TIME to FREQUENCY FFT

March 20, 2016, 9:36 pm

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Dear all,

I am simulating pulse echo technique using ultrasound. I have my results in time domain. Now I need frequency spectrum of my time dependent study result. In study 1 "time dependent " time range is = range( 0, 5.0e-8, 9.0e-5 )s and generated ultrasound frequency is 1 MHz.

So I add study 2 "Time to frequency FFT" in my COMSOL model. Now I dont know what to do. I tried once and get 0 Hz frequency result.

Thank you

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Integrate an Isosurface

March 23, 2016, 1:54 pm

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Hello,

I am wondering if there is any way to find the area of an isosurface, or integrate a function over such a surface.

For example, if I am working with electrostatics, is there a way to check the area of the surface where es.V is equal to 100 V?

One technique I can think of would be to actually perform a volume integral of the divergence of a vector field over the volume enclosed by the surface. I can integrate over the right domain using boolean functions, and if the vector field were chosen to be orthogonal to the surface of interest, the desired area would follow by Stoke's Theorem. However, I suspect the divergence calculation would be very noisy and not yield a nice result.

I've done similar things in MATLAB, and there I found that a much more accurate technique was to export the graphics data from a plotted isosurface and sum up the areas of the tiny triangles composing the surface individually. I know I can plot and color isosurfaces in COMSOL, so could I get the area this way somehow?

Importing data in 3 dimensions to COMSOL

March 25, 2016, 3:31 pm

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Hello everyone, I have a question about importing data into COMSOL which I would then like to impose on my model. I have an excel spreadsheet that contains Width(x) and Length(y) and then x*y amount of data points for Power Density(z). I'm not quite sure how I can important this amount of data in to COMSOL. Do I need to come up with a function for the power density z and then apply this separately to my model? Any help or hints in the right direction are greatly appreciated; thanks in advance. I should include that I have been trying to use the interpolation function without much success.

Nick

Viewing post-processing expression equations

March 25, 2016, 2:42 pm

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Hello,

I am using the laminar flow physics and I was wondering how to view the equations for the post-processing expressions (specifically total stress and viscous stress). I would like to see the equation that the software is using. I have turned equation view on in my preferences, but that seems to only be helping me view the governing equations.

Any help is appreciated.

Thanks,
-Mike

How to plot time-dependent temperature profile at a point

March 27, 2016, 6:59 am

≫ Next: How do you plot error in COMSOL Multiphysics V5.2?

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Hi all,

I have 1 question over here, i have been using the SAR head tutorial (Provided by comsol tutorial) to do research for my school project and i have 1 problem over here which is, i cant seems to plot temperature VS time graph. i had searched though the internet for solution and i did follow their steps but still i cant manage to get it. whatever i try to plot, the time will varies but the temperature remains the same throughout which makes my curve looks like one horizontal straight line.

I will really appreciate if someone can take a look at the attached picture (i can't seems to upload my model file here) and suggest to me what should i do next.

Thanks,
Boon

How do you plot error in COMSOL Multiphysics V5.2?

March 27, 2016, 5:31 pm

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While doing the simulation in COMSOL Multiphysics V5.2, I found that there were errors being plotted (for my case, it was from Convergence plot 1 and 2 I think).

I would like to get this graphs for my thesis. Do you know how I can plot these errors? 

How do you plot error in COMSOL Multiphysics V5.2?

ResearchGate. Available from: www.researchgate.net/post/How_...ror_in_COMSOL_Multiphysics_V52 [accessed Mar 28, 2016].

Partition Function Derived Values

March 28, 2016, 5:42 pm

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Hi,

I am deriving some values from my model which I use for calculations in a thermodynamical ensemble. For this reason, I have one derived value, the partition function, which several other derived values need to reference for appropriate normalization.

The partition function is a triple integral over a certain volume in my case, while some of the other derived values are fluxes- double integrals over some area with a velocity integral. For this reason, I can't calculate the partition function as part of the flux calculation, as the integrals have different domains.

Is there any way that I can use the scalar result of my triple-integral partition function calculation in my other derived values?

An obvious workaround is to do the partition function integral first and put the number in by hand, but I always strive to use the cleanest technique to solve problems in order to minimize mistakes.

Thanks,
David

Superimposing results

March 29, 2016, 10:35 am

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Hi,
i just wanted to know if i could get help with comsol v5.1. Im new to this program but managed to design and compute far field characteristics of a patch antenna against my proposed antenna array design. im finding it a bit challenging to plot the 2D far field plot against the 2D far field plot of the array. be grateful for assistance.

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Way to speed up stopping of simulation? / way to keep COMSOL from disappearing after error?

July 4, 2012, 9:04 am

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Hello dear colleagues,

Currently I am working on a model for reversed combustion of biomass using an Equilibrium model based on Gibbs Free Energy for the gas reactions. I have made a connection with Matlab using livelink. Unfortunely sometimes things go wrong for unclear reasons.

When I want to stop the simulation, it often takes hours and hours while it is apparently doing nothing. Why?
How to force COMSOL to stop without losing the solution?

Also, it seems sometimes Matlab gets a vector containing only NaN's and the script I made can't return anything useful then. Instead of just stopping, COMSOL completely disappears entirely, not a trace left. WHY?
In the log it says it can't find the matlab function. This is not true, it just can't handle the crap from COMSOL.
Why is COMSOL and everything with it completely removed from memory? It there a way to prevent this?

Thanks and kind regards,

Ray

How to compute the derivative of magnetic field?

March 30, 2016, 10:07 pm

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Hello everyone.

I am learning "scattering nanosphere", a model in the library of wave optics module.
I try to acquire the derivative of magnetic field, e.g. , d(ewfd.By, z).
The result is zero everywhere. But ewfd.By is not homogenous in any direction at all.
I find that this problem happens in many 3D models, but 2D models are fine.

Please, anyone tells where I am wrong.
Thank you very much.

Accessing eigenvalue of solution in a material sweep

March 31, 2016, 8:36 am

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Hi,

I want to access the 2nd eigenfrequency in a material sweep to calculate the frequency difference between two materials. I managed to access the eigenfrequency with

withsol('sol1',freq,setind(lambda,2)).

How can I access the solution for a special material in this solution.

Kind regards,
Levin

Function

April 2, 2016, 3:02 pm

≫ Next: Optical Eigenfrequecy study leads to only one result

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How to enter summation of a series ?

I am trying to enter prony series(generalized maxwell model) in which there is summation of series for a viscoelastic model. SO if anyone can help me.

Optical Eigenfrequecy study leads to only one result

April 4, 2016, 5:39 am

≫ Next: How to set up a temperature profile?

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Dear all,

I've set up a first rough approach for calculating the optical eigenmodes of a dielectric disc on a substrate. The problem is, that even though I ask comsol to look for 20 eigenfrequencies around a certain frequency, it will always only calculate one. The same happens if I account for symmetry and cut my design in quarters together with adding perfectly magnetic conductors. I am confused since I would have expected to find many parasitic solutions which live within my substrate or pml layers but to find only one which is off from the expected values by three orders of magnitude.
Did someone maybe experience the same issue in the past or has some hints for me how I can improve the study?

Best regards,
Philip Heringlake

How to set up a temperature profile?

April 4, 2016, 8:46 am

≫ Next: Current conservation issue when integrating charge density

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Hello together,
i am an absolute beginner with comsol and hope you can help me with a small problem. I have a running 2D model with heat transfer and now want to set up a temperature profile over time relating to a certain point.
So as a x-axis, i want to have the time and as a y-axis, i want to have the temperature.
I have searched in the user's guide and in the forum, but i couldn't find anything concerning this. In fact, it should be a very trivial problem.

I hope you can help me.
Thanks a lot!

With best regards;
Maximilian Maußner

Current conservation issue when integrating charge density

April 4, 2016, 2:30 pm

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I am working with an AC/DC, 3D, Stationary, Electric Currents model.

I am currently studying charge density between deep brain stimulation electrodes of different surface areas and geometries. To represent an oversimplified electrode, we created a cylinder with high conductivity (1e10 S/m) and a point current source at it's center. The cylinder was in a conductive medium similar to the brain, 0.2 S/m.

When evaluating this simplified problem, I encountered some error that seems violate current conservation. Specifically, I am finding 47% error between the point current source (0.001 A) and the integral of current density (ec.normJ) over the surface area of the cylinder.

Note: I did conduct a mesh convergence study - below the current mesh size (0.00002), the error does not notably improve.

I then explored this issue with different geometries (spheres, ellipsoids, cubes, rectangular boxes, etc). See page 1 of the attached TroubleshootingComsol.pdf for the results. To quickly summarize, symmetric geometries - spheres and cubes - integrate to a current within 2% error. However, non symmetric geometries, ellipsoids, cylinders, rectangular boxes - have unacceptable integration error (11-63%).

*****Question: Any ideas of the possible sources of error in the simple problem? Any suggestions to further troubleshoot?

An an alternative method, I also explored representing the electrode as a cylindrical shell with floating potential of 0.001 A. In this case, the integral of normJ is 0.0004- less than half than expected. The integral of nJ was closer - 0.0009. To troubleshoot this, I went back to a sphere, with similar results - integral of normJ was 0.00048 and integral of nJ was 0.00097. See page 2 of the TroubleshootingComsol.pdf for more details.

*****Question: I originally believed that normJ, the total current density norm, was the output of interest when studying charge density on a surface, but I am confused by the fact that it is substantially lower than nJ in the floating potential situation.

I greatly appreciate in advance any tips, explanations or corrections.
Sincerely,

Ashley

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How to reduce the model file size

October 26, 2012, 5:27 pm

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Hi,

I built a 3D heat transfer, time dependent model. After simulation, the model file goes beyond 5G. Is there anyway I can reduce the file size? I only care how the maximum temperature changes with time. The detailed temperature information in the 3D structure is useless to me.

Thank you very much.

Derived Value Slower, Less Accurate than Plot

April 5, 2016, 11:56 am

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Hello,

I am trying to find the minimum value of a function evaluated only on a subset of a given surface, where that subset is specified by a constraint.

I've defined a minimum derived value, and given an expression of the form:
myfunction(params) + (mycondition==0)*largenumber

This way, unless mycondition is met, the expression won't come close to the minimum value, so I restrict attention to the aforementioned subset of interest.

Now, here's the rub- when I calculate the derived value, it takes a full minute, then returns a minimum value that is greater than largenumber. BUT, when I do an isosurface plot of my data set, colored with the expression above, it evaluates in 10 seconds and I can clearly see with my eye that the minimum is much less than largenumber (because there ARE regions where my condition is met) and what the derived minimum value was returning cannot be correct.

Why would plotting my expression on the surface of interest be so much faster than finding the minimum value on that surface? Why is the minimum value calculation way off?

Should I just make plots for all of my parameters of interest and use the minimum value on the color bar to get what I want? Seems like a silly way that shouldn't be more efficient and accurate than the derived value approach, but here I am.

Acoustic post processing.

April 5, 2016, 4:52 pm

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Hello, This is Venkat Ganduri, Clemson graduate student. I am presently working on Comsol Acoustic module as part of my research. After I get the pressure plot I have given a condition in which the resulting plot gives me only the values higher than the specified value. Resultant graph after the condition, is displaying the scale which is from 0 to 1. Could you please give me information of what the scale is about and what it actually implies. I have attached a snapshot of the result.



Thank you,
Venkat Ganduri

Table surface for Optimization study (?)

April 7, 2016, 12:36 pm

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Hi,

Is it possible to create a Table Surface plot to visualize the results of an optimization study, or are Table Surface plots specific to Parametric studies ?

I'm running an optimization study depending on 2 parameters and created a 3 columns table of [ Param1 | Param2 | Obj value ( Param1, Param2) ] but find it impossible to plot a Table Surface using this table, instead getting the error "Table does not contain filled data".

If it's not possible, would anybody know an alternative plot where I could visualize the objective value depending on the parameters values ?

Thanks a lot !

K'

design a nano structure solar cell in comsol

April 10, 2016, 8:08 am

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Hi,

I need some help to design a solar cell structure in comsol because my aim is to model a highly efficient nano structure of solar cell and this software is just a new product for me. I am not very well familiar that how to simulate and design solar structure and to measure its dimensions, efficiency and performance etc. Can anyone help me how to take a start and how to model. The comsol tutorials for solar cell design could be useful for me. Please help me in this regard.
Thank you.