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PML Error: Could not find any cartesian regions

May 14, 2013, 7:19 am
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Hi

I am new to PML. I am using Structural mechanics module to implement PML in a cantliver to determine the waves propagating to the PML. I am getting this error: Could not find any cartesian regions. My dimensions are in micro meter but using conversions to use it in meters. Please suggest what am I missing here.

Is there any model on PML using SM module?
Thank you.
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export mesh data for an explicit selection

May 15, 2013, 6:01 am
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Dear all,

I am a relatively new user COMSOL with a lot of Finite Element experience both on the programming and theory level. I started with COMSOL today which went actually quite fine and fast.

I would like to use that to interface assemled system matrices and mesh data to my own solvers. And it was successful for simple shell models.

At this point, I am also wondering if that is possible to export some selected node data or not. What I am directly interested is 'can I create node groups in COMSOL' which can be exported in some kind of text data file format or in binary file format?

Let me be more specific, lets say I have two fixed edges of a square plate and I would like know which nodes are associated with this selected fixed edges so that I can extract the constrained dof infornation myself. These kinds of information might give me more freedom for simple FRF analysis that I would like to conduct.

I learned that I can extract the matrices Kc and K, for instance, for the system stiffness matrix which represent the matrix on which the boundary conditions are applied and the full system matrix without any boundary conditions applied, respectively.

I would like to know or compute the fixed dof information by either getting it from COMSOL(which would be nice since no extra computations will be necessary.) or by iterating over the nodes myself.

Any ideas or pointers are appreciated on this matter.

Best regards,
Umut

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Weak Constraint thermal flux

May 15, 2013, 11:30 pm
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Hi every body

I am using Weak form to model strongly coupled thermal-structural equations.My model consists of two layers.
I used Weak constraint to ensure that my thermal flux will be continuous but
Comsol gives me an error that "Expression must be linear in test function"..and i dont know how to solve it.
What does it mean?.Do I have to change a shape function?if yes,how can I do that

I used (q22-q2)*test(u3), where u3 is dependent variable for temperature and q variable for heat flux of each layer.

Coud you please help me?

Thank you



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Adding points and lines to 3D models with parametric surfaces

May 16, 2013, 5:35 am
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Hi all,

I was wondering if there is a way to add a point or line to a model in which I have a 3D block with the top surface replaced by a parametric surface derived from a DEM (digital elevation model).

I guess this can be done in either the geometry section so it is there for the entire model run, or in the results section under data sets.

The end result I am after is the displacements along a line that traverses the parametric surface or at a certain point on the parametric surface.

Any help in this matter would be greatly appreciated, and let me know if I haven't explained the problem sufficiently.

Thank in advance!

James.
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System of PDEs with mixed order

May 16, 2013, 1:03 pm
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Hello,

I am trying to solve the attached system of PDEs using the general PDE interface. However, I always get a singular matrix error. What am I missing?

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Elasticity matrix,voigt notation of PDMS

May 9, 2013, 3:28 am
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Hi,

I am currently working on bio-compatible material based sensor, where i want to use PDMS (Polydimethylsiloxane) as a membrane.But I can not get Elasticity matrix,voigt notation(mechanical properties) of PDMS for simulation.

Do any of you know the Elasticity matrix,voigt notation of PDMS?
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Buoyancy effects for turbulence model

May 9, 2013, 7:46 am
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In nitf (non-isothermal flow), the way for setting the Buoyancy force is only can set the volume force(-rho*g)?
However in somecase it is not accuracy for compare to the experiment result (the buoyancy effect is low).

I know some CFD software has own defluat Buoyancy model in turbulence model or in laminar model , how about in comsol?

Any comparsion for comsol and experiment for buoyancy has done to know the accuracy?
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2D Arrow Surface Plot -- Arrow Length Logarithmic

May 9, 2013, 8:02 pm
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Hello,

I am plotting a 2D Surface with an arrow surface. I need to scale my arrows logarithmically, but I am unsure whether this scaling is a natural logarithm or a base 10 logarithm. Which is it? The COMSOL Multiphysics User Guide states:

"Logarithmic, so that the length of the arrows is proportional to the logarithm of the
magnitude of the quantity they represent. This makes arrows representing small
values relatively larger. The value in the Range quotient field (default: 100)
determines the ratio between the smallest and largest values in the range of values
for the logarithmic arrow length."

If this is a natural logarithm, is there any way that I can scale my arrows with a base 10 logarithm? I will have some negative values in my vectors, so taking a log10 before plotting is not an option.

Thanks,
Lauren
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simple solidmechanics

May 9, 2013, 8:50 pm
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Hi

I have a simple model that a piston presses down brain tissue.

The piston moves with a prescribed displacement.

A problem is that the piston always moved the same distance, even I have changed numbers in prescribed displacement.

Can anyone help me with this problem?

I attached the file.

Thanks,

Jay
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find the coordinates of a point in which the magnitude of derived variable at a specific time is minimum?

May 9, 2013, 9:41 pm
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Hello,

I am working on a 2D transient problem. I have the results and want to post processing them.
I want to plot my results like this:

Detect the minimum of a derived variable all over the domain at a specific time.
then, again detect that variable in another time (and repeat it for my time period)

I could do that. Now I wanna to have the coordinates of that point in which the derived variable is minimum at that specific time.
Any idea?

Thanks
Hooman
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Modelling a Lumped Capacitor in COMSOL (Frequency Domain EM simulation)

May 10, 2013, 3:30 am
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Hello,

I am trying to critically couple a resonator to a microstrip transmission line using a lumped coupling capacitor. Does anyone know how I can do this in COMSOL. I know it is possible in HFSS. Can COMSOL do the same?

Thanks you
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Oscillating electric dipole source Comsol RF module

May 10, 2013, 9:17 am
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Dear All,

I have question.

I do simulations of plasmonic gap nanoantennas, and I want to excite antenna with dipole source which is placed and oscillate in the middle of the gap.


I have questions.

First, as I know unit for electric dipole moment is Cm (coulomb * meter), and in Comsol there is electric CURRENT dipole moment and unit is Am (amper * meter). I will mark electric dipole moment as P, and electric CURRENT dipole moment as PI.

My first question is how to set up equation in of “electric point dipole”? Is that for example just 1 [mA], or cos(emw.omega*t) [mA], or something else?

My second question is what is relation between P and PI. Is that PI = P/t (time), or PI = dP/dt, or maybe PI = P*emw.omega??

My third question is about Mie scattering. I want to calculate scattering and absorption cross section and I need electric field to calculate that.

Do you maybe know relation between electric current dipole moment and electric field??


Thanks for every reply and suggestion.
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Simple example to solve ODE

May 10, 2013, 2:17 pm
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Hi,

I am a complete newbie with COMSOL (I have the MultiPhysics module, version 4.3a), and I have only followed some initial tutorial examples in the manual (e.g. "the wrench").
However, as I understood, there are also ways to solve ODEs (ordinary differential equations). I wanted to start with something really simple, and on the internet I found some example involving the predatory model:
#rabbits: dr/dt = a*r - b*r*f
#foxes: df/dt = -c*f + d*r*f
However, the example I found is clearly meant for a different COMSOL version because the description of buttons/menus to use does not correspond to my version.
To give it a go anyway, I used the "Global ODEs and DAEs" node, and in the "Global Equations 1" settings window, defined:
r*(a-b*f)-rt (initial value r: 100, initial value rt: 0)
-f*(c-d*r)-ft (initial value f: 100, initial value ft: 0)
My question is (and this might come from a complete misunderstanding of how COMSOL works):
- Where do I define the upper and lower bounds for t? (In the geometry, but how?)
- how do I get a plot showing r and f versus t? (Do I need to use "parameter sweep" for this?)
- Is it possible to just solve a 1-D ODE without selecting any kind of other physics models?
- Is there any example around for version 4.3a?
- (I actually tried to just click on "Study1->Calculate" and while the program does not complain, I also have no idea how to get the plot I want)

Best regards,

Machiel
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Initial negative concentrations

May 10, 2013, 2:19 pm
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HI All,

I am trying to simulate a A+B-->C chemical reaction. I dont know why my reactants (A and B) concentration is going negative. How do I fix this?



I have attached my model as well as pic of the concentration profile.

I saw in the knowledge base they give you certain options, but they have not worked for me. I tried adaptive meshing as well introducing A>0, B>0 in my reaction terms in the variables part. This is what I have in my variables section.

A=1 mol/m^3
B=0.5 mol/m^3
C=0 mol/m^3
k1=0.01
rA=-k1*A*B*(A>0)*(B>0)
rB=-k1*B*A*(A>0)*(B>0)
rC=k1*A*B*(A>0)*(B>0)

If someone can guide me please.

Thanks,
Romani
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Electric Field / Potential distribution

May 10, 2013, 9:31 pm
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Dear all,

I am trying to simulate a structure which consists of different material layers. I would like to see Electric field and Electric Potential distributions of a certain potential applied to the boundaries of substrate (poly-Si). My multilayer structure includes two different metall layers. When I press compute, Comsol asks me to enter Relative Permittivity values for these metals, which is actually incorrect. I set them to 0.01 but also set all boundaries of metal layers to 0V potential. Is this approach is correct?

After computing Electric Potentials (image is attached), instead of V I type sqrt(es.Ex*es.Ex+es.Ey*es.Ey+es.Ez*es.Ez) in Expression field to get Electric Field distribution, however I am getting an error (image is attached), which says "Failed to evaluate variable ... Domain 1, 10". Material of Domain 1 and 10 is Air.
Does anyone can explain what am I doing wrong?
Thanks in advance.
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Assigning a sinusoidal electric potential to a boundary

May 13, 2013, 1:37 am
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Hello,

I am relatively new to COMSOL and I am building a transformer model for use in researching different hysteresis losses and associated heat energy with using different core materials. I have built a geometric model of the device. I am trying to assign an electric potential to one end of the primary winding on the transformer (the other end will be ground). I can give it a DC potential, but I want to assign a sinusoidal waveform to the voltage across the primary winding. How can I do this?

I included the Electric Currents physics engine and I defined a waveform for the primary coil voltage, but that is all the further I have gotten. I feel like if I could just define the "Equation" setting in the Electric Potential phenomenon under the "Electric Currents" physics, it would allow me to specify a sine wave potential.

I would really appreciate any input.
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Inlet outlet mass flux is not identical

May 14, 2013, 3:24 am
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Hi all,

I am trying to measure the bulk temperature, but initial testing on the conservation of mass failed as it gave me different results between the inlet and the outlet mass flux. This result makes me doubt all my simulation results so I would like to know what's going on. The model is a rectangular duct flow with 1) heat transfer on one side and 2) prescribed temperature at one of the walls and the channel inlet.

I used two 'general projection' planes to measure the mass flow rate over the entire channel length. As you can see in the graph, there is a sharp rise at x=0 (not to mention the fluctuations. I would say it's a numerical uncertainty due to the meshing). I don't understand why this is happening as this happens even if I refine the mesh, which is very concerning.

Any suggestions on why there is a discrepancy in the mass flux along the channel? Please let me know if there is any mistakes in my model.

Thanks!

Best,
Tae Jin Kim

ps: I also quantified the bulk temperature based on some discussions threads and extensive trial and error. At this point, I think it's almost correct, but please let me know if there is an error.
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Mass extraction and saturation of bulk solution

May 14, 2013, 8:22 am
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Hi all,

I am fairly new to COMSOL. I am after solving a non steady-state packed bed extractor with fluid flow convection in the axial direction of the extractor and diffusion of soluble material from the solid particles.

The first thing I am trying is to solve the diffusion equation in 2D with different boundary conditions, as if the particles were in a stirred vessel.

My question is:

When a forced convection boundary condition is imposed, how can be the saturation of the bulk fluid be accounted for?

Manually the process would be

1) Bulk concentration of the fluid is 0 at time 0
2) After the first time step has elapsed, the solution of the diffusion equation gives a total flow (mass released) out of the particle. That mass divided by the volume of the vessel gives the bulk concentration for the next time step.

In COMSOL, how can I update the bulk concentration by imposing that it would be the volume integral over the particle divided by the volume of the vessel for each time step? Is there a way to create a for loop of should I combine this problem with Matlab?

Thanks in advance

B
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Particle Selection

May 14, 2013, 9:48 am
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Hello,

I running a simulation to release particle in an electric field and observe their tracks. I wish to find out how many particle and how fast they reach a certain boundary. I have been able to determine the drift time for each particle, but have not found out how to select particle based on the boundary they first collide with. Is there a simple way to do this in COMSOL?

Many thanks!
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PML Error: Could not find any cartesian regions

May 14, 2013, 1:19 pm
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Hi

I am new to PML. I am using Structural mechanics module to implement PML in a cantliver to determine the waves propagating to the PML. I am getting this error: Could not find any cartesian regions. My dimensions are in micro meter but using conversions to use it in meters. Please suggest what am I missing here.

Is there any model on PML using SM module?
Thank you.
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