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variable determined by integral formula

February 13, 2014, 12:36 pm
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Normally when people talk about integration, they refer to coulpling variables, which could be integrated over space or time.

In my variable definition, I have a variable determined by a integral formula.
For example, one varialble is the integration of f(r) over r with up and low limits, r can be particle size or whatever, other than space coordinates or time.
I tried to search from the website for information, but still have no idea to figure this problem out.

This qustion was also posed by others 1 year ago. Please refer to:
www.comsol.com/community/forums/general/thread/28492/

Could someone give me some nice advices? I will appreaciate a lot for your answer!
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Issues calculating magnetic field using multi-turn coils

March 10, 2014, 9:18 am
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The end goal that I have is to produce a magnetic field map for a superconducting magnet. In my first attempt, I tried to model the magnet coil packages (there are 11 of them) and the result was approximately 0.5 Tesla lower than the measured map and the calculated map from the manufacturer (it should be 5T maximum, but comsol gives something like 4.5T).

I then tried to model a simple system to see if I get an answer that I expect, so I tried Helmholtz coils. I again got a result that was approximately 10% low for the field at the center between the two coils. I then thought that perhaps I was doing something else wrong and tried to use the Helmholtz coil tutorial, but found that it produced a similar result (10% low).

The entire helmholtz geometry is contained inside a 1m sphere with 20cm coils. The sphere is centered on the center between the coils. I added a 10cm thick layer to this outer sphere and then added an infinite element domain to this layer. I re-ran the simulation and the result was once again 10% low. Is there something wrong with my implementation of the infinite element that prevents it from being correctly interpreted?

Thanks for your help.
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PML width for Longer wavelength

March 20, 2014, 4:27 am
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I generally use the wavelength(in case of shorter wavelength)as the starting PML width. This does not create a large geometry to solve, saving some memory. But, my recent models have longer wavelengths. These are several magnitudes higher than the resonators actual size which I am modelling. Creating a PML as large as the wavelength does not make any sense. What could be a better way to find a starting PML width?

Thanks.
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post processing parameter sweep?

March 27, 2014, 6:46 am
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Hi

Is it possible to run a sweep of surface integrations on a set of parameterized surfaces that are defined

as data sets? More convenient than doing it one at a time. I guess I could define a set of surfaces in a parametric

sweep before running the model.

thanks Dave
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step function

March 28, 2014, 12:18 pm
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Hi,

I am trying to simulate heat transfer problem. I have the values of voltage V_in and Watt density Q_in of the heat source and the heat source is attached under a lid. Now, I want to assign a step function such that when I apply current to the heater, it gets heated to certain temperature Tmax and at this particular time I would like to turn/switch off the current supply, so that the heater temperature goes down eventually. I just want to check the temperature distributionon over the lid to which the heater is attached.

Please let me know how to go about it.
Thanks in advance!!
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Suggestion: Membrane Vibration Simulation

March 30, 2014, 10:26 pm
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I am planning to build a simulation which is about the thin ( 30~50 nm) membrane mechanical vibration due to the electromagnetic radiation pressure (EM force on the membrane surfaces). I am just learning COMSOL Multiphysics 4.3b. I hope some of you recommend me a suitable module for my system. I have planned to use electromechanics(emi), however, it seems to hard to generate EM (TEM) field with this module. I hope someone will suggest me a useful module, thank you.

Abdu
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Heat transfert in a bounded medium

March 31, 2014, 12:06 am
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Hi all,

I work with the Heat Transfert module (comsol 4.2). My work consists to find the temperature of a small sphere of radius R = 10 cm and made of plastic embedded in air or water. The convection and radiation are take into account. The initial temperature of the sphere is 80 °C and the temperature of the external environment is 20 °C (when air) or 50 °C (when water)

These are my settings : Heat Transfert (ht) / Heat Transfert in Solids
The only geometry is a sphere of radius R = 10 cm.

- Solid enbedded in air
Materials (properties of the solid) :
k = 0.1 W/(m.K) (Thermical conductivity)
rho = 100 kg/m^3 (Density)
Cp = 1000 J/(Kg.K) (Heat capacity at pressure constant)

Initial values : 80 °C (initial temperature of the sphere)
Convective cooling : h = 2 W/(m^2.K) (heat transfert coefficient defined at the boundary of the sphere)
External temperature : 20 °C (temperature of the external environment when air)

I do not understand...Never, the properties of the external environment (air in this case) are defined. Therefore, I cannot to do it when the solid is embedded in water (or an other liquid). Also, when the external environment is bounded (for example when the sphere is embedded in 1 m^3 of water or 1 m^3 of air...), I cannot defined the heat transfert coefficient at the surface of the sphere since COMSOL "forces" me to define it only at the surface of the external environment.

Have you an idea to take into account the properties of the external environment and/or the finite size of this environment ?

Thank you !!









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diffraction orders for 2d nano pillar grating structure in comsol 3d modeling

March 31, 2014, 2:54 am
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Can you help me in this regard?
I would like to find the electric filed intensity of reflection(or reflectance) from gold nano pillars from gold substrate, with the consideration of diffraction orders in case of reflection((1,1 ), (1,0 )reflection grating orders), for the different wave lengths starting from 200 nm to 1000 nm, for a fixed angle of incidence (normal incidence, or 30 degree angle of incidence)

I had made a 2D grating structures in 3D modeling Wizard of Comsol 4.4 version, While I am putting Diffraction orders( m=0, n=0 ; m=-1, n=0 ; m=+1, n=0) with "port reference points" as diagonally opposite end of Unit square cell with periodic boundary conditions on all four sides( by doing copay face). Unfortunately it gives me zero electric filed while i am running., If I run the same model without diffraction order it gives the result but only from one order diffraction (normal reflection) . Since I need to calculate reflection from at least two orders (0 th order and 1 st order) I have to include those diffraction orders to my model

Can you help me in this regard?
can you tell me how to put diffraction orders (with proper port reference points)in 2d grating on 3D model?

Also How can i implement PML layer over the port ( or it is really needed ), since if i add PMLl layer on the top of port, selecting port reference point is declined by default. I follow the pyramidal absorption model for making the infinite array of my 2d Grating nano structures( there a PML layer added over the port)

the plasmonic wire grating model available in Comsol library is just a 2D model , so we can't able to follow the same thing in here in our model.

I used this blog for making port reference points and putting periodic ports
www.comsol.co.in/blogs/modelin...tic-waves-periodic-structures/

Here I am attaching the COMSOL model named as 2port_modified_Au_nanocylinder.mph , please go through this model ( it do not giving expected result)

Also another pdf attachment in which the model ( result by FDTD) which i am trying to reproduce it by COMSOL FEM also attached with this mail.

In this paper figure-2 (a) is the graph which i am trying to reproduce it. also in the , figure 2.b is they component magnetic filed intensity in y-z cross section for separate modes, figure 4 is the electric filed intensity which is corresponding (1,0) grating order ((1,0))plasmonics mode) for peak

Please help me in this regard..
Thank you


Aneesh M Joseph
Indian Institute of Science, IISC
Banaglore
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Flow rate integration over a period

March 31, 2014, 3:18 am
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Hello,

I am using 2D to model periodic flow through an open boundary with zero normal stress. I have added an integral over the surface of this boundary, as well as a global probe that reads "intop1(u) *z", where z is the hypothetical depth of my (rectangular) device. What I am trying to do is find a way to integrate the results of the probe over time so as to get a net flow rate over said boundary. Is there a way of doing this, given that the results of the probe are not produced in a set time step since they depend on the solver's time steps.
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How to get deposition rate of particles in fpt module? Or should I use an auxiliary Dependent ?variable

March 31, 2014, 3:49 am
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Hi
I have been doing a model using the "fluid flow" and "particle tracing for fluid flow "module. A bell type insulator is placed across the flow channel. I have implemented the boundary condition to stick , so whenever a particle hits the insulator, it get stick with the insulator wall.
Now, If I want to calculate how many particles are stick with the insulator wall by the time I'm interested and the rate of particle deposition from the result section, how I am supposed to do that?

Thank you.
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Desalination in electrodialysis - constraint force expression

March 31, 2014, 5:23 am
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Hello,

I have one question about the manual "Desalination in electrodialysis" for Comsol 4.4 - I tried to simulate it with version 4.3b.
The boundary conditions at the electrolyte-membrane interface are defined using Pointwise constraints.
The constraint expression field is mostly clear (phil-phil_m = Nernst(E0, T, aNa/aNam)). What does the constraint force expression mean? In "Pointwise constraint 2", the force expression is phil-cCl/F_const-phil_m, although phil and phil_m are potentials (volts) and cCl/F_const is a charge (1/As). On the other hand, in "Pointwise Constriant 1", the force expression is just phil-phil_m (without any term containing cNa).

thank you and best regards
Carsten
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definition of boundary condition

March 31, 2014, 5:47 am
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Hi all,

How should I define a boundary condition which will classify two possible or more boundary condition to right and left hand boundaries of a rectangle in a 2D to create a cross section like attached picture?
(except of considering constant value)
I also attached my project.
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Effective Young's modulus used by comsol for simple uniform loading

March 31, 2014, 10:40 am
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Hi,

I have some fundamental problem with the solid mechanics problem. I have a square and would like to exert uniform load on the top surface. The simulated results give the correct y-stress (which is expected since this is y input), but the y-strain obtained is slightly smaller, which means that the modulus used is not the same as the material input young's modulus.

Hand calculated: y-stress:26250Pa ystrain: 8.47e-6 given E=3.1e9Pa
Comsol: y-stress: 26250Pa ystrain: 7.43e-6

Could it be that my boundary conditions are wrong and over-restricting the model. I have checked the Young's modulus after simulation and it is the same as the input value. Also, the strain in the x direction is wrong as I would expect it to be y-strain multiply by the poisson ratio, but it is not the case too. I have use a point fixed constraint at the bottom left point and also a "roller" for the bottom line.

Please advise. Thanks.
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how can I plot the resistance(x) vs voltage(y) relation in 1D without error

March 31, 2014, 12:18 pm
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I am trying to plot the resistance(x-axis) vs voltage(y-axis) 1D relation by fixing the vibration frequency at 72Hz ,but it gave me error: "Failed to evaluate variable "

I have added 2 physics : AC/DC circuit and piezo-electric device

I have set up "Parametric sweep" of resistor from 1e5 to 1e6 ( 10 steps)

I have added frequency domain study by fixing the vibration frequency at 72Hz and select fully coupled

I am not sure if I set up studies correctly , should I add "stationary study" after "frequency domain" study?
or should I use "frequency domain model" study?

I am confused
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Global Equations defined variable has singular matrix problem?

March 31, 2014, 1:00 pm
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Dear support,

Does anyone ever meet the following error when using global equations to define a variable as source? I am trying to simulate in 4.2a the enclosed volume of fluid following the COMSOL Blog: www.comsol.com/blogs/computing-controlling-volume-cavity/
Following the guideline in this blog, I can simulate incompressible fluid by using global equations to define the inner pressure in 2D, but the error comes out when I translated it into a 3D model.

Could anyone please give some hint on possible reason? Thanks.

****************************************
Failed to find a solution.
Singular matrix.

There are 1 void equations (empty rows in matrix) for the variable comp1.Inner_pressure.
at coordinates: (0,0,0), ...
Returned solution is not converged.
****************************************
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Help with free convection

October 22, 2010, 6:25 pm
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hi everyone

I'm quite new to comsol modelling, and here I've run into this problem trying to model a free convection in air
being a test model the geometry is simple, just a heated plate surrounded by air, which is subjected to a buoyancy force.
the problem is this error message" failed to evaluate expression" every time I try to solve it using constant damping, but when I try automatic damping it cannot converge, maybe I linked the two physics wrong?
I've attached my model here, would be much much appreciated if anyone would look into it and tell me what's wrong..

best regards
Eric
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ERROR: Cannot open native files

March 31, 2011, 10:13 pm
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hi,
version: 4.1
error: cannot open native files

This popup warning happened usually these days, and i found the solver related to this in the comsol knowledge:

www.comsol.com/support/knowledgebase/1095/

But now, i have no access to the web page, could you please give me your hand?
Thanks.
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Difference in results of a model with 3.5a and 4.2a versions

January 4, 2012, 8:56 pm
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Hai,

I have tried to model my problem using PDE module in both COMSOL 3.5a and 4.2a versions. It have 4 dependent variables. I got reasonable results using 3.5a and verified them analytically. When I tried to do the same model using 4.2a , I am getting different results. I have difficulty in finding the reasons for this behavior. Although results are following the same behavior qualitatively, their quantitative values were different. Analytical verification tells me that results were not correct.

I used Identity pairs(IP) and periodic boundary conditions(PBC) in my model.

I suspect that problems with 4.2a as follows.

a). PBC doesn't override the default zero flux condition, it only contributes with it.

For this, I applied a 'continuity' using IP at both boundaries, so that it overrides the default zero flux and contributes with PBC. Results has not changed.

b) Bondary conditions using IP:
In 3.5a, boundary conditions using pairs shows like n.(gamma1- gamma2)= *. If this * equals to 0, it means continuity at given IP. If * equals some value, it means some jump in flux is there at that boundary. (For eg. withdrawing current at electrode).

But in 4.2a its showing n.(gamma)= * instead n.(gamma1- gamma2)= *, I have no other option then to give the same value for * as in 3.5a. It could be the one reason for other results.

Moreover, I have tried linear extrusion coupling parameter alternative to PBC, still I am getting the same results.

I believe that all my inputs are correct and verified many times.

The reason I came to conclude that the problem is with new version is I tried the 3.5a model file with 4.2 and i have some other results than 3.5a, but I remodeled everything new in 4.2a, I am getting the same results as upgraded results, not the results obtained in 3.5a.

It would be great help for me if someone throws light on my issues.

Thank you
Ganesh

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Material property dependent on solution variable

February 7, 2014, 3:45 am
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Hi,

I'm trying to solve a 1D flow problem with Darcy's Law. The permeability of my material is supposed to be dependent on the pressure gradient in the medium. So I would first have to create a study to solve for the pressure gradient in the medium and then use the solution from the first study as an input to the second study, which calculates Darcy's law right? How would I go about setting up a study that only solves for the pressure gradient in the medium?

Thank you!
Alexandra
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Importing One *.mph geometry into another *.mph file

March 17, 2014, 8:10 am
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Hi All;

I want to import *.mph geometry into another *.mph model.

Example: You create a model for single phase flow. Now you want to use the same geometry for two phase flow model. You do not need to spend time to create the same geometry into another *.mph file.

How can you get the geometry from the first file and use it for another practice model. Instead to create the complete geometry again.

Thanks
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