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How can I modify equations of a built-in physics model ?

March 21, 2016, 11:31 am
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Hello,

I am working on a piezoelectric device and I would like COMSOL to take into account the electrostrictive effect.
I know that it's possible to build your own physics but here I just need to add a single term to one equation.
So I'm looking for an option in a menu, or a .mphphb file that I could modify; but so far I haven't found it.

Do I need to rebuild the Solid mechanics physics? Can I add an equation that override the one COMSOL currently using ?

I really appreciate any help you can provide.

Thank you,

Pierre
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Self-consistent Schrödinger-Poisson - Comsol

March 23, 2016, 8:04 am
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Hello,
my question is how I can coupled two equations PDEs (schrodinger-poisson) such as the 1st equation depends on two variable ( u1 and V) or V is a variable in the 2nd equation (poisson).
thank you
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Integrate an Isosurface

March 23, 2016, 1:54 pm
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Hello,

I am wondering if there is any way to find the area of an isosurface, or integrate a function over such a surface.

For example, if I am working with electrostatics, is there a way to check the area of the surface where es.V is equal to 100 V?

One technique I can think of would be to actually perform a volume integral of the divergence of a vector field over the volume enclosed by the surface. I can integrate over the right domain using boolean functions, and if the vector field were chosen to be orthogonal to the surface of interest, the desired area would follow by Stoke's Theorem. However, I suspect the divergence calculation would be very noisy and not yield a nice result.

I've done similar things in MATLAB, and there I found that a much more accurate technique was to export the graphics data from a plotted isosurface and sum up the areas of the tiny triangles composing the surface individually. I know I can plot and color isosurfaces in COMSOL, so could I get the area this way somehow?
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AC current simulation

March 23, 2016, 8:28 pm
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Dear COMSOL community,
I am kind of new to COMSOL and I am building a Lorentz force based MEMS sensor. Basically the current passing through an Aluminum wire on top of the cantilever will interact with the magnetic field from the permanent magnet thus causing the cantilever to deflect accordingly. I manged to simulate it but currently i am using DC current (using single turn coil and terminals) and didn't got any deflection so i think that i should use AC current with a frequency equals to the resonance frequency of the cantilever, could anyone please tell me how i can simulate this ? I already got the eigenfrequencies from another study so i know the resonance frequency. I just need to simulate the AC current now. By the way , i am using stationary study now, should i use frequency domain ?

Looking forward to your answers and discussions,
Best Regards,
Amal
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How can I implement an incremental constitutive model with external material

March 23, 2016, 9:22 pm
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Hi, I want to implement an incremental constitutive model such as dσ=E(σ,α)dε, σ,ε,α are vectors and E is tensor with external material. dε can be calculated by increment of independent variable u. So, how can I get the increment of independent variable in iteration step and how can I build the incremental constitutive model? Thank you very much.
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Level set/Phase field with mass addition

March 23, 2016, 9:35 pm
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Hello all,

I am trying to combine the level set/phase field method with mass addition, but I couldn't find a way to implement it. I saw that the level set/phase field examples in gallery basically follow mass conservation, but I wonder how could I apply boundary shape change at the same time? I've tried to add a moving mesh or deformed geometry module, but failed to evaluate. Anyone could share your experience?

Thanks a lot in advance!
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Global equation expected unit

March 23, 2016, 9:57 pm
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Hi guys,

I'm using a global equation to drive a prescribed mesh rotation.

The equation I'm using is for angular velocity. It has the form

ut = x (a long expression, with units of rad/s, as ut. all variables/parameters are correct, I've triple checked)

In global equations I have the following:

name: u (plane angle)

f(u,ut,utt): ut - x

initial value (u_0): rot_0 (parameter defining initial rotation

initial value (ut_0): 0

Dependent variable quantity: plane angle (rad)

Source term quantity: plane angle (rad)

The equation is in orange and the mouse-over tool-tip says "deduced unit is [1/s], expected is [1]"

Given that the equation is for the angular velocity, was does it not expect [1/s]? I can't see any syntax errors, and it seems logical to me that if the units of 'u' are [rad], then COMSOL knows that the units of 'ut' are [rad/s].

I've tried changing the dependent variable and source term units to angular velocity, but this doesn't help.

I feel like the solution is probably simple but I can't figure out how to fix it and the manual is no help at all. Any guidance welcome.
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ALE technique to large deformation about mold blowing

March 23, 2016, 11:45 pm
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Hi I am planning to use COMSOL for modeling Moldblowing. This model has been described in the paper of Demonstration of 1 Million Q-Factor on Micro-glassblown Wineglass Resonators With Out-of-Plane Electrostatic Transduction . I really need you help hurry, thank you.
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ALE technique to large deformation about mold blowing

March 23, 2016, 11:45 pm
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Hi I am planning to use COMSOL for modeling Moldblowing. This model has been described in the paper of Demonstration of 1 Million Q-Factor on Micro-glassblown Wineglass Resonators With Out-of-Plane Electrostatic Transduction . I really need you help hurry, thank you.
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3D contact Problem

March 24, 2016, 12:28 am
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Hi everyone,

I confronted with a tough problem about contact pairs. I attached my model below. The upside is a structure, the downside is the other structure. Both has semi-sphere roughness, At the beginning, the two semi-sphere just contact, no penetrate. With a downward pressure on the upside structure, the upside has great vertical displacement, resulting the large penetration. However, the software didn't alarm any warning or error. I used the identity pair for this contact. What's the reason for that?
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ALE technique to large deformation about mold blowing

March 24, 2016, 12:32 am
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Hi I am planning to use COMSOL for modeling MOLDBLOWING. This model has been described in the paper of Demonstration of 1 Million Q-Factor on Micro-glassblown Wineglass Resonators With Out-of-Plane Electrostatic Transduction . I want to know which physics do I select, and how to parameters do I set up. I am a new for COMSOL and I REALLY NEED YOU HELP , THANK YOU.
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ALE technique to large deformation about mold blowing

March 24, 2016, 12:42 am
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Hi I am planning to use COMSOL for modeling MOLDBLOWING. This model has been described in the paper of Demonstration of 1 Million Q-Factor on Micro-glassblown Wineglass Resonators With Out-of-Plane Electrostatic Transduction . I want to know which physics do I select, and how to parameters do I set up. I am a new for COMSOL and I REALLY NEED YOU HELP , THANK YOU.
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Problems with High Mach Number laminar flow

August 9, 2012, 12:52 pm
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Hi,

My case is not so common for an application of hmnf that I have not been able to make it work. The main idea consists on modeling the flow physics of an electric arc by considering the arc channel heat as the main energy source for the surrounding gas, using a 2D-axisymmetry model. This gas at the same time will start moving inside an enclosed volume creating shockwaves that will reach the container walls several microsec. after the energy stimulus has been already applied to the one of the solid electrodes in less than 1us.

The problem I've facing is that the simulation does not start and I get the following error message:

Attempt to evaluate real square root of negative number.
- Function: sqrt

I've tried to simplify my model so the initial condition of temperature and pressure (6kK and 0.6MPa at the geometry axis, decreasing to 293K and 0.1MPa within the 0.5mm from the axis) IS the ONLY energy input to the model, on the gas volume (cylinder of r=4mm height=6mm). The materials properties used (copper-electrodes, nitrogen-gas) are mainly constant, avoiding problems with those that might not function for temperatures >1.5kK.

I would really appreciate any comment or suggestion.

Thanks.
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Using matlab function in comsol model

December 14, 2015, 7:05 am
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Hi. I do need help with using m-function in comsol 5.1. after all procedures decribed in tutorials(define function, adding path to m-function i need) comsol cant open matlab even to plot function. comsol shows error:
Can not start Matlab
- Library: csmatlab.dll
Error in multiphysics compilation.
I do sth wrong?
Could sbd who has expirience in such a problems explain where mistake is and what sequence of actions should i execute to succeed.
P.S. I dont need Comsol_with_matlab to use external m-function just to transform some results from comsol, do i?
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Custom Linear Solver

March 17, 2016, 5:24 am
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Hi,
I'm wondering whether it is possible to use custom linear solver for my model on comsol.
For example, can I use my original solver implementation instead of MUMPS or PARDISO at Study ->Solver configuration -> Solution -> Direct Solver -> Solver?
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Relative tolerance vs Relative Error

March 18, 2016, 12:19 am
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Hi all,

having read many relevant threads and also searched in the comsol manual , i still dont understand the difference (if there is indeed some) between relative error and relative tolerance.

for example:
1)in this thread (also in manual)
www.comsol.com/community/forums/general/thread/13157

''The Relative tolerance value is the relative error tolerance that the ODE solver uses.''
''Use the Relative tolerance edit field to enter a positive number. This number controls how accurately the nonlinear system of equations is solved in each time step. In general, the desired relative error in the solution should be entered here.''

2)in comsol manual
''The software stops the iterations when the relative tolerance exceeds the relative error computed as the weighted Euclidean norm''
suggesting a difference

i need some clarification
V4.4

Thanks in advance
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About calculation of the electron trajectory in magnetic lens(2D Axial Symmetry)

March 21, 2016, 6:34 am
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Hello,
I am a beginner of COMSOL.

I want to calculate the electron trajectory in magnetic lens(2D Axial Symmetry).
I can calculate the magnetic flux density.
Then I try to calculate the trajectory by using Time Dependent calculation.
However, electrons do not be affected by magnetic field and just go straight.

Could you check the attached file?
Any help would be greatly appreciated.

Best Regards,
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Boundary conditions for electromagnetic waves

March 24, 2016, 6:35 am
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Dear comminity,

Can you help me with the following:

What should be the boundary conditions if my wave changes its polarisation during the propagation from linear to elyptically - polarised? I suppose ports will not work, because we should define the polarisation of the wave, but in my case it can vary.

I tried PML, but it doesn`t work properly.

So I have a linear - polarised plane wave that is propagating through anisotrpic medium and changes its polarisation to elyptic.

Thanks for help!


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How can I build a elasto-plastic constitutive model with external material model

March 24, 2016, 11:14 am
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Hi, I want to build a elasto-plastic model with external material model. However, there is no strain increment input within external material model. How can I carry out?
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Time-Dependent initial solution violates specified initial conditions

March 24, 2016, 3:19 pm
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I am trying to build a dye-washout model (using Reacting Flow in Porous Media physics to predict the time-dependent concentration of a dilute species). I'm able to run simulations for varying lengths of time, but I'm having issues with my initial conditions.

I specify what the initial concentration of the dilute species should be with an Initial Values node, but when I run the time-dependent study the t0 concentration does not match what I specify. For example, I specify concentration should uniformly be 0.1 mol/m^3 at t=0, but instead the solution will show a non-uniform concentration distribution with an average ~5-50% lower than the Initial Values node indicates. The discrepancy from the specified initial concentration also changes with study length (i.e. ~5% error for study from t=0-0.1sec, ~50% error for study from t=0-10sec).

Am I specifying initial conditions incorrectly? Is there a way to strictly enforce initial conditions?
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