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Sensor Positioning

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Hello all,

Can anyone give me a hint on how to use vibration as an indicator to find the best positions of sensors on a simple geometry?

Please someone help, i need urgent help :(

Initial value-Initial guess at a time dependent solver

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Hi all,

I was wondering whether Comsol is using an initial guess for every time step at the time dependent problems.
I know that it needs an initial value (at time=0) to start solving the problem.
My understanding is, that an initial value is required for the stationary solvers(I suppose this is just an initial guess, that will later on be overwritten). Is there such an initial guess for every time step at the time dependent problems?

Any thoughts?
Thanks a lot!

Nancy

Radio wave antenna calculation, solution doesn't converge

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Hi,
I was trying to study the radiation from an nano-scale antenna as well as its impedance. However, the solution doesn't converge. I tried all iterative methods, unfortunately, none of those actually worked.
I tried to use the Gauge fixing for A-field. it gave my an error saying that "a 0 element in the matrix". I knew I may be able to fix it by increasing the conductivity of air and other high resistance components. But I don't really want to do so, because the antenna has a really low conductivity too~10^4 S/m

Any help will be appreciated.

Variable WAr in microwave plasma simualtion & other

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Hi everyone,

I'm trying to re-do all the steps of the following model : inplane_microwave_plasma.mph, available in the model library. I encountered two different problems that i don't understand.

1) When i use an union at the end of the design of the geometry (the 3 rectangles), i am not able to set properly the boundaries for the "Wall" and "Ground" : it says "not applicable" on the common border with the upper & lower rectangle ! Why is that ? In the model, it works... I got round this problem by using an assembly with contcat pairs instead, and everything work then. But it's not what the model was using... Any clue ?

2) More serious : when i try to compute the solution, i get an error message a few seconds after the beginning of the time dependant solver phase. It says :

Failed to evaluate variable jacobian
- Variable: WAr
- Geometry: 2
- Boundary: 8
- Feature: Time-Dependent Solver 1 (sol4/t1)
- Error: Failed to evaluate variable Jacobian.

So i displayed the equation and started looking for the WAr variable. Never able to find it. I just found this :
mwp.wAr=1-mwp.wArs-mwp_wAr_1p
mwp.wArs=exp(Re[WArs])
mwp.wAr_1p=exp(Re[WAr_1p])

But no hint of the localisation of those WArs nor WAr_1p and above all, WAr... Any idea on this ?

Thanks in advance,

Alexandre Bondoux

Tutrial help

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Hello,

I am having trouble doing the stress analysis of a rotating disc with some angular velocity, varying density and varying thickness.Please help me to solve this problem urgently.If possible please send a tutorial on the rotating disc analysis problem.


Thanks,
Chakradhar

weak magnetic field converges, strong magnetic field does not converge

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The simulation should analyse the following problem: A magnet is passing by a piece of iron. (sketch.jpg) As Study Step I chose Time Dependent.

At first I tried a simple problem, where the magnet was not moved at all. The magnet is just slowly switched on and off by using a smoothed rectangle function. This simulation takes about 2 Minutes on my PC, the solutions are mostly converging very well and the time steps are rather large. (convergence weak field.jpg; step size weak field.jpg; graphics weak field.jpg)

But if I increase the strength of the magnet by a factor of 10 (magnetization.jpg; 1000 A/m works, 10000 A/m does not), without changing anything else, convergence gets worse and time steps get very small until the simulation ends with an error after 20% of the time. (convergence strong field.jpg; step size strong field.jpg)

COMSOL Error
The following feature has encountered a problem:
Repeated error test failures. May have reached a singularity.
Time: 0. 1956460958724214
Last time step is not converged.
- Feature: Time-Dependent Solver 1 (sol1/t1)
- Error: Repeated error test failures. May have reached a singularity.

I searched for this error in the forum and could not find the mistake in my model. The mesh looks good (mesh.jpg), the magnetization is smoothed both in time and space, there are no sudden changes in the magnetization, there are no sharp start conditions.

Any ideas why it does not converge?

(The magnet is not modelled as a real mass, because later I do not want to move a real mass in my simulation. Instead I defined a magnetisation, which is dependant of the coordinates of space and time. I did not use a moving mesh, because it is not suitable for complex geometries in 3D.)

Tracing a single particle in an expending pipe in a fluid

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I would like to module an finite pipe inside a fluid that expands in time and the effect this expansion has on a near by particle.

In the attached illustration, the wired tube is the pipe, and the blow arrows mark the movement of the pipe. the black disk remains stationary during the expansion, so the gap between the the disc and the pipe grows with the expansion. all of this is inside a fluid.

I would like to trace the motion of a particle ( the red arrows).

Will appreciate any help on how to build and execute this.

Electric field distribution - Point Source

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I want to simulate the Vertical and Horizontal polarization of dipole source.

Its Au-GaAs-Air structure with electric dipole source. You can find that in attachment. For this we need to import n and k values for both GaAs and Gold right ? I did it and imported via interpolation. Since I m dealing in Lambda I made arguments in nm. The data source file for GaAs and Au can be found in mph file. I got that from refractive index.info site. I think for each wavelength, the values are not listed.So this would be fine for simulation?? And I had defined the relative permittivity in GaAs and Au. Is thats right ?? Can we use Scat B.C or PML for this ?

For Vertical and Horizontal Polarization, I chose Dipole : Magnitude and direction - x = 0 and y=1 for V-Pol and vice versa for H-Pol right ? Correct me if I m wrong.

Thank you.

A SMALL DOUBT ON PORT POWER VALUE !

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Hi all,

I am new to this software and I am trying to simulate a multi-layer structure on which plane electromagnetic wave is allowed to fall at normal incidence. I kept the port power at the default value(1 W) and after running the program I tried to check the power at the port 1. For that I use line integration to calculate sqrt(emw.Poavy^2 +emw.Poavx^2), but to my surprise the values I am getting is smaller than 1.

Can any one help me.
Thanks in advance...

Eigenvalue problem with Dirichlet Boundary condition

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Hi all,

I am a newbie of comsol and try to solve eigenvalue problem in a finite region using the PDE module. When I try to impose Dirichlet boundary condition, it seems not working. The eigen-wavefunction's value at boundary does not equal to the value I set. Even when I set u(at boundary)=1, the eigen-wavefunction appears to be 0 at boundaries. What kind of stupid errors have I made in the attachment? I am really looking forward to your response.

Thank you for your time!

jason

Electrical Circuit, Pulse source

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Hello Comsol users,

I want to model a simple electrical network.
But the voltage source is modeled as a pulse source.
And I have big problems with the solver.
Any idea how the solver has to be set?
I attached the model that I did.

Thanks.

Missing Voltage

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Hello all,

I seem to be having a problem with the electric currents model. I have a boundary current source defined for an object in a poorly conductive medium. Over the domain of the object the current ranges from negative to positive, and what I'm seeing is that after solving, (in some circumstances) the voltage everywhere will either be positive or negative (IE. the scale bar showing potential will stop at 0)

Any help with the issue will be greatly appreciated.

Thanks,
Aaron R. Shifman

Initial conditions (problem) - Fluid Structure Interaction

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Hello,

I am working with the FSI module and I am trying to simulate the pumping phenomena in a micropump, by means of a moving plate. I have successfully obtained results for certain design parameters of the micropump, but, when I try to slightly modify those parameters (for example: plate thickness from 20[um] to 22[um]). The simulation doesn't work and the study reports an error in the initial conditions.

Any input is appreciated!

Thanks,
Mauricio

jacobian error

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Hi all,
I am running more than one model in the same file, separately. I do it because in one model I need the results of another model. I tried to refer to a solution typing, for example mod1.mf.Bx. The call works but when I solve it Comsol generates an error saying " failed to evaluate vaiarble Jacobian" of the variable mod1.mf.Bx.
What does it mean? Is there a solution? Is what I am trying to do possible in comsol?

Many Thanks,
Andrea

1D Schrodinger equation

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Dear Comsol users,

I want to solve radial Schrodinger equation for Hydrogen atom in comsol. So I use 1D axisymmetric coefficient form PDE and solve for eigenvalues with boundary conditions as u =0for r----> 0 and Infinity. I cannot get the degeneracy of energy states as in 2D H atom example.

I have attached my model here. Could anyone please tell me what am I doing wrong here ?

global equations (ODE) and global variables

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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!

Variable modifications between solution iterations

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I'm developing a transient model using the Livelink interface. The model first calculates an initial solution for the first time window, then I would like to modify the state variable values due to some external disturbance using MATLAB, then proceed to rerun the model for the next time window using the newly adjusted values of some of the variables, but the other variables should be retained from the previous solution iteration.

When I prescribe the use of the previous solution for subsequent iterations as:

model.sol('sol1').feature('v1').set('initmethod', 'sol');
model.sol('sol1').feature('v1').set('initsol', 'sol1');

it seems that the specification of initial conditions through either

model.variable('var15').set('X0', num2str(X0));

or

model.init('init4').set('position',num2str(X0));

are ignored completely and the unchanged values from the previous solution are retained. Is there a plausible method to effectively update some of the variables this way? Perhaps by modifying the dataset of the previous solution?

Thanks.

frequency domain using reduced basis (modal)

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Hi all,

I'm trying to make a frequency sweep on a (big) model of an acoustic cavity, with specified surface impedance as boundary conditions.
If I use the "Frequency domain" study, everything goes well, except that it is really, really slow.
Considering that 1) few eigenmodes participate to the vibrations at these frequencies (20-200Hz), that 2) I would like to have a good frequency resolution in my sweep, and 3) I need to test a lot of combinations of parameters for the boundary conditions, I figured that using the "Modal frequency domain" study type is perfect for me.

However, it does not seem to work : I always get a null solution, for all frequencies, when I use this type of study.
The predefined study contains two step : one for finding the eigenfrequencies and eigenmodes, and the second to use these modes as a reduced basis for the frequency sweep. The first step seems to work well (I can visualize the modes, they seem ok to me), but the second gives mysteriously a null result everytime (without any error message).

If anyone knows that problem and how to overcome it, I'd be really glad to know!

Regards,
Sami

PS : I'm using Comsol 4.3a.
PS2 : I use a point volume flow source, which works as expected with the traditional "frequency domain" study.

acoustic pressure, frequency response on modal basis

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Hi all,

I am using Comsol 4.3a and I can't find what's wrong with my use of the frequency response study using reduced model. This is my second post on the subject, as the first one didn't attract much answers...

Attached is a very simple example, where we first compute the first ~10 eigenmodes, then use it as a reduced basis for a frequency sweep.

The model is composed of a rectangular box with a volume flow point source near a corner, and a point were we would like to measure the pressure near another corner. The walls are slightly lossy, with a finite, real impedance.

When running "Study 1", the eigen modes are properly computed, but the output of frequency sweep on modal basis is the null solution for every frequency. No error are notified. The log seems clean too...

Any idea ?

Thx,
Sami

Re: maximum (or minimum) value in a time domain

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Hi
To tr to obtain starch gelatinisation after cooking and cooling, I tried to use what describe at www.comsol.com/community/forums/general/thread/28107/

I did not succeed. Perhaps I did not find the right “non zero” coefficients.
And when I try to use derivative, I have under and overshoots and a bad value at end.

Some suggestions?

Jean-Michel
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