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CNT Based Resonator

September 25, 2017, 11:02 pm
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Hello Team,

I'm Using Comsol 5.0 Version and Wanted Help from this forum for designing a CNT Based Resonator.
I wanted to actually calculate the Q Factor and Sensitivity of the device with displacement modes. As I'm new user in my learning stage. So can anyone help for designing of CNT Resonator. Its same as MEMS Resonator, Only CNT is suspended between the two electrodes.
Or can provide links for the reference.

Dharmesh Mishra
Research Intern
CSIR Ceeri Pilani
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asimmetry or error for different mesh

February 13, 2018, 2:26 pm
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Hi all, I am trying to model a ultrathin sandwich plate with holes, contrained in the center, where residual stresses are applied on different layers. for the mesh I put the top and bottom as free triangular, and the sides with the swept.

If I model with min-max size of 2-5 um, it runs fine but I have the problem that the deflection of the edges it's not the same on all the corners (3 over 4 are the same, but not the last). I thought that have a finer mesh would have helped but instead I have an error, or it doesn't converge (error higher than 0.40) or it gives me this. " There are 1 void equations (empty rows in matrix) for the variable comp1.u. at coordinates: (-1.35185e-005,5.69688e-006,3.35e-008), ... There are 1 void equations (empty rows in matrix) for the variable comp1.v. at coordinates: (-1.35185e-005,5.69688e-006,3.35e-008), ... There are 1 void equations (empty rows in matrix) for the variable comp1.w. at coordinates: (-1.35185e-005,5.69688e-006,3.35e-008), ... and similarly for the degrees of freedom (empty columns in matrix). Returned solution is not converged. "

Can you help me to understand where it is the problem? Thanks in advance.

Emanuele

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Releasing rays tangential to geometry curve

February 13, 2018, 5:49 pm
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Hi everyone,

I would like to be able to use the Release from Edge node in Geometrical Optics to release rays tangentially to an interpolation curve that I've drawn into a 3D geometry.

Does anyone know if it's possible to use geometric variables such as "tcurv1x", "tcurv1z" etc.. to define the ray direction vectors? So far, I'm getting errors that these variables are unkown. Thanks for any help and please let me know if you need additional information.

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Spherical Wave

February 13, 2018, 8:30 pm
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Hi, all

I want to introduce spherical wave in my model by ports. How can I set the incident wave to be spherical incident wave?

Thank you! Chen Gong

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Piezoelectric gyroscope setting

February 14, 2018, 5:05 am
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Hello, I'm simulating the Coriolis force effect on a piezoelectric vibrator in order to detect the angular velocity motion. I think I set propoerly the demanded boundary conditions to provide Drive mode (ossilation/vibration mode) using AC signals (Vsin(2pift) defined in Terminals with domin frequency study. However when applying the rotating frame considering Corilois force and centrifugal force , Effect of Corilois force does'nt appear only under very hight input value of angular velocity (500 rad/s). Using 500 deg/s input rotationg frame, sensing voltage do not change as if Corilis force doesn't been generated.

Do we need a special setting of the solver under domain frequency study when detection angular velocity? or there is something wrong in boundary conditions?

I would be really grateful if somebody could help with this. Sincerly

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Use mef physiscs to calculate the capacitance

February 14, 2018, 7:14 am
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Hello everyone, I have a problem with calculation of capacitance by using the physics "mef".

My model is a conductor disk isolated with paper placed in surrounding air. I want to calculate the capacitance of this disk at different frequencies, so I choose "frequency domain" as solver.

After computation, comsol does not provide such a variable of capacitance in global evaluation. Can someone tell me how to calculate the capacitance of the disk in this case?

Before, I always calculate the capacitance with this expression, 2*mef.intWe/real(mef.VCoil_1)^2

However, in this case, a disk is comprised of several independent conductor and I need to exciate each conductor separately. It means, I need to define a "single turn coil" for each conductor. So I can not use the equation above anymore, because there is only one total electric energy.

Thank you very much

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Coaxial Cable connection to Antenna

February 14, 2018, 7:35 am
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Hello Guys, I´m new with COMSOL and I try to simulate a quarter wave antenne which is excited by a SMA connector. I designed the coaxial cable like person did it in this tutorial https://www.comsol.com/video/intro-to-electromagnetics-modeling-with-comsol-multiphysics. But now I don´t know how to connect my antenna with the coax cable. Which boundaries do I have to use and is there a gap between the antenna and the coax cable? Thanks for your help

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Releasing only a direct beam radiation into a 3D V‐trough concentrator geometry

February 14, 2018, 10:27 am
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Hi everyone,

How can I set the incident angle of 0° using solar radiation node to release only a direct beam radiation into a 3D V‐trough concentrator geometry?

(Rays that are traveling on a straight line from the sun down to the surface of the earth)

kind regards

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How to create a conditional in a definition?

February 14, 2018, 3:05 pm
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I am doing a two-phase flow analysis in Comsol, based on the model found in the Comsol application gallery called "two-phase flow in column". I have some problems because when I do the analysis I get the following error "Attempt to evaluate the power of zero". According to this i think that the error is due to one of the definitions that is used to calculate the effective saturation degree of the liquid phase.

The definitions that I have in the retention model are:

p_c p_nw-p_w Capillary pressure

Hc p_c / (rho_w * g_const) Capillary pressure head

Se_w (1+ (alpha * rm1 (Hc [1 / m]) [m]) ^ N) ^ (- M) Effective saturation, wetting phase

Se_nw 1-Se_w Effective saturation, non-wetting phase

theta_w theta_r+Se_w*(theta_s-theta_r) Volume fraction, wetting phase

theta_nw theta_s-theta_w Volume fraction, non-wetting phase

kr_w (Se_w^L*(1-(1-Se_w^(1/M))^M)^2)+eps Relative permeability, wetting phase

kr_nw ((1-Se_w)^L(1-Se_w^(1/M))^(2M))+eps Relative permeability, non-wetting phase

Cp ((alphaM/(1-M)(theta_s-theta_r)Se_w^(1/M)(1-Se_w^(1/M))^M))/(rho_w*g_const) Specific capacity

i think that the problem is in the third definition. The question is that there is the possibility of applying a conditional to the definitions in comsol to restrict the analysis to that if Hc <= 0 then Se_w = 1.

Thank you for your attention.

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Thermal/mechanical simulation of two layers: how can COMSOL 'seperate' them due to different CTEs?

February 15, 2018, 1:08 am
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Dear Community,

I am trying to simulate the following case with COMSOL 5.3a. I have a 2D design of a Si wafer, with a layer of SiO2 on top of it (as shown in the picture). I want to apply different temperature values and find out at which T the two materials start to seperate from each other due to different CTEs; hence it is a thermal-mechanical study case.

However, I do not know how can I simulate that with COMSOL; the program shows me how the two materials expand due to elevated T as a unity, there is no 'seperation' between them.

Any help is welcome. Thank you in advance.

Kind regards,

Dimitrios

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Acoustic-structure boundary among sold and fluid in pressure pulse generation

February 15, 2018, 1:20 am
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Dear all, I'm trying to simulate the pressure generated by a thin film that is expanded by temperature variation. In order to take into account the physics at the interface between the film and fluid domain where the pressue propagates (water in this case), I have used the acoustic-structure boundary multyphysics option. The model seems to work but I can't understand the numerical version of boundary condition imposed.

In particular, analyzing the condition expressed in the "equation" of

multyphsics -> acoustic-structure boundary

it seems to me that boundary condition studied in the book(i.e. continuity of the displacement and equality among stress tensor contracted with the normal to the surface and pressure) are applied. But if I plot, as a function of time, at the interface, the acceleration in solid (solid.u_ttZ) and the acceleration of pressure domain (actd.az) they don't match. Since the displacement is the same, I expect the acceleration too is the same. Analogously, the stress tensor in the direction of the normal to the surface (solid.sl33 or solid.sz) doesn't match with the pressure p at the interface.

Is there a particular reason why? I made some mistake (maybe mesh not enough refined)? Thanks

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Challenge. I do not think that someone in this foro can solve this.

February 15, 2018, 2:11 am
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I am currently working in a classical EWOD simulation (I have attached the model). The model uses the following modules: ES (Electrostatics): the voltage is applied through a needle. A thin Low Permittivity gap simulates a hydrophobic layer. SPF (Laminar Flow and two phase laminar flow). The ES and SPF modules are related by a Maxwell Stress Tensor calculation. In the "Volume force" boundary the components of the force in r and z coordinates (calculated from the MST) are introduced. After I performed many (like 60 ) simulations the results did not agree with the theory. I am obtaining a strange result. The drop contracts and mass is lost. I am sure that it is not a problem with the equations. I think that with the current configuration I should see how the interface moves correctly, but that is not the case.

Am I making a mistake in the configuration?

Kind Regards, Cristian

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Stress distribution

February 15, 2018, 2:32 am
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Hello everyone,

Is it possible to generate diagrams of the stress distribution in the cross section? I am working with a 3D model in the Structural mechanics module.

Kind regards,

Jovana.

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Computing stucks at 71%

February 15, 2018, 10:04 am
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Hi I am now trying to simulate a 3d model for plasmonic response from a cylinder. During computation, it reaches 70% within a minutes or so, but then it stucks there for hours. Could you please suggest me the necessary actions to rectify it.

Thanks Mahfuj

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What is L_exiit in Laminar Outflow boundary condition in Laminar flow (spf) physics?

February 15, 2018, 10:26 am
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In spf physics, I like to give a flow rate as an outlet boundary condition. But I have to insert a value called 'L-exit'. What is the physical meaning of this and how do I determine this?

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Convention of pressures

February 15, 2018, 10:26 am
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Hello good morning

I'm new using Comsol. I would like to know if it is possible to know the convention that software has by default for signs of pressures. I would like to know if NEGATIVE INPUT pressure values will be understood by the software as compression values and pressure values that the software receives as positive, which will be understood as traction values.

Thanks for your help.

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How can I display a curve length in result section

February 15, 2018, 11:48 am
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Hello all.

I'm a new user in comsol and now I have a problem. If you explain me this moment I will be very gratefull for you. I'm trying to calculate a helix edge tension after loading. It's the simplest model. It's a cylinder with interrior helix. After loading helix will be streched. I need to know what his length will be. I've displayed a line graph (total displacement), but It isn't what I need. What I should choose in a gap expression for showing edge length?

Thank you for your attention and help. Vladimir.

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Boundary Condition Solid-Liquid Interface

February 15, 2018, 2:14 pm
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I am stuck in a simple problem. It is 2D problem about heat exchange between solid-liquid. Initially I have a square box (1m x 1m) contains 280K water. Inside the box I have a 400k iron block (0.1m x 0.1m). The problem is I dont know which boundary condiction in solid liquid interface I must use in order to study the convection in the liquid and the temperature rate in the iron.

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Flux of a concentration using weak contributon

February 15, 2018, 7:20 pm
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Hi,

I am solving the advection-diffusion equation of a speceis inside a Domain. Now I want to put a flux of the same species on a boundary using weak contribtion inside that physics. What expression should I enter in the weak expression column to set a flux for that species ?

Thanks Apoorv

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About adding my own physics

February 16, 2018, 12:33 am
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Dear sir/madam;

currently i'm using comsol multiphysics 5.3, i want to model heat and mass transfer of beer fermentation , then i selected ' Laminar fluid flow, Heat transfer in fluids and Transport of diluted species" physics for the model but i have two additional my own equation to represent the phenomena but i couldnt enter my own equation in addition to seleced physics.

therefore i would like to ask you how to enter my own ordinary differential equation

or how to add my own physics with comsol 5.3

my two equation are;

     dT/dt= (-XHfgmiu/rhocp)- UA(T-Tj)/rho1Cpv-------(1)       dT/dt=(Fc(Tc-Tj)/Vj)+(UA(T-Tj)/rho2VjCpc)-----------(2)

Thank you in adsvance for your support

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