Latest Discussions - COMSOL Forums

Using FMincon with Comsol

Wed, 23 May 2012 08:48:44 +0000

Hi i am solving time dependent PDE's in comsol and I have a parameter called the dispersion . I need to fit this parameter to match my experiment and theoritical profiles. For this i am thinking to CALL fmincon to optimise. Can some one suggest how to call FMINCON in a comsol file
Thanks

How to use a (Global Variable) Probe to plot variables calculated using system variable 'timestep'

Wed, 23 May 2012 08:09:02 +0000

Hello dear colleagues,

I have a question about the 'timestep' variable. I use it to calculate certain concentrations into source terms (like: C_out-C_in)/timestep [mol/m^3*s] ).
The simulatution runs, but I am not able to actually plot any source term or variable that is calculated using the timestep variable. COMSOL is just unable to do that.

A guy from support gave me the tip to use Probes to achieve this. I have indeed been able to make a probe, use 'timestep' as the probed variable and then, after simulation, plot the timestep.
However, I am not yet able to plot any other variable (dependent on the standardly unplottable timestep variable) using probes.

Does anyone know how to do this? How do I plot calculated variables dependent on the system timestep using probes (or any other way) ?

Thank you and kind regards,

Ray

Basic doubt about comsol

Wed, 23 May 2012 07:43:40 +0000

Hi ,
Is there any place where i can read about each of the options in the results section ? I mean i want to know what each of the functions calculate , for example von mises , stress tensors etc I don't really understand what they mean , i would like to read about each of the functions and choose the appropriate one.
Looking for a speedy response
Cheers

Plotting result

Wed, 23 May 2012 05:31:15 +0000

Dear COMSOL Users
I have evaluated the electrostatic potential on a 2D disc and I find a circular symmetrical profile. Now I would like to plot the potential against the radius. How can I obtain a cross section through the disc?

Thanks for hints.
Martin

minimizing the number of meshes

Wed, 23 May 2012 02:56:18 +0000

I am trying to do a stress analysis of Material. There are two phases in my model: one is imported from a real material (using imaging technique) and the other one is created in COMSOL 4.2a. But when I am trying to do mesh for this model it seems I keep getting error messages (e.g. Failed to respect boundary element edge on geometry face or edge is much shorter than the specified minimum element size). Then I tried to put the maximum element size at 5 and minimum element size 1. This time I did not get any error and the mesh is done. But when I try to solve it, at a certain period it shows the message- Out of memory during LU factorization. So now the problem is the number of element is too high to solve the problem, though I have a RAM of 16 GB. Can anybody suggest me how I can minimize the mesh elements so that I can solve this problem? I have also attached a doc file with screenshots of both the mesh settings and solving error's windows.

A complicated boundary condition

Wed, 23 May 2012 01:52:31 +0000

Hello,

I am solving a viscoelastic problem, where the in-plane flow 'talks' to the surface deformation. I am running the simulation on a square and the height field is clamped on the boundary, i.e. z=0 ans z'=0 on all four sides.

The boundary condition relating velocity and z is k*zxx^2+p-2*mu*v1xx=0, where k, mu and p are parameters. What is the best way to implement this boundary condition? I am currently using the constraint node and a bidirectional symmetry setting. However, I am unable to get convergence. Any help would be much appreciated.

Thanks,
Padmini

Problems with S-parameter calculation

Tue, 22 May 2012 21:01:36 +0000

I do not understand the behavior of the transmission parameter S21 in a rectangular wave guide excited with two Port, there are differences in behavior between S21 and dBS21, these are the following differences:

-S21 has peaks. Evident when exporting data and analyze it presents oscillation between 1 and -1

-dBS21 plot in COMSOL, the oscillatory beahavoir of S21 is not presente and what is more strange is when calculated
with the formula (dBS21 = 20 * log (abs (S21)), I get different values from those used in the graphic COMSOL dBS21 and the oscillatory beahavoir is present.

If Comsol used otherwise for caculation of dBS21 parameter please enlighten me, that the results did not close.

Please help me!

Which type of stress should i use

Tue, 22 May 2012 18:13:02 +0000

Hi everyone ,
In my model pressure is being applied to the top surface of a cube. ( say pressure is applied along z axis )
I want to study the the variation in stress perpendicular to the pressure ( x axis ). To make it clear , pressure is applied along z axis , to the face of the cube in xy plane . I am trying to find stress variations along x axis.
Now i don't understand which which how to find the stress along the x axis. The options available are First Principal stress , piola kirchoff stress , von mises stress etc.
Plz tell me which of these is applicable in the above case i have mentioned.

I have another basic doubt , wats the difference between surface stress plot and volume stress plot ? And to study variation of stress along x axis i have to use 'Cut line' in the surface stress plot or in the volume stress plot ?

Thanks in advance

Fixed Temperature Boundary Condition

Tue, 22 May 2012 13:31:03 +0000

Hello, I'm new to COMSOL and am trying to run a non-isothermal flow problem where I have basically a rectangular solid object immersed in a volume of air flowing across the object. I placed a fixed temperature boundary condition on one of the sides of the rectangle, a temperature and velocity boundary condition on the inlet and a pressure boundary condition on the outlet. What I'm trying to accomplish is to determine the heat loss from the object to the air (without radiation for the time being). After I run the scenario, I plot the temperature and to my surprise the temperature on the wall that I placed the fixed boundary condition on is somehow lower than the fixed value I applied. What have I done wrong, or is there something else I need to do to the model?

Thanks

Cotinuous Release in Particle Tracing Module

Tue, 22 May 2012 13:27:54 +0000

I would like to continuously release particles for a set amount of time using the new particle tracing module. Under hte Inlet settings there only seems to be the availability of step-wise release times. Is there any way to implement continuous release in this module? Or should I so a quasi-continuous release, ie: setting the interval time to a very small value?

Which port type to use with RF module

Tue, 22 May 2012 10:48:41 +0000

Hello everyone,

I am trying to simulate a CPW resonator in 3D. Can anyone help me with the chioce of the ports? In 'real life', the rf signal is fed in by several aluminum bond wires. I saw that the tutorial models cpw_bandpass_filter and coupled_line_filter (which are the closest to what I want to do) use lumped ports with the 'cable' option. However, when I do this I get errors, except when I define length and width of the lumped ports. Can anyone provide a short overview when to use lumped, rectangular or numeric ports...?

Best regards,
Diana

How can I avoid the interference between the deflected structures

Sat, 19 May 2012 20:23:15 +0000

I'm using comsol to do static analysis for a cantilever beam. I used a relatively large force to press the beam toward the substrate, I expected the deformation of the beam tip would stop at the substrate, but according to the value I got, it already penetrated the substrate.
Has anybody had this problem before?
How can I avoid it?

The attached picture shows another structure. A load is added at the upper surface in the middle, the deflection will cause the upper inner surface of the channel touch the bottom when the force is large enough, but in my simulation, it goes beyond the bottom, how can I solve this problem?

the domain of fluid

Fri, 18 May 2012 21:35:07 +0000

Hi,
I am using the Conjugate Heat Transfer Model to model a structure with water cooling. I imported the solidworks drawing file to COMSOL. My question is what is the easiest way to define the domain of the fluid for the structure? The fluid region wasn't previously defined and the shape of the cooling pipe is very complicated.
Please help. Thanks a lot.

Define PH

Fri, 18 May 2012 16:38:10 +0000

Hi

Anyone has any idea how I can define PH of my solution in ACDC module?

Best
Bahar

Trouble coupling heat transfer physics for both fluids and solids

Wed, 25 Apr 2012 03:19:20 +0000

Hey all,

So I'm getting stuck with my model where I try to couple three different physics. The three physics are: Electric Currents, Heat transfer in Solids, and Heat transfer in Fluids. Basically what I'm trying to model is a silicon microbridge which is doped, which makes it so that it has a temperature gradient accross it when heated. I am able to couple and run the electric currents and heat transfer in solids physics, where the power dissipated from the ec physics is inputed into the ht physics as the heat source, and then the temperature that is calculated is used to calculate a new resistivity value for the silicon, until the model converges to a value for the temperature at the given voltage.

The problem arises when I try to incorporate the ht2 physics to mimic the effects of natural convection from the sorrounding gases. I set a temperature boundary condition within the ht2 physics which tells the program that the temperature of the microbridge is given by T, which is calculated in the ht physics. However, when I run this, I get the following error:

"Constraing found for variables in different Segregated groups. Try to merge these groups.
-Feature: Stationary Solver 1 (sol1/s1)
-Error: Constraint found for variables in different Segragated groups. Try to merge these groups."

Any ideas? Attached you will find my model. Thanks.

pulse and heat flow

Wed, 14 Mar 2012 06:04:17 +0000

Dear Sir

I need urgent help !!!

i have to apply pulse train voltage with( 13kv and 1khz) on cable termination to get its effect on temp. after 3 hours.

i can not solve by time dependent solver (take very long time)

i can solve for 10 pluses and get the heat produced Q(W/m3) after 10 pluses then i can get the total energy produced by multiply (Q* (time of 10 pulses =0.01 sec). afterthat , how can i use the value of Q or the value of the energy to get the temperature after 4 hours

Variable material properties

Mon, 21 Nov 2011 13:49:08 +0000

Is it possible to make my material properties temperature dependend? I'm talking about a Cp value for example. Also do I have a MATLAB license, so if that has to be incoorporated; that's not a problem. Thanks in advance.

Undefined Value found in Boundary Condition Analysis

Wed, 12 Oct 2011 20:34:30 +0000

Hi,

I am doing boundary condition analysis in RF module (plus frequency analysis). I defined frequency as a parameter. For certain frequency it works without any problem. However when I change the frequency from the parameters, it gives an error:

"Undefined value found.
- Detail: NaN_or_Inf_found_when_solving_linear_system_using_SOR
"

What could be the reason for this? I did not change anything else in the file...

I would appreciate any help.

Thanks,
Damla

voltage induction due to changing magnetic field

Tue, 24 May 2011 12:14:58 +0000

hello,

I'm a relative newbie to comsol. what I'm trying to do is (I think) relatively easy, but I have no idea how to tackle it. what I need to do is model voltage induction in a coil due to a changing magnetic field. I know how to how to create a coil in a fixed magnetic field, but I don't know how to set up time varying field. but, more importantly, I don't know how to measure the induced voltage. should the coil be open and then specify two points where the voltage would be measured? thanks for any suggestions.

r.

Comsol 4.2

Wed, 18 May 2011 14:34:09 +0000

For your informations,

I just received an email for a free workshop on COMSOL Multiphysics Version 4.2 at montreal in one week. We should receive the cd soon!