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Inlet

September 5, 2017, 8:06 am
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Hello everybody!

I'm new to the forum and glad about any help.

I have been working on a 3D cfd/heat-transfer problem consisting of a nozzle which ejects nitrogen gas into a "vacuum chamber" (diffrent chamber geometries). The geometry is quatered.

I'm using a compressible single phase flow with the Algebraic yPlus turbulence model (or k-eps).
At the chamber walls I use the Outlet pressure boundary condition (p_out = 1 mbar, suppress backflow).
At the Inlet I tried the mass flow (SCCM = 1250) and the pressure (p_in = 164 mbar, suppress backflow, normal flow) boundary conditions. (From experiment it is known, that at the particular flow rate of 1250 SCCM corresponds to an inlet pressure of 164 mbar).
The reference pressure is p_ref = 1 mbar.

1. What do you think is the better (more accurate) choice for the inlet bc (I experienced the mass flow bc is harder to converge)?

2. Do you have a hint for effective lowering of the reference and the outlet pressure at the bc since I try to approach 0.1 mbar absolute pressure in the chamber?

3. Downstream the nozzle exit the flow becomes supersonic with some oscillation in pressure and velocity (propably due to shock waves). There is a region were the pressure drops and stays constant at its minimum value of 1 mbar (p_ref). It seems that the pressure "would like to" drop further but can't because of the lower limit which I placed in the segregated solver. Does this constant minimum region negativly affect my simulation or is it "physically correct"? If not, do you have a hint how to avoid this behavior?

Thank you for your help.

Philipp

PS: Please led me know, if you need further information.
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Wall Shear stress

September 5, 2017, 11:51 am
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Hi everyone,
How can I plot the wall shear stress on the 3D CFD model in Comsol?
thanks
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Creating insulated electrodes attached to scalp / brain

September 5, 2017, 12:27 pm
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Hi there, I am completely new to Comsol. I am attempting to model the field density and distribution within brain tissue by means of insulated electrodes on the scalp, similar to the concept of transcranial stimulation. Although I would like to use complex multilayer models in the future, right now I am trying to just figure out how to set up insulated electrodes on the surface of a sphere with voltage of 2V and 1-3 Amp currents. I am also wondering, if there is a way to set up alternating current system between two electrodes. Thank you so much.
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trouble creating geometry using boolean operations

September 5, 2017, 2:03 pm
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Hi,
I was trying to create a geometry in COMSOL with boolean operations but do not know how to proceed. The geometry I want includes everything in the big circle, including the boundary of the big circle, as shown in the attached file original.png. If I do a intersect of the big circle and the union of small circles, it gives the graph after_intersection.png, which excludes other areas inside the big circle. I guess what I am trying to do is a partition of A(big circle) based on B(small circles), then delete parts of B that is outside A. Could anyone give me some instructions on how to do this in COMSOL? Thank you.
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Streamer modelling in Transformer oil

July 26, 2010, 6:46 am
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Hello,
Can anyone help and check on my model, am trying to simulate streamer in liquid dielectric. my major problem is that the model is not converging after 400ns and needed about 1000ns. I have four equations Viz poisson equation and three continuity equations for the transportation of the charges. i built the model on 2D axialsymmetry with needle-plane electrode separated by 1mm gap.

Thanks
Gaambour Bawa
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Thermocouple model

October 7, 2010, 10:03 am
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I am trying to model the effect of two metals that are in contact with a heat source at one end and isolated from each other at the other end. It should be similar to the Seebeck effect in thermocouples. I am trying to obtain the voltage difference produced between the two metals when they both come in contact with a heat source.
Does anyone have any ideas on how to obtain this?
The only info I found was that it is necesary to use the PDE Application Mode.
Any help is appreciated.
Thank you
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Absorbing boundary condition for transient wave propagation problem

March 4, 2013, 12:14 pm
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Hi!

I am wondering if I can define an absorbing boundary condition for a transient ultrasonic wave propagation problem (let's say in a 2D plane strain transient module). The option certainly doesn't come up in the boundary conditions pulldown menu. I wonder if there is a trick.

Thanks!

Deb
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Ionization of air.

May 2, 2013, 6:47 am
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Hello.

I have a model that comprises a container filled with air.
From the outside there are two coils
number of turns 1e6
supply current is 1e3 A
in the container are two electrodes voltage + - + e7 V
the pressure in the container was 3000 per annum and the temperature 1E7 K.

The device should break down air to the electrons and ions.
But nothing is doing.

Does anyone advise me why?

Thank you.
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How to calculate the scattering corss-section of a gold nanorod concave structure

September 5, 2017, 2:35 pm
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Dear All,
I am new to Comsol, and I am now dealing with the stimulation of the Scattering/Absorption/Extinction cross-section of a Gold Nanorod concave structure（Fig.1）. My work is based on the APPs: Scattering nanosphere & scatterer on substract.

The absorption cross section as a function of wavelength was calculated by integrating the resistive heating losses over the volume of the nanoparticles. The scattering cross section as a function of wavelength was calculated by integrating the dot product of the scattered intensity vector and the normal vector pointing outwards from the particle surface across the entire particle surface. The extinction cross section was calculated by summing the absorption and scattering cross sections.

Here are some of my questions:
Someone mentioned me that we should use a sign function:
ofact = if(nx*x+ny*y+nz*z>=0,1,-1) ; onx = nx*ofact; ony = ny*ofact; onz = nz*ofact;
to make sure the normal vector is outward. But my structure has a Concave inner surface, so the situation can be complicated.

1. Does the vector (nx,ny,nz) just mean the outward normal vector?

2. How to define the ofact sign function of the complicated structure?
nx*x+ny*y+nz*z>0 for some parts of the concave inner surface, while the others <0. （Fig.2）

3. While I define ofact=if(nx*x+ny*y+nz*z>=0,1,-1), the scattering value sometimes be negative, does a negative value of scattering corss-section meaningful? (Fig.3)

4. While I define ofact=1; The scattering value could be so low compared with the Absorption value, is it right?
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Electromagentic field from charged particle

September 5, 2017, 11:19 pm
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Hello everyone,

at the moment I have a 2D model with different layers (each with different electromagnetic properties (permeability, conductivity, permittivity)) which build an upper and a lower layer-system with a vacuum cap in between. Now I would like to add a charged particle (or better: a electron beam with Gaussian shape) flying with a velocity of almost c through this vacuum cap. For the moment I do not want to consider any back-reaction from the produced fields to the particle - I am only interested in the produced em-field (i.e. the wakefield) in the layers due to the charged particle. Does anybody know how I can add such a moving, charged particle? At the moment I use the electromagnetic wave, Frequency Domain for my model. For licence issue I should use the AC/DC or the RF module (I do not have access to particle trajectory module).

Thanks in advance.

David Rohrbach
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Operators not allowed in user-defined functions

September 6, 2017, 3:56 am
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Hello,
if I try to define an analytic user-defined function like integrate(s,s,0,x), I get the above error. Is it indeed not possible to define a function as the indefinite integral of another one? Is there a workaround?
Thank you
Daniel
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Java Out of Memory Error

June 10, 2010, 1:56 pm
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Running a parametric sweep of a 2d electrostatic model. Sweeping two parameters results in 80 configurations.
The computer this runs on has 12 Gb of ram.
The analysis never goes over 1 Gb for the Working Set or Virtual Memory
Comsol Version 4.0, Windows 7 64 bit.

Near the end of the run, sometimes, get an error message relating to memory. Here is the last bit from the log:

00611:005 ERROR Exception:
java.lang.OutOfMemoryError: GC overhead limit exceeded
(rethrown as com.comsol.util.exceptions.FlException)
Messages:
Error in sweep: GC overhead limit exceeded
[com.comsol.logger]
00611:005 ERROR Stack trace: [com.comsol.logger]
00633:891 ERROR Exception:
java.lang.ArrayIndexOutOfBoundsException
(rethrown as com.comsol.util.exceptions.FlException)
Messages:

[com.comsol.logger]
00633:906 ERROR Stack trace: [com.comsol.logger]


What is going on, and How do I fix this issue?
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Math operations with different solutions

September 5, 2012, 3:08 am
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Hello. I hope someone can help me with this question.

Is it possible to operate with different post-processing solutions? For example, I want to obtain a ratio between a temperature field calculated with initial thermal parameters of the materials and the temperature field obtained with some parameters modified (with no changes in the geometry and the mesh). This way I want to obtain a surface plot to see where the temperature has decreased from the initial situation (ratio>1).

It seems this is not possible with Comsol, so I have tried to export both temperature fields to files (in spreadsheet and sectionwise formats) to operate them y Matlab, but these format don´t seem to be easy to use in Matlab.

Any suggestions, please?
Thanks in advance.
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timestep in stationary study?

April 29, 2015, 12:21 am
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I am modeling laminar flow at steady-state. Error message:

Undefined variable.
- Variable: timestep
- Geometry: geom1
- Domain: 2
Failed to evaluate expression.
- Expression: nojac((0.5/sqrt(((((comp1.spf.rho^2)/(timestep^2))+(((6*comp1.spf.mu)^2)*emetric2))+((comp1.spf.rho^2)*emetric(comp1.u,comp1.w))))))
Failed to evaluate variable.
- Variable: comp1.spf.tau_m
- Defined as: nojac((0.5/sqrt(((((comp1.spf.rho^2)/(timestep^2))+(((6*comp1.spf.mu)^2)*emetric2))+((comp1.spf.rho^2)*emetric(comp1.u,comp1.w))))))
Failed to evaluate temporary symbolic derivative variable.
- Variable: comp1.spf.streamlinens@VDN${test@30}
- Defined as: (((-comp1.spf.tau_c)*comp1.spf.res_p)-((nojac((comp1.u*comp1.spf.rho))*comp1.spf.tau_m)*comp1.spf.res_u))
Failed to evaluate expression.
- Expression: d((2*comp1.spf.streamlinens*pi*r)*(dvol),{test@30})

It might be related to a very thin domain in the geometry.

br
Lasse
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Transport of diluted species in porous media

September 10, 2015, 5:30 pm
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The equation for the porous media transport properties has various parameters. Can I get explanation for those parameters? for example what is P1i, P2i and other parameters? Thank you.

Kailas
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Export comsol file( mph) to GDS FILE

March 23, 2017, 2:00 am
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Hi
Does anyone have idea on how to export (geometry) mph file (Comsol) to GDS file.

Need your help please.
thanks
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Shock wave simulation in fluid structure interaction

September 6, 2017, 6:15 pm
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Hello,
I am trying to model the circular moving wall with prescribed mesh displacement inside the closed chamber. The amplitude and the period of velocity are 30 m/s and 200 micro seconds, respectively. I am using the fluid structure interaction. the closed chamber has 3 no slip walls and one of the chamber walls is a membrane. However, I got an error. When I reduced the amplitude to 0.1 m/s and increased the period to 1s, the problem was solved. Could you please let me know how can I solve the issue when the amplitude is 30 m/s and period is 275 micro seconds?
Do you have a better suggestion to model the shock wave?
I tried to reduce the mesh size and time step but it is not the solution.
Thanks
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Modification of variables in parametric sweep

September 6, 2017, 7:35 pm
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Hi community, I'm trying to get solved my model with LiveLink.
Is it possible when study runs with parametric sweep and
in each parameter(voltage in my model), change the variable?

I've done this before.

Before runs my model I already set up variable(x1 as "a")

and After runs my model.sol('sol1')
I'd reset up my variable x1 with other value(b) at certain parameter(voltage)

and runs it again.
But when I looked it up in comsol, x1 has only "b" in every parameters.
I meant it has to be different value in different parameter.

simplifying it.
V=0~1V , x1=a;
V=1.1~2, x1=b;
like this.

But What I got is
V=0~1v, x1=b;
V=1.1~2, x1=b;

Any suggestions would you give?

Best regards,
Min
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How to connect electrostatic module to creeping flow

September 6, 2017, 10:29 pm
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As you know we have a volume force term in navier stokes equation which is due to electric field or electric potential gradient (ρe*E ,E = −∇ϕ ) I want to introduce this force in volume force but I don't know how to write gradient term, I mean I cannot find the annotation or syntax to introduce gradient to COMSOL.
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Getting Error "Low Minimum Element Quality" during computation

September 7, 2017, 12:16 am
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Hi there. I'm trying to animate flow of blood through a membrane with lots of pores. I have followed the particle tracing physics example from youtube as guide. However, during the computation, I kept getting the error message as stated in the subject. How do i solve this? Is it because the design was too complex? thanks for any and guides n recommendation. I include the mph file link* for references. Once again thank u.

*the file is quite big @ 55MB. so i cant attach it here. sorry so here's the link
(drive.google.com/file/d/0B8HbW...JoMFk5bmd1ek0/view?usp=sharing)
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