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How to apply ac voltage

January 29, 2010, 4:45 am
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Hi all
I want to apply ac voltage(using piezoelectric excitation) to an accelerometer to perform transient analysis.
I have tried the SPICE circuit editor, but it supplies only dc voltage.
Is there any way to apply a periodic input to the device.

Indhuja
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Rayleigh Damping in v4.2

August 12, 2011, 8:12 am
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Hi,

I'm trying to analyze a cantilever beam and I need to change Rayleigh damping parameters (alpha and beta) but I am not able to find them in Comsol v4.2! I knew how to do it in 3.5!

Please advise

Thanks
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Tube Heat Exchanger

April 3, 2012, 6:59 am
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Dear COMSOL community.

I am new in COMSOL and I have some problems with my simulation. Mayby anybody could help me.

I try to simulate a heat exchanger. Steam is condensing on the outside. The heat is transfered through the tube to a cooling fluid inside the tube. For my problem I use the paper "On Teching Chemical Engineering Fundamentals Using FEMLAB" by William M. Clark.

In the attachement you can see my model. Given paramters are: geometry of the tube, temperature of condensate, temperature of incoming cooling fluid and velocity of the cooling fluid.

It consists of four domains: an inner cooling fluid, an estimated thermal boundary layer, the tube wall, and an estimated condensatefilm. The thermal conductivities of the two layers are determined by trial.

Now i would like to tell you my problems.

First i would like to scale my model in length. The real tube is 0.38 cm but i would like to simulate a 0.038 cm long tube without losing any information. So the fluid flow and the heat transfer must adapt to that scale.

The second problem i have is the biggest. I think the results of my simulation are false. As you can see in the attachement the temperature profile is not continouos. There is a jump in temperature between the fluid flow and the thermal boundary layer. But the aim is to have a continous temperature profil along the radius.

My last problem concerns to the results. I would like to determine the total heat flux along the tube. So I have to integrate the heat flux along the length but I do not know how to do it.

Maybe anybody can help me or anybody has a better idea to solve my problem.

A big thank to all of you!

Alex
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How to export data in excel format?

May 31, 2012, 10:35 am
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Hey Ivar,
I want to export my figure into excel format. and replot it in matlab. Is it possible ?
Thank you very much!!

Baofeng
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'Turbulent Two Phase Flow, Level Set' Module

June 20, 2012, 3:45 pm
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Hi,

We are designing a two phase (Air and Gas) burner model using 'Turbulent Two Phase Flow, Level Set' module with Transient Initialization. We are not getting any error when we compute the model for studying but the study does not compute any dependent variables other than phase field variable (mod.phils). I was not able to select 'Solve for this field' in any other dependent variables section. We are having Air and Syn Gas (User Defined Material) as the two fluids in the model. In the results, the volume fraction of gas is shown as zero even where it has to be one. May be the problem exists because the model is not taking into account the physical properties of Syn Gas(No green tick mark shown to the left of property column in syn gas material section) inspite of specifying them and we don't know how to rectify this problem.

It would be grateful if anyone shares his/her knowledge on this.

Thank you.
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Ivar, could you please say something about the solver error?

July 3, 2012, 4:32 am
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Hi,
I would like to model friction stir welding process. I used general heat transfer module and incompressible navier stokes module together. I do not have any problem with the heat transfer module, when I solve seperately, the heat transfer model gives reasonable results. But when I add the navier stokes model I get an error. NaN or failed to find.....(I can give later on detailed error message)

I checked all the boundary conditions, all of them are consistent with the real world. I added some pressure boundary conditions to ease the numerical calculations.

I think the problematic thing is the way I define the viscosity. The viscosity is a function of flow stress and strain rate. And again the flow stress is a function of temperature and strain rate. So the viscosity changes with the strain rate and temperature.

I ticked higly nonlinear problem alternative, I ticked complex number alternative, I tried several different solvers. But I could not manage to avoid this error.

I looked up for the error code in the manuals. I realized that this error occurs because of the viscosity equation. This equation has first derivatives of velocity components. In the manuals it says either change the expression or the initial values of the velocity components. I did change the initial values, but I got the same result. I also have to use the viscosity expression. What can you recommend? What can I do to eliminate this error.

Ivar, could you please say something?

I want to continue to have fun comsoling...but it is hard to do it now.:)

Atilla.

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Internal error in geometry composition, Comsol 4.2a

July 3, 2012, 6:10 am
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Dear Fellows,

I want to make blocks with round edges in 3D. I use standard procedure, I define workplanes, make geometrical objects in 2D, do "extrude", end on the end - INTERSECTION.

When I do these steps in Comsol 4.2a version 4.2.1.110 (older version) everything is OK, but when I do the same in

Comsol 4.2a version 4.2.1.166 (newer version) I receive message:

"Internal error in geometry composition"

Please if someone have a solution for this problem because I need to solve this in newer version because on workstation on which I work is the newer Comsol. I tried to make this module in older version and then to open it in newre version, but then I receive some message about problems with CAD import module.

Thanks in advanced.

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Combustion Flame

July 9, 2012, 8:26 pm
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Does anyone out there have any experience with combustion modeling? I'm trying to model the combustion of methane and air. Any input would be appreciated.
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How to draw a spherical dome?

August 7, 2012, 1:52 pm
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Hello all,

I am very new to Comsol. I am trying to draw a three dimensional spherical dome and mate it with a rectangular box. I have successfully drawn the box with a circular opening on one face. I would really appreciate it if someone could point me to the right direction.

Thanks in advance

Amar
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zero induced current density at conductor boundary, is this normal?

August 14, 2012, 9:16 pm
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I'm modeling a coil-over-steel-plate system using 2D Perpendicular Induction Currents, Vector Potential mode (emqa), time harmonic analysis (f=1.0 MHz).

The calculated induced current density Jz at the plate surface (please see the attached picture) is exactly 0. However, 0.001 mm into the plate will give a induced current density of the order of 10^8 [A/m^2].

Why am I getting zero induced current density at conductor boundary? I checked the 2D eddy current model in the model library (axi-symmetric mode), the induced current density at conductor boundary is not zero and is continuous to its inside values.

Any hint will be greatly appreciated!
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PML and scattering boundary condition in v4.3

August 15, 2012, 6:33 am
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HI,

Is it necessary to define the outer boundaries of a PML as scattering boundary layers to obtain the losses of a leaky waveguide? In v3.5, I have defined the outer boundaries as perfect electric conductors (PEC) and still obtained the correct loss values. With v4.3, if I set the outer boundaries as PECs, I get only real valued effective index solutions and not complex ones, implying that the losses are zero.

Thanks.
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How to have correct expression for stop condition

August 15, 2012, 7:05 pm
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Dear all,

I would like to set the stop condition so when the internal time step below 1e-9, it will stop.
I set it as timestep-1e-8, so, so when it is below 1e-8, i will expect it to stop.

However, instead of pop up stop condition fulfilled, it give me an error.

May i know what's wrong with my stop condition expression?
I attached the file for your information.




Thank you

p/s: referring to comsol documentation
"Another example is timestep-0.04, which makes the solver stop when the internal time step drops below 0.04 s (when the time-dependent solver hits a sharp transient, for example)."
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Integration with upper bound depending on x

August 15, 2012, 7:34 pm
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Dear All,
I am trying to integrate a function of x in 1D between 0 and x for all x and create a new continuous function, varying with x. It seems the integration coupling (intop) does not have the option to specify this variable upper limit and only works over the whole domain, giving a constant value for the integration at every point? Is there a different way that I could try to do this in COMSOL?

Any help much appreciated,
Thanks.
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Complex eigen frequencies

August 15, 2012, 8:39 pm
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Hi there
I used to solve eigen frequency of a piezoelectric diaphragm with Comsol 3.5 and obtain real values for the eigen frequencies as real numbers. But now that I use version 4.3 for the same problem Comsol gives complex values for eigen frequencies. I am sure that I did not include any damping in the system. I can imagine the complex part of the eigen frequency is related to phase difference for higher modes when different locations on the diaphragms may have out of phase vibrations with respect to each other and that is why only for the first mode the phase is zero and the eigen frequency is real. But I am now only concern about the eigen frequency. How can I change the setting so that I see the real part of eigen frequency only?
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LiveLink matlab, how to plot field distributions?

August 15, 2012, 11:51 pm
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Hi,

I am considering an optical fiber in 2D, where I calculate eigenmodes using the RF module and do a sweep of the wavelength in Matlab using a for loop. Inside the for loop I only have the parameters that changes with wavelength. The geometry does not change, and therefore I put it outside the for loop. I have the impression from the user guide that this is the way to do it. Do you agree or are there some problems related to this? It seems to find the right solution.

My problem is that I would like to plot the field distribution for every wavelength. But I cannot seem to figure it out. I have tried to create a plot group, but I get an error when setting the solnum regardless of this is done inside or outside the for loop. I have also tried with mpheval where I then have the field distrbution as a data set, and plot it with mphplot. Also this is not optimal, since I cannot see the geometry structure, put on a colorbar or remove the mesh from the plot.

Does anyone know how to do a plot when having a for loop in matlab?

Thank you in advance.

Best regards
Mette
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Bending stresses due to vibration

August 16, 2012, 1:01 am
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hi, this is rotor dynamics. A shaft supported by two ball bearings is under vibration due to misalignment of the shaft with the coupling. now i need to calculate the bending stresses in the shaft due to vibration. I was provided with the bending vibration data of the rotor system. [Vibration is being measured at the bearings, rpm of shaft and all dimension is known] . So, how to find the stresses of the shaft.?
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Setting a Constant Temperature on Edge of Cylinder

August 16, 2012, 5:27 am
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Hello,

I am modeling the flow of a liquid in a pipe, with another liquid being injected and mixing, they are at different temperatures.

I imported a Rod I drew in Solid works, I'm injecting a fluid into the rod. The rod is a fluid but my understanding was I need to model it as a whole solid domain (Though this seems dubious to me).

When selecting the boundary conditions I would like the outer edge of the rod to be a constant temperature, but it appears it makes the entire rod always that temperature not just the outside. Do I need to create a very thin layer in my model on the outside of the rod? Or am I fundamentally doing this incorrectly?

Thanks.
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Elasticity Matrix

August 16, 2012, 6:22 am
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Hi

I have a question regarding material parameter of Si3N4.
In the library it is mentioned that "Elasticity Matrix" of Si3N4 is 100 Pa.
I just wondered How they calculate this value. Because I need to fill this for other material (Poly-Si) But I have no Idea where I can find It.

Regards
Bahareh
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Works on one computer, Damaged or Invalid on another?

January 24, 2012, 9:06 pm
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I have a 13.8MB ComSol MPH file that opens just fine on my ComSol 4.2a (4.2.1.110) Trial software.

However, when I upload it to the internet and download it into our university's Remote Desktop server to use our school's version of ComSol 4.2a (4.2.1.110), I get an error "COMSOL Multiphysics model file is damaged or not valid." It opened another model file just fine.

I need to use the Remote Desktop computer because it has more RAM!

What's going on?

Thank you in advance,
Arjun.
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Interpolation and parametric sweep

June 18, 2012, 5:24 am
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Hi,

i have a problem in my project. I use v.4.2 and AC/DC module in frequency domain.

I have a model built by some materials, that they depend of the frequency (electrical conductivity, relative permittivity) .
I would like to built a function to inteporlate the values in frequency and then to make a parametric sweep on frequency values.

For example:
In material --> new material --> functions --> interpolation
Function name: a
t f(t)
1 10
2 20
3 30
....

In study --> Parametric sweep: f= range (10,10,100)

It is corrects?
I have an error: 'Failed to evaluate variable' .
How can I define the function?

Thank you so much for your help!
Chiara

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