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How to get the derivative of stress along r direction in 2-D axisymmetric model

April 7, 2015, 1:38 pm
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I found if I use 'd(solid.pm,r)' to calculate the derivative of pressure along the r direction, the result is very weird, like the Figure 1.
However, the 'd(solid.pm,z)' can give me the right distribution of derivative of pressure along the Z axis. as shown in the Figure 2.
Can anybody tell me the reason?
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How to apply normal stress on a solid box cube ?

April 20, 2015, 11:40 pm
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The thing is that we have to study deformation in solid cube . How to we study ?
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how to implement absorbing boundary in wave propagation in saturated porous media(biot govern equation)

April 21, 2015, 6:35 am
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how to implement absorbing boundary in wave propagation in saturated porous media(biot govern equation?

though low reflection boundary in solid module ,it do not work very well
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Advanced photonic bandgap (triangular lattice)

April 21, 2015, 9:17 am
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Hello,

I'm trying to expand the photonic bandgap model (www.comsol.com/model/bandgap-a...ysis-of-a-photonic-crystal-798) to a slightly different structure, but since I'm new to photonic crystals, I'm struggling with this.

In the model, a square lattice is simulated, with lattice constants a1, a2 = a. The primitive cell is a square with the circular column in the centre. I want to simulate a triangular lattice. The cell changes into a parallelogram, and . Now, if I just want to sweep from k=0 to (M point in the Brillouin Zone), it works fine out of the box. I don't know how to proceed from here, in order to calculate the whole bandgap (Gamma -> M -> K -> Gamma). I thought I'd simply redo the initial eigenfrequency step for M point, and then move on to the k sweep. K values are defined as follows:

kk1*(k1*b1x+k2*b2x)
kk2*(k1*b1y+k2*b2y)

Where b1, b2 are reciprocal lattice vectors and kk1/k1/k2 are my parameters. For the initial eigenfrequency sweep at Gamma I set:
kk1=kk2=k1=k2=0
For the subsequent sweep kk2=1 and I'm sweeping k1=0...1.

Now, to get from M to K I wanted to set kk2=k1=1 for the initial eigenfrequency, but I'm getting the following error:

Singular matrix.
- Detail: There are 1 void equations (empty rows in matrix) for the variable comp1.freq1.
at coordinates: (0,0), ...
and similarly for the degrees of freedom (empty columns in matrix).


Where might this be coming from? freq1 is defined for the second study and I don't know why Study 1 would complain about it. I'm attaching the model.
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Plotting a 3D Mirror Data Set to view Particle Trajectories- ERROR

April 21, 2015, 9:23 am
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Hi,

I have modelled half a pipe with particle trajectories in it, but I want to view the particle trajectories in the whole pipe. I tried doing so by creating a 3D Mirror Data Set.

I created a 3D Mirror Data Set and selected the particle trajectory (Particle 3) as the Data Set. The 3D Mirror of the pipe is plotted and is fine. The problem is when I change the data set for Particle Trajectories (fpt) into "Mirror 3D 2" instead of "Particle 3" and then in the sub-section, Particle Trajectories 1, I change the data set into "From Parent". I get an error:

Error: Operation cannot be performed on data set mir2(Mirror 3D).

Does anyone know the reason for that?

Thanks a lot
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How to define the gradient of pressure in large deformation

April 21, 2015, 12:32 pm
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Recently I encounter a problem about define the gradient of pressure in a large deformation ball. I want to get the derivative like this: d(solid.pm)/dx. Is it right to use the operator like d(solid.pm, x)? I found the d(solid.pm, x)and d(solid.pm, X) is total different, like one is positive while one is negative. I am very confused about it. I just want to make sure which one is right?

Meanwhile, I also found the derivatives from the d(xxx, x) is weird. Figure (a) shows the variable 'sm' and figure(b) shows the derivative of this variable 'sm' . It is against with our observation. why?
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COMSOL API and Eclipse

April 21, 2015, 1:07 pm
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Hello all,

I'm trying to get COMSOL API working with JavaSE 1.8 in Eclipse.

Following the directions in the COMSOL API documentation I've created a project and imported the *.jar from COMSOL/COMSOL50/Multiphysics/plugins as an external library. I have also copied the test code which I'll reproduce bellow.

package comsol_test_2;

import com.comsol.model.*;
import com.comsol.model.util.*;

public class hello_world {
public static void main(String[] args)
{
run();
}
public static Model run()
{
Model model= ModelUtil.create("Model");
model.modelNode().create("comp1");
model.geom().create("geom1", 3);
model.geom("geom1").feature().create("blk1","Block");
model.geom("geom1").feature("blk1").set("size", new String[]{"0.1", "0.2", "0.5"});
model.geom("geom1").run("fin");
return model;
}

}


However, when I try and run the test code I get a bunch of java.IOExceptions like the following:

java.io.IOException: javax.crypto.IllegalBlockSizeException: Input length must be multiple of 16 when decrypting with padded cipher
at javax.crypto.CipherInputStream.close(CipherInputStream.java:321)
at com.comsol.model.util.BuilderUtil.getBuilderFileType(Unknown Source)
at com.comsol.model.util.BuilderUtil.loadGeneralDesgin(Unknown Source)
at com.comsol.design.util.BuilderContributor.a(Unknown Source)
at com.comsol.design.util.BuilderContributor.getBuilders(Unknown Source)
at com.comsol.design.migration.DeployedMigrationHelper.getMigrationHelpers(Unknown Source)
at com.comsol.migration.g.e(Unknown Source)
at com.comsol.migration.e.d(Unknown Source)
at com.comsol.migration.e.f(Unknown Source)
at com.comsol.model.util.ModelInternalUtil.create(Unknown Source)
at com.comsol.model.util.ModelInternalUtil.create(Unknown Source)
at com.comsol.model.util.ModelUtil$1.a(Unknown Source)
at com.comsol.model.util.ModelUtil$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at com.comsol.model.util.ModelUtil.create(Unknown Source)
at comsol_test_2.hell_world.run(hell_world.java:13)
at comsol_test_2.hell_world.main(hell_world.java:9)
Caused by: javax.crypto.IllegalBlockSizeException: Input length must be multiple of 16 when decrypting with padded cipher
at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:913)
at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:824)
at com.sun.crypto.provider.AESCipher.engineDoFinal(AESCipher.java:436)
at javax.crypto.Cipher.doFinal(Cipher.java:2048)
at javax.crypto.CipherInputStream.close(CipherInputStream.java:314)

Any help would be appreciated.
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Open domain

April 21, 2015, 4:32 pm
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Hello everyone

I am trying to implement a model in COMSOL with this characteristics:
- the model is 1D with a domain defined between 0 to 1
- in each boundary (x=0 and x=1) are defined one ODE
- also are defined one boundary condition in each of this boundary
- inside the domain there is a EDP that exist in 0<x<1
- and finally, there are 2 more EDP that are defined in x=0 and x=1

My question is, how can I define the 3 EDP's? is it possible in COMSOL? I am particularly confused for the domain condition in this equation.
And because of this I wonder how is defined the domain of each equation in COMSOL (I mean, there are open boundary, closed...?)


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2D Dielectrophoresis Separation

April 21, 2015, 6:51 pm
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Hi! I'm new to Comsol and I'm trying to model a microfluidic channel lying on top of a system of interdigitated electrodes in 2D. The channel has 2 inlets on the right (the top right one is for the sample and the bottom right one is for a stream of buffer solution) and 2 outlets on the left. The electrodes are supplying an AC voltage so that the cells will separate by dielectrophoresis. So far, I've modeled a stationary study just looking at the Creeping Flow physics and I'm trying to add a frequency domain step to compute the Electric Current physics for an AC voltage. However, the voltage plot I get only has the electrodes at a voltage and everything else is at 0V (attached picture). Can anyone else me figure out how to get a good voltage plot?
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Restore a Model to a Previously Simulated Version After Reopening

April 21, 2015, 7:37 pm
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Hello,

I have a question regarding restoring a Comsol simulation to a previous version. I successfully ran a Comsol simulation, but I must have made a change to the conditions and then saved it. When I reopen the simulation, the results appear to be the results I would like, as the charts are correct, but if I hit "compute", then I get new results that are undesirable. I am trying to figure out exactly what my conditions were when I was successfully running the simulation.

I have already tried restore saved and clicked reset history, but these do not work. Is there any way I can view a log of all the changes I have made and at which points I have run the simulation? How can I recover my file so that when it computes it gives me the previous results?

Thank you!
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Density in a Deforming Bar

April 21, 2015, 7:44 pm
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When I apply a force to a rectangular prism with one fixed end such that it deforms under tension, it appears that volume is not conserved. The bar lengthens, but there is no other visible deformation and the density across the structure appears to remain constant (when I do a volume plot of the density it remains constant). Does anyone know why Comsol doesn't appear to conserve mass in this case and what I must do to "convince" it to do so?

I have attached an example.
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Membrane Deformation without Convergence or Mass Conservation

April 21, 2015, 7:55 pm
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In the attached file I deform a spherical membrane filled with water with a force applied across a boundary. The applied force appears to cause a deformation that does not converge, but just continues to expand in the direction of the force without stopping. Furthermore, although I have specified the inner fluid to be nearly incompressible, the object appears to constantly increase in volume, suggesting that mass is not conserved. How can I set up this problem such that a deformation at one point on the object causes a complementary deformation elsewhere on the object such that volume is conserved?
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Ultrasonic

April 21, 2015, 8:05 pm
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Excuse me, where can I find this model?
"Ultrasonic Beam Steering in Isotropic Media".
"Finite element models for the solution of underwater acoustics problems"
This from the annual meeting of the comsol,2006.
Thanks !
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How comsol solves a stationary studyr in single phase flow with a parametric sweep

April 21, 2015, 8:07 pm
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Hi,
The problem which i am dealing is flow past a sphere in 3D. I have used a parametric sweep with different inlet velocities. I have found that instead of solving for the velocity directly, it is calculating the flow for intermediate values of velocity. The solutions which got with a single velocity value is different from that of the same value in a parametric sweep. Could any one help me why this is happening.
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Divergence cleaning failed for linear system.

April 21, 2015, 9:39 pm
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When I try to couple the both magnetic field and solid mechanics I encountered following error...
Divergence cleaning failed for linear system.
-Feature: Stationary solver1(sol2/s1)
-Error: Divergence cleaning failed for linear system.
How to get rid off this error????
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Current Source and violation of Maxwell-Ampère’s law

January 2, 2013, 8:29 pm
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Hi,

I have setup a model using both Electric Currents and distributed ODE and PDE interfaces using a simple 2D square geometry.

One boundary of the geometry is set as ground and another used to apply an external current density. I'm also injecting a current into the domain using the "Current Source" option. This current is formulated using a set of ODE equations.

The model solves but I am getting very large errors when validating the model using Kirchhoff's laws.

COMSOL's documentation state that the current source option should be used "with caution as it may violate the current conservation law that is inherent in Maxwell-Ampère’s law".

Has anybody had any similar experience using the Current Source? Did you observe any violations in Maxwell-Ampère’s law? If so how to work around this problem?

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Frequencies on a squared plate (membrane)

January 6, 2015, 5:48 am
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Hi guys, i'm sorry if i'm doing this wrong but i'm kinda new on COMSOL and I'd like to know if you guys could help me on this exercise:

i need to see how is the behavior of the frequencies on a squared membrane, like chladni figures experiment. There's a model for circular ones that works pretty well but i can't adapt for myself, because there's data like pre-tensions that i won't need right now. My plate is only fixed on the center by a screw on a base made by me. I just would like to see how would be this behavior on some frequencies (6 is fine).

Plate:
size - 16 cm
thickness - 3 mm
poisson - 0.33
young - 69 GPa
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Build 2 port SAW

January 15, 2015, 8:33 am
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Hello all,
I am very new to COMSOL. I am trying to set up a 2 port SAW device model using COMSOL. I down loaded the existing tutorial (in ppt format) from the COMSOL site. I also downloaded the mph file, but unfortunately it does not open because I have an older version.

Here are some of the challenges.
- I am not sure if everything is explained and illustrated in the ppt. www.comsol.com/model/two-port-piezoelectric-saw-device-19155

- I followed the steps. However, I cannot visualize the device that they have pictured in the slide deck, slide 14. I see that there is no object added. I tried to add. However, it does not read all the parameters, including IDTs that were specified.

I am right not stuck. If some one can help me with details on setting up of the problem, it would be of tremendous help.

thanks very much
Kamala
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Can S11 or S21 be less than 0?

April 15, 2015, 4:45 pm
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Hi,
All

In the module 'Frequency Selective Surface, Periodic Complementary Split Ring Resonators' on the website of COMSOL, we can get analysis of S-Parameter by 1D plot group. The module has already got results of S11dB and S21dB, what I want to analysis are S11 and S21, so I add emw.S11 and emw.S21 in a new 1D plot group without any other operation. After plot, what I got makes me confused: I saw part values of S11 and S21 are LESS THAN 0.
I wornder if I understand this physical significance in a correct way, because when I repeat paper 'oe-21-5-6153,' I got negative value, too.
Please help me out!
THANK YOU!

BEST,
Y,Zou
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change the numerical values for the physical quantities

April 20, 2015, 2:35 am
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Hello COMSOL community :)

I am new to COMSOL multiphysics program , i am trying to build a simulation of a cantilever in the mm length scale. I am using Magnetic fields (mf) , Electric currents (ec) and Multiphysics modules.
I found this in the User's guide book
"It is important to be careful using physical constants if the unit system is not in SI units.
For example, the default values for the permittivity of vacuum and the permeability of
vacuum require that you provide all other quantities in SI units and that you use meter
(m) for the geometry length. If you draw the geometry using another length scale, you
need to change the numerical values for the physical quantities accordingly. "

Can anyone tell me how to change the numerical values for the physical quantities according to mm length scale ?

Thanks in advance,
have a nice day,
Amal
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