Latest Discussions - COMSOL Forums

Periodic boundary conditions

Tue, 22 May 2012 02:55:47 +0000

Hi, all

Does any one knows what are the suitable boundary conditions for heat transfer scenario applied at the inlet and exit of a domain when the boundary conditions for turbulent kinetic energy (k) its dissipation (e) and axial velocity (w) are set as periodic conditions. Note that the temperature of the fluid always increases in the stream wise direction if a constant heat flux acts at the wall of the pipe. Since periodic conditions are used to reduce the computational burden only a small segment of the domain is modeled in this case which means we cannot set a Dirichlet boundary condition for the temperature at the inlet and exit.

Thanks for any sort of help

How to apply diffusion coefficient of PDE at node point

Tue, 22 May 2012 01:46:59 +0000

I am working on an inverse problem solving.

1)I used coefficient form PDE for forward solving
2)Obtained the solution in matlab and calculated new updated diffusion coefficient at each nodes
3)Now I want to apply this value of diffusion coefficient the nodes and solve the PDE again.

Please help me in solving this problem

Stationary and Time-dependent study for Joule Heating

Mon, 21 May 2012 20:14:35 +0000

Hello,

I am working on a rather simple simulation of Joule Heating on a Pt resistor deposited on AlN substrate. Although I can perform the stationary study and have some of the results I want, I cannot perform a time-dependent study. The execution fails with error message "Failed to find consistent initial values. Last time step is not converged.".

Any ideas what might be wrong?

Cheers!

-O.

step function in diffusion equation

Mon, 21 May 2012 18:21:48 +0000

Hello,

Can anybody help me !! I am solving diffusion equation in 2D geometry, initial concentration in domain = 0, I gave Dirchlet boundary condition with some constant number. I am trying to look at the phenomenon of stuff diffusing into the geometry. When I solve I see some negative values for first few seconds and read that it can be corrected using STEP FUNCTION.
Please can anyone tell me how to implement step function in V4.2a!

Regards
Harkirat

What are emw.wee1.input.Ex and model.input.E1 in Electromagnetic Waves of 4.2a

Mon, 21 May 2012 14:45:14 +0000

Hi all,

I'm trying to figure out the equation system there. Among all the variables, I don't know what these two mean. Since we already have emw.Ebx as the background field, what are these two?

Thanks a lot!

Definition of convective heat transfer coefficient

Mon, 21 May 2012 09:41:58 +0000

Hi,

I have modeled phase change process (PCM) coupling with forced convection boundaries (air). Now I'd like to define and draw variation of convection heat transfer coefficient along the air channel. But I can't find it.

Could you please help me about this problem?

Kind regards,

help on structural mechanics

Mon, 21 May 2012 08:16:54 +0000

How do you put the following expression on comsol global definitions : Awcos[wt]

Rectangular pulse train

Mon, 21 May 2012 03:35:02 +0000

hi Ivar ,
i know this topic has been discussed on this forum but i still have a problem.

i want to generate a rectangular pulse train of pulse width = 28e-9 , frequency = 1 Hz

i used the rect function along with mod function to get an analytic function an1= rect1(mod(t[1/s],1e-9))
but when i plot this for time t = 0 to 1 sec i get zero magnitude , after solving i also found out that heat flux is zero throughout the time . I also tried 'make periodic' option but still not getting the plot.

As discussed earlier on some other thread , i made sure that the solver is selected as strict but still no luck .

i have to model 2 pulses each of 28 ns width in every second (t = 0 to 2 sec ) please help me with this .

Hirendra

How can I build the structure with sinusoidal-shape interface in comsol?

Mon, 21 May 2012 03:10:46 +0000

Hi! I am a novice and have a question that how to construct a geometry object with the sinusoidal shape interface,e.g.sinusoidal grating in comsol 3.5a? Thank you very much.Here is my email:hujigang@hfut.edu.cn.

Does COMSOL treat the nodes at the boundary also as unknowns?

Sun, 20 May 2012 05:53:18 +0000

Hi,

Does COMSOL treat the nodes at the boundary also as unknowns?

I am trying to solve colloid transport inside a pipe with constant concentration boundary condition at the inlet. The unknown is concentration. There is sharp gradient in the concentration between the boundary node and the node next to that in the flow direction. I use very fine mesh.
If I plot the concentration at the inlet boundary using domain plot parameters, it is showing whatever the boundary condition I have given (Dirichlet) . But when I use cross-section plot parameters dialogue box to specify the line plot and plot the concentration at the inlet boundary, it is showing wrong values of boundary condition. Also the plotted values at the boundary changes with time where as the original B.C. is constant. In the data exported also the values at the points in the inlet boundary are different from my boundary condition.

So does comsol interpolate from the results to find the value of variable at the boundary if we give the co-ordinates?

Does anybody know why is this problem happening?

Thanks in advance.

Regards,
Seetha

Temperature decline in autoclave model while rise expected

Sat, 19 May 2012 18:53:59 +0000

Hi there,

I've made a model for an autoclave with 2 blocks in it that should heat up during time. They however first cool down to like 200K and after they start to heat up again. That's not what i suspect and what is realistic. Since the model with results is to big i will upload only the model. It isnt that big so should be computable in short time;). (comsol 4.2a)

See the attachted image.

hope all is clear.

Thanks in advance!

RF - Implementing Perfectly Matched Layers (3D)

Sat, 19 May 2012 10:18:58 +0000

Hi all,

I want to simulate a 3D RF system infinite in 1 direction (eigenvalue analysis). I realize I need to use PML's, but am unsure on how to configure them. It seems that simply setting the scaling factor to lambda* isn't enough - anyone know if I need to configure the material itself in a special way? Does the size/permittivity/permeability not matter?

What about the "PML order"?

If it matters, this is a cylindrical structure infinite in the +z direction.

Thanks!

*Took that tip from here: http://www.comsol.com/community/forums/general/thread/25860/

P.S. one more related question: let's say I want to calculate the power radiating out of my model (i.e. in the infinite PML direction), by integrating over emw.Poavz. However, emw.Poavz is not in the list of available "quantities" (such as normE,etc.) in my results. What simulation must I do to get Poav for the resonant frequency?

how to cut or trim a parametric curve ?

Thu, 17 May 2012 14:31:32 +0000

Hi,

I'm trying to make a 3D coil using parametric curve and sweep, but I need to cut a part of my curve, I can't change it by equation, so i just need to cut a part of it. In solidworks we used to do this by trim. Any Idea how can i do the same in COMSOL.

Indeed is there any changes or difference in results, when we do 2D modelling on MEF module for coil design with 3D modelling?

Thanks in Advance

Airfoil in Air using FSI

Wed, 16 May 2012 22:31:14 +0000

Hi,

I am trying to compute an FSI simulation of an airfoil with a constant wind flowing from the right to the left with slip wall condition. The model will converge if the Re number is low (less than 10^5). As I increase my Re, the model will not converge. In additional, my inlet ramp flow velocity will need to be increase very slowly before it reaches constant. I am wondering if there is something wrong in my setup? Or, can FSI be used for high Re.

I am wondering if anybody can point me to the right direction.

Thanks a million in advance!

Ting

Question regarding a PID controller

Wed, 16 May 2012 13:36:21 +0000

Hello,

I have a question regarding the use of a PID controller. The idea is to control de power of a laser beam using as input the value of the temperature at the surface of the material. I have managed to do it by integrating the surface temperature over the interaction area and using a global ODE for the "I" term of the controller. This was done by setting the solver time step equal to the experimental measuring device integration time, and it worked fine. Now I have a slighlty different problem, where the time step must be smaller, so I cant tune it to the pyrometer (temperature measuring device) integration time. I would like to know if there is a way to integrate an expression, via an integration operator, but using a different time step than that of the solver.

Best regards!

Coplanar Waveguide Resonator with RF Module

Wed, 16 May 2012 08:45:06 +0000

I have only little experience with COMSOL and consequently I appreciate any kind of advice:

Eventually I would like to simulate a metallic sample on top of the resonator, separated by a slab of dielectric. Right now I am trying to get the S-parameters for the empty resonator (my model is attached, I am using COMSOL 4.2). I tried to keep close to the Coplanar Waveguide Bandpass Filter Example, just using my dimensions. However I am not sure what kind of ports to use (in 'reality', the signal comes in through a bonded aluminum wire) and also how to optimize the mesh in order to save resources. Also when I try computing the model with the current mesh, the program crashes after a while without any error message.

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.

Thermoelastic damping - nonlinear geometry - Jacobian variable problem

Thu, 19 Apr 2012 05:20:03 +0000

Dear all,

I try to do a modal analysis thermoelastic damping in 4.2 within a membrane spring. I adapted the model
www.comsol.com/showroom/gallery/1439/

and it worked quite well with the equation for the thermal source (even though I thought in 4.x it should be different variables)
-smsld.iomega*(rho*Cp*T+T0*smsld.Ent)

My problem now is that I need to include nonlinear geometry and I always get the error that the Jacobian variable cannot be evaluated.
- Modal analysis including nonlinear geometry but without the thermal source does work.
- Modal analysis including the thermal source but just linear geometry does work.

Does anybody have an idea what the issue might be?

Thanks a lot!
c

Acoustic Module Simulation

Thu, 12 Apr 2012 16:49:08 +0000

I am just wondering what would cause the damping effect on fluid-structure interaction simulation based on Acoustic and Structure modules. For instance if you run two simulations, one on air and the other on water. You'd see the higher damping effect from frequency response. You can even calculate the damping using Half Power Bandwidth Mehtod. When I run fluid-structure interaction simulation using Acoustic module without adding any damping, I could still see the extra damping effect of water compared to that of air. What caused this? Thanks!

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