Discussion Forum

Posted: 1 decade ago Mar 4, 2011, 1:08 a.m. EST

Hi indeed MF/MEF are not set up for time dependent solvers. "My way" around, is to consider the physics and identify the time frames for the different items, i.e. take a magnet falling down in a Cu tube: The eddy currents induced in the tube will "hold" back and make the magnet fall very slowly. How to model that in time dependent ? If we consider the magnetic field you can solve it in stationary mode, extract the Eddy current forces related to a velocity displacement (note: in v4.1 the Lorentz velocity term is not yet set up in 2D, you must complete the BC with the equations, good exercise to write out the curl ;) and then add a global equation GE solving the velocity of the falling magnet expressed as derived from canceling out / minimising gravity forces with the magnetic Eddy current forces. Then you solve this independent ODE in the time domain and at each step you run a stationary for the MEF. You have hence a coupled problem and you solve it in the time domain, with MF/MEF -- Good luck Ivar

Posted: 1 decade ago Mar 4, 2011, 1:56 a.m. EST

Dear Ivar thanks for your helps, i understand your explanation, it is similar to the magnetic brake example in the model gallery: http://www.comsol.com/showroom/documentation/model/2014/ but it is done by version 3.5 behrooz

Posted: 1 decade ago Oct 4, 2011, 3:56 p.m. EDT

Hi, In version 4.2, should the Magnetic Fields module be capable of time dependent solutions in 3D? The specifications table (http://www.comsol.com/products/specifications/acdc/) seems to say that it is, but I haven't been able to get it to work. I tried starting with a very simple problem and checked that the stationary solver could handle it, then included the variable t in a boundary condition. I got the error "Failed to find consistent initial values. Matrix has zero on diagonal. Last time step is not converged," but I don't see a reason for the error. Thanks, Phil

Posted: 1 decade ago Oct 5, 2011, 12:54 a.m. EDT

Hi i think time-dependent solver should be selected, similar to the following example in the model gallery: http://www.comsol.com/showroom/documentation/model/2014/ Behrooz

Posted: 1 decade ago Oct 5, 2011, 3:59 p.m. EDT

The problem turned out to be that the conductivity of air was set to 0 (i.e., I didn't change it from the built-in value). Ivar explained this in a different thread: http://www.comsol.com/community/forums/acdc-module/thread/21628/ The time-dependent solver does work in 3D. -Phil

Posted: 1 decade ago Nov 21, 2012, 11:17 p.m. EST

Hi, I,m doing my project on stator bar winding testing (insulation testing),I want simulate by Comsol.But still i don't know can i do with comsol or not? as example how can measure PD(partial discharge, tangent delta, leakage current, field...) is it possible simulate with Comsol? is there any tutorial or example that have been done before. thanks in advance

Posted: 9 years ago Dec 17, 2015, 9:00 a.m. EST

[QUOTE] Hi indeed MF/MEF are not set up for time dependent solvers. "My way" around, is to consider the physics and identify the time frames for the different items, i.e. take a magnet falling down in a Cu tube: The eddy currents induced in the tube will "hold" back and make the magnet fall very slowly. How to model that in time dependent ? If we consider the magnetic field you can solve it in stationary mode, extract the Eddy current forces related to a velocity displacement (note: in v4.1 the Lorentz velocity term is not yet set up in 2D, you must complete the BC with the equations, good exercise to write out the curl ;) and then add a global equation GE solving the velocity of the falling magnet expressed as derived from canceling out / minimising gravity forces with the magnetic Eddy current forces. Then you solve this independent ODE in the time domain and at each step you run a stationary for the MEF. You have hence a coupled problem and you solve it in the time domain, with MF/MEF -- Good luck Ivar [/QUOTE] Dear Ivar, I am using a coupled 2D MF + Structural model (version 5.1) in order to simulate a pitch-catch setup of an electromagnetic acoustic (EMAT) transducer. I managed to get my model to partially work, i.e. I can see the Rayleigh wave generation and propagation in real time but I failed to correctly model the reception process. I am using a single-turn coil domain (meander-line coil) above the conductor and beneath a dc magnet domain and have added a velocity Lorentz term to my conductor domain where the components come from the structural mechanics module (solid.u_tX & solid.u_tY). Unfortunately, I cannot see the corresponding voltage across my RX coil which should have a profile similar to that of my Rayleigh wave ( see attached picture). Having read your comment above, I am wondering if this is in fact due to the incapability of the MF module for the time dependent analysis, as you pointed out. If so, how can I work around this issue ? Your advice is much appreciated in advance, Regards, Hamed

Attachments:

• rayleigh-wave velocity.tif

Posted: 9 years ago Dec 17, 2015, 12:15 p.m. EST

Just a quick update, I seem to have managed to get the reception also working, at least qualitatively. It turned out that my model is very sensitive to the value of magnetic permeability (ur), thus to see a signal that compares with the experimental measurements a more precise value of ur is needed. There are a few more parameter that need fine tuning too. Regards, Hamed

Posted: 9 years ago Jan 5, 2016, 2:00 a.m. EST

[QUOTE] Hi indeed MF/MEF are not set up for time dependent solvers. "My way" around, is to consider the physics and identify the time frames for the different items, i.e. take a magnet falling down in a Cu tube: The eddy currents induced in the tube will "hold" back and make the magnet fall very slowly. How to model that in time dependent ? If we consider the magnetic field you can solve it in stationary mode, extract the Eddy current forces related to a velocity displacement (note: in v4.1 the Lorentz velocity term is not yet set up in 2D, you must complete the BC with the equations, good exercise to write out the curl ;) and then add a global equation GE solving the velocity of the falling magnet expressed as derived from canceling out / minimising gravity forces with the magnetic Eddy current forces. Then you solve this independent ODE in the time domain and at each step you run a stationary for the MEF. You have hence a coupled problem and you solve it in the time domain, with MF/MEF -- Good luck Ivar [/QUOTE] Hello Ivar, I have done all the geometries, replicating the 2d axisymmetric version of the falling_magnet.mph in 3d but I do not have a velocity, lorentz term that I can use for my copper tube domain. But I do not know how to "complete the BC with the equations" .I know that the equation I have to add is : Ji=sigma*E + sigma*v x B where Ji is the induced current density. How does one give such an equation and where? How are the vectors "v" and "B" defined? I have defined sigma as a parameter and hence, I have no problem with it. Please tell me as to how I have to proceed, in a detailed manner.