Latest Discussions - COMSOL Forums Most recent forum discussions Wed, 20 Feb 2019 01:08:00 +0000 COMSOL Forum: Latest Discussions Carga Pontual <p>Criar um modelo 2d com uma carga pontual el├ętrica ou magn├ętica.</p> Wed, 20 Feb 2019 01:08:00 +0000 4.2019-02-20 01:08:00.227932 The direction of current density vector in Secondary Current Distribution <p>Hi,</p> <p>I'm trying to solve the Butler-Volmer equation using Secondary Current Distribution physic. The values of electrode potential and electronic potential are calculated as what is expected. But, the direction of both electrode current density and electrolyte current density vectors are in the reverse side (in comparison with the case I'm trying to validate my results by) Is it by accident that the magnitude of the current density vector is correct but the direction is not? or there are any other approaches to determining the direction? does anyone have enough experience using this physic who can help me?</p> <p>Thanks in advance, Shahide</p> Tue, 19 Feb 2019 23:41:10 +0000 4.2019-02-19 23:41:10.227921 How to Plot a Graph of Amplitude versus Time in Ultrasound Study <p>Hello Team Comsol, I was assigned to a project that acquired me to simulate or modelling the ultrasound. However, I have a problem in showing the results. I desired a graph of amplitude versus time but I don't know what is the expression for Time.</p> <p>I also wondering if the Study chosen is the right one. Can I just use Frequency Domain Study or use Time-Dependant Modal because it's involving time?</p> Tue, 19 Feb 2019 17:59:07 +0000 4.2019-02-19 17:59:07.227911 Unexpected unit of input. :Function- exp <p>I am trying to add an equatiom ii definition node under variable section. It turns yellow showing error message as: **Unexpected unit of input - Function: exp***</p> <p>Why it is so? What is the mistake in input, can anyone tell? Screenshot attached for reference. Please help.</p> Tue, 19 Feb 2019 13:52:31 +0000 4.2019-02-19 13:52:31.227881 Photonic band-gap analysis queries <p>I am trying to <em>further</em> develop the Band-Gap Analysis of a Photonic Crystal [application ID: 798] model to conduct for a certain photonic crystal I'm designing. And I would appreciate any help or advice that anyone can provide. Thanks in advance!</p> <p>The original model is found <a href="">here</a>.</p> <h3>Context</h3> <p>This model investigates the wave propagation in a photonic crystal that consists of GaAs pillars [70 nm radius] placed equidistant from each other [375 nm square lattice primitive cell side length]. Because it has a repeated pattern, only one pillar is needed for the simulation.</p> <p>I want to use the same model but for a photonic crystal made of SiO2 that consists of Air pillars placed equidistant from each other. Again, because it has a repeated pattern, only one pillar is needed.</p> <p>The complications for this [that I'm aware of] or the aspects I'm unsure of, raise the following questions:</p> <h3>Q1. Dispersion function</h3> <p>The dispersion relation for the refractive index of GaAs, in the original model [application ID: 798], is defined as an analytical function [as it used more than once]. This is expressed, using Global>Analytic, as: -3.3285e5[1/m]*c_const/f+3.8359.</p> <p>Where, [I believe] 'c_const' is the speed of light in vacuum constant [approximatley 300E6 m/s] and 'f' is frequency [Hz]. However, I'm unsure what the values -3.3285e5[1/m] or +3.8359 represent.</p> <p>As I understand it, this equation is used to determine the dispersion of light in the medium. Commonly, the optical dispersion of a medium is estimated using the Sellmeier equation. But the usual form of the equation is very different than the above equation.</p> <p>Of course, my material is SiO2, with Air holes. And so I will need to define an expression for this arrangement for the analytical function. Can anyone advise further?</p> <h3>Q2. Photonic crystal configuration</h3> <p>The original model [application ID: 798] uses a unit cell composed of a stiff inner core and a softer outer matrix material. As stated above, I want to model a photonic crystal made from SiO2 with Air pillars, thus, a soft inner core and a stiffer outer matrix material. Can anyone advise if this arrangement causes any impact that requires a setting/parameter/etc change to the model?</p> <h3>Q3. Band numbers</h3> <p>The original model [application ID: 798] uses a parametric sweep to calculate the dispersion relations for the five lowest bands [1 - 5]. Can anyone explain why? And perhaps advise what bands I should consider for my photonic crystal?</p> <h3>Q4. Wave vector, k, magnitude</h3> <p>Lastly, the original model [application ID: 798] uses an Auxiliary sweep to scan the fraction of the wave vector, k, magnitude from 0 to 0.5 (half the Brillouin zone). And later, using a Stationary Solver>Paramteric> Continuation the Tuning of step size is selected to make sure the solver takes small enough steps when starting the sweep. With values set as: the initial step size = 0.0001; minimum step size = 0.0001; and maximum step size = 0.01.</p> <p>Considering the photonic crystal I want to model, can anyone advise what k value and what step sizes to use for the sweep?</p> Tue, 19 Feb 2019 11:41:28 +0000 4.2019-02-19 11:41:28.227871 Statistical pit formation <p>I am working on a pit corrosion model and was able to describe the growth of a single pit. Is it possible to include a statistical approach into Comsol so the pits appear as a function of time e.g. as some probability distribution function and then grow?</p> <p>Thanks!</p> Tue, 19 Feb 2019 09:15:51 +0000 4.2019-02-19 09:15:51.227861 Microstrip Patch Antenna Radiation Pattern <p>I examine the "Microstrip Patch Antenna" example in the application libraries. I just want to see the radiation pattern of the antenna. I selected "emw.gaindBEfar" expression to see the far-field gain pattern in dB. Eventually, I can see it now. But the grid values around the pattern are not fit with the real gain values as you can see in the following figure. What are these grid values and how can I switch them to the proper values?</p> Tue, 19 Feb 2019 08:44:19 +0000 4.2019-02-19 08:44:19.227851 Difference between current density and induced current density <p>Hello, I'm using the AC/DC module and I need to know the difference between "current density mf.Jx, mf.Jy,mf.Jz"(apparently it's the quantity from Faradays law of induction) and the "induced current density mf.Jix, mf.Jiy, mf.Jiz" Knowing that both quantities have units of A/m^2. Thanks,</p> Tue, 19 Feb 2019 07:24:01 +0000 4.2019-02-19 07:24:01.227841 Secondary current distribution <p>In the steady-state research case of high-temperature proton exchange membrane fuel cells, a secondary current distribution model was adopted, where a fixed boundary potential E_cell condition was added. Is the secondary current distribution suitable for transient research? If so, how do I set it up? If not applicable, how do you simulate the current distribution of a proton exchange membrane fuel cell under transient conditions?</p> Tue, 19 Feb 2019 06:24:58 +0000 4.2019-02-19 06:24:58.227833 Challenge simulation <p>COMSOL colleagues and users:</p> <p>I have two challenge simulations I would like to propose in using COMSOL. I would like to use this forum to hear additional proposed simulations from other folks.</p> <ol> <li><p>tornado - I have always heard that the tornado velocity was so high it could not be measured. A simulation could reveal some amazing information.</p></li> <li><p>hummingbird in flight - perhaps the most challenging fluid-structure interaction example on earth. Could you imagine if man could develop the ability to fly like a hummingbird?</p></li> </ol> <p>What challenging simulations can you think of?</p> Mon, 18 Feb 2019 21:09:10 +0000 4.2019-02-18 21:09:10.227822 Errors in Computation <p>Hello, I am a new user to COMSOL and I see this in one of my studies Undefined variable. - Variable: exp - Geometry: geom1 - Domain: 1 Failed to evaluate Jacobian of expression. - Expression: ((((-nojac((comp1.spf2.betaT<em>comp1.spf2.rho)</em>comp1.v2))<em>comp1.spf2.tau_c)</em>comp1.spf2.res_p)-((nojac(comp1.spf2.rho)<em>comp1.spf2.tau_m)</em>comp1.spf2.res_v))<em>dvol Failed to evaluate Jacobian of expression. - Expression: (comp1.spf2.streamlinens)</em>(dvol) What does this mean? I am conducting a study to patterns of blood platelet aggregation and fibrin formation under complex shear using a multi-bypass ladder as the model</p> Mon, 18 Feb 2019 19:25:40 +0000 4.2019-02-18 19:25:40.227812 How to find out the shear stress distribution of the fluid from velocity profile ? <p>I need to find out shear stress distribution profile of the fluid in my geometry. How do I go ahead from velocity profile ?</p> <p>It is laminar flow, stationery/time-dependent study with water as material.</p> <p>Kindly reply.</p> Mon, 18 Feb 2019 14:34:28 +0000 4.2019-02-18 14:34:28.227811 Sweat Duct Modelling COMSOL <p>Hello,</p> <p>I am trying to model an eccrine sweat duct on COMSOL. The model must have a source of water with NaCl concentration and an outflow through the pore to the skin.</p> <p>There must be reabsorption of water and Na+ ions along the walls of the duct.</p> <p>Any ideas on how to proceed?</p> Mon, 18 Feb 2019 12:47:37 +0000 4.2019-02-18 12:47:37.227801 Activate Geometry During Simulation <p>Hi,</p> <p>I have a question about how I can activate parts of a geometry after a certain time during my simulation. So I have two layers, start with the bottom layer and want to activate the next one after some time.</p> <p>For every answer I am very thankful</p> Mon, 18 Feb 2019 12:19:31 +0000 4.2019-02-18 12:19:31.227791 Calculation capacity of a parallel plate capacitor by varying the permittivity <p>Good morning, I am recently using Comsol multiphisycs 5.1. I created a model for the study of a parallel plate capacitor with mica dielectric. Now to test the sensitivity of the design I would like to change the value of the permittivity in a range between 0.5 and 70 and calculate the capacity. Could someone direct me on the steps to be taken? thank you</p> Mon, 18 Feb 2019 09:17:53 +0000 4.2019-02-18 09:17:53.227761 Deformed Geometry + Chemistry <p>Hello,</p> <p>I am working on a model which describes a chemical dissolution process. My idea was to use the chemistry module which calculates the kinetic data of the surface reaction and couple it with the deformed geometry and transport of diluted species modules. The chem and tds module with flux boundary conditions communicate just fine. But there seems to be a problem with the dg module as the solution does not converge. It does if I put some number into the Prescribed Normal Mesh Velocity box. What could be the reason?</p> <p>Thanks</p> Mon, 18 Feb 2019 08:57:09 +0000 4.2019-02-18 08:57:09.227751 NaN value in Maxwell capacitance <p>Hi to everyone,</p> <p>I'm trying to perform a simulation of a simple capacitor following this example: and applying the dimensions of my system. I attach my model in this message: it is based in two metal plates (one changed and another grounded) in contact with two plastic plates and an air space in the middle. I've created "blk6" in the geometry to try if this way it worked but my initial model did not have it.</p> <p>However, all I'm getting is a NaN result in the Maxwell capacitance (es.C11) and I do not know why. Also, no matter how I change the initial voltatge on the terminal, the value of the "Terminal voltatge" (es.V0_1) remain constant to 1.6364, which I found disturbing.</p> <p>Am I missing any BCs? As I mentioned, I followed the steps of the tutorial above but with my geometry...</p> Mon, 18 Feb 2019 08:45:24 +0000 4.2019-02-18 08:45:24.227741 recovery file can not be detected by the comsol <p>Dear All, I already have two recovery files in the "recoveries" folder of my Comsol. but Comsol can not detect the later one. As I tried to replace the newer one under the name of the older, it now can not detect both of them. do you have any suggestion?</p> <p>I should say that I need the newer one as it is from a long running time model so it would be a huge waste of time if I want to re-run from the beginning! thanks in advance,</p> Mon, 18 Feb 2019 07:46:51 +0000 4.2019-02-18 07:46:51.227733 how can i insert rpm to the electrode in electrochemical module? <p>To simulate ECM i am using electrochemistry module, comsol 5.3a so the problem encountered is to rotate the electrode at a certain rpm in 3D.</p> Mon, 18 Feb 2019 06:42:13 +0000 4.2019-02-18 06:42:13.227731 Microstrip patch antenna <p>Hi,</p> <p>I am trying to design a microstrip patch antenna in comsol multiphysics software. I saw a video on youtube in which they were designing it in CST software. I had the student version of CST software, so i followed the step in the video and designed the antenna. The results obtained from the antenna in my CST software were good. However, now i am trying to design the same antenna with all same parameters in comsol multiphysics and getting awful results.</p> <p>My problem is in CST they were using wave port for exciting the antenna, which was very big and almost floating in the air. In my case, in comsol, i am using lumped port for it. which is very small in size than the waveport of CST.</p> <p>Can anyone tell me how should i get the good results in comsol software, the only difference which i can see in designing the antenna in CST and comsol is waveport.</p> <p>Please help</p> Mon, 18 Feb 2019 06:23:56 +0000 4.2019-02-18 06:23:56.227723