Latest Discussions - COMSOL Forums

the domain of fluid

Fri, 18 May 2012 21:35:07 +0000

Hi,
I am using the Conjugate Heat Transfer Model to model a structure with water cooling. I imported the solidworks drawing file to COMSOL. My question is what is the easiest way to define the domain of the fluid for the structure? The fluid region wasn't previously defined and the shape of the cooling pipe is very complicated.
Please help. Thanks a lot.

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

Need help with Magnetic and Electric Fields Error

Fri, 18 May 2012 15:44:27 +0000

Greetings,

I'm trying to generate a model with a cylindrical anode, two metallic electrodes, and two magnets with the MER Module so I can model both the E and B Fields together so I can look at ExB drift effects. However, I keep getting this same error:

Failed to find a solution.
Singular matrix.

There are 213295 void equations (empty rows in matrix) for the variable mod1.V.
at coordinates: (-0.308496,-0.261123,-0.294355), (-0.299595,-0.278459,-0.287582), (-0.289647,-0.29545,-0.280739), (-0.288168,-0.272107,-0.304823), (-0.278712,-0.28882,-0.298166), ...
and similarly for the degrees of freedom (empty columns in matrix).
Returned solution is not converged.

I've double-checked all of my material properties and I've input the electric potentials using the Initial Values, but it's still having a hard time looking for the voltage (?). I'm not sure what boundary conditions I've missed to get a singular matrix and nothing's popping out at me. Does anyone have any ideas?

Live Long and Prosper,
JAY

P.S.: I can't attach my file because the size of the file is too large.

Continous first spatial derivative?

Fri, 18 May 2012 15:43:17 +0000

Hi,
I'm trying to model a chemical reactor. In my model I'm using a boundary PDE module, solving a concentration profile in one dimension.

Unfortunately, my solution is behaving very erratically. One of the terms in my model uses the convective coefficient for the forced flow of the fluid. I believe that since the first spatial derivative is not continuous between elements the convective term is causing 'bumps' in my solution. Each iteration makes these bumps worse and they eventually become very significant.

It seems like this would be a pretty common problem. Is there a known solution? I've tried a google search and I've turned up "Argyris elements" which have continuous first derivatives, but when I try them it get an error "Failed to evaluate variable Jacobian" "Variable mod1.CTx". Is there something else I should be trying?

mass transfer coupled with two phase flow (level set)

Fri, 18 May 2012 13:30:38 +0000

Hello Everyone,

I am trying to study mass transfer with two phase flow. My question is; how to implement mass transfer boundary conditions (no flux or finite flux) at the interface, where interface is defined as a level set function (phi =0.5). I have tried varying Diffusivity (D) as a function of level set parameter 'phi' such that it takes very small values in other phase but no success.

Please share your experience, if you are/were engaged in similar study. Adding mass transfer to two-phase flow problems is not supported in the current interfaces based on the level-set and phase field methods (as told by support).

Thanks in advance.

Greetings! Question regarding Heating Tissue with Magnets via particles inside said tissue (2D, 4.0a)

Fri, 18 May 2012 12:16:02 +0000

Greetings COMSOL users!

I have just started my endeavours with COMSOL 4.0 in 2D, and after reading some very interesting documents, examples and articles I consider hitting a wall to which I require some assistance.

My model aims at studying the influences a nearby magnet can have on tissue if we have several randomly distributed particles that are influenced (if any) by the magnetic field of said magnet.

So far I believe that I set up the model correctly with the use of the physics Magnetic, No Currents - I have a magnet, and a square nearby in which I have some circles of various dimensions and distance - I see some effect on them.

After this, I found interesting the library model RF Cancerous tissue heating (or somewhat to this nature), but I am unable to use the same means found there to get results here (basically since there is a different cause, the antenna that causes the loss of power) - while typing, I also came to the realisation that perhaps my model - Magnetic, No Currents is unfit for this process, since from literature the said particles should be able to be heated from using an alternating field, so perhaps a re-done model is in order.

However, my question is - are there any tips that I can get, to succesfully finish my model? Is the simple model in Magnetic, No Currents (attached) correct? Does it actually do what it claims to do, or just seeing what I wish to see? How can I do the alternating field thing, to influence the (magnetic) particles inside the tissue? Also, from the Heat Sources menu, I only get User Defined input, whilst in the example from the library there are other options selectable (I assume because of the other physics, RF - and my Magnetic, No currents physics gives no other option than to manually give the W/m3 number)

Thank you for any possible help on this matter!
Lucian

Stationary study failed to converge

Fri, 18 May 2012 12:06:45 +0000

Hello, I am trying to do a stationary of a MEMS device, but Comsol keeps failing to converge.

Some background on the model:
The model is a MEMS consisting of a plate attached to 4 thin cantilever beam springs, which are in turn attached to a fixed substrate. The plate is also attached to a set of combs whose teeth act as capacitors with a set of corresponding combs attached to the fixed substrate (see attached image. Everything not in purple is attached to the plate and should be free to move). A voltage is applied to the fixed combson one side of the device, while the plate itself is grounded. This causes the plate to be displaced until the capacitive force from the comb capacitors is balanced by the restorative force of the springs.

Comsol Physics setup:
I am using the Electromechanics physics node in Comsol 4.2a to do a stationary study on how much the plate is displaced. I placed all domains in Linear Elastic material model, fixed the boundaries shown in purple in the image, applied a voltage to the three fixed combs in the right of the image, and grounded the center plate. I then created a stationary study and computed. Unfortunately, the solver fails to converge. Is there something wrong with the way I have set everything up? Does anyone have suggestions about what may be going wrong?

Josh

Best Beginner Textbook for Comsol

Fri, 18 May 2012 11:23:12 +0000

Hello,

I am an aspiring chemical engineer in my junior year going for my Bachelors. This is the first summer I have free from taking classes and did not receive any internships so I want to learn how to use this program. I have taken classes on Transport Phenomena and my professor told me this would be a good program to learn in the field. I have not purchased a copy yet but I will try to see if my school will be able to get me a copy(of comsol). So back to the main question.. what is the best textbook to buy for a beginner like me? I have no idea where to start and I still have my transport phenomena textbook by Bird Stewart and Lightfoot that I can use for the math/physics in this program I believe. Thank you.

Ideal mesh for a regular grid export

Fri, 18 May 2012 10:09:42 +0000

Dear Comsol community,

I have a simple 3D electrostatic configuration of two separated point charges in free space. What I want to do is solving the poison-equation and exporting the data in small cubic region between the point charges on a regular grid.

What is the optimal mesh for exporting on a regular grid with about 201^3 points?

Since the error of the FE-method is minimal (vanishing) on the nodes of the mesh-elements I think it would be perfect to have a regular mesh with nodes exactly on the points of the regular grid. I found no way to achieve this yet and I would be thankful for any kind of help giving me a good exported result.

Designing Voice Coil Actuator with Flexures

Fri, 18 May 2012 00:54:41 +0000

Hi,

I am new to comsol and am trying to design a linear voice coil actuator by fixing each end to two diaphragm flexures, therefore keeping everything axisymmetric. I have designed the flexures with the structural mechanics and have placed loads and found eigenfrequencies. I am wanting to know how to incorporate a voice coil actuator to drive the flexures? I searched for some similar models without any luck. The ideal setup would be applying some step function to the voice coil and get the response of the system, and tune it with a PID controller. Any help whatsoever would be great as I am just getting used to the new software.

Thanks!

Fluid Shear Stress in Thin Film Fluid Module

Thu, 17 May 2012 13:49:04 +0000

Hello,

I'm currently working with the journal bearing model (Comsol 4.2) from the model library and I'm trying to calculate the shear stress within the fluid film with is an essential for the torque calculation. Unfortunatly the shear stress is not one of the output values of this module.

I tried the surface integration and using the simple formula:
tau = eta*du/dh
with dh = tffs.dh
and eta = constant value

I used du = d(tffs.uy,y)+d(tffs.ux,x)

Unfortunately the results don't seem logical. I believe my expression for du or my usage of the integration (under results, derived values) is wrong.

Please help!

Thanks,
Toan

simulation of eddy current damper

Wed, 16 May 2012 03:59:16 +0000

hi all
i am working on eddy current damper....in which beam (cantilever type) vibrations (free) are to be damped...the material of beam is aluminium....i have simulated the two electromagnets..but i am not able to simulate the beam vibration with initial conditions @t=0 displacement of free end is 0.003m and velocity =0. i have used the eigen frequency study but the tip displacement is very large as per our initial condition....when i used the time dependent study it gives an error of singular matrix...and one thing more should i use the moving mesh for beam...

plz help

i've attached the model...

Thanks and regards
Mohit

90 degrees phase shift

Tue, 15 May 2012 22:21:41 +0000

There are two inputs and two outputs to microring resonator, in that two inputs, one input i will give directly and i want ton give another input with 90 degrees phase shift ,how can i give

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

Temperature Sensor Model

Tue, 08 May 2012 21:11:51 +0000

Hello,

I am designing a temperature sensor using an interdigitated capacitor. Using the electrostatics modules, I was successfully able to design the sensor with the starting capacitance value I wanted. However, regarding changing capacitance with temperature is where I run into problems. Thus far, I have been using the joule heating and thermal expansion module. Can anyone tell me if this is the correct module for my goal?

I have had some measure of success using it. However I find that when I remove the surrounding rectangle with air associated with it in the positive z direction and silicon in the negative z direction, the study completes, but with that rectangle, I get an error of singular matrix/solution does not converge and cannot complete the study. Why is this?

Also, without the surrounding rectangle, and without the proper dielectric in place, it seems that I cannot calculate capacitance correctly, and the values are all very small, approaching zero.

I have attached my model.

I would greatly appreciate help understanding this, thank you.

microwave heating(waveguide)

Sat, 05 May 2012 00:48:55 +0000

Hello,

In microwave heating, I want to use port boundary condition, how can I calculate incident electric?
(I known input power, and frequency.)

Thank you!

Need some help in meshing, inverted mesh elements warnings

Tue, 27 Mar 2012 07:23:37 +0000

I have an FSI problem, by moving a membrane I would like to see the fluid move.

When I setup the simulation, I've played around with the settings, looked at how other models were setup, but I can't get rid of the 'inverted mesh elements' warnings.

Does anyone have any advice on this model?

Object rotation in a microwave oven

Sat, 17 Mar 2012 01:43:55 +0000

I am trying to model the temperature distribution in foods rotating in a microwave oven. My problem can be best described as a stationary food object experiencing a time-variant heat source distribution. Can such movement be done in a straightforward manner or is something more complex called for?

Thank you in advance for any help.

Dielectric heating between parallel plate electrodes

Fri, 09 Mar 2012 23:25:06 +0000

Hello all, I'm fairly new to Comsol. What I'm trying to do is model the heating of a dielectric in between two parallel plate electrodes at a frequency of 27.12 MHz. I would appreciate any suggestion as to which physics to use. Thanks in advance.

Stress gradient

Mon, 11 Jan 2010 08:25:59 +0000

Hello,
I am trying to simulate the deflection of a cantilever due to stress gradient, how can I define an uniform stress gradient through the cantilever thickness??
Thanks for your help!
Andres