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COMSOL Error for Initial Values(RF Module)

June 16, 2011, 12:23 am
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I am defining a model with different concentrations of oil and water and trying to measure the heat distribution by using the microwaves. But I obtained the error mentioning Initial values:

THE FOLLOWING FEATURE HAS ENCOUNTERED A PROBLEM
-FEATURE:TIME DEPENDENT SOLVER
-ERROR:FAILED TO FIND CONSISTENT INITIAL VALUES

Failed to find consistent initial values.
Undefined value found.
NaN or Inf found when solving linear system using SOR.

Although the initial values defined are by default and I did not change any thing.Please see the attached file.

Can anybody suggest me some solution of these problems?

Thanks in advance.
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PDE solving or defining force in terms of displacement

February 18, 2013, 7:56 am
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Hi,
I hope you are doing well,
I am a new user of Comsol.
At this moment, I am trying to model a cantilever beam with an applied force on its edge as a function of displacement.
So, what I need is either:
define the force on the boundary as a function of displacement. I am trying to do this in equation view but it seems that it does not take expression and it asks me just values in x and y directions.
or:
write a PDE and try to solve it. The equation is derived from the Euler-Bernoulli theory as:
EI uxxxx=F(u)
I tried to find a predefined PDE and change the coefficients but I could not to do it.
Could you please help me on this issue?
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about heater simulation in the room

April 11, 2013, 6:31 am
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Hello!

I am use comsol 4.3a , and use the non-isothermal flow physics to simulate the heater in the room.
And the room has a door , and I want to set the door is open, and it is connect to the enviroment. So the heat and air can go out through the opened door , also the air from enviroment can supply the room by the opened door.

So I select the open boundary to the door (I don not know it is correct or not correct),
1.if the heater is remove, the simulation can sucessful, but when I see the velocity field , the result the opened door have a very high velocity inlet to the room, and it is not resonable.


2.When the room has heater,and the simulation will fail , and the result is not convergence.

So anyone know how to set the open boundary for open door?



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electrical potentials

April 24, 2013, 9:42 am
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Hi,
I´m analyzing the distribution of electrical potentials through different geometries with varying conductivities. An active electrode is firing with 1V under quasi-static conditions. To analyse the behaviour I modeled a big sphere which also contains the ground electrode and encloses the 'structure of interest'.
Now I want to manually define that the electrical potential on the big sphere should be 0, or how can I define Ve in a certain distance??

Hope someone can help!

Best, Manfred
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Single turn boundary domain ACDC module

April 29, 2013, 3:24 am
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Hello
I am trying to simulate magnetic field around a inductor and I am using single turn coil domain. In the start i am trying to simulate some part of inductor not a complete circular coil. I have analysed single_coil_boundary_feed.mph but the issue i am facing is that I can't apply boundary feed and ground at the input and output of current. I see something like this when I open right click single turn to get boundary feed (in the attached image).
I also have attached my build model.
Any suggestions, i have already respected the norms such as surfaces touching the external walls of the air domain surrounding the conductor.
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Multiphase Flow LS - Air Bubble in Water Cylinder - Conservative forms fails

April 29, 2013, 4:56 am
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Hello everybody and thank you in advance for any kind of help you may give,
I am trying to simulate a simple kind of scenario, an air bubble in a water cylinder rising due to buoyancy force.
I made my model axisymmetric and used the level set method (but I want to be able to simulate the same scenario also with the phase field and moving mesh method in the end). I am using v4.2a and I use the stokes flow approximation. I initialize the pressure inside the bubble with young-laplace equation assuming outside pressure is 0.
This example is very similar to the rising bubble model of comsol with an oil droplet in water, but here the second fluid (air) is only in the bubble and not also in the top layer, which is closed by a wetted wall.
What happens is that if I use the incompressible and non conservative form I have a converging solution and the bubble rises, even though the volume fraction seems to decrease in time (mass is not conserved apparently) and if I use the conservative form it inevitably fails to converge.
If I try the compressible Navier Stokes, which is intrinsically conservative, it fails as well.
Since I am not expert with mesh refinement and solver's advanced options, could somebody point some of these things out for me? Or if you find any wrong BC (which I checked over different tutorials and should be fine), could you please highlight me where the errors are?
If I am missing something with the physics, please let me know too, I tried to make it as simple as possible.
I attach the file model if you have the opportunity to check.
Thank you again for your help

Stefano
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Problem: "Variables not coupled to any dof" in multiphysic sequence

April 29, 2013, 6:01 am
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Hello to all.
I have a problem with a multiphysic sequence on Comsol 4.0.

In order to simulate the electromagnetic behavior of a rectangular waveguide with a dielectric block inside, that can be warmed up, I built a model which contains three physical modules:
- thermal stress,
- moving mesh,
- EM waves.

The model should perform the following steps:
-simulate the displacement due to the deformation induced by the heating,
-move the mesh to a new position,
-calculate the electric field and the S-parameters using the moved mesh.

When I launch the simulation, I experience the following error:
"Variables not coupled to any dof
- Detail: mod1.Ex, mod1.Ey, mod1.Ez".
However, if I disable the moving mesh and the Thermal stress or if I disable the moving mesh and the RF module, the only Thermal stress model and the only RF model run correctly.

What problem does my model encounter ?
Probably it resides in the solver sequence.
I attach the file.
Thanks in advance.
Alberto
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Control Mesh element distribution over the surface

April 29, 2013, 6:10 am
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How is it possible to control the mesh element distribution over a surface (boundary) between two 3D domain? I know "Distribution" that is used to control the mesh element distribution over an edge in 2 and 3 D. But I want to control the distribution of mesh over the surface (boundary) of a 3D domain. [To clarify the problem take cylindrical waveguide with length 2*d and radius R and apply a circular boundary or surface (in my case vibrating plate with zero thickness, I am using Acoustic-shell interaction) at point d with radius R. Now how is it possible to apply extremely fine mesh over this surface and the remaining domains normal or coarse mesh]
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Integration boundary condition that is probably of circular dependency (dye-sensitized solar cell)

April 29, 2013, 6:19 am
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I tried to model the ionic exchange in a dye-sensitized solar cell. The concentration of cations in the electrolyte follows a Boltzmann distribution:

Nc = N0 * exp(-Q/kT). N0 is a constant.

That is, Nc is a variable dependent on N0.

To fulfill ionic conservation, one of the non-local boundary conditions is:

∫Ω Nc*dΩ = N0*Ω , where Ω is the volume of the cell.

That is, the integration of Nc over the entire domain = N0 * volume of the domain.

My question is, how can I fulfill it in Comsol? I know how to add an integration coupling variable (intop1), but what to do next?

Thanks.
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Object moving with the magnetic force

April 29, 2013, 8:26 am
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Hello

I simulated an electromeagnetic system in which there is a moving object affected by the magnetic field. results seem correct; however, when I extract an animation, I cannot see the motion of the object.

Any clue?

Thanks
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Density dependent on temperature and solute concentration

April 29, 2013, 8:31 am
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Hello,

I am trying to couple fluid flow (Darcy's Law), temperature (Heat Transfer in Porous Media), and solute transport (Species Transport in Porous Media). I need the density of water to be dependent on temperature and solute concentration, but I am not sure how to modify the density term under the material properties.

For example, what would I add to this term to make the density dependent on concentration as well as temperature:
rho(T[1/K])[kg/m^3]

Thank-you for your help.
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3D simulations and meshing problems

April 29, 2013, 9:24 am
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Hello,

I am doing some 3D simulations. I have basically some very small structures on a bigger substrate. I have to use the ignore faces operation to remove the common faces between the substrate and those small structures. And This make the whole geometry as just one domain so I won’t be able to refine the mesh just on those small stretchers, is there any way to solve this problem please?

I do appreciate any suggestions
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Current not being conserved in AC/DC.

April 29, 2013, 3:42 pm
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Hello all, I am having an issue with current conservation using the AC/DC module and a stationary 3D model.

The model is two parallel pieces of copper and a small piece of aluminum between them. I add either a voltage or current source at the end of one piece of copper and a ground or current sink at the end of the other. Basically I am making current from through one piece of copper, through the aluminum block and out the other copper piece.

I have been setting the normal current density source at the ends such that exactly 100000 amps (arbitrary) runs into and out the ends of the copper. I then solve the system and do a surface integral of the normalized current density at the interface between the copper and aluminum pieces. I get a total current of over 100000 amps, Something 1.5 times as much. If I do a cut plane in the middle of the aluminum piece however and integrate over the current density there I get something like 99999 amps (good enough since its a numerical solver). I tried cranking up my mesh at the boundries of the aluminum and copper interface but it is still the same.

According to Kirchhoff I should be getting 100000 amps at any slice through my model. There is no way I can have 150000 amps running through anywhere in the system if all I input is 100000. Or if I do a voltage source at the end of the pieces I should get the same integrated current through any slice of the model.

Any ideas? Thanks
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Changing the relative repair tolerance seems not to be effective in any way

April 29, 2013, 8:25 pm
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I need to define a geometry that is very awkward but it's necessary, It's a sphere with a very big radius compared with the thickness of a single layer (1000 times smaller, e.g. s/r = 1e-3, where s = layer thickness and r =sphere radius).

The problem is: When I define a sphere with such dimensions, it will give me an error giving a minimum value for layer thickness, witch respects the following equation, independently of the relative repair tolerance value: s_min/r=5e-3, It was tested for relative repair tolerance between 1 and 10e-12 and for some radius values around 1 and 1e-3 in COMSOM 4.2 and just keeps giving the same value with the error.

Then I tried to define this layer by creating two spheres: one with the desired internal radius (r-s) and another one with radius equals r, and at this time, the geometry was created with no errors, and I could see that both spheres where created by hiding a boundary from the external sphere and zooming in a bit. And again, the relative repair tolerance seems to have effect in the created geometry. But I have already defined a very complicated geometry with several of those spheres with layers and it was working fine, but now I need to reduce this thickness and I don't really want to redefine the entire geometry.

By the way, as this parameter (relative repair tolerance) appears both in geometry and form union's settings, it set the same value on both every time.

I also tried to created new clean mph, just with this sphere and nothing changed, the same errors, so it's not something I defined in my model that is causing trouble.

Am I missing something here?
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Is it possible to show cavitation phenomenon in liquid due to ultrasonication?

April 29, 2013, 9:49 pm
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Is it possible to show cavitation phenomenon in liquid due to ultrasonication?
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Any sucessful study for fire simulation by comsol?

April 29, 2013, 11:26 pm
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Hello!

Is the comsol suitable for fire simulation?

Another question is any sucessful case for high temperature (100c - 900c)environment air flow simulation with inlet and outlet condition.

Since When I use non-Isothermal flow(nitf) to simulate high temperature air wiht inlet and outlet, it is sucess in 2D case, however convert the 2D case to 3D case, the simulation is not convergence.

So I want to know whether has sucessful case in high temperature environment air flow simulation for comsol?
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COMSOL 3.5a has already stopped working, please connected to the net for a solution.

April 29, 2013, 11:37 pm
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Hi,
There is a question puzzled me for a long time. In my model, thermal field, flow field, strain-stress module and ALE module are coupled. I want to make a transcient model which based a steady result. Errors happened (see the upload file), and i think that these phenomenonto may be caused by computer configuration, thus,i computed the same .mph file on six different computer (i5 2320 3.0GHz 3.2GHz ,i5 2310 2.99Ghz, i3 2100 3.1GHz, i3 3120 3.3GHz). The result illustrates that no error as shown in the upload file happened on i5 2310 2.99Ghz but happened on the others. I just don't know why,and how to deal with this question.
Could you please help me on this issue?
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Squirrel cage induction motor - external circuit?

April 30, 2013, 1:14 am
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Hi!
I am trying to model a 2D short circuit squirrel cage induction motor in COMSOL v4.3a using the rotating machinery module. My goal is to get a nice distributed electromagnetic field in the motor. It works when I am making a stationary simulation. I have also tried to make a time varying sweep at no load which means that no current should be induced in the rotor bars and the rotor is moving synchronously. The field only gets nice distributed if I force the current to become zero by typing in 0A.

The problem is that I am not taking care of the short circuit ring as I am modeling in 2D. In order to include these effects, I want to couple an external circuit just as Hartman and Escarela-Perez did and couple it to the FEM model using a single turn coil domain. I have uploaded the links for their papers, see below.

Escarela-Perez, figure2 shows the equivalent circuit of the squirrel cage. The ciruict is short circuited in the figure. Is it enough to couple the gray fields (38 in my case because my motor has 38 rotor bars) to a “single turn coil domain”? I’ve got to put ground somewhere also, right?!

Hartman, figure4 shows a similar circuit. This circuit is harder for me to understand. I guess they are only modeling one pole but I don’t understand how to implement this at all.

I want to do this as simple as possible, I would prefer to use an external circuit instead of the “Global ODEs and DAEs” interface. I think that my main problem is that I don’t completely understand the circuit and how to implement it.
Hartman: www.comsol.com/offers/conferen.../file/14397_hartmann_paper.pdf
Escarela-Perez: www.comsol.com/papers/4959/

Thanks in advance, I really appreciate your help.
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about 'Warning : Re-ordering of null vectors failed"

February 16, 2013, 6:02 pm
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Dear all,

I'm trying to simulate the dynamic behaviors of the linear control solenoid valve.
The spool motion is determined by the force balances, which are calculated from each field, and a spring-mass-damper system.
So, I am using the AC/DC module and Creeping flow with the ODE module for ALE.

When the spool in the valve starts to move, I got a message ' Warning: Re-ordering of null vectors failed".

I want to know what the meaning of the warning message is.

Does anybody have any advice?

Regards,
Lee.
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Photonic crystal fiber

March 28, 2013, 9:21 pm
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I am trying to model a photonic crystal fiber with liquid crystal filled holes..

I am a starter and it would be really helpful if any one can help me regarding the resources from where i can learn the modelling steps and methods...

Thanks
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