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A contact model with exceptional force related to the distance

January 8, 2015, 2:54 am
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Hello, everyone. I'm a new user of COMSOL. I want to bulid a contact model with Structural Mechanics Module that the hemisphere contacts a flat plane. The contact pair is used between the spherical surface and the plane. However, I want to add another exceptional force between two surfaces. This force is a kind of surface force existing on the non-contact area next to the contact area. The interactional surface force is proportional to the distance between the calculated point on the spherical surface and the corresponding point on the plane with same values in X and Y axis. The force exists when the distance is close enough. The value of this force on the spherical surface is different due to the different distance to the plane. On the other hand, the flat plane is going to deform elastically so that the distance is a variation in the calculation. I sincerely want to ask how could I exert this force on my model. This would be great helpful to my work.
Thank you!!
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Charge accumulation

January 8, 2015, 3:13 am
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Hello,

We want to simulate charge accumulation on a surface. The surface has a penetrating electric field on in and there is a non trivial charge density outside the surface. As the charges accumulate on the surface they will change the electric field until the field becomes parallel to the surface at which point charge accumulation should stop.

But how to do this?

We would be grateful for any hits how to proceed. Also if there is a existing COMSOL model that can do this or something similar.

Thank you in advance.
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resume parametric sweep from crash

January 8, 2015, 10:40 am
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I'm running some big simulation with parametric sweep. Before the crash, a portion of the sweep is completed, how do i resume the parametric sweep instead of restarting it after the crash? If i press f8, the sweep just start over...
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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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COMSOL error

January 8, 2015, 1:42 pm
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Hi,

When I do the coupling calculation for laminar flow module and transport and diluted species module (concentration), it always shows an error:

Undefined value found.
- Detail: Undefined value found in the stiffness matrix.
There are 11 equations giving NaN/Inf in the matrix rows for the variable comp1.c.
at coordinates: (-0.00124375,0.00168601), (-0.00124375,0.00168501), (-0.00124375,0.00168401), (-0.00124375,0.00168301), (-0.00124375,0.00168201), ...
and similarly for the degrees of freedom, NaN/Inf in the matrix columns.

Do you know what it is? How to avoid this happen?

Thanks,

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random function

January 8, 2015, 8:11 pm
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I want to disperse 300 ellipses randomly oriented in a rectangle. Is it possible to do this in comsol?
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Particle tracing module for 1D problem

January 8, 2015, 8:16 pm
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Hi everyone,
I am trying to use the particle tracing for 1D problem, but the manual of particle tracing says that the module is applicable for 2D and 3D problem.

Is it possible to deal with 1D problem in comsol? thanks!
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Cut point 3D

January 8, 2015, 10:47 pm
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Hi,

Simple question: my model is 3D, and I defined 5 points inside the results-Data Sets sections in order to evaluate them.
But how can I be sure that the points that I defined are located inside my geometry?
with visualization is not possible to understand for me.
Attached my model

Any answers will be appreciated, and many thanks in advance.
Best
Sepideh
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comsol 4.0 how to have unequal scaling on different axes

June 9, 2010, 1:52 am
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In comsol 3.5a, i could selected under
options->axes/grid settings the option that the axes not be equal.

I have problems where dimension along one of the axis are microns while along the other axes they are cm.
in 3.5a after unticking axes equal, I could use the zoom extents tool to display my results so that the short dimension could be expanded to fill the screen.

I am struggling with 4.0 now. In geometry (model builder) such a possibility exists.
I have not been able to find this possibility once I have results.
When I extrude a 2D object 5 microns in the z direction to get a 3D object from a 2D drawing made in a working plane, I do not know how to view that.
It is also a necessity to be able to define correctly some of my boundaries.

It is also necessary to be able to view the results!!!

I do not understand where this possibility is hidden. I am not sure it even exists.
if not, please include this ASAP.

Can someone help??
Thanks


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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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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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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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