Latest Discussions - COMSOL Forums

open java model

Wed, 23 May 2012 19:51:57 +0000

How to make someone without Comsol to see a Comsol model simulation? I don' t arrive, for instance, to open a model saved as .java file. Can anyone help me ?

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i46

Urgent : Which stress should i use ?

Wed, 23 May 2012 19:40:45 +0000

Hi everyone plz help , this is urgent.

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

Newton's law of cooling

Wed, 23 May 2012 19:11:04 +0000

I am trying to model a cilinder at 200 ºC that its cooling following the Newton's Law of cooling.

What I want is to model how the temperature decrease with time. I need help., Someone can help me to model this?

Thank you.

Noise due to vortex shedding around a cylinder

Wed, 23 May 2012 18:47:36 +0000

Hi all,
I would like to analyze the noise generated by the vortex shedding around a cylinder immeged in a flow.
I've already done some 2D Laminar flow and Turbulent Flow time dependent analysis in Comsol to determine the flow field around the cylinder at different inlet velocity.

Now the problem is to use the results of these analysis to compute the acoustic pressure field at a certain distance from the cylinder: do I have to use the Aeroacoustic module? And how do I insert the CFD flow field as input for my acoustic analysis (I mean... if it is possible...)?
I already tried to insert the "u" and "v" or "mod1.u" and "mod1.v" variable into the "MEAN FLOW VELOCITY" section in "Aeroacoustic Model 1", but it seems that nothing happens...

Thank you for your suggestions... I hope this discussion could be useful for other Comsol users

Ps. I put in attach to this discussion an image of the geometry and flow field in exaam

contact discretization linear vs quadratic

Wed, 23 May 2012 18:39:00 +0000

I am trying to solve contact problem. And what I found on this forum, someone suggests that sometimes it is helpful to change displacement discretization from quadratic to linear with still contact pressure linear. But as I know, with same meshing size, the displacement of model is different between linear and quadratic setting. And I test with several combination like the following this:

finer mesh linear case, extreme mesh linear case, fine quadratic and finer quadratic.

All results are different.

I am very confusing about this.

Is there someone to explain about this?

Thanks in advance.

No speedup when using distributed memory cluster

Wed, 23 May 2012 17:54:43 +0000

I wanted to see if someone could help me determine why I am not seeing any speedup when I submit jobs on multiple nodes of my HPC cluster (using the distributed memory capability).

I understand that speedup is highly model dependent. Per the suggestions of previous discussion threads, I have tried numerous different models with different physics (both linear and non-linear problems). Also, I have tried large memory models and small memory models (also per previous thread recommendations). Below are my results using the COMSOL Model Library Example "Micromixer Cluster Version":

Micromixer_cluster.mph (mesh as given):
1 node; 1 proc.: Run time = 123 sec
1 node; 12 proc.: Run time = 38 sec
4 nodes; 12 ppn: Run time = 99 sec
8 nodes; 12 ppn: Run time = 223 sec

I see speedup when I go from 1 proc. to 12 proc. running on 1 node (shared memory), but I don't see speedup, in fact I see slowdown, when I try to distribute the job across multiple nodes (distributed memory). The step-by-step instructions said to try refining the mesh for better speedup (this was also COMSOL support's recommendation). Here are the results for 2 different refined meshes:

Micromixer_cluster.mph (refined mesh):
1 node; 1 proc.: Run time = 566 sec
1 node; 12 proc.: Run time = 130 sec
4 nodes; 12 ppn: Run time = 259 sec
8 nodes; 12 ppn: Run time = 501 sec

Micromixer_cluster.mph (super-refined mesh):
1 node; 1 proc.: Run time = 1169 sec
1 node; 12 proc.: Run time = 414 sec
4 nodes; 12 ppn: Run time = 614 sec
8 nodes; 12 ppn: Run time = 896 sec

Still no speedup. Only slowdown.

Has anyone seen any speedup on this COMSOL Model Library example? If so, I'd be interested in your results.

One thing that I am doing that is different than the COMSOL recommendation is submitting jobs through the command line rather than through the Desktop. Does anyone know why COMSOL recommends submitting batch cluster jobs through the Desktop and not through the command line? Could this be my issue?

Any help would be appreciated.

Plane wave propagation inside a duct (Vibroacoustic)

Wed, 23 May 2012 17:42:17 +0000

Hello everybody,

I me dealing with an interesting vibroacoustic problem and wanna model it WIth COMSOL.

The subject is : A duct (Air inside)of one meter length and cross sectional 0.2m*0.2m (width=1m)
Boundary conditions : unit velocity at one end
zero velocity Boundary condition is applied at the other end (rigid wall)
The 4 lateral sides are assigned rigid wall.

Frequency = 100hz.
Material (duct) = Any (wood, alu,etc...)

My questions are :
- COMSOL doesn't contain Velocity boundary ? (Drive me mad)
- How to handle that with COMSOL ?

Your help would be very appreciated .......
Thanks in advance.

reactions, concentration and fluxes

Wed, 23 May 2012 17:21:20 +0000

Hi friends,

I have a problem with flux boundary condition. I am using the electrolyte-electrode boundary interface as the boundary condition. The flux of ions is proportional to the current passes at electrodes.

What is surprising is that when the concentration of leaving ion reaches zero, comsol just continiues the flux condition without stopping that. so we have negative concentrations. Does it mean we cannot model diffusion-controlled kinetics with COMSOL?

Does anyone know a way to avoid this problem or to limit the current at certain conditions?

King regards,

Mohsen

Using FMincon with Comsol

Wed, 23 May 2012 14:48:44 +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 14:09:02 +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 13:43:40 +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 11:31:15 +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 08:56:18 +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.

Problems with S-parameter calculation

Wed, 23 May 2012 03:01:36 +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!

problem in meshing

Tue, 22 May 2012 15:54:39 +0000

Hi Ivar,

I am trying to solve a model where i get this error:
Failed to respect boundary element edge on geometry face.
although i mesh the geometry in parts and using finer mesh in critical areas but still it doesn't work? Do you know what is causing this error in meshing?

Thanks,
Rabia

Trouble coupling heat transfer physics for both fluids and solids

Wed, 25 Apr 2012 09:19:20 +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 11:04:17 +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 19:49:08 +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

Thu, 13 Oct 2011 02:34:30 +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 18:14:58 +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.