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Coefficient Form PDE and periodic boundary conditions

August 2, 2011, 6:08 pm
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Hi,

I'm new to COMSOL so I would like some help regarding the following -

I'm trying to perform a phononic crystal simulation using coefficient form pde. I have a hexagon with a circular hole shape removed - this is my geometry.

I want to apply a zero flux boundary condition at the circular shape and periodic boundary conditions at opposing edges of the hexagon.

So I end up using 3 periodic boundary conditions, for the 3 pairs of opposing sides of the hexagon. Unfortunately COMSOL sets up zero flux boundary conditions by default on all the boundaries and I don't see how I can remove this boundary condition on the hexagon's edges on which I only want the periodic boundary conditions.

Any help would be greatly appreciated!

Kartiek.
I'm using COMSOL 4.0a
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Variable defining/refering

August 28, 2011, 5:24 am
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I am going through some of available examples of models and and I have some difficulty understanding syntax of certain variables' references such as tpf.G0i, chds.ndflux_c, chns.G0i, nitf.rho or chns.Vf2. The bit before dot is obvious (physics ID), but how do you identify the part after it?
I went through the part of manual on variables (from COMSOL Multi. User Guide) but somehow it did not clarify it. For instance in one place I found what ndflux is (Normal conductive heat flux), but what does it mean with _c at the end? Or where does rho come from (no such variable appears in the model, only symbolic). Same for G0i or Vf2.

I know what some of you may think right now (RTFM), but I kindly ask for a more specific answer. Actually the relevant part of manual would be nice, but one that actually DOES mention such construction and variables above.

Thank you,
Mike
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Outlet boundary condition in PDE coefficient form

February 12, 2014, 1:08 pm
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Hi guys,

I'm trying to simulate the charge transport equation for 3 species (electron, positive and negative ion) using the PDE module coefficient form.

Here's my question, How can I simulate "outflow" boundary condition for any one of these species in the model?

From mathematical point of view, I understand that "Outflow" means Neumann boundary condition. The problem is, Neumann BC in "PDE coefficient form" couldn't be selected for each variable separately, in contrast to the Dirichlet BC which gives us the choice for choosing the variable on any boundary .

It seems to me that the only solution for this problem is to have 3 coefficient form modules, one for each species. I think there should be easier way for this problem.

Any suggestion is appreciated. Version: 4.3a

Regards,
Peyman.
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Solution is not converging for solar cell problem.

January 9, 2015, 7:35 am
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Hello everyone,

I'm trying to simulate a solar cell with four layers, and I got success with the one dimensional problem. But when I tried the same in two dimensions, the solution is not converging. I'd done the things I can do with my little knowledge in Comsol, still I am not getting positive results.

Thanks in advance for your help.

Regards
Binoy
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Motion of a Deformable Solid in a Fluid

January 9, 2015, 12:08 pm
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Hi there,

I want to study the motion of a deformable solid in a fluid; the red blood cell's motion in a capillary. I don't know if COMSOL is a good choice for this kind of problem. I would be grateful if anyone could please help me.

In addition, for the very first step, I want to study a very simple case; something like motion of a rigid sphere (circle) in a cylinder (2D channel).

Then, I'll try to add the elasticity of the cell, which does not seem to be easy!

Any suggestion, reference, tutorial, or sample, is appreciated!

Thanks
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Laminar Flow in Mixer Tank

January 9, 2015, 1:35 pm
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Dear Sir,

I am trying to simulate a mixer tank with impeller (cubic tank with center impeller with four blade rotating around z-axis). I am using "Rotating Machinery, Laminar Flow (rmspf)" to simulate my mixer tank. The problem is when I am using a Frozen Rotor to compute the velocity filed and pressure filed the study going well and converged but when I try to use Time Dependent study the solution didn't converged and I got many problems. Please any one can help me solve this problem??

Best Regard's,

Eng. Mohammed A. Alameedi
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Prestressed piezoelectric analysis in frequency domain

January 10, 2015, 12:46 am
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Hi everyone,

I'm trying to simulate the prestress effect on the resonance frequency of a piezoelectric material using the "piezoelectric devices" physics.

I'm trying to do it in two steps :
1. Apply the static force to the structure and solve the problem using a stationary study and store the solution.
2. I use the stored solution to initialize the variables of the frequency domain step. I disable the "boundary load" condition and apply voltage to the piezoelectric material.

The result seems not good. when i look at the 3D stress for different values of the prestress the result remain unchanged.

I believe i'm missing something here, in particular the transition from step 1 to step 2.

Could you please help ?

thank you
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Complex swept meshes

January 10, 2015, 2:57 pm
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Hi, I would like to build a "relatively" sophisticated swept mesh which would involve the step of building a nice 2D mesh and then mapping that. In particular, on that 2D mesh I'd like to have some boundary layers. Maybe that 2D mesh would itself have some swept or mapped components.

But once I'm making a 3D mesh and choose triangular on boundary then I don't have any options for doing much other than size and corner refinement.

Am I missing something - should I make a 2D geometry somewhere (in a different component) and mesh that and import? Any case studies showing this?

Thanks, John
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Geometry primitives

January 10, 2015, 9:35 pm
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Does Comsol have a primitive for a block with rounded edges? I am using the software for teaching and do not want to see sharp edges in electrostatics. I also do not want to spend a lot of time standing live before class defining a complicated geometry so a single primitive would be ideal. Is there a library of primitives one can download?
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failed to generate mesh for domain; intersecting face elements

January 10, 2015, 9:57 pm
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Hello,

I'm getting the errors stated in the title, i.e. the mesh failed to generate and there are intersecting face elements. How do I fix this?

I wish I could attach the file but I get a "file size error."

Please help
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Help needed with model to find acoustic transmission loss of a membrane.

August 18, 2014, 9:57 pm
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I've been trying to 2 weeks now to get this model working with no joy so far.

It's a membrane with a mass attached to the centre. I have YM, PR & density for membrane & mass.

I need to calculate the transmission loss the membrane+mass provides to acoustic waves that pass through the membrane+mass.

I've tried to replicate an impedance tube within comsol using a long cuboid shape with the membrane-mass half way along. I've used 26 PMLs surrounding the cuboid to get rid of reflections.

I've been using intop1 & intop2 with either a plane wave radiation source on the source side wall, or a background field (the domain before the membrane-mass).

I've also read somewhere about adding face loads to the membrane-mass.

Any help in getting this model to work would be hugely beneficial,

Thanks



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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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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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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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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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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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