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convective heat flux magnitude

January 21, 2013, 2:02 am
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Hi ,

I'm computing heat transfer from a rotating hot sphere in a fluid medium. The flow is creeping type.

I have issues related to the various COMSOL options for convective, conductive and total heat transfer. Is there any documentation about how COMSOL computes these quantities. For instance the "convective heat flux" gives a zero value while "convective heat flux magnitude" gives close to 20 kw. In principle the correct value should be zero. I even looked for this in the documentation but failed to find any. I would appreciate if someone can share some light on this.
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Building a mesh having in a thin gap in between two beams

January 21, 2013, 2:02 am
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I am using the piezoelectric devices interface (pzd). Some layers are linear elastic and other - piezoelectric. The surrounding block is air and I defined it with the electrical material model. I want to employ the electrostatic attraction across the thin gap between the two beams. Therefor I need an electrical material to perform force calculation and apply this force as boundary load on the cross section of the gap.

At the moment I am stuck on building the mesh, because the gap in the middle is interrupting the symmetry (at least I suppose this is the difficult point here). I tried to build a mesh on some surfaces and then make a Swept, but I had little success.

What is a good strategy to build a little bit more complicated mesh like this here?

Thanks!
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LiveLink

January 21, 2013, 3:04 am
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Dear sir,

Is there a LiveLink extension in COMSOL 3.4 version for importing files from AutoCAD to COMSOL??
Please help me out how to find it in my installation??
Thank You...

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solvers in 4.3a/4.3 version

January 21, 2013, 3:16 am
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Hello

I am interested in accesing solver information for ODE/PDE in Comsol.

So far I was using the Matlab - COMSOL connection in 3.5 version which was showing a sequence like:

fem.sol=femtime(fem, ...
'solcomp',{' ',' ',' ',' ' }, ...
'outcomp',{' ' , ' ',' ',' '}, ...
'blocksize','auto', ...
'odesolver','genalpha', ...
'tlist',t:dt/4:t+dt, ...
'tout','tlist', ...
'incrdelay','off', ...
'nlsolver','manual', ...
'ntolfact',1, ...
'maxiter',4, ...}

I would like to know if and where this information is available in newer version.

What I could understand untill now id that all the solving is unde (for example):

model.sol('sol1').runAll

Thank you in advance for your support.

Dana Ofiteru
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Ice melting

January 21, 2013, 6:26 am
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Hello,
I am new to comsol and i am using comsol 4.2a. I have to create a model for melting ice on steel block of dimensions 0.1 X 0.1 m. This steel block is placed over a slab of dimensions 0.3 x 0.5m.

A thin layer of ice is formed between the block and the slab due to external weather conditions. The ambient temperature is -20 C. A heat source is available at bottom of the slab.

So how should i proceed with selecting the physics and setting the boundary conditions. Someone please help me.

Regards
Bax
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Unexpected results with "Ground" BC

January 21, 2013, 6:32 am
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Hello,

I do most of my modelling using electrostatics steady-state solutions.

One thing that comes up in my models alot is a grounded metal object in a linear, vertical electric field as shown in the attached figure (sorry for MS paint, I am not at a comsol workstation right now).

I have two large horizontal steel plates, 1m apart in an air domain. In the diagram, you can see how the ambient field is set up (by setting the top plate to 100V and the bottom to ground).

In real life, assuming the object and the bottom plate are all connected and made from the same metal, it would take only one connecting wire to ground the whole assembly (i.e. set it all to 0V).

When I apply the BCs (100V to top plate, ground to bottom plate) my metal object does not come out at 0V, but appears 'transparent' to the field like a floating electrode would (it matches the potential of the air around it).

I can of course manually set every boundary of the object to 0V, but it is counter-intuitive to me that I should need to do this. In practice, you would ground a metal assembly just by connecting a wire to a single part of it.

I intend to go on to study electrostatic induction (bringing charged objects up to the metal assembly) and study changes in charge distribution (and thus electric field) in the assembly.

I hope the question is coherent. Thanks in advance for any help.
Kind regards,
Dominic.
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post-processing (plotting)

January 21, 2013, 6:33 am
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hello, after a biphasic flow simulation "drop flows in other fluid" i hoped plotting my results, but i could not, beacause what i wanna draw it's a user difined characteristic, and i don't know how to implant that in comsol!!
CAN you help me please!!!!
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VariableLlimitFvar

January 21, 2013, 8:29 am
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Hi,

I get this error LlimitFvar vhay i s wrong?

Thank
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Effect of sound pressure on refractive index/density

January 21, 2013, 8:43 am
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Hello fellow Comsol users,

Most of my experience comes from structural mechanics and fracture and would like to ask a little help regarding pressure acoustics.

The presence of high and low pressure in air will change it's refractive index due to a change in the density. I would like to model this change in refractive index, but am a little lost.

Could anyone tell me how I could go about modelling/solving for a change in material property?

I am using a time dependant pressure model in air. Within the materials section I can see that the density is a function of temperature and pA [which I assume to be absolute pressure], but when I plot the change in density it says that there is no change in density even if there is a change in pressure.

I'm sorry if this question sounds a little dumb, I'm sure that I am just being stupid today.

Also, I have had a look at the forum and a little in the documentation but could not find anything to help.

Anyone have some ideas out there? I would greatly appreciate your help.

Rob
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Two interdependent studies - but I cannot use results from frequency domain study as input for stationary study

January 21, 2013, 10:35 am
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Dear all,

How can I use the result from a parametric (parameter 'b') sweep-frequency domain study (e.g. emw.Ez(b) ) in another function defined under Definitions (e.g. f(emw.Ez(b)) ) which is used as an input for a stationary study?

According to the results that I am getting, what is actually passed to the second study is ZERO.

Could you please send me a reference?
Or tell me how to shall I input the results from a study to another study?

Thanks a lot!

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Radiation in conjugate heat transfer

January 21, 2013, 12:24 pm
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Hello all,

I have the CFD module and am using conjugate heat transfer. This means that I don't have surface to surface radiation but I do however have surface to ambient radiation.

I am modelling external natural convection in 2D using a rectangle of air with open boundaries. I am trying to add surface to ambient radiation on the boundary between the air and the solid part. COMSOL wont let me do this however. I don't know whether it is because it thinks that the air is opaque or because it wont let heat flux's be defined at boundaries between materials.

Would anyone be able to advise me on how to define this surface to ambient radiation flux at such a boundary.

Thank you all very much,
Josh
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Scattering Cross Section

November 1, 2009, 9:24 am
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Hello All,

I am trying to find the best way to calculate the scattering cross section in COMSOL for a 2-D cylinder. I eventually will move to 3-D spheres and cylinders. What I have is a circle of 30 nm in radius and my circular PML layer drawn 10 Lambda away, where at this boundary (far field) I want to calculate the scattering cross section. I am doing all of this in the RF Module->In Plane Waves->TE Waves->Scattering Harmonic Propagation.

Scattering cross section to me seems to be defined differently depending on what text book you are looking at. The one I see most is:

Csca = (1/|Eincident|^2) Integral (|Escattered|^2) dr

So here are my more specific questions.

1. Would you take Eincident to be E0iz_rfweh or Ez?
2. Is scEZ the Escattered filed I am looking for or do I have to subtract out the incident field?

Any input on your experiences calculating scattering cross sections would be greatly appreciated.

Regards,
Jacob
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2D Model Problem...

August 3, 2011, 2:13 am
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Hi all,
I'm working on a model which is a 2D representation of a solid container in a multiphase-flow section. what I'm trying to model is to study the relation between the contact angle and the ratio of the viscosity of the two different fluid inside the tank. The model is really simple and it's working in 5-20 second but still I'm not sure the whole model is right or not.
The main problem that I have is that when i increase the time of simulation for about 100 Sec what I could see was that some of the fluid inside the tank will disappear which is quite strange and I think it has to do with the boundary condition on the side wall. Interesting thing is that still the flow structure is kind of similar to what we are looking for, the only thing that makes me worry is that something might be wrong with the model and i wouldn't see it and then the whole result that I'm getting might be wrong.
The model that i uploaded here is the one im trying to work on but in case u wanted to run that model you need to increase the mesh size to extra-fine so that it would run without any error.
any help or anything that you think will be helpful for me will be deeply appreciated.
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Surface Integration with different modes

June 7, 2012, 11:35 pm
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Hi all,

I have calculated the results for the different transverse modes in an optical waveguide using the Mode Analysis solver in the RF module. Now I want to perform a surface integration across one domain, say I want to integrate the electric field in x-direction (emw.Ex) inside the core. I can do that for one of the found modes.

Is there a way to get the surface integration for the multiplication of the electric field for two different modes, i.e.

emw.Ex(of mode1)*emw.Ex(of mode2) ?

Thanks a lot
Patrick
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Structure with 2D and a 1D truss...

November 26, 2012, 6:20 am
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Hello all,

I am trying to implement this (see attachment).

I guess it should be relatively easy, and for sure lots of people know how to do it :).

After "spending" quite a lot of time, I decide to share my problem with you. I hope you can help me with that.

I guess the "problem" is with a boundary condition, or with the definition of a Pair there...

Thanks a lot in advance

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One way multiphysics coupling within a transient simulation

January 14, 2013, 12:16 pm
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Hello Fellow Users,

I am solving two physics (fluid flow & species transport) in a transient simulation. The fluid flow is not affected by the concentration distribution. As of now, i have implemented the above using 'fully coupled' feature.

Is there any way to implement one way coupling within a transient simulation i.e. at every time step i want to solve for fluid flow first and then use that estimated velocity field to calculate the concentration distribution. It is not clear to me whether segregated step would do the above mentioned? I tried segregated node separating these two physics (still using direct solver) for both physics but the total simulation time goes up (more than double) compared to the fully coupled case.

my understanding is : if segregated step implements one way coupling then the simulation time should be less than the fully coupled case? any thoughts/comments on this.

Any guidelines or similar example for this kind of coupling in dynamic simulation is appreciated.

Thanks in advance for your suggestions.
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Diamond Young's Modolus Property confusion

January 21, 2013, 3:25 pm
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Hi Comsol Community,

the Young's modulus property of diamond in the Comsol material library is

105 x 10^9 Pascal

whereas the proper property value in literature is an order of magnitude larger:

1220 x 10^9 Pascal - www.chm.bris.ac.uk/motm/diamond/diamprop.htm

1220 x 10^9 Pascal - www.sciencedirect.com/science/article/pii/0925963593902506

1220 x 10^9 Pascal - en.wikipedia.org/wiki/Young%27s_modulus#cite_note-27

I understand that this number can vary for different crystal orientations, quality, etc. but I found no data justifying the big discrepancy of the Comsol value with other values found.

Any idea why that is?

Cheers,

Erik
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How to implement Valveless Piezoelectric Micropump

January 21, 2013, 3:47 pm
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hello everyone. Is there any way to have a hold on the model presented in this paper for a piezoelectric micropump?
"Design and Simulation of Valveless Piezoelectric Micropump"

I have difficulty linking Piezoelectric to the Membrane in Comsol.

Thanks
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Material properties in axialsymmetric form for 2D axialsymmetric simulations

January 22, 2013, 12:40 am
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I have the following problem:

I'd like to perform a 2 dimensional axialsymmetric simulation of a piezoelectric thin film on a silicon bulk and a round metal electrode on top. Since the coordinate system of the problem is already axialsymmetric, I'd like to enter the material properties (the material tensors) in the same fashion, meaning I'd like to define the material properties in r and in z direction. When I add a material from the library, however, all the material tensors are in xyz configuration.

Is there a easy way to achieve this? Or does COMSOL calculate the appropriate material parameters from the cartesian configuration? And if yes, how does it this calculation?

Thanks, already appreciating the help :)

Greetings
Michael
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Issue with temperature distribution

January 22, 2013, 4:34 am
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I am using COMSOL for simulating heat generated by current through a metal bar for my 3rd year dissertation project.

I am having an issue with the output following the simulation.

I have applied a terminal current to one end of the bar, and a ground to the other, along with appropriate heat transfer values for convective cooling etc. I would expect a linear heat distribution across the bar, with it being hottest towards the terminal end, and coolest at the ground end. Instead of this, the output shows sort of a concentric circle based distribution moving out from the centre of the bar, so that the corners are coolest.

Any help as to how I am going wrong, and how to solve this issue would be greatly appreciated, as I am on a tight deadline to complete this for my project.

Many Thanks,

Best Regards,

Spencer Mintram.
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