Latest Discussions - COMSOL Forums

A problem in far-field calculation

Sat, 11 Feb 2012 17:45:37 +0000

Hi there,

Now I have a problem when I do the far-field calculation in RF model. What I wanna do is to calculate the power outflow of an optical antenna in the far-field. But I found I only can get the electric field for the far-field calculation.

Does anyone know how to solve it?

Thanks and best regards,

Guanghao

Problem in assigning boundary and initial conditions

Sat, 11 Feb 2012 17:14:18 +0000

Hello,

I am new to comsol & i want to simulate a very simple system; water in a pipe with a spherical ball kept as a disturbance.

I am using comsol multiphysics 4.0a. I opened comsol, clicked on 2D> fluid flow> single phase> laminar flow, then I chose stationary study and finished the initial settings by clicking on the checkered flag.

In geometry, i created a rectangle and a circle inside it. Under laminar flow, i clicked on wall properties to assign boundary conditions.

Here is the problem. By default, comsol has selected all lines & curves as boundary and the 'Selection:' has been disabled, so I cannot change it manually. I face same problem while assigning initial value conditions. I cannot give 'Initial Values' to my model. The problem remains same even when I switch between 'Form an assembly' to 'Form a union' under Geometry.

The problem remains same even when I try to simulate structural problems or any other physics.

I am sure this problem has a very simple answer but as a biginner I am unaware of the features and it'll take me life long to explore each feature all by myself. I appreciate anyone who would like to help me.

Thank you in advance.

comsol 3.5a

Sat, 11 Feb 2012 11:12:44 +0000

what is the expression used in comsol 3.5a to get the output of a wheatstone bridge.
plzz help

results from frequency response is not matching the eigen v

Sat, 11 Feb 2012 10:26:34 +0000

Hello everyone,

I am modeling a structural mechanics with ale mesh frame and electrostatic in this frame.. in an axi-symmetric 2d model (MEMS module)..

The problem is ... When I solve for the eigenvalues I got a resonance at 1MHz and then at 4.1MHz and the third at 9MHz

but when I solve for the frequency response analysis I found different result

thanks you so much for your support

yours
Mark George

Out of memory during LU factorization

Sat, 11 Feb 2012 08:30:49 +0000

Hi to all,

i trying to solve transient heat conduction problem. During the solving "Out of memory during LU factorization" message has come . i also tried spooles solver. but i could not solve the problem.

Time-dependent solver (BDF)
Number of degrees of freedom solved for: 184391.
Symmetric matrices found.
Format not changed since SOR line uses nonsymmetric storage.
Solution time: 6 s.

can you help me?

with regards
parma

problem on getting frequency responce

Sat, 11 Feb 2012 07:47:04 +0000

Modeling rough walls

Sat, 11 Feb 2012 07:35:48 +0000

Hi

There is an equation available in comsol as lograthmic wall function which can be applied easily to model smooth walls with C+ parameter that can also be tuned if required, however does any one knows how to model a rough wall in comsol is there any other wall function available for this option?

Thanks

Conjugate heat transfer boundary conditions

Sat, 11 Feb 2012 06:25:54 +0000

Hi,

Does any one knows wether it is possible to aaply logarthmic wall functions in Comsol on two walls simultaneously in a model. I mean lets us consider two concentric cylinders and let us assume that the inside cylinder is a solid rod with some internal heat generation and within the annulus some liquid flows, then in this case can we apply the logarthmic wall functions on the inside solid-fluid interface and as well at the external wall. what boundary conditions shall we use if the flow remains laminar?

Thanks in advance

Modeling free gas channel fllow inside porous media

Sat, 11 Feb 2012 00:31:11 +0000

Hi!

I am looking to model gas channel flow within a porous reactive element. Is it possible to model this as an assembly of two different domains and specify boundary conditions at the interface connecting free gas flow and the medium? To be noted is that there is no physical channel inside the medium.

Comments/thoughts/suggestions, please?

Thanks!

Cylic Voltammetry

Fri, 10 Feb 2012 21:30:27 +0000

Hi

I want to model a 3 electrode electrochemical cell that I am using in cyclic voltammetry. I wanted to know if anyone had any suggestions on how to get start doing this using COMSOL as I am new.

I would like to be able to plot current vs potential and current vs time.

Thanks
J

Material properties dependent on two parameters

Fri, 10 Feb 2012 20:51:47 +0000

Hi,

I'm familiar with using say a function to set a material property that is dependent on one variable (i.e., temperature), but how would I go about setting it for two variables? Would I simply write two functions under the same material? Would Comsol then use both functions for its output?

Kind regards,
Jeff

How to fix the shape of a boundary

Fri, 10 Feb 2012 20:49:30 +0000

Hi,

In my simulation, one of the boundaries are evolving with time due to creep. Is it possible to fix the shape of the boundary? (thus the displacement of the boundary in the moving direction is uniform for a flat boundary).

Thank you very much!

Best regards,

Save comsol file as M.file, but without any parameter of geometry in script

Fri, 10 Feb 2012 20:46:47 +0000

Hi everyone,

I builded my model based on comsol 3.5a version, and then save it as to M.file. But when I save comsol file as to the .m file, the geometry file was produced as flbinary file .mphm (the example I list below). I do not want to get this script (flbilary), since the geometry parameter can not be changed. is there anybody know how to solve this problem?

Thanks.

% The name of this file is given by the variable 'flbinaryfile'.

flclear fem

% COMSOL version
clear vrsn
vrsn.name = 'COMSOL 3.5';
vrsn.ext = 'a';
vrsn.major = 0;
vrsn.build = 603;
vrsn.rcs = '$Name: $';
vrsn.date = '$Date: 2008/12/03 17:02:19 $';
fem.version = vrsn;

flbinaryfile='1.mphm';

% Geometry
clear draw
g247=flbinary('g247','draw',flbinaryfile);
draw.s.objs = {g247};
draw.s.name = {'CO1'};
draw.s.tags = {'g247'};
fem.draw = draw;
fem.geom = geomcsg(fem);
fem.mesh = flbinary('m1','mesh',flbinaryfile);

% (Default values are not included)

I want the form like this so that I can change the geometry parameter.

g1=ellip2(0.2,0.2,'base','center','pos',[0.4,-0.4]);
g2=ellip2(r,r,'base','center','pos',{'0','0'},'rot','0');
garr=geomarrayr(g2,pitch,pitch/sqrt(3),5,8);
[g40,g41,g42,g43,g44,g45,g46,g47,g48,g49,g50,g51,g52,g53,g54,g55,g56,g57,g58,g59,g60,g61,g62,g63,g64,g65,g66,g67,g68,g69,g70,g71,g72,g73,g74,g75,g76,g77,g78,g79]=deal(garr{:});
g80=ellip2(2.0E-4,2.0E-4,'base','center','pos',[-0.0010,6.0E-4]);
g81=ellip2(r,r,'base','center','pos',{pitch/2,pitch/(2*sqrt(3))},'rot','0');

One basic and fundamental question about boundary conditions

Fri, 10 Feb 2012 09:40:36 +0000

I feel confused about the introduction of Lagrange multipliers in generalized Neumann boundary condition. Why do we need to have extra dependent variables? I checked relevant items in documentation, and found the following description:

"When Dirichlet boundary conditions are introduced, the finite element algorithm makes a corresponding change to the Neumann boundary conditions so that the resulting model becomes solvable."

It sounds like there exists certain inconsistency within Neumann and Dirichlet BC's, making the equation system unsolvable.

If anyone knows about these details, could u please explain it? Thanks a lot! I really feel unsafe to run simulations with doubts in underlying systems.

simulate temperature rise due to current flow in copper track

Thu, 09 Feb 2012 21:21:20 +0000

Hi all

I am new to COMSOL and I would like to simulate the temperature rised of a short and straight copper track on PCB due to current flow. How should I start with? I have a fixed voltage and current to apply on it.

Does using solely the joule heating physics sufficient to simulate this? I can see that there is electric current -> electric potential for me to enter the voltage but no where to enter the current. How am I suppose to simulate this?

Thanks
Ron

timeint (Time Integral) gives wrong answers for mass balance

Thu, 09 Feb 2012 12:54:58 +0000

Hello everyone!

I hope the COMSOL experts and enthusiasts here can help a fellow user out :)

The situation is that I have a tube (L=1m) with a porous medium in it and air flows in and out again and at the same time water is being evaporated from the porous solid phase to the gas phase. This 1D model uses "Species Transport In Porous Media" and "Darcy's Law". The first one calculates the concentrations, the second one gives the species mass and calculates the density and velocity. The 'drying' is modeled using a mass source of H2O that is dependent on temperature and the amount of water left (first order Arrhenius type equation). These I modeled myself and seem to be fine.

The problem is that I want to make a mass balance over the reactor. So, the time integral over all mass that flows in + the time integral over all mass that is released from a source term into the gas phase, is equal to the time integral over all mass that flows out at the exit. When the temperature is constant, this seems to be true with an 0.5% error, which is quite ok, but if the temperature changes over time (simulated using a step function) the difference in the time integral over what goes in and out is more than 50% off!! And so is the integral over the source flux, it is ridiculous. I may be doing something wrong here, but I have no idea what. Can anyone please help me?

I have included the file. It is not as complicated as it may seem. Just O2 and N2 (air) flow in without doing anything and there is 1 kg of water available for release. No water flows in, so the end point should 'see' 1kg of water come by over time. However, according to the timeint function the answer is 0.49 kg.. same for the integrated source flux.

Things I already tried:
1) Increasing the accuracy of the timeint function ( timeint(START,END,dl.rho*dl.u*A_flowthrough,1e-10) )
(1e-8 is standard), usually I use 1e-5 since it doesn't matter much and takes a lot longer to calculate).
2) fiddling around with the time stepping. Tried all kinds of settings, it converges well now, but still no good answer.
3) Changed Tolerances: Abstol is now 1e-3, Reltol is now 1e-8, quite okay I think.
4) Conservative vs Non-conservative formulation (compressible vs incompressible gas), does not matter much.
5) Changed amount of elements, currently 2000, just to be sure. But increasing # elements does not help.
6) Changed discretization order from Quadratic to Quartic and back again, no effect.

It must have something to do with temperature, because it did work when T was constant and now it doesn't since T is a function. Included in this post is the actual model (4.2a) and some graphs.
The first graph shows my own calculation of the amount of water left in the solid, this goes to 0, like it should.
The second one show the time integral over the exit and this is 0.49 kg while it should be 1 kg.
The third has the same problem.
The last one shows the H2O concentration in the entire reactor vs time. Most important is that is goes to zero, so apparently all mass flows out of the reactor.

I would be very happy with any useful input! Thanks in advance!

Time-step dependent reaction rates

Wed, 08 Feb 2012 10:51:31 +0000

Hi,

Help me how to set a "time-step" dependent "reaction-rate constant" for a time-dependent solver.

Regards,

animation vieweing from different reference frame

Mon, 06 Feb 2012 11:43:53 +0000

Hi.
I have used 2d spf cfd flow simulation. I have solved and found velocity of a moving wall BC. Is it possible to change this body-fitted movies/animations to view through other frame?
I mean I solved simulations from boat and I want to see the solution from water. Is that possible?
Thanks in advance
Siva

Stability in a Rotating Machinery models

Mon, 30 Jan 2012 18:17:49 +0000

I am running a model in the rotating machinery module of COMSOL. Anytime I increase the time of my model, I am obtaining different answers. Does anybody know if the rotating machinery models become stable in some point, or because of the rotational nature, there is always a time dependency?

Thanks

ht_pem

Thu, 12 Jan 2012 22:40:32 +0000

Hello,

I am new to comsol 4.2 and I started with the built in model of ht_pem in the model library.
It seems that changing the dimension of this model (L) to make the channel longer result in a problem for solver that can not evaluate some of parameters.

I have no idea why such a change result in this error so any comments is appreciated.

"
Failed to find a solution for all parameters,even when using the minimum parameter step.

Segregated group 3

Attempt to evaluate negative power of zero.

Function: ^

Returned solution is not converged.

Thanks,
Siamak