Latest Discussions - COMSOL Forums Most recent forum discussions Thu, 21 Feb 2019 01:21:07 +0000 COMSOL Forum: Latest Discussions Comsol <p>I need to create 3 different media in 2D comsol, and I need to learn how to center an environment the source coordinate, how do you find the expression?</p> Thu, 21 Feb 2019 01:21:07 +0000 4.2019-02-21 01:21:07.228092 Creeping, dilute, surface reacting flow - Negative concentration fixes? <p>Hi all, new to COMSOL</p> <p>I'm modeling creeping flow and transport of dilute species through a series of ribs, with an electrochemical reaction occuring at 10 mA/sqcm on the bottom surface of the under-rib flow. The initial concentration of the dilute species is 6.25 mM. It reacts away to form water. My model results show concentrations varying from 6.29 mM to -63.3 mM, but almost all the variation occurs within the last few millimeters of the outlet channels.</p> <p>The inlet channel is the upper right face of the middle trunk, while the outlets are the two left faces of the outer drains (top and bottom of geometry)</p> <p>I'm looking to either improve the calculations so that I don't get any negative concentrations, or limit the range of plotting values (to only show concentrations between 0 and 6.25 mM). Also, I'm looking to improve the visualization of the concentration distribution, so any tips there would be appreciated.</p> <p>Please let me know if there is any additional information needed, thanks!</p> Wed, 20 Feb 2019 21:53:16 +0000 4.2019-02-20 21:53:16.228082 Plotting / Viewing Predefined Physics Variable Reflection Coefficients <p>Hello world,</p> <p>I haven't been able to evaluate the predefined physics variable for the Fresnel reflection coefficient gop.matd1.rpf or gop.matd1.rpr in an Evaluation Group in the Results node of my model. To test out if it was something wrong with my model, I went back to the <a href="http://"></a> example and tried to do the same thing from its results and got the error shown in the image attached. This is the same error I got with my model. I've evaluated predefined physics variables in other models in the Wave Optics module, and this is my first time using the Ray Optics Geometrical Optics module, but I wouldn't expect it to be any different.</p> <p>In my model I need to generate a surface plot of the imaginary part of the reflection coefficient against frequency and wavenumber.</p> <p>Any advice on what I'm doing wrong would be really appreciated!</p> Wed, 20 Feb 2019 20:42:37 +0000 4.2019-02-20 20:42:37.228072 Data From A Cut Line With MATLAB Livelink <p>The below post is related to an <a href="//">archived discussion</a></p> <hr /> <p>Hello,</p> <p>I am having a similar experience as Pierre Guilleme (linked above), but with an acoustics model. I wish to export the imaginary component of acoustic pressure along a cut line to MATLAB and am finding some sort of jitter (supplementary points as Pierre calls them) in my MATLAB data. Two figures are attached for reference.</p> <p>My model: Frequency Domain: An idealized 3D pipe acoustics model straight out of a textbook (no crazy boundaries, etc). A Cut Line has been defined along the center axis of the pipe. Using a Line Graph in COMSOL, I can easily plot imaginary pressure the expected smooth curve.</p> <p>In MATLAB, I am using the command</p> <pre><code> data = mphinterp(model,{'imag(acpr.p_t)'},'dataset','cln1','solnum',1); </code></pre> <p>to retrieve the values for the first solution of my frequency sweep.</p> <p>What are these extraneous points? Is there a more efficient way to export this data into MATLAB?</p> <p>Regards,</p> <p>Michael Rouleau</p> Wed, 20 Feb 2019 19:18:41 +0000 4.2019-02-20 19:18:41.228062 Domain probe <p>What is the meaning of the integral order 4 (default) in the integral setting option when setting the domain probe? Does the setting of the integral order affect the result when the probe type is average?</p> Wed, 20 Feb 2019 14:07:11 +0000 4.2019-02-20 14:07:11.228033 Modelling using darcy law for multiphase flow in porous media <p>I am trying to model a system having 2 phases. I am using physics Multiphase Transport in porous media, which by default adds two physics - <em>Phase Transport in Porous Media</em> and <em>Darcy Law</em> interface coupled by multiphysics. I want to solve pressure for both two phases, water and gas which gives velocities for both water and gas. But this interface only solves for one pressurre and only one velocity. But in <em>Phase Transport in Porous Media</em> solves for 2 saturation, one gas and one water. How can I do so?</p> Wed, 20 Feb 2019 12:18:30 +0000 4.2019-02-20 12:18:30.228021 About Heatline <p>How to draw heatline using comsol multiphysics ?</p> <p>Regards</p> Wed, 20 Feb 2019 11:20:56 +0000 4.2019-02-20 11:20:56.228011 Restrict displacement for Solid in FSI simulation <p>Hi!</p> <p>Anyone kindly let me know the procedure to restrict displacement for solid in FSI simulation. My model is 3D symmetric and the problem is simialr to check valve where the valve deflects when the fluid pressure increases. But, I have to restrict the opening of solid valve displacement to 0.3mm in y direction for that I have used integration coupling operator (definitions--> complement coupling--> integration) over a point which is experience maximum displacement.</p> <p>Under solid mechanics, from global section I have added global constraint if((intop1(v_solid)&lt;0.3[mm]), intop1(v_solid), 0.3[mm])</p> <p>Here if the condition is true, it should use actual displacement caused by the fluid pressure, else 0.3[mm] of displacement should be allowed.</p> <p>But my model is showing some conflicts. Stresses on where constraints points weredefined got very high as shown in the image. Aim was to restrict the solid displacement in y direction unfortunately stress got much higher and also the deformation patterns does not seems logical. Please help to understand am I going in the right way or are they any other way to restrict displacements for solid in FSI simulation?</p> <p>Thank You. Suraj Sukumaran</p> Wed, 20 Feb 2019 09:43:14 +0000 4.2019-02-20 09:43:14.228001 Modeling a mixture of organic-aqueous (liquid-liquid) flow (10%-90%) <p>I would like to model liquid-liquid flow. one liquid is organic phase, and the other aqueous phase.</p> <p>I thought using <em>volume averaging method</em>, but I couldn't find papers on this topic. Why?</p> <p>I know Comsol suggest using other methods to deal with this flow, which focous more on the changing interface between the phases.</p> Wed, 20 Feb 2019 09:37:37 +0000 4.2019-02-20 09:37:37.227991 Heat flux and thermal resistance <p>Good morning to whom it may concern,</p> <p>I would like to know if there is a feature on COMSOL Multiphysics 5.4 that allows the discovery of R_th and Q_dot on a model where there is only the phenomenon of conduction on several layers of solid materials through a temperature different between the outer surfaces.</p> <p>Thank you very much. Best regards, Juan Hernandez</p> Wed, 20 Feb 2019 09:31:25 +0000 4.2019-02-20 09:31:25.227981 Spectral settings in ray optics <p>Hello everyone, I want to ask how to input the spectral data of the known light source in geometric optics? That is, how the distribution of the wavelength of the light should be input into the light source, thank you.</p> Wed, 20 Feb 2019 02:18:58 +0000 4.2019-02-20 02:18:58.227943 External Property Package in Reactor Engineering <p>Hi! I was trying to implement a external package in the properties of sustances in the reaction. However, it can't run and appers unknown function (repp2hF_mixture) and the properties of the packages disappers. Could you help me?</p> Wed, 20 Feb 2019 01:20:28 +0000 4.2019-02-20 01:20:28.227942 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