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Calculating Rayleigh Damping Parameters

November 4, 2010, 9:41 am
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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


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tutorial model - step index fibre

January 26, 2011, 4:49 am
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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..?
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Acoustics - Large models convergence

August 26, 2015, 10:32 am
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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.
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Importing ASCII Geometry

March 16, 2017, 4:34 am
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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,

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Sheet metal bending - spring back effect

March 17, 2017, 9:26 am
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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)
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charge particle tracking assigning experimental magnetic field values to a magnet model

March 22, 2017, 11:38 pm
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Dear All,

I have model of my magnet. I want to give it my own measured magnetic field and want Comsol to interpolate all missing values if it needs for the particle tracking. Can somebody tell if this is possible?

Regards,

Noaman
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Export comsol file( mph) to GDS FILE

March 23, 2017, 2:00 am
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Hi
Does anyone have idea on how to export (geometry) mph file (Comsol) to GDS file.

Need your help please.
thanks
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Longer simulation study

March 23, 2017, 4:24 am
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Hello everyone,

I'm facing an usual problem with a comsol time dependent simulation study. In the study settings, upon changing the upper limit of the time range by a decade, the simulation doesn't show any physical response. While it was completely fine when the study setting was up to a certain limit in range, considering I didn't make any further changes in resolution or meshing in my physical model, going to one more decade level in the range doesn't produce any results. Could it be related to any other setting I might be unaware of?

To give you context I'm running a time dependent study of 0-10ms with a resolution of 1e-7 and relative tolerance of 1e-6. And I'm being able to run it when this range is 1ms but after that it just doesn't produce any result.
Any help would be appreciated.
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Propagation constant using port excitation

March 23, 2017, 5:56 am
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Dear All,

I have a question regarding the usage of propagation constant parameter for port excitation in the EM frequency domain simulation.

I tried to use the port functionality to excite an EM wave in a 2d geometry. I used a very simple rectangular geometry with PEC boundary conditions on the sides parallel to the x axis (length). The input excitation port was used on one of the side parallel to the y axis, with the user defined electric field as:
Ex=0, Ey=0, Ez=1.

1) Is the propagation constant defined always in the x direction (normal to excitation side).
2) If I assume that the propagation constant is in the x direction, I expect a phase shift in the Electric field vector for varying value (real) of propagation constant at any point in the x axis. However, I observed a change in magnitude (for emw.normE field) without any expected phase shift. Could you please help me to understand this?

Thanks,
Rahul
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selective laser melting_interface heat transfer

March 23, 2017, 7:42 am
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Dear all,

I am working on the simulation of SLM. I have a question, firstly laser irradiates on the surface of the metal, and I want to use the average temperature of the surface from coupling as the initial value of the surface temperature, in order to calculate the temperature driven flow effect. So actually how to realize this? Thanks in advance.

Greetings
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Two phase Liquid Liquid extraction

March 23, 2017, 10:59 am
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Hello,
I am trying to model mass transfer through interface of two liquid in a two phase model. I believe comsol by default applies flux continuity at the interface. However I also want to an equilibrium boundary condition (governed by partition coefficient) at the interface. I used pointwise constraint to tie concentration c2-K*c1, where c1 and c2 are variables. As a results interface is not tracked properly and I get anomalous results for examples the phase fragments and no concentration value.

Can you please help me with this?
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Simple Easy Resistance

March 23, 2017, 12:10 pm
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Hi,

I am looking to find the resistance of two cylinders joined end to end.
Each cylinder is a different material.
I vary just the conductivity of one the cylinders and do a parametric sweep.
1 V is applied to the top, and GND is applied at the bottom.

The results I am getting do not make any sense at very low conductivities. My simulation severely underestimates the resistance.

Any ideas? It should just be the bulk resistance of the two material
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MicroFluidic channel with Shape Optimization

March 23, 2017, 12:23 pm
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Hi all,

I was having some problems getting my shape optimization for a microfluidic channel
working. The idea is that the channel should have constant strain rate through its center.
The strain rate equations and constraint is set up correctly, however the shape
optimization on its top and bottom walls is not working. I am trying to set it up similar
to the beam optimization problem here:

www.comsol.com/blogs/designing...tures-with-shape-optimization/

The entrance height is Wu and the exit height is Wc, I am trying to use a berstain polynomial
to model the top and bottom walls. My current iteration is really close, however the right side
sags a little for some reason. the channel itself is centered on 0 and shifted in the x so that it begins at x=0. I create a prescribed deformation and outline the rectangular channel. The equation I enter for the displacement in the y is "(Yg/(Wu)-1)*((Wu)*((A2)*Xp^1*Xn^2+(A3)*Xp^2*Xn^1))+(Yg/(Wc)-1)*(Wc*Xp^3)" with Xp=Xg/Lc and Xn=1-Xp. This is the berstain polynomial. Under optimization, I control the length, a2, a3, and the entrance height (the entrance height is coupled to the exit height through and equation) while constraining the constant strain rate through the center. Attached is a picture of the output, for some reason it sags on the right side, as mentioned previously.
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comparison between two different fields

March 23, 2017, 2:08 pm
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Dear all,

suppose you have two different fields and you want to compare them in order to obtain a numerical value that reflects their similarity. Do you know a way to do this?

My first approach was to export the two fields (A and B) as matrices in Matlab. Then use a function as xcorr2(A,B) to compare them. The problem with this approach is that I cant get to export A and B.

Other way could be to use a 'Join' dataset and then try to implement the cross-correlation function inside the comsol interface. The problem with this approach is that I cant find a way to sum over all the domain.

Any help is greatly appreciated.

Best,
Felipe Beltran-Mejia
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How to resolve warnings about relative error in comsol?

March 23, 2017, 3:09 pm
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I have a model that consists of two squares that are one on the other separated by 10 mm. The bottom square measures 60 mm, is pine wood with tangential behavior (with a poisson coefficient of 0.3) and has a fixed constraint on the lower edge. The above square is 40 mm, it is made of structural steel (material pre-configured in Comsol) and is centered on the lower square, to which a prescribed displacement is applied with the value of "-f * max" where the value of Max is 10 mm and the value of f varies between 0 and 1.5.
A contact pair is defined between the upper edge of the steel square (source) and the upper edge of the wooden square (destination).
The mesh consists of free triangles with a size defined as "finer" and it is calibrated for general physics, and the edge defined as "target" is "extra fine" in size.
In the study, an auxiliary sweep is configured with parameter f and the maximum number of iterations per segregated group is 60.
When giving the model to execute, it gives me a series of warnings which say: In the segregated group 1: There was an error message of the linear resolver. The relative error (8.9) is greater than the relative tolerance.
I do not know how to resolve these warnings would appreciate any help
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Band diagram of 3D periodic photonic crystals

February 2, 2014, 8:53 am
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Hi,

I try to calculate band diagram for 3D photonic crystal using RF module (version 4.4). At first, for the sake of simplicity, I plot a cubic unit cell and set Floquet boundary conditions (3 pairs). Then I solve eigenvalue problem using Bloch vector components as parameters.

When I start the calculation Comsol gives me an ERORR "Inconsistent uni-directional constraint".

In 2D case (square unit cell) all works. What am I doing wrong?

Many thanks in advance!

Andrey

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Liquid-Vapor interface drops very fast

March 18, 2016, 11:01 am
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Hi

I have encountered a problem in COMSOL when using the “laminar two-phase flow, moving mesh” physic.
A simple model indicating what I am trying to solve is provided in the attachment.

As you can see in the attached file, the liquid evaporates at a very low pressure (300 Pa) with a uniform mass flux of 1e-3 (kg/m2s). When the contact angle is 90, everything goes well. However, as I change the contact angle even a little bit from 90 (i.e. pi/2.1), the interface drops very fast, which is physically meaningless. I have had this problem with both COMSOL 4.3b and 5.1.

I could have been able to improve it by making the contact point meshes extremely fine. However, this gives me weird values of pressure (absolute negative pressures) in these regions.

I appreciate if you could help me sort out this issue.

Amin
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The relation between mesh size and time step in time-dependent study on heat transfer problem

May 13, 2016, 8:03 am
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Dear all,

I am simulating heat transfer problem and when it comes to set up the mesh, is there a limit on the max mesh size according to the time step I set up? Say I compute the heat transfer and want to see the temperature distribution in the material each 1ms, is there a requirement for the mesh size due to this condition (or the velocity that heat travels)?

Thanks.
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Boundary condition is a PDE, rather than a constant!

February 23, 2017, 3:15 pm
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Dear all:

In my model for fluid flow in porous media, I want to set gas pressure as boundary condition at one end of the soil specimen. In the experiment, the gas pressure is applied on the specimen by compressing a certain amount of gas, just like a syringe. At the same time the gas will flow through the specimen following Darcy's law.

According to the condition, I derived a differential equation to solve the gas pressure, but I have no ideas how to applied it on the specimen in Comsol. The equation is like: d(PV)=APv, where P is the gas pressure, V is the gas volume, A is the area of the cross-section of the specimen, v is the Darcy's velocity, which is v=k*grad(P). This seems complicated to apply it on the boundary. If you have any ideas or any experience about such a problem, could you please share them with me? I really appreciate your assistance and time.

Thank you!

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mass loading on quartz resonator

March 29, 2017, 9:54 am
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hi
i am working on the modelling of sensor for detection of fuel adulteration based on the shift in the eigen frequency for that i am trying to employ TSM based quartz resonator, the main problem i am having is that the fuel sample should be having interaction with the resonator with density of the sample acting on the resonator as mass on it. but i amnot getting how to add the mass in the model. i will be grateful if anyone tell me how to add mass on the resonator model
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