Latest Discussions - COMSOL Forums

repeat cycles of simulation

Fri, 10 Feb 2012 06:34:13 +0000

Hi, I would like to ask:

Let says now I have velocity profile for 1s (velocity profile at inlet.png) at the inlet, and i wish to run the simulation for 5s with this inlet condition repetitively. How can i do this in COMSOL?

Thank you so much. Looking forward for your help.

COMSOL expression evaluation

Fri, 10 Feb 2012 01:03:53 +0000

Hi all,

In COMSOL I am using two expressions to calculate a certain parameter, and the objective is to pick the one that under given conditions gives a result, which is a real number and 0<=x<=1). I can easily plug a MATLAB routine to do the assesment and pick the value calculated from either of the two expressions, but I always consider the use of MATLAB from COMSOL as last resort, so maybe there is a way to do it with some smart formula in COMSOL?

Any help will be very much appreciated,
Mike

I have a trouble on calculating electromagnetic force

Fri, 10 Feb 2012 00:30:28 +0000

I want to simulate how much force is induced to the permanent magnet when I use helmholtz coil and put a permanent magnet in the middle of coil.

I simulated and get some results.
However, eventhough I changed current which flows into the helmholtz coil, the force is not changed.
In addition, the result value does not make sense.

I attached my simulation file.
Coudl you help me?

Boundary current source

Thu, 09 Feb 2012 23:37:22 +0000

Hello, I am using ACDC module and I want to know how to create a high frequency current source to observe the current flow in the model.
Basically, I drew two surfaces inside the solid and apply a positive boundary current source to one surface and a negative current source with same amplitude to the other surface. But the result didn't show any electrical potential change or current flow inside the model.
Instead, if I apply a voltage potential to one surface and make the other one as the ground, the result came out well.
can anybody help me?

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

Stretching+Inflation

Thu, 09 Feb 2012 19:30:51 +0000

Hi,

I have a 2D problem in which an arc of a circle undergoes two consecutive deformations (Please see the attached mph and PDF files). I use a two-step study which also happens to have two Physics in order to change some of the boundary conditions throughout the deformations. Also because the geometry is symmetric, I only simulate its upper half. The Steps try to deform the geometry as follows:

Step 1:
The right end is a symmetric boundary and the left end is stretched in circumferential direction to create a closed cylinder. Through this deformation, the central angle of the arc will change from 71.4 to 180 degrees.

Step 2:
After the solution is saved by “Store Solution”, the left end will change to Symmetry and the surface on the inner radius will go under a pressure to inflate the cylinder. I do this by a parametric study by setting an Extension in my “Step 2: Inflation” node.

I have run into a few problems:

1- Although I have used a “Store Solution” in my Solver after the Closing solution, inflation does not use the stored data and therefore does not start from the first deformation (i.e. closed cylinder); in other words it starts from the original unclosed arc. However, it correctly enforces symmetry condition for the left boundary before it solves for the inflation.

2- In the second step of the solution which is inflation, although the boundaries are symmetry, the solution never converges as if the problem is indetermined. This obviously does not occur if I wanted to simulate a full cylinder and not half of it. Adding at least one constraint such as fixing one node on the geometry makes the problem mathematically determined but the physics of the problem will be compromised. Is there any approach that can resolve this issue?

3- I have set the program so that it plots the results while solving it. I have experienced that sometimes I do not get one of those plots during the solution. I mean sometimes, the result of Step 1 is shown during solving, but not the results of Step 2, and sometimes the other way around. I was wondering if this is a bug or I am missing something.

Please see the attached mph and PDF files and let me know how I can fix these problems.

Thank you for reading my posting,
Jessica

2D and 3D boundary pair?

Thu, 09 Feb 2012 18:55:11 +0000

My basic question is how does one match a solution on boundaries between a 2D model and a 3D model? I feel like this was possible on a much older version of COMSOL, but I can't seem to figure it out now.

In detail: I'm working on a simple electrostatic model to solve for the capacitance of a system. The majority of the geometry is rotationally symmetric and is being modeled in 2D (it also has drastic changes in aspect ratio, which 2D is able to handle easily). One surface, however, has radial spoke features that I need to model in 3D. I am currently imposing the "zero charge" condition at the boundaries that I would like to pair and running the 2D and 3D as independent models, but this is not necessarily physically accurate.

two-phase fluid flow in a PEM Fuel Cell

Thu, 09 Feb 2012 16:48:24 +0000

"Hello all

I'm trying to model two-phase fluid flow in a PEM Fuel Cell,
The "Free and Porous media"physics is a tool combining laminar flow and Brinkman Equations to model fluid flow in free and porous media, but as you know it models single-phase fluid flow, I need to use physics to model two-phase fluid flow in free and porous media
I think that I should use both "Laminar two-phase flow" and "two-phase Darcy's law" to achieve my goals, but I do not know how to relate free media(gas channel) inlet velocity to porous media innlet velocity,
As you know, the gas inters from the channels and flows over porous surface and diffuses through it

Please help me on this,
Any help will be appreciated"

Please mention these keywords: PEM Fuel Cell, 4.2a, Fluid Flow

Damping in Transient

Thu, 09 Feb 2012 16:24:04 +0000

Hi,

It seems that in transient solid mechanics the damping is not taken into account. I tried to introduce a structural loss factor but whatever the value i put in the subnode "Damping" i still have the same results.
I also introduced a complex young modulus E*=E(1+i*eta_s) but i got the same results. Did you face this problem?? i'am on Comsol 4.2 and 4.2a
In frequency domaine evrything is ok!

Thanks

Meshing of Disproportionate Geometries

Thu, 09 Feb 2012 16:01:13 +0000

Hi,

Small structures are often disproportionate Geometries with orders of magnitude difference in height to width. E.g. There is a thin film 10nm high (y) and 0.5cm(x) long.

There was a thread on a similar topic in 3D which suggested the use of sweep meshing.
http://www.comsol.de/community/forums/mesh/thread/19108/
However I am usually working in 2D since that simplifies the model and there this isn't possible.
In the upper example when creating a standard normal mesh I get very many meshpoints even when there are only 4 "layers" of mesh points. So perhaps longer a meshpoint could be longer in x direction and smaller in y direction. However how does this influence the accuracy of the simulation.

I also found some other approaches used.
(1) "scaling the problem": E.g. in an electric only problem: A diode which is only about 100nm thick is scaled to 1 or 10um with 10 or 100 times less resistance.

(2) "cutting the problem" Since sometimes most of the lenght of the "layer" is not important another geometry is created which just simulates the interesting part and the other part is taken with a boundary condition in account.

Especially I am looking for some literature about handling handling disproportionate kind of geometries in kind of meshing. Perhaps were such (simple) methods as the upper 2 are used and put on a scientific ground.

Cheers
Robert

edit:
I found a thin film paper which describes the problem and the solution (1) as follows:
http://de.arxiv.org/abs/1001.4447v2
To confirm the scenario discussed above, we calculated
the spatial distribution of the electrical potential and the
current in ring-shaped MTJs using finite element method
(FEM) simulations. The geometry of the ring-shaped junc-
tions is shown in Fig. 6. The in-plane conductivities used in
the simulations for the various layers of the junction stack
are summarized in Table 3. Since the simulation tool did
not allow to represent the real dimensions of the sample be-
cause of its large aspect ratio we scaled the geometry and
the conductivity in the out-of-plane direction by a factor of
100.

Helmholtz Coil Simulation with frequency

Thu, 09 Feb 2012 15:27:27 +0000

Hello

i have the problem to simulation of Helmholtz Coil in the frequency domain, when i would to use this domain appeared different errors.

i would be grateful if anybody help me

Thank you very much in advance!

Animation with Parametric sweep

Thu, 09 Feb 2012 13:18:50 +0000

first, is it possible to have all the parametric sweep values configurations playing at the same time in an animation ? i can have either one parameter in a gif file, or several but consecutively, not simultaneusly (at the same time).

Second question : how to change an gif animation file from only one loop to several... my real problem is that it doesn't run in Power point (it stays as an image)

thanks in advance for your answer

Time-dependent problem-convergence issues

Wed, 08 Feb 2012 22:32:33 +0000

I am working on a time-dependent FSI problem. The problem converges well when I model the solid as a linear elastic material. However I end up with convergence issues and inverted mesh elements when I use a hyperelastic material model to represent the solid.

I am wondering if someone can explain why this is happening and how do I fix this?

Thanks,
Sirisha

Hagen–Poiseuille flow

Wed, 08 Feb 2012 09:52:50 +0000

I would like to use Comsol for a microfluidics project I'm working on. As this is my first time working with Comsol I am trying to simulate a simple problem to compare the simulation with analytical results.
The Hagen-Poiseuille flow problem is a solution to a simple fluidic problem, the flow of a liquid through a circular tube. More infor here: http://en.wikipedia.org/wiki/Hagen%E2%80%93Poiseuille_equation

I've set up the simulation as follows:
The geometry is 2D axisymmetric (radius is 15, length is 800)
The material is water
The physics is laminar flow (spf)
I use incompressible flow as my physical model
And define the inlet and outlet
The inlet pressure is 100 Pa, the outlet pressure is 0 Pa
The mesh is 'physics controlled' and normal

I run the simulation and to obtain the flow rate I apply a surface integration spf.U with Dataset 'solution 1'.
The result for this particular simulation is: flow rate = 5.593e-11 M3/s

The problem is that the analytical result is different. I've put it in Matlab to make sure it isn't me. The result is a factor of 22 too high.

r = 15e-6; % radius
dP = 100; % Pressure
Nu = 0.001; % viscosity
L = 800e-6; % Length
Fr = (pi*r^4*dP)/(8*Nu*L) % Hagen-Poseiulle equation for flow rate

Fr = 2.4850e-012 M3/s

I've triple checked these results but I cannot find why they don't match up. A factor 22 indicates that something's wrong, either on the theory side, or on the simulation side. Because I've just started with Comsol I would like to ask anyone to look over this model and see if something is evidently wrong. If anyone can give me suggestions why the analytical model is not correct I would like to hear it as well.

Thanks in advance

integration coupling variable (unit problem)

Thu, 02 Feb 2012 18:34:59 +0000

Hi,

I just have a quick question about evaluating a scalar integral over an area. In the 3D RF simulation, I have:

1) integration operator: intop1, integrating over an 2D boundary;
2) global variable: power_z = intop1(emw.Poavz), which integrates the power density 'emw.Poavz' over that area to give me the total power in the z-direction 'power_z'.

The expression shows up as orange, so I am guessing that there is an issue with the unit. But I don't understand what is wrong. The power density has a unit of 'W/m^2', so if the integration is over an area, it should give me a value in Watts.

I also would like to plot this power_z over a parametrized variable to show the power_z fluctuation with respect to the variable, but unable to find a way to do it. In a 1D plot, I used a 'Global' plot. In the y-axis, I select "power_z" in the expression. In the x-axis, I used "Expression" under the Parameter choice, and entered name of the parametrized variable. But I got an error saying "unknown function. Name: intop1. Plot: glob1(Global)".

Any help would be greatly appreciated. thanks.

Yan Wang

Error in multiphysics compilation

Wed, 23 Nov 2011 06:27:21 +0000

I got error message "Error in multiphysics compilation"

I am working with Heat transfer in solid module.

Can you help?

Thanks

Drawing rotator with parametric curves

Fri, 07 Oct 2011 13:43:58 +0000

I am looking to draw the concentric shells as shown in the uploaded figure, it is a rotator design. I am having difficulty drawing the layers with parametric curves. In polar coordinates , the equation is

r = a * exp( theta/cot(tau/2)) ; where tau = 0.5* (sqrt(t^2+4) -t); and t = rotation angle / log(r2/r1)
where r1 and r2 are inner and outer radii.

with parametric curves I just get straight lines.

Advice and help in drawing the curves ( to construct the layered material is appreciated).

Ref : H Chen and CT Chan,Electromagnetic wave manipulator by layered systems using transformation media concept, PRB 78,054204(2008)

thanks in advance.
supra

Parametrized curve in COMSOL 4

Sun, 12 Jun 2011 20:09:13 +0000

Hi everybody !

Hi have a 2D ring and i would like to plot the pressure in the interior boundary.

In comsol 3.5 i could do it very easily but with the new version I don't manage to do it anymore.

I tried to use a parametrized curve like this :
x = ri*cos(theta)
y= ri*cos(theta)
with theta a function of x and y.
But all i have is an error.

I also tried to use "phi" from cylindrical coordinates but i don't manage to use this variable.

How could i resolve this problem ?
Thanks :)

Vince.

Post-process two different solutions from a Parametric Sweep...

Fri, 29 Apr 2011 19:38:53 +0000

Hello,
I am using COMSOL 4.1 and I am facing a conceptually simple problem with no luck. Basically I have a simple 2D Eigenvalue Problem which gives me 20 eigenvalues (and 20 eigenmodes) and a Parametric Sweep with 50 parameter values. So, in a nutshell, I have 50 outer solution and 20 inner solutions.
Now I need to estimate a surface integral of a product of two variables through the derived value tab. The only problem is that I need the two variables to be picked from two different solutions (different parameter).
I have looked at the with operator but it lets you choose one inner solution (of the 20 in my case) for a FIXED outer solution (parameter of the Parametric Sweep).
Is there a way to combine these two solutions so that I can define a product like U(outersol(3))*U(outersol(10))?

Thanks very much.

Claudio

Internal Damping in Transient Solid Mechanics

Mon, 04 Apr 2011 06:14:38 +0000

Hello All,

In Comsol version 4.1, I would like to know how we can assign Rayleigh damping or loss factor damping coefficients to the material properties in the Solids Mechanics while performing transient analysis, like we used to do in 3.x versions.

Thanks in advance,
Onur