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high mach number flow simulation (CFD)

January 6, 2015, 11:52 pm
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Hi
i want to simulate an airfoil in high mach flow .
i am using high mach number flow module k-epsilon method .also adaptive mesh method employed to finer meshing near shock wave region .
for the special case of airfoil, simulation for all velocity converged(even for higher than 10 mach number flow) and result are acceptable .but when i change slightly dimension of the airfoil ,simulation cannot be converged,even in very low mach number(nearly 2 mach ). I am really confused.nearly two month i am working on this simulation but i can not identify the problem .
to solve the problem please help me.
best regard
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Cannot evaluate expression : undefined variable - Comsol 5

January 11, 2015, 9:44 am
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Hi,
I am new to comsol, I am using Comsol 5 as apart of my project. I am trying to run simple electrolysis model and it is showing the following error

Cannot evaluate expression.
Undefined variable.
- Variable: comp1.phil
- Geometry: geom1
- Domain: 67
Failed to evaluate expression.
- Expression: comp1.phil
- Plot: slc1 (Slice)

Can you please help me?

I am attaching the model
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Assigning different material property to a solid in COMSOL

January 12, 2015, 12:23 am
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Please can anyone answer my question?

Assume you have Young modulus (E) date of a degrading material saved as a spreadsheet E(x,y,z). That means the material has different E all over the volume in a specified time. The question is that how can I assign this property data to a solid material in COMSOL. I tried to use the interpolation function, but it shows the error of 'failed to evaluate variable Jacobian'.

Many thanks
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Hydrogen storage system

January 12, 2015, 1:26 am
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anybody have the solved model of hydrogen storage system in comsol ?
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how to export the deformed geometry

January 12, 2015, 3:21 am
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Hello.
I try to modify the geometry after the sold mechanics(solid) simulation.
(I want to add and cut some blocks to the deformed body, and do other physics!!)
But, I can't export the deformed geometry.
I can export deformed mesh, but it can't be modified in the geometry section. Cutting, and differenciating and so on.

How can I export the deformed geometry?
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Multiphysics coupling

January 12, 2015, 4:12 am
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I am trying to solve a coupled model using the PDE Coefficient Form. The computational domain (x:0,RR) includes 3 subdomains . The mathematical model is comprised of Poisson and Helmholtz equations (ODEs). The dependent variables are the same for the whole computational domain, but each subdomain has ODE of different analytic structure. This makes the model a bit special – the dependent variables are the same for the entire computational domain, but the subdomain and boundary equations differ from one subdomain to another.
What solving strategy should fit this type of problem in order to generate the numerical solution?

The problem was solved using a simple 1D computational domain by the shooting method for numerically evaluating some unknown boundary values. Given that the shooting method is suitable only for 1D problems, what type of method should be used for 2D/3D computational domains?

Any advice or hints would be much appreciated.
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Undefined value found in the equation residual vector.

May 13, 2011, 9:05 am
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I am trying to model a pem fuel cell. Although I have entered all parameters and values, i have some errors as follows;

Undefined value found.
- Detail: Undefined value found in the equation residual vector.
There are 4 degrees of freedom giving NaN/Inf in the vector for the variable mod1.wH2Oc
at coordinates: (0.0022813,0), (0.0022432,0), (0.0022813,7e-005), (0.0022432,7e-005), ...
There are 4 degrees of freedom giving NaN/Inf in the vector for the variable mod1.wO2
at coordinates: (0.0022813,0), (0.0022432,0), (0.0022813,7e-005), (0.0022432,7e-005), ...

Can you help me please?
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Recoverable error in residual, but solver could not recover.

October 17, 2011, 6:20 pm
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Recoverable error in residual, but solver could not recover.

What is the meaning of this error ?
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Failed to evaluate variable Jacobian

August 4, 2014, 11:20 am
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Hi,
I'm trying to implement Lithium-ion battery 2D tutorial on a 3D structure. I followed all steps but COMSOL gives me this error:
Failed to evaluate variable Jacobian.
- Variable: comp1.liion.cs_surface
- Geometry: 1
- Domain: 6
Domain 6 is the separator between anode and cathode. Its thickness is 0.1mm.
I can't figure out what this error is about. I appreciate any advice.

Best,
Mohad
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Determine the damping coefficient

October 30, 2014, 7:24 am
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Hi,

I am designing a cantilever with specific dimensions in the micro scale, and using the electromechanics module in order to study the cantilevers behavior in the stationary and eigenfrequency cases. After getting the eigenmodes, I am interested in knowing how will my beam displace w.r.t. time (i.e. the transient analysis). Since there is no a transient analysis, I am planning to carry out the frequency domain analysis and then inverse Fourier transform the results to get the change in displacements w.r.t time. In the MEMS module, it mentions that in order to carry out the frequency analysis, I should define my damping coefficients alpha and beta. Can these coefficients be determined through comsol (although I don't think it is possible) or shall I determine them analytically?
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Frequency response of displacement at a point -- maximum amplitude varies with frequency resolution

November 13, 2014, 4:45 pm
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I am studying an example in the Model Libraries, the 'shear bender' under 'Structral Mechanics Module\Piezoelectric Effects'.

The example has only Stationary study.
I added a Eigenfrequency study, and found one of the resonant frequencies to be 19057.945Hz.
I then added a Frequency Domain study to see the ampliftude of displacement around 19057Hz.
I chose a point on the tip of the shear band to generate a 1-D plot of the displacement at that point.
I started with a frequency resolution of 50Hz, the plots looks fine, I can see the displacement peaks around the resonant frequency.
The problem is when I increase the freqeucny resolution from 50Hz, to 10Hz, 5Hz, 1Hz, the maximum value keeps increasing. This may be fine, if the system has high Q. At better resolution, I expect to see the peak increase to some extent. However, when I change frequency resolution from 0.1Hz to 0.5Hz, the amplitude jumped from 0.35mm to 4.5mm, more than 10 times! (I have attached a pdf file with the plots.)

The shear bender is used as example here. I noticed this issue in one of my own projects, in which a metal plate is being studied. At finer resolution, the maximum displacement also jumps like discribed above. Eventually the value obtained becomes unrealistic given the thickness of the metal plate. We are trying to compare the displacements at several resonant frequencies, but now it seems those results are questionable.

Can anyone share some thoughts on this? Thanks.


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Cooling Rate at equilibrium condition

December 10, 2014, 1:34 am
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Hi everyone,

I'm calculating the solidification of the melting of a plastic when passing through a cooling die with a certain velocity. The prozess is transient and at a certain point it came in an equilibrium condition. The problem is, when I calculate the cooling rate "Tt", it has positive and negative values (must to be only negative), although the temperature profile is descendent (T2<T1, so to say).

How can I solve this problem?

Thank you in advance!
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Create an electric terminal in physics builder

December 30, 2014, 9:26 pm
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Hi,

I am trying to create a boundary condition feature in physics builder that functions similar to an '(electric) terminal' in AC/DC physics module.

At least, I would like to know if it is possible to make an 'electric terminal node' through physics builder.

This is because I would like to use an electric current as an input in the thermoelectric physics interface provided in COMSOL 4.3. In the thermoelectric physics interface (COMSOL 4.3), only an electric potential is an input. And I am having trouble with creating terminal node through physics builder.

Any help is appreciated.

Thank you

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Magnetic Flux Function

January 8, 2015, 11:27 am
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Hello,

I'm working with the magnetic fields, no currents module to create an axially symmetric model. I'm verifying past work done by a colleague. He used another program to plot the flux function of the model (see attached image).

Does Comsol have similar functionality? I have modelled the same item, but I only seem able to plot the flux density, not the flux function (see second attached). Ideally I would like the two attached images to match fairly closely.

My understanding is that the flux function, Phi, is a function of the flux density, B:
Phi = Integral( B dA)

Any help is greatly appreciated!

Thanks,
Chris
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failed to evaluate variable, failed to evaluate expression, division by zero

January 12, 2015, 9:24 am
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The error messages "failed to evaluate variable", "failed to evaluate expression", and "division by zero" can occur when you add a *new* physics node to a model that you have already solved and then you try to re-solve.
The problem can be that earlier you have set the following in your solution node:
Values of Dependent Variables > Initial values of variables solved for
Method: solution
Study: study 1 (for example)
Selection: Automatic

The errors are coming since COMSOL is looking for "initial conditions" for the new physics variable to come from the stored solution, which doesn't contain any information on the new physics variables that have just been added to the model.

One solution is therefore to set
Method: Initial expression
Study: Zero solution
Then COMSOL will be able to solve the model including the new physics node that you have added.
-DW
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Subtract data sets in transient RF ewt simulation

January 12, 2015, 11:56 am
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I would like to extend the second harmonic generation tutorial offered by Comsol (www.comsol.com/model/second-ha...ion-from-a-gaussian-beam-14701)

I want to run two simulations: one with d33 = 1e-17 and another with d33 = 0. Then I would like to subtract the results for Ez. It would be great if I could also subtract the results of the domain point probe that is defined in Comsol's model (in order to FFT the transient data at that point).

I have tried to define a parametric sweep over d33 with two values (0 and 1e-17). Then I attempt to join the data sets. The issue is that Comsol will only join data sets at one time step, so I would have to create a join data set for every time step. The other problem is that the parametric data set does not encompass the domain point probe, so I would not be able to FFT the data sets unless I store every single time step taken by the solver.

Perhaps Matlab? I have no problem extracting the data from Comsol and then subtracting the results, but I am not sure how to convert the result into data Comsol can use for functions like mphplot.

Thanks for your help.

Comsol 4.4
Wave Optics Module: Electromagnetic Waves, Transient (ewt)
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geometry optic (GO)

January 12, 2015, 12:43 pm
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hi
how can i find Ray intensity in GO(geometrical optic module) for example along a line ?
thanks
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Plate heated by external power source

January 12, 2015, 4:10 pm
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Dear everyone,

Please help me. I am trying to do a simple simulation of plate heated by lamp in 50 second. The heat source power is 1000W and located 150mm above of the plate.

I use Heat Transfer in Solid Physic surface to surface radiation and apply source power in External Radiation Source. However, there is no temperature increment in the plate.

Could anyone please to share your opinion to solve this problem.

Thank you

Nazirah
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imposing changing boundary condition based on a point inside the geometry

January 13, 2015, 12:24 am
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Hi,

I have a 3D model solving Richards equation, (my goal is to catch the behaviour of a floodplain which is inundated periodically)
How is it possible to impose a boundary condition( for upper boundary of the geometry)
that is able to impose hydraulic head boundary condition for the flooded regions; which these flooded regions are changing during time based on F5 that is a point inside the geometry.
And the other parts of the domain which are not flooded regions: imposing "Inlet boundary condition"?

I mean how is it possible to impose boundary condition which is related to the spatial changing during time? Is there any tutorial like this situation?
Attached also my model.Please let me know if my question is not clear.

Thanks in advance for your answers.
Sepideh
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Multigrid failure during solver

February 1, 2011, 1:56 pm
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Hey all,

I'm working on a 3D model of a microfluidic system using the laminar flow (spf) and transport of diluted species (chds) physics models. I've got everything modeled with all the physics settings done, and the mesh applies quickly, but when I set it to solve, it errors out saying:

"Error: Problem setting up multigrid"

Looking through the model the error appears to be when the solver creates a coarser mesh in order to get a simpler solution, the "coarse" mesh has elements too large for the geometry and so it errors out. To give an idea of what I'm working with, the microfluidic channel is 301um x 6000um x 110um with microcantilevers that are 300um x 45um x 1um in the middle.

I know this will be a very intensive model; I have a Core i7 system with 16GB of RAM dedicated to this (as well as about 200GB of virtual memory). The problem is it never even gets to needing any of that RAM (gets to about 500 MB working set and 500 MB virtual) and just errors out trying to simplify the mesh. (Or at least that's my understanding.)

Is there anything I can do to get it to stop with the multigrid preconditioner, or get it to push through it? I'm using Comsol 4.0a, and have a copy of Comsol 4.1 that I haven't upgraded to yet, so if there's a solution that needs 4.1 I can update my system.

Thanks for any advice.
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