Transport of diluted species and electrostatics

March 1, 2011, 5:33 pm

≫ Next: Weak form for 2-D elasticity

≪ Previous: Optimization problem

$

0

0

Dear Community,

I am trying to combine the physics of the poisson equation (electrostatics) with the nernst-planck (transport of diluted species) in Comsol 4.1 for like 6 weeks now, and all I get is errors. Does anyone have experience with these two models combined?

Eventually I wanna model the concentration and potential and flux fields in a nanopore, but I took a step backwards, and just try to get the physics right, by modeling an electrical double layer next to a charged surface.

Some details on my model. I use the potential field from the electrostatics as input for the potential in the nernst-planck, and in return I defined a space charge density of F*(c-c2) (c and c2 being the electrolyte ions) for the electrostatics. That should be enough to couple the two physics, or am I wrong?
Moreover I have a -1 mC/m2 surface charge on one side and ground on the other side (just rectangle). Electrolyte concentrations are fixed on the ground side to 1 mM.

All I get as error is that the maximum number of newton iterations is reached and the solution is not converged. I tried a whole bunch of different initial conditions and solver configurations, but nothing is working.

Does anyone have any ideas?

Best
Nils

---

Weak form for 2-D elasticity

April 15, 2011, 8:03 am

≫ Next: scaling of variables

≪ Previous: Transport of diluted species and electrostatics

$

0

0

In an effort to learn COMSOL, I decided to model the two-dimensional elasticity equations using the weak form. I was surprised looking through the documentation that there are very few example problems, so I didn't have much to go on. I think I'm deriving these correctly, but I could use another pair of eyes:

-(lamb+2*mu)*test(ux)*ux-mu*test(uy)*uy-(lamb+mu)*test(ux)*vy

-(lamb+2*mu)*test(vy)*vy-mu*test(vx)*vx-(lamb+mu)*test(vy)*ux

lamb and mu are the lame coefficients. u=u(x,y) and v=v(x,y) are the displacement fields.

One question that I can't figure out is how to apply stress boundary conditions. Can anyone help with that?

scaling of variables

October 9, 2011, 1:00 pm

≫ Next: the free vibration of the cantilever

≪ Previous: Weak form for 2-D elasticity

$

0

0

What is scaling of variables and how does this influence the final solution?

What should be chosen for scaling of variables from "manual, initial value based, automatic, none"?

Thanks for the help.
Sirisha.

the free vibration of the cantilever

April 2, 2013, 6:15 pm

≫ Next: initial Values of Variables Solved For

≪ Previous: scaling of variables

$

0

0

Hi everyone:
The initial displacement D=1.05mm, is placed on the end of the cantilever beam, to solve the velocity response of the beam end. First, I select space dimension 2D, and add physics Beam, but the simulation result is wrong, when I select space dimension 2D, and add physics Solid Mechanics (solid) , the simulation result also is wrong, the velocity curve is increasing against time. Now I select space dimension 3D, and add physics Beam, but the simulation result is not the same to the experimental result and analytical result. I am confused. Hope some one give me a advice, thank you! Details see attachment.

initial Values of Variables Solved For

April 18, 2013, 11:27 pm

≫ Next: Can I pause the calculation and check the intermediate result?

≪ Previous: the free vibration of the cantilever

$

0

0

Hi, I have someting to know.

As you know, the initial condition set is as follows:

initial Values of Variables Solved For
Method: initial expression, Solution
Solution: Zero, Solver1, Solver2

in magnetic problem solving,
I made study1 for low frequency, and study2 for high frequency for the better convergence.
So I'd like to solve at first study1 and after that study2 by using initial condition from study1.
How do I set the initial values?

in study2
initial Values of Variables Solved For
Method:Solution
Solution: Solver1

is it right?

What's the difference between initial expression and Solution in Method set?
and when can i use Zero for Solution?

In addition, I changed Solution to Solver2(from Solver1), and I got the very diffenrent calculation result.
Could you explain the reason?

Thank you in advance.

Can I pause the calculation and check the intermediate result?

September 1, 2013, 9:33 pm

≫ Next: Weak Form PDE: how to impose Neumann BCs?

≪ Previous: initial Values of Variables Solved For

$

0

0

Hi All,

Can I pause the calculaiton of COMSOL and check the intermediate result? I would like to check if the intermediate result is reasonable or not. I don't want to only depend on the convergence plot.

Thanks and regards,

Jiuan

Weak Form PDE: how to impose Neumann BCs?

May 1, 2014, 11:45 am

≫ Next: Joining Edges (results)

≪ Previous: Can I pause the calculation and check the intermediate result?

$

0

0

Hi everyone,
I'm new in Comsol, so I apologise in advance if my question is trivial. I'm exploring Comsol capabilities in solving PDEs, and my current unidimensional equation is the follow:

A(u) u'' = f

where u = u(x). I'm trying to use the weak form, then by using a test function v, it becomes:

[v A ux] - (vx A ux + v A' ux^2)- v f = 0
-test(ux)*A(u)*ux - test(u)*dA(u)*ux^2 - test(u)*f(x) in Comsol

where [ ] denotes the boundary terms, and the apex ' is a derivative wrt the argument of the function (i.e. A' = dA/du = dA). Here (A ux) should be the flux term.
In order to check if what I do is right, I'm using two benchmark case:
- CASE (1): A(u) = u and u(x) = 2 sin(x) + (x/pi)^2 + 0.5
- CASE (2): A(u) = u and u(x) = x^2-x^3 then f = (2-6x)(x^2 - x^3)

By imposing Dirichlet BCs (by means of "Dirichlet Boundary Condition" modules and selecting "prescribed value of u") Comsol computes a solution that (1) does not match the benchmark function, even if the BCs are satisfied. In case (2) it finds the benchmark solution.

By imposing Neumann BCs, instead I have some trouble. A "flux/source" module is added, and then the equation is

-n flux = g - qu

I set q = 0. At the right side, for instance, I compute g as follows (R denotes right side, i.e x = xR):

flux(R) = A(uR) ux(R) = uR*duR = g

where uR denotes the value of u at x=xR whereas duR is the value of derivative at x = xR.
When I compute a solution using these settings (for instance a Dirichlet BC at the left side and a Neumann at right), in the case (1) Comsol reaches the maximum number of iterations allowed (even if I use as initial guess the exact function), whereas in case (2) the final solution breaks the BCs I wanted to impose.

Where is the mistake?
Thanks in advance for all your help.
Pietro

PS
In which direction is the normal vector n assumed to be positive? When does it point inside or outside the domain?

Joining Edges (results)

May 2, 2014, 1:51 pm

≫ Next: How can I manage individual cell values of a mesh in a convergence process?

≪ Previous: Weak Form PDE: how to impose Neumann BCs?

$

0

0

Hello All,

I have a situation where I have a parametric sweep, and I get a couple of solutions (all having the same geometry). I can use the join operator to subtract the solutions and I get something that makes sense. The issue is when I try to apply the same approach to one of the boundaries. I define it via "edge 2D" and I can plot the any variable over the arc length of the edge, however when I try to subtract two edges (via join), they don't even show up in the "data" drop down.

Does any one have any insight into why this isn't working, or how to go about subtracting 2 line plots (asides from exporting them)?

Thanks,
Aaron R. Shifman

---

How can I manage individual cell values of a mesh in a convergence process?

May 3, 2014, 3:50 am

≫ Next: Convergence problems simulating a thin film - structural mechanics coupling

≪ Previous: Joining Edges (results)

$

0

0

Using module Semiconductors for computing Electric Potential for a certain geometry, sometimes during convergence process one can observe how few cells close to both exterior limits of a domain and contacts (ohmic or Schottky), assume values clearly far from the expected. These sets of cells constitute a sort of splodges which prevent convergence uselessly. How to eliminate them? Can I manage the individual cell values in those cases when I am completely sure of a more appropriate ones?

Convergence problems simulating a thin film - structural mechanics coupling

May 5, 2014, 4:40 am

≫ Next: Heat Transfer

≪ Previous: How can I manage individual cell values of a mesh in a convergence process?

$

0

0

Hey everybody!

My goal is to simulate the stresses during a load cycle in a bearing shell.

The problem:
I assembled a journal bearing in con-rod from which i cut off the major part so that the calculation time can be reduced and defined this as union. So I don't have to bother about contact problems. On the surface of the journal bearing I solve the hydrodynamic with help of the thin film module and a PDE which handles the equilibrium of an external force. On the top of the con-rod I fix the structure what seems to me as the best way. Assuming a rigid structure in the first case the simulation works fine until it reaches the physical borders when the time dependent load becomes to high and the fluid film breaks. Therefore I coupled the hydrodynamic with elastomechanic so that the bearing shell can deform and a higher load carrying capacity is possible.
Due to initial value problems I set up an extra study where I calculate an inital status, which acts as an inital solution for instationary time dependent main study.


I made first tests with small loads, checked how to link both physics best,... and it worked fine. Now I have the problem, that I can't reach convergence with the load I'd like to simulate.
The calculation takes very long and it stops at 4 %. Until this point the convergence behaviour looks very good but then it changes, see attached screenshots, and I can't figure out why. In my opinion its not a physical problem because at the time, when the calculation stops the maximum fluid pressure is about 20 bar, the minimum filmthickness at about 20µm whereby the stresses are quite low. I think the problem must be the way the solvers work. I dont have enough experience to cope with this kind of problem. Thats why I would like to ask help from u.

I add some screenshots of the model so u can imagine how it looks like and a screenshots of one of the two solvers. (both have the same specification)


With kind regards,
Philipp

Heat Transfer

May 6, 2014, 2:18 am

≫ Next: Cyclic Voltammetry (CV)

≪ Previous: Convergence problems simulating a thin film - structural mechanics coupling

$

0

0

Hello

I'm a student on the project, and I'm messing in COMSOL 4.0 a short time and have no notion of "heat transfer", I need to get the temperature to stabilize so you can compare with the real model!
We apply a certain power and determine how long it will work, but as time increases the temperature to stabilize her investment keeps rising gradually not getting any results!
Material I am using is to dispel "Al" and the power part'm using "Tin"!
If someone can help me be grateful, and what commands I can use to stabilize the material the right power exercised

Thank you!

Cyclic Voltammetry (CV)

May 6, 2014, 9:26 am

≫ Next: Convergence problem in a simple geometry with Joule heating module

≪ Previous: Heat Transfer

$

0

0

1) I am using the electroanalysis physics module with the built-in CV study only. How does one define the two electrodes which the CV is being applied, in the case of a two-electrode setup?

2) Is there a way to conduct the CV using a three electrode setup (2 electrodes + reference point)?

Convergence problem in a simple geometry with Joule heating module

May 7, 2014, 8:40 pm

≫ Next: Mhd model of a crucible furnace

≪ Previous: Cyclic Voltammetry (CV)

$

0

0

Hi guys,
I am modeling a simple geometry (some stacked layers) with um and nm dimensions in comsol 4.3b.
The convergence plot goes down rapidly at first and after some iterations it seems that the software changes the value of parameters. Then the convergence plot goes up and the cycle continues. I have many upward and downward trends in my convergence plot and can not get fully converged solution. The plot is attached as well. Could anyone suggest me what should I do? Should we play with different solution methods, which I did and couldn't get satisfactory results? I appreciate for any help.
Thank you.

Mhd model of a crucible furnace

May 8, 2014, 7:59 am

≫ Next: Successive simulation in COMSOL 4.3

≪ Previous: Convergence problem in a simple geometry with Joule heating module

$

0

0

Dear collegues,
i´m setting the procedure to evaluate the movement of the molten metal in a simple crucible furnace: reading the papers from comsol ( www.comsol.asia/paper/download...3709/quintana_presentation.pdf , www.comsol.com/paper/download/83615/roy_paper.pdf )and from internet (www.comsol.eu/paper/download/14619/Ernst_pres.pdf) i understood that the ac/dc, the moving mesh ale and the turbulent flow k-epsi module should be used. in few hours i hope to start the project by myself, so in the way to let you have a look at it. In any case, if you have knowledge about how to set the solve processing , are well accepted

Thanks in advance
Gianmarco

Successive simulation in COMSOL 4.3

May 10, 2014, 1:06 am

≫ Next: Optimization problem

≪ Previous: Mhd model of a crucible furnace

$

0

0

Hello everybody!
I am curently doing simulations of fluid flows in microfluidic channels. The total simulation domain is quite big and a mesh that could provide a solution with resonable precision would take too much memory.

Can I somehow partition the simulation domain so that I run computation only on a part of it and when it is done I use the outlet data as input to the next channel section and run the simulation on this section only, etc.? If yes, how? Has anyone experience with such tricks?

Thanks in advance,
Daniel

---

Optimization problem

May 10, 2014, 4:35 am

≫ Next: Fresnel Equations Model Frequency Sweep

≪ Previous: Successive simulation in COMSOL 4.3

$

0

0

Hi everybody!
Please give a hand.
Knowing the experimental value of the function (for example, the maximum temperature of the body at a specific point), I need to find the optimal value of parameter (for example, coefficient of thermal conductivity) at which calculated body temperature should be as close to experimental. This parameter can be in a certain range of values. How can I solve this task in Comsol?
Thank you.

Fresnel Equations Model Frequency Sweep

May 11, 2014, 2:21 pm

≫ Next: how to optimize the result of comsol with matlab??

≪ Previous: Optimization problem

$

0

0

Hey guys,

I would like to change the COMSOL Wave Optics/RF model, the Fresnel Equations model, to change the transmission and reflectance vs. alpha to transmission and reflectance vs. frequency/wavelength, of light.

How would I change the frequency settings / parametric sweep to do this?

Thanks

how to optimize the result of comsol with matlab??

May 12, 2014, 12:04 am

≫ Next: Generic Error

≪ Previous: Fresnel Equations Model Frequency Sweep

$

0

0

Hello everyone!

I want to know how to optimize my problem.
already i got the result from matlab using the m.file which is from Comsol.
Afterthat, I don't know how to solve that...

Because I didn't use the function of optimization in matlab.
it's first time. So i want to know how to do.

And I checked some pdf file of optimization tool in matlab.
if i want to optimize something i need the equation to solve.
But in my case, I don't know how to determine my equation in my problem.

if you guys know my problem, let me know....
Or if there is a tutorial or guideline, let me give please.

Generic Error

May 12, 2014, 10:46 pm

≫ Next: Time dependent solver of unsteady Navier Stokes

≪ Previous: how to optimize the result of comsol with matlab??

$

0

0

Hello everyone,

I am using the COMSOL 4.4 AC/DC Module to reach the saturation of an electrical steel block with the help of (two) current coils. Therefore I implemented a HB-curve in the materials data for the steel block and created two coils (type: "numeric") with a certain amount of current.
When I start to "compute" my simulation I immediately recieve a error message and the calculation stops:

"Interner Fehler im Auxiliary space Maxwell Löser.
- Detail: Generic error."

Does anybody have experience with this error massage or know the reasons why it appears?

Thank you very much.

Regards,
Richard

Time dependent solver of unsteady Navier Stokes

May 13, 2014, 7:00 am

≫ Next: Global ODE or General form PDE

≪ Previous: Generic Error

$

0

0

Hi,
I'm simulating flow of incompressible fluid in periodic media.
I have implemented dimensionless Navier Stokes equations with Comsol PDE module.

1- I resolve my problem with a steady solver (for high Reynolds number which leads to unsteady flow). I get a good solution.

2- I use this steady solution as initial values for my unsteady solver. The problem is that the unsteady solver is giving me a steady solution (for high Re !). The solution is still the same as the steady solver solution.
The unsteady solver : Direct MUMPS solver, Times: range(0,0.002,1), Time stepping method: BDF, Steps taken by solver: Strict, Error estimation: Exclude Algebraic.

Can you advice me please how can I fix this problem ?
Thanks in advance.

Mehrez Agnaou