement of the magnet in my problem so that it could be possible to show the changing of the field distribution in the resulting plots?

Can I do this only with the Physics “Magnetic Fields, No Currents”, "Solid mechanics" and "Moving Mesh" or do I need another physics?

Thank you very much!
Serg
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Animated Magnetic Field of 3-Phase Cable

March 22, 2017, 9:47 am
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Hello,

I tried using the model "www.comsol.de/model/electromag...rrent-carrying-wires-131" to create a 3-phase cable carrying current with a 120° phase shift. I use the time domain and first I tried to set three different current parameters like " 6[A]*sin(2*pi*50[Hz]*t[s]+120[deg]) " and now I tried to use directly the term " 6[A]*sin(2*pi*50[Hz]*t[s]+120[deg])/(pi*r^2) " as external current density. But unfortunatley the resuting magentic field does not behave as expected. The attached .gif shows the magentic field, which is not correctly in phase. I also attached the .mph-file.

I want to use the time domain, because I want to animate the magnetic field. But I was wondering what the frequency domain can show me after calculation because when I shuld get an alternating field it showed e stationary field around each of the three wires.

Can someone help me?



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Phononic crystals

March 22, 2017, 10:23 am
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--i want to calculate the transmission spectrum for 2D solid solid phononic crystal and i am using solid module but i do not know what boundary condition i must use …
can any one know what boundary condition that help me to calculate the transmission coefficient ???
ahmed
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Analysis of mixed convection flows within a square cavity

March 22, 2017, 11:54 am
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Hi , I am new to the topic of solving mixed convection problems in COMSOL multiphysics.
I want to validate Fig. 8 of the paper(attached). Stream function and temperature contours for uniform bottom heating case with Pr = 10, Gr = 1000: (a) Re = 1, (b) Re = 10, and (c) Re = 100.
Which physics is to used here. Is it Laminar flow(SPF) and heat transfer(ht)?How to take care of Reynolds number in the governing equation? What is the body force or volume force to be used here? There is no Rayleigh number here but Grashoff number is the parameter of study here.
Kindly suggest for a non dimensional case.
Thanks in advance.
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How to add predefined materials & physics in my own physics builder in Comsol 5.2a ?

March 22, 2017, 1:55 pm
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Hi Everyone

I want to contruct my own model which contain 2 physics and 2 materials already predefined in the wizard model from comsol and my own complex adsorption - convection - diffusion equation. But I don't know how do that!!

I want to add phase field and lminar two-phase flow for physics and oil, water for material.

Thanks in advance.
Marc
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