global equations (ODE) and global variables

December 9, 2010, 6:27 am

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!

↧

PCF design

January 9, 2012, 3:21 am

≫ Next: Parameter estimation

≪ Previous: global equations (ODE) and global variables

How can I get imaginary part of effective mode index using comsol4.0 ? Help me.

↧

Parameter estimation

April 24, 2012, 5:26 pm

≫ Next: Input Port Excitation Power for 2D RF Simulation

≪ Previous: PCF design

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!

↧

Input Port Excitation Power for 2D RF Simulation

January 15, 2013, 8:50 am

≫ Next: rigid particle carried by a channel flow

≪ Previous: Parameter estimation

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.

↧

rigid particle carried by a channel flow

April 15, 2013, 4:16 am

≫ Next: Damped Eigen frequency Analysis

≪ Previous: Input Port Excitation Power for 2D RF Simulation

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

↧

Damped Eigen frequency Analysis

March 5, 2015, 12:57 pm

≫ Next: Far field of dipole antenna on substrate

≪ Previous: rigid particle carried by a channel flow

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

↧

Far field of dipole antenna on substrate

December 8, 2016, 6:46 am

≫ Next: COMSOL 5.2 Linux version installation

≪ Previous: Damped Eigen frequency Analysis

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

↧

COMSOL 5.2 Linux version installation

March 21, 2017, 1:17 pm

≫ Next: The transition zone is too large. Two transition zones overlap

≪ Previous: Far field of dipole antenna on substrate

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.

↧

The transition zone is too large. Two transition zones overlap

March 21, 2017, 3:22 pm

≫ Next: Couple heat and mass transfer of porous media

≪ Previous: COMSOL 5.2 Linux version installation

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

↧

Couple heat and mass transfer of porous media

March 21, 2017, 9:16 pm

≫ Next: Graphis Display Overlap

≪ Previous: The transition zone is too large. Two transition zones overlap

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.

↧

Graphis Display Overlap

March 21, 2017, 10:21 pm

≫ Next: Unsteady incompressible N-S solver

≪ Previous: Couple heat and mass transfer of porous media

How to fix this

↧

Unsteady incompressible N-S solver

March 22, 2017, 12:10 am

≫ Next: Magnetic fields of moving objects

≪ Previous: Graphis Display Overlap

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

↧

Magnetic fields of moving objects

March 22, 2017, 4:20 am

≫ Next: Animated Magnetic Field of 3-Phase Cable

≪ Previous: Unsteady incompressible N-S solver

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

↧

Animated Magnetic Field of 3-Phase Cable

March 22, 2017, 9:47 am

≫ Next: Phononic crystals

≪ Previous: Magnetic fields of moving objects

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?

↧

Phononic crystals

March 22, 2017, 10:23 am

≫ Next: Calculating Rayleigh Damping Parameters

≪ Previous: Animated Magnetic Field of 3-Phase Cable

--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

↧

Calculating Rayleigh Damping Parameters

November 4, 2010, 9:41 am

≫ Next: tutorial model - step index fibre

≪ Previous: Phononic crystals

Hello,
How can one calculate damping parameters (alpha & beta) for a given structure in Comsol?

there is some information about damping in 'smeug.pdf' (pages 140-148, 199, 372). However, it seems that the procedure of the constant calculation is not clear. They address a reader to the Elbow Bracket example (smeug.pdf' pp 36-37). In this example, they assume that "the structure has a constant damping ratio of 0.1."...Actually, if we assume another number we will get different damping parameters.

Please advise,
Stephan

↧

tutorial model - step index fibre

January 26, 2011, 4:49 am

≫ Next: Acoustics - Large models convergence

≪ Previous: Calculating Rayleigh Damping Parameters

I am a beginner in comsol. could you please explain how the electric field pattern is being displayed even when we are not giving any input to the fibre.... is it just assuming a field..?

↧

Acoustics - Large models convergence

August 26, 2015, 10:32 am

≫ Next: Importing ASCII Geometry

≪ Previous: tutorial model - step index fibre

Hi,

I'm driving crazy about this subject. I think don't know many things about this model, but regardless how mesh, solver configuration (multigrid, iterative, direct, etc.) or any other thing I change doesn't matters, it can't reach the convergence.

I'm trying to simulate the noise generated by a solid source that emits noise in all frequencies, like a power transformer, in an electrical distribution substation. I'm using the acoustics module in the frequency domain.

I don't know very well how to set the sources pressure (or the pressure level) with the boundary conditions. I'm using the "pressure" boundary condition setting an arbitrary pressure in order to perform a test.

I'm using an iterative solver with multigrid method (2 or 3 levels), with Krylov preconditioning. Then I use SOR or Jacobi methods.

No matter if set or not a parametric sweep (for frequencies), the solver not converge.

There's any "trick" or something that guides me to reach the convergence in these type of models?

Please, see the attached images in order to understand the described above.

Thanks in advance.

↧

Importing ASCII Geometry

March 16, 2017, 4:34 am

≫ Next: Sheet metal bending - spring back effect

≪ Previous: Acoustics - Large models convergence

I am completely new to Comsol so I really am struggling.

Can anybody explain how I can import some flat panel polygons I have defined as a series of x, y, z position vectors in ASCII into a Comsol file geometry.

I have used Matlab to create some CAD dfx files and tried doing an import. Everything looks like it is going fine then just get a "no geometry imported" error message. (See attached screenshot)

I also have Matlab LiveLink.

Any suggestions would be greatly appreciated,

↧

Sheet metal bending - spring back effect

March 17, 2017, 9:26 am

≫ Next: A Swept Quad Mesh Within Free Tet - Quality Concerns

≪ Previous: Importing ASCII Geometry

Hi

I am trying to make an elastoplastic 2D simulation of sheet metal bending to observe different spring back in various materials. I want to use V-bending proccess since we have a fixture to do the same experiment in the future to check for validity.

I would like to keep it as simple as possible. I am using some data that I have from tensile experiments for different materials and setting simple stress-strain curves with 3 points to vary the hardening. I also set the load/load removal along with auxiliary sweep for the load stepping so the spring back could be observed after the removal of the load.

I am not sure how I should define the contact between the parts. I tried to use contact pairs but I don't know if it is correct. Unfortunately cannot find much about this specific topic on the internet or in this forum (tried to use 3 point bending that someone used which could be similar but didn't succeed much)

↧