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Creating (and using) an involute coordinate system

May 1, 2013, 6:48 am
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I've defined an involute coordinate system using an interpolation function. For an unstructured grid of x and z coordinates, I have defined two new coordinates t_coord and h_coord which have vectors everywhere parallel to a circular involute and everywhere perpindicular to a circular involute, respectively.

I can't figure out to make COMSOL recognize them as a new reference frame however. For instance, in the general projection model coupling feature I would like to be able to use t_coord and h_coord as my directions of value storage and integration. Also, in output plots I would like to be able to enter them as expressions to use along my x and y plot axes.

I've tried making variables out of the interpolation functions (t_coord_var = t_coord(x,z) ...) but that still does not seem to work. I've also tried making a new coordinate system with the option under "definitions" using these new coordinates, but that doesn't seem to work either. It's also possible that one of these methods have worked but I am calling the resultant variables by the wrong thing when attempting to access them later. But, for instance, if the variable t_coord_var is valid then I would think I should be able to access it simply by typing "t_coord_var" into the coordinate expression field.

Any help is appreciated.

Thanks

Adam
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Store data at certain time intervals

May 1, 2013, 8:01 am
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How do I set up my model so the data are not stored at every time step, but at certain intervals. That way I don't end up with huge file, and my plot files for exporting are small as well.

Thanks in advance.

Kaushik
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Magnetically Sprung Electromagnetic Generator

May 1, 2013, 8:40 am
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Hi guys,

I have built a magnetically sprung electromagnetic generator for harvesting energy from machine vibration. Its basically three magnets in a tube, one fixed at either end and one floating in the middle (as all the like poles face each other). There is a copper coil around the middle of the tube. When the tube oscillates up and down the center magnet passes through the coil generating a voltage.

I'm trying to use comsol to model the movement of the floating magnet as the tube oscillates in the z direction (vertically).

I created a 2D axisymmetric model with magnetic fields and moving mesh. I used predescribed mesh displacement with a sine function to oscillate the fixed end magnets in the z direction. I used Amperes law to define the remnant flux density of the magnets. So far I've only been able to plot the change in electromagnetic force exerted on the floating magnet over time if I fix it in one place.

Is there a way to allow the floating magnet to move freely based on the force exerted on it by the other magnets, its mass and gravity? and then plot its displacement?

Any help would be greatly appreciated.

Cheers

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how can set up the inital field velocity for the fluid?

May 1, 2013, 7:08 pm
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Hello

I want to simulate the motion of a nano ball in the liquid. the liquid has a stale field velocity, angular velocity W(r) and has nothing with the angle theta due to symmetry. I would how to set up the inital field velocity. since there is only Ux and Uy. here, the U(x)~ r*w*sin(w*t) and U(y)~r*w*cos(w*t). there w is the angluar velocty and t is the time.
but t is the a variable.

I atttched the picture of this modle. The big circule is the confined space for computation.


Thanks for your time.
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How to solve a derived PDE equation with coefficient form?

May 1, 2013, 7:20 pm
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Dear all:
I had derived a PDE equation in attachment.
But the result seems to be not correct.
Please give me a help.
Thanks a lot.

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How to assign automated input values?

May 1, 2013, 8:37 pm
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Can we assign input values to faces of an object via .csv file or through any web based tools?
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Meshing a surface with an interior point

May 2, 2013, 12:14 am
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I need to apply a point force on a point inside a shell surface. For this purpose I draw a point in geometry section. However, because of that point I can't mesh the surface with free quad or mapped. It is giving the error "isolated vertices found".

In some other softwares it is possible to pick nodes from the mesh for this purpose. But I couldn't figure out how to do that in COMSOL.

I appreciate any help.

Thanks
Bugra

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Taylor Hood or Crouzeix -Raviart element

May 2, 2013, 12:25 am
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Hello all

I am using COMSOL for solving the velocity and concentration field. I am using quadratic elements. But I don't know what kind of quadratic elements COMSOL uses. Is it a Taylor Hood or Crouzeix -Raviart ? Can I choose between these two options? Is there a way to find it out?

I really appreciate your answers
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applying a periodic biphasic square waveform to a surface electrode

May 2, 2013, 12:44 am
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Dear friends,

I want to apply an AC voltage/Current to a surface electrode placed on the human skin. I need a periodic waveform with unequal on and off time. How can I apply this stimulation pulse to a surface electrode at specific frequency?(I built the desired waveform from Definition node-> Function-> Piecewise, and set the electrode voltage from Electric Potential at 5V but when I change the duration of the pulse nothing changes. Is it correct way to apply desired waveform to an electrode?)

Please help me to solve this problem.

Best regards,
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Effect of sound pressure on refractive index/density

January 21, 2013, 8:43 am
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Hello fellow Comsol users,

Most of my experience comes from structural mechanics and fracture and would like to ask a little help regarding pressure acoustics.

The presence of high and low pressure in air will change it's refractive index due to a change in the density. I would like to model this change in refractive index, but am a little lost.

Could anyone tell me how I could go about modelling/solving for a change in material property?

I am using a time dependant pressure model in air. Within the materials section I can see that the density is a function of temperature and pA [which I assume to be absolute pressure], but when I plot the change in density it says that there is no change in density even if there is a change in pressure.

I'm sorry if this question sounds a little dumb, I'm sure that I am just being stupid today.

Also, I have had a look at the forum and a little in the documentation but could not find anything to help.

Anyone have some ideas out there? I would greatly appreciate your help.

Rob
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Thermal expansion model

April 14, 2013, 5:22 pm
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I am trying to examine the stress distribution in a SiO2 film coated on a thick Si sample with a small Al bump on it, all heated to 700 K. I thought I built the model correctly, but the resulting stress distribution diagram is quite different from what I expected. Specifically, the stress gradient is not existent at the boundary of Al and other metals. Can anyone tell me what may be the problem with my model? Especially concerning the boundary conditions? Thanks.
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Simulation of Stirling Engine

April 22, 2013, 2:07 am
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Hello everybody

I'm currently trying to simulate the fluid flow in a stirling engine (beta) which I've built from the following video:
www.youtube.com/watch?v=X2qssmxA82U
You should try building it;-)

My problem is that I'm not very experienced using COMSOL (only made one model before using the CFD-module) so I'm having quite a bit of difficulty setting the model up. I'm following a CFD course (no heat transfer course though) at my university but we don't use COMSOL but MATLAB. I wanna start off slow with the COMSOL model by trying to just model the free convective flow which occurs when heat is added to the bottom of the "can" and water cooling to the sides at the top.

My questions are as follows:

I) Which physics should be used? Conjugate heat transfer or non-isothermal flow or some other?
II) In the model I've tried using the Convective Cooling but should I just use a negative heat flux instead?
III) Why is the wall condition (no slip) not satisfied?
IV) Do the initial values act as a boundary condition or are they just a guess for the solver to initiate the calculations?

I've added a model which shows the geometry and what I've tried to add to the model but pay no heap to the number/parameters as I've just been playing around a bit.

Thanks

Mikael

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Moving mesh 2D

April 23, 2013, 10:00 pm
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Hello,
I am trying to simulate the moving boundary for a 2D geometry.

I started with a simple 2D geometry where 1 boundary is moving in the x-direction and 1 boundary in y-direction, but because the boundaries are connected in one point, that point cannot move in x and y in the same time. For this reason i did a filet and the added boundary is moving in x and y.

My problem is that after specifying the velocities for which the boundaries are moving and hit compute, inverted mesh elements appears after 11 seconds. Even if I change the mesh quality the problem is not solved.

Mai I know why is behaving like this and what should I do in order to move the boundary further?

I attached the model for a better view.


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Using a Gaussian Pulse

April 25, 2013, 11:47 pm
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Hi everyone,

I'm actuallay working on a modeling of a laser pulse which is heating a front of a material in order to determinate the diffusivity, my current issue is that i would like to model my heat source by a pulsian gauss but i don't really know how to integrate the fact that the heat source is a gaussian pulse type.
Thank you for reading this topic and i hope some of you could answer to my question.
Have a nice day !

Cordially,
Samy
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time interval for automatic remeshing

April 30, 2013, 4:55 am
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Hi,

I would like to know the COMSOL keyword for the time interval when using the automatic remeshing module. I want to define a criterion based on the time that's why I need to know the actual number of the mesh (which correspond to the number of the time interval)

Thank's.

Julien

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Why is the gradient of the voltage potential field not equal to the electric field?

May 2, 2013, 3:55 am
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Hi,

I have been working on a 3D model of a resistive current sensor, and I am trying to verify the high frequency (50-500Mhz) response. Essentially, i want to make sure that at a fixed current excitation of 1A, I measure as close as possible to 0.1V, corresponding to a resistance of 0.1Ohm. The challenge is to avoid coupling the measurement loop to the rapidly changing magnetic flux.

I initially used boundary probes to measure the average voltage potentials at the two boundaries corresponding to the sensor output, but this gave erroneous result. I then tried to do a line integral of the electric field between the two sensor outputs, and I was surprised to find the expected voltage of 0.1V. After further investigation, I have discovered that the electric field in general corresponds well with my expectations, in that the electric field is strong across the resistive elements, and weak at the copper. The voltage potential field does not make much sense in this regard - the total voltage between the terminal and ground is close to the expected 0.1V, but by plotting the 3D voltage potential field over the model, it seems that the entire voltage drop appears in an area very near the terminal, with apparently nearly zero voltage drop over the resistive elements.

I have attached screenshots of d(V,y), and Y component of E field, respectively, plotted in the same plane. From my understanding, these should be equal, which is clearly not the case in my simulation.

Is the V field somehow calculated based on the bulk impedance seen at the terminals, ignoring the geometry? This is the only explanation I can come up with...

I would be grateful if you would help me solving this discrepancy. I have attached the model file with all cleared as the uncleared file is too big, but I would not expect anyone to run the simulation themselves as it took about an hour to run for me with a modern workstation. Let me know if you want me to upload a screenshot of a plot of anything.

Best regards,
Kristian
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Moving Mesh - Thermal stress coupling

May 2, 2013, 4:44 am
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Hello to all,
I need to perform an RF frequency domain analysis on a waveguide which has been previously deformed by a Thermal Stress stationary analysis.
In order to do this, I need to calculate the new coordinate x,y,z (after the thermal stress induced displacement) using a moving mesh module.

I configured a Thermal stress module with all the needed conditions, to compute temperature T and deformation u,v,w.
In order to verify the used settings, I computed this step and the the deformation has been well calculated so that the waveguide has been deformed by the heating (excluding the fixed constraints boundaries) showing deformation and, using a boundary probe, I verify the computation of u, v, and w.

Then I inserted a Moving Mesh module, in which I set "Prescribed mesh displacement" on all boundaries, with dx=u, dy=v and dz=w (in order to use the previous step results). I insert a "free deformation" block in which I set to zero the initial values: dx0=0 dy0=0 and dz0=0 for all domains. In the default "fixed Mesh" block, all domains have been overridden.
This module has been assigned to a Stationary step in the same study.

After few seconds, it generates the following error:
Failed to evaluate expression.
- Expression: X+mod1.u-x
Failed to evaluate variable.
- Variable: mod1.u
- Geometry: 1
- Boundary: 3 10

Have you any suggestions to help me configuring the right coupling between these modules?
Thanks in advance.

Alberto
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Ionization of air.

May 2, 2013, 5:47 am
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Hello.

I have a model that comprises a container filled with air.
From the outside there are two coils
number of turns 1e6
supply current is 1e3 A
in the container are two electrodes voltage + - + e7 V
the pressure in the container was 3000 per annum and the temperature 1E7 K.

The device should break down air to the electrons and ions.
But nothing is doing.

Does anyone advise me why?

Thank you.
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Base vector coordinate system

May 2, 2013, 10:05 am
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I am modeling the anisotropic behavior of a plate that have fibers oriented in different directions. For this purpose, I am using the structural mechanics feature. I am orienting my fibers using a based vector system. Does this system work for angles other than 0,-45,45 and 90? I have not been able to rotate my coordinate system at other angles ( I would like to try 10,15, 25 and so on)

Any advise
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parameter sweep error when loading a geometry

May 2, 2013, 10:36 am
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I am applying a load to the top of the geometry shown while the bottom is fixed but i keep getting an empty parameter sweep error can u help me??
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