Definitions and operators - Latest Discussions - COMSOL Forums

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

HOW to integrate between two boundaries

Fri, 18 May 2012 13:28:40 +0000

hi david, i am fatemeh, my Previous account dont work so with this account comment for you :)

thank you so much for your attention.

my model is a layer of a fuel cell that i have on a boundary influx that a chemical reaction accure in this layer.
i want to calculate the total flux over this layer,ie i want to plot total flux over the x direction. when i choose 1D plot. comsol want me to specify a boundary for it
when i choose top or down boundary, the flux is not continuous.
so i want to calculate the total flux for each x and all y amounts related to this x and then plot the average of y according to x
because i think with this idea total flux according to x direction will be continuouse.

i hope that i could explain my perpuse.
do you have any idea?

thanks again

HOW to integrate between two boundaries

Fri, 18 May 2012 06:47:14 +0000

hi david,

i have question and hope that you can help me :)

i have a 2D geometry and i want to calculate a parameter such as flux in the direction of X.
I want to integrate all values of Y for each X and calculate them for n values of X.
I refered to integrate section but in this section we can choose just boundaries while i need to integral for all values of X, not just boundries!!!

i use comsol 4.2 and i dont find your direct that propose ' option- ...."

thanks a lot

analyze movement of particles in ACDC

Thu, 17 May 2012 12:22:26 +0000

Hello everyone

I have a question and I wondered if anyone can give me some info.
I am using AC/DC module right now. However I want to add movement of A charge in my model and analyze the effect of its movement on electrical field of whole system.
Do you think it is possible to do these kind of simulations by using time dependent analysis? Because I have checked different models here that use time dependent analysis but their goal was totally different from mine.
I think in order to do this I should have access to particle tracing module. Am i right?
I would be really grateful if someone can give me some information or give me a link of example related to this.

Best
Bahar

What does the c in chcs.c_wH2 mean?

Wed, 16 May 2012 11:43:46 +0000

I know chcs refers to the transport of concentrated species module and wH2 is referring to a set parameter but, what does the c_ mean?

defining a variable for each element

Tue, 15 May 2012 08:47:01 +0000

hi ,
i would like to know how can i define a variable (which i am using in calculating other properties of the material) such that it is solved for each element separately ?
Right now what COMSOL does is solve for a time step and use that value to calculate material properties for all elements together , whereas i would like COMSOL to solve that variable for all elements differently (much like temperature in heat transfer) in each time step.

- Hirendra

Accessing solution of one study into another one ?

Mon, 14 May 2012 13:29:30 +0000

Hi,

I been modeling hyperelastic material models in comsol which revolves around three different studies on the same geometry, with different boundary conditions. I would like to access results from the second study to give input to some of the variables in the third study. Well, i could use the second solution as initialization to third study, but i want to access, for example, shear stress at every point in the material domain, which in turn affects inelastic deformation gradient matrix in third study.

Any one has any idea how to do this ? Any help is greatly appreciated !!

Thanks !

/Krishna

boundary settings- Thin layer (surface thickness)

Fri, 11 May 2012 17:28:17 +0000

Hello everyone

I have a question about settings for boundary conditions. In most of physics, when you use 3d and you want to define a boundary condition like dielectric shielding for one surface there is one option called "Thin layer".
For this "Thine layer" you should define "Surface thickness" which is by default 1m.
My question is that what exactly this surface thickness means? because it seems it can affect my results a lot.
I know it is not thickness of my domain (if the domain is a cube with dimension of 0.1 um). So what exactly is this? I went through some tutorials and i couldn't find any explanation.

Best
Bahar

Metal-Insulator boundary

Thu, 10 May 2012 12:39:16 +0000

Hello everyone

I have a question about boundary conditions.
I have 3 layers (Insulator-metal-insulator) and this structure is placed in a solution that i want to sense its voltage with that metal layer.
Insulators are SIO2.
The problem is that when i choose dielectric shielding for insulator boundaries, the voltage on the metal, insulator and the solution will be all the same (and i was expected that it should not be the same on insulator layer!)
So i thought may be i should choose ground for insulator (!) and then the problem is solved. I have zero on insulator, different voltages on metal, and in solution.

But i beleive there is something wrong and i shouldn't work like this. Anyone has any idea?

Best
Bahar

Divergence of gradient of a variable calculation

Thu, 10 May 2012 09:04:52 +0000

Dear Support,
I need to calculate, in the post-processing stage, the divergence of the gradient (it is the laplacian) of a known variable, in a specified volume.
May you help me, please?

Enquiry on Livelinks for Matlab

Mon, 07 May 2012 11:26:32 +0000

Dear Mr Lars,

I'm using COMSOL 4.2a in my lab, and I have no problem with simple examples in extracting data from COMSOL to matlab.
The thing is, when I try to use a more complicated model, I just cannot get any values of any variable!!!
I posted it on this page: http://www.comsol.dk/community/forums/general/thread/28413/#p77051
The mph and m file is also attached there.
Could you please spend some time to help me out？
I'm looking forward to your reply.
Thank you very much!

Best regards,
Di Kai

electro-thermal analysis of micro-heeater

Mon, 07 May 2012 04:07:32 +0000

sir,when i tried to simulate it it is showing some errror, and i could not find the error .can u please help me to solve this structre..

Floating Potential

Wed, 02 May 2012 18:35:25 +0000

I have a question about floating potential condition. I have 2 metals (like electrodes) which are not directly connected to voltage source and i want to use them to measure the signal (or voltage variation in my medium). According to some of the posts here i should use floating potential option as my boundary conditions for these metals. Is that right?
And if it is correct, then what should i assign in its terminal charge value section (in floating potential option)?

Best
Bahareh

Could you please tell me the name of the variable for time steps in comsol?

Thu, 26 Apr 2012 19:21:37 +0000

Hi

I am working in comsol 4.2a version for hydrogen storage system modelling. Could you please tell me the name of the variable for time steps in comsol?

calling variables in COMSOL

Sun, 08 Apr 2012 00:38:20 +0000

Hi,

I am creating a heat transfer model. I have imported a file as a parameter under Global Definitions, but COMSOL automatically labels the first column t and the second column f(t). In reality, the first column is actually T, temperature.

I am mainly confused because I extract data for the properties for thermal conductivity and Cp. I want certain values to only be used during specific temperature intervals. For example, I have imported files called, Cp_material1, Cp_material2, Cp_material3, k_material1, k_material2, and k_material3. Each of these files have the first column referring to temperature (not time) and the second column referring to the material property. When the temperature is below 200K I want the model to be using properties for material1, when the temperature is between 200-500K I want the model to be using properties for material2, and when the temperature is above 500K I want the model to use properties of material3.

I currently have something like this for k: k_material1(t<200)+k_material2(t>=500)+k_material3(200<=t<500).

What is COMSOL referring to as t and T? Should the expression above be using T instead of t?

Thanks

How to define a surface charge that depends on electrostatic potential (stationary solver)

Fri, 21 Oct 2011 17:08:23 +0000

Hi There,

I am using COMSOL 4.2 and I have a 1D electrostatic model. At one boundary, I have a simple fixed potential boundary condition, which is no problem to implement. At the other boundary, I would like to define a surface charge density that depends on the potential at that same boundary. For example, it would be similar to the following equation: Q[C/m^2] = Q_0*exp(-phi/E_A). I would then like the solver to calculate the appropriate value of phi (electric potential) at the boundary and elsewhere in the model, based on the constraint between phi and the surface charge defined at the boundary (which depends on phi).

Does anyone know how I can do this? It seems there should be an easy, straightforward way to define a self-referencing boundary condition.

Thanks in advance,

George

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.

How to define two surface reactions having same flux or reaction rate?

Thu, 12 May 2011 13:10:40 +0000

Hi Everyone,

I am using Comsol 4.1 doing a simple simulation on a 1D geometry. There two simple reactions are happening on both end of the line. On the left, the reaction is A to B. On the right, the reaction is B to A. The reaction rate constant is different though. Whenever reaction A to B happens on the left, the reaction B to A will happen on the right. I just simply using the Transport in Diluted species to have the reaction happen. However, I do not know how to make the connection between these two reactions.

This has bothered me for a few day. I am new to Comsol. Anyone can help me out with this?
Really appreciate.

Thank you in advance,

Jay

128° YX Lithium Niobate

Tue, 22 Mar 2011 18:58:45 +0000

Hi,

I´m interested in doing and eigenfrequency analisis of a SAW based on the model "SAW GAS SENSOR" available for COMSOL 4.1 an 3.5a. Mi interest is to obtain free velocity. If I simulate with YZ-Lithium Niobate, I get a velocity of 3425 and the reported one is 3488. This result is acceptable. But, when I try to simulate 128° YX Lithium Niobate, I shuould get a velocity of 3996 and Im not even close.

For the simulation I've used a rotated system, based on Euler Angles (0,38°,0). I've calculted the elasticity matrix, coupling matrix and permitivitty matrix taking into account the transformation available in the COMSOL userguide (Page 158). I include the matrixes obtained:

{2.02897e+011[Pa], 6.99850e+010[Pa], 1.93975e+011[Pa], 5.78425e+010[Pa], 9.03296e+010[Pa], 2.21158e+011[Pa], 1.28464e+010[Pa], 9.31243e+009[Pa], 8.00285e+009[Pa], 7.53232e+010[Pa], 0[Pa], 0[Pa], 0[Pa], 0[Pa], 5.68597e+010[Pa], 0[Pa], 0[Pa], 0[Pa], 0[Pa], -5.09155e+009[Pa], 7.79193e+010[Pa]}

{0[C/m^2], -1.88047[C/m^2], 1.71492[C/m^2], 0[C/m^2], 4.44672[C/m^2], -2.69210[C/m^2], 0[C/m^2], -1.52214[C/m^2], 2.31358[C/m^2], 0[C/m^2], 0.06738[C/m^2], 0.63381[C/m^2], 4.47243[C/m^2], 0[C/m^2], 0[C/m^2], 0.27875[C/m^2], 0[C/m^2], 0[C/m^2]}

{43.6, 0, 38.1267, 0, -7.0055, 34.63333}

My question is, do I have to include any additional consideration besides a rotated coordinate system so I can have an adequate result for this material? Does anyone know where can I get the real matrixes for this rotated material?

volume change

Thu, 24 Jun 2010 17:30:16 +0000

I have a simple question,

I want to calculate the change in the volume of a deformed body . I am trying to get it using the boundary integration of the total displacement in the post processing option. but i couldn`t be sure whether it is really representing the change of the volume or not. or if there is any other way, please inform me.
tnx in advance