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Fluid structure interaction and damping ratio

May 2, 2013, 12:10 pm
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Hello,

My model is a vascular cantilever beam (the first mode is pictured below). I work simultaneously on COMSOL and on my experiments. The channel is to be filled with a silicone fluid which will increase the damping of the structure (confirmed by experiments).

Note that I can perfectly model the beam with an empty channel on COMSOL, the results for damping and natural frequencies are identical to the experiments (I use the frequency domain analysis while applying a force at the tip of the beam). However, I still try to make the fluid structure interaction working (ie: adding the silicone fluid in the channel):

- I tried different approaches to model the silicone in the Solid Mechanics module: bulk + shear modulus, added mass + shear force, Young's Modulus + Poisson's ratio... Without good results.

- The "Fluid structure interaction" module in COMSOL looks perfect for this model, the inputs are the viscosity and the density of the fluid, no need to estimate bulk, shear modulus, etc... But I cannot make the simulation running since 1) there is no obstacle in the fluid flow -> error message and 2) frequency study is not available in FSI.

Thanks,
Matthias
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Mathematical particle tracing: Diffuse scattering wall condition

May 2, 2013, 4:37 pm
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In Comsol 4.3a the diffuse scattering boundary condition is incorrect. it generates an angular distribution with respect to the surface normal of cos(theta), while the correct distribution is cos(theta)*sin(theta). In equation view you see that theta is is chosen by
theta = acos(0.5+random(xxx))
the correct expression is
theta=acos(sqrt(0.5+random(xxx)))
see J. Greenwood, Vacuum 67 (2002) 217–222
Comsol support showed me how to fix this in equation view. The original expression is incompatible with detailed balance
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Cylinder section modelling

May 2, 2013, 5:02 pm
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Hello all

I wish to model structural mechanics in a section of a cylinder-like geometry consisting of longitudinal bars and radial rigid hoops supporting the bars. The full cylinder is too memory intensive to model an so I have drawn a section of the cylinder only. My question is: what contraints should be used on the end of the hoop sections to simulate the fact that they form a closed loop. I have tried a number of fixed constriants and prescribed displacement conditions but they do not result in reasonable solutions.

Any ideas?

Thanks a lot

Dan
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generating the graph

May 2, 2013, 8:34 pm
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I am designing a model of saw resonator and i want to see the admittance vs frequency graph but i dont know how to generate this graph , i am new to comsol . please help.
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managing plots

May 2, 2013, 11:32 pm
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Is there a way to stop refreshing a plot? I mean, if you realize that you're plotting something wrong, can you give some command to stop the plot immediately?

--
Mr. Ezio Cosatto
Ansaldo Energia S.p.A.
PDE/ISV/TMC/TCD
Building 34, 2nd floor, room 22
via N. Lorenzi 8
16152 Genova (ITALY)
Tel.: +39 010 655 3930
email: ezio.cosatto@aen.ansaldo.it ezio.cosatto@physics.org
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Mesh Swept Problem

May 3, 2013, 3:25 am
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Dear All,

I need to reduce the elements of the mesh in the fluid domain.
At the beginning I used a tetrahedral mesh but it is too dense in the axial direction, while I need it mainly in radial direction.

For this reason I'm trying to create a triangular mesh on the boundary of the domain 2 and sweep it in the axial direction. (Attached 1.png)
The source and the destination faces are not the same, how can I build a lighter mesh in this domain (2.png) ?

Thank you in advance for your support!
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silencer duct

May 3, 2013, 4:31 am
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Hi

I want to simulate insertion loss for ventilation ducts. I need to define inlet and outlet pressure so i can calculate insertion loss by its difference.
There is a similar sample "perforated muffler" for this situation.
there is a definition for acoustic power
P_in = intop_in(2*p0^2/(2*acpr.rho*acpr.c)) Total acoustic power at the inlet

P_out = intop_out(2*p*conj(p)/(2*acpr.rho*acpr.c)) Total acoustic power, outlet

TL 10*log10(P_in/P_out) Transmission loss

Are these formulas suitable for calculating sound pressure or i need to use anything else?

Regards
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Uniform Magnetic field?

October 16, 2009, 1:12 pm
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Hi everyone,
I am wondering how I can generate a uniform magnetic field in Comsol. Basically, I want to simulate a metal particle moving in this uniform field. Now I am thinking put all the particles inside a very big magnet. The field is almost uniform, but I don't think it is the best way!

Anyone any ideas?

Thanks for helping!
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Total magnetic Energy

April 10, 2011, 11:40 pm
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Hello,

im Using COMSOL 4.1 and I have got heavy problems simulating the cogging of a synchronous machine. The calculated Torque using the stress tensor is correct. But if I use the method of virtual work something ist going terribly wrong . If I use a stationary solver and add "Parametric" to the solver the magnetic energy has the wrong periodic time and is not sinusoidal (the cogging torque is more or less sinusoidal). Using the timedependant simulation tends to result in a sinusoidal form after a ramp.

What can I do to improve the calculation of the total magnetic Energy? And why are the results so different?
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Error: Internal error: cannot convert ElemStruct to int.

October 19, 2011, 6:32 am
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Hello,
last week I got a new computer (Windows 7 system) and just tried to run my Comsol models and an error occurs.
Whenever I try to run the study I get the following error:

Error in sweep: The following feature has encoutered a problem

- Fearture: Compile Equations: Frequency Domain (sol1/st1)
-Error: Internal error: cannot convert ElemStruct to int.
-Value: f6
Error in multiphysics compilation.

This happens with all my models (all ran well on the old computer which was also Windows 7 and also Comsol 4.2).

But when I start a the same models from a USB device no error occurs.
I attached a sceenshot of the error message if someone is interested.

Could anybody help me with this problem?

Thanks!
Annika
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AC field Simulation

October 28, 2011, 10:42 am
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Hi,

I have to do a simple Simulation in comsol multiphysics 4.1 and i am just a beginner in this work,i have to build an electrode geometry and apply various frequency and voltage ac signals and see the resulting electric field profile
which ac model shalll i add (electric currents(ec)?) and to apply potential i shall assign ground and potential values?
While frequency domain is selected from study1? if yes then what is the purpose of global definition and various functions placed there?

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Voltage Induced in a Coil by Moving Magnet

April 18, 2013, 6:41 pm
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Hi,
I am trying to learn how to use moving mesh. And I created a project same as "Voltage Induced in a Coil by Moving Magnet", which can be found in Comsol website. But the problem is: when I run it, an error always occurs like: Nonlinear solver didn't converge...

I am very sure I follow every instruction from the Documentation. So, does anyone get this model run?
And I have a question about the "study settings" described in the documentation, in which it is stated that: Step 1: Stationary; Step 2: Time dependent...' Could anybody show me more detail about this part? Thank you!

My comsol version: 4.3
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How to define variable regard to time dependent steps?

April 26, 2013, 10:26 pm
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Hello All
I'm running a time dependent AC/DC model,
The conductivity of material must be changed regards to voltage differences on two coordinate in each step .
I wonder How can I define a variable or parameter which can access the voltage of these two coordinate in each step and change the conductivity of material for next other step?
thanks in advance
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How to set boundaries as continuity ?

April 30, 2013, 10:10 pm
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I want set the boundary as a continuity but I don't know how?
I know that in Comsol by default the boundaries are define as continuity and it follow this equation: n.j=0
but I want change it to n(j1- j2)=0 (i want the boundary between two domain will be as continuity like n(j1-j2)=0)
how I can do this?

Any help would be appreciated.
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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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Point Evaluation using Biot-Savart Law.

May 3, 2013, 11:26 am
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Hello everyone,
I am currently attempting to do a point evaluation with the expression for Biot-Savart Law.

The set up is like this:
I have an array of points that I would like to get the value at using this expression:
1/(4*pi)*intop1((ec.Jy*(z_P-z)-ec.Jz*(y_P-y))/r^3)

where:
intop1 is the integration operator,
z_P, y_P, x_P are the field points, and x.y.z are points within the domain
r = ((x_P-x)^2+(y_P-y)^2+(z_P-z)^2)^0.5

I can get the values with parameter sweeps, but this involves solving the model over and over again for each point I use. (No point in solving for the current density N times, when it only needs to be solved once.)

Ideally I would like the expression to be something like:
1/(4*pi)*intop1((ec.Jy*(z_coord_at_selected_point-z)-ec.Jz*(y_coord_at_selected_point-y))/r^3)
r = ((x_coord_at_selected_point-x)^2+(y_coord_at_selected_point-y)^2+(z_coord_at_selected_point-z)^2)^0.5

I have looked pretty extensively on the internet, and in manuals, but cannot seem to find a command for such a general function. I have seen dom==particular point, ie 7, but I need it to take a general selected point, as well as only taking the x, y, or z coordinate of the point.

Does anyone know of a command I may have missed? Or offer an alternative method to getting this information without having to input an insane amount of parameters into comsol?

I realize I can use the magnetic package to get the field from the current density, but I am needing to do it this way for now, so the magnetics package isn't an option for me.

Any and all help is much apprecated, thanks!
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Induction heating

May 4, 2013, 1:11 am
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Hi to all!
I'm getting some trouble about induction heating using frequency-transient solver.
In the 2D-axisymmetric model, I have an aluminium workpiece surrounded by 12 turn of coil (each one setted in "single turn coil domain 1").
In the "single turn coil domain 1" I set the power of the coil, and here is the problem! If i set very low power (10-60 W) the simulation works so good, but if I set the power I need (at least 6000 W) I get the error
"Time-Dependent Solver 1 (sol1/t1)
Failed to find consistent initial values"
I think I have to change some BC, but i don't know which one!

Anyone knows?

Thank you!
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what is "circular dependency " error?

May 4, 2013, 3:35 am
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Dear All
I,m doing simulation partial discharge by Comsol and Matlab but during simulation I faced this problem, I would be thankful if any one help me for solve this problem.
I set two probs under model , definition on two boundary of my geometry and the average of the voltages is measured by them.then the measured data is transferring to a function of Matlab for calculating conductivity , ( there is no error untill this part) after that I define another prob on DOMAIN for measure integration of current density norm .this is new variable that I want transfer to function of Matlab , but the problem is when I define this new prob I get the error that say: circular dependency (Circular variable dependency detected.
- Variable: mod1.ec.Jir
- Geometry: 1
- Domain: 3)
I want know how can i solve this error? and what is the meaning of this error?
thanks in advance
Hadi
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Electrostatics: relative permittivity

May 4, 2013, 5:00 am
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Hi,

I am solving an electrostatics model which includes conducting and insulating material. What is the relative permittivity value of conducting material used? All the in built materials use the value as '1' for epsilon; where it should be infinity in theory. Also, I am getting the correct answer using it as '1', but that would be incorrect as I am ignoring the shielding in conductors in this case.

Do the relative permittivity have its usual meaning in COMSOL?

Thanks,
K
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Extract data

May 4, 2013, 5:24 am
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Hello,

I ran a Comsol program with parametric sweep. I want to extract data using MATLAB. How can I extract special data for all values of swept parameters. I can extract data for the first value of swept parameters but for others I don't know how to do.

thanks in advance for your help.
Saeed
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