Latest Discussions - COMSOL Forums Most recent forum discussions Tue, 21 May 2019 01:40:22 +0000 COMSOL Forum: Latest Discussions Selection of physics <p>What physics needs to be used to analyse the stress/strain experienced by a pipe due to fluid flow?</p> Tue, 21 May 2019 01:40:22 +0000 4.2019-05-21 01:40:22.235642 convective heat flux boundary condition <p>Hi,</p> <p>I'm confused by the result I got by using convective heat flux boundary condition. Hope I can find the answer here. Thanks a lot.</p> <p>I'm working on a time dependent heat transfer model, one of the boundaries is defined as convective heat flux, q0=h<em>(Text-T). Based on the definition of heat flux, q0=-k(dT/dx), I checked my result after running a few time steps. I plotted ht.q0 line graph along the boundary, as well as -material.k_iso</em>Tx along the boundary, they are not the same.</p> <p>I also plot integration(q0) and integration(-k*Tx) along the boundary with respect to time, they are getting closer as time, but I'm still confused about this, is it because my mesh is not good enough? I'm checking all these because I didn't get a good result for my model and trying to find out the reason.</p> <p>Really appreciate your help. Thanks a lot.</p> <p>Jing</p> Tue, 21 May 2019 01:31:58 +0000 4.2019-05-21 01:31:58.235632 How to use the result of each time-step as input to next time-step? <p>Basically this is how I would do it in hand calculation:</p> <p>T_out_0 = 22 [degC] %% Guessed initial value.</p> <p>%% at t=1:</p> <p>COP(1) = 5[1/K] * <strong>T_out_0</strong></p> <p>T_out(1) = T_avg_Domain(1) - COP(1)/7[1/K]</p> <p>%% at t=2:</p> <p>COP(2) = 5[1/K] * T_out(<strong>1</strong>)</p> <p>T_out(2) = T_avg_Domain(2) - COP(2)/7[1/K]</p> <p>and so on ..</p> <p>My problem is how to calculate COP from the <strong>previous</strong> T_out? When I define this in Variables node (As shown below), I got this warning: "circular variable dependencies".</p> <p>COP : 5[1/K] * T_out</p> <p>T_out : T_avg_Domain - COP/7[1/K]</p> Mon, 20 May 2019 20:06:26 +0000 4.2019-05-20 20:06:26.235612 How to simulate AC Multi-turn coil numeric <p>Hi,</p> <p>I want to simulate the result as changing inductace of coil in an active eddy current sensor with different material target. The Multi-turn coil numeric fuction is selected to simulate regtangular coil geometry. I use two steps in simulaiton; step1 Coil Geometry Analysis and step2 Frequency Domain. However, I cannot find the coil inductance parameter in results. There are only coil current and coil resistance DC.</p> <p>Can anyone guide me to solve this problem?</p> <p>Note that I follow the example of using Multi-turn coil numeric with Alternative Current in However, there is only DC simulation case in that example.</p> <p>Thank</p> Mon, 20 May 2019 17:09:33 +0000 4.2019-05-20 17:09:33.235601 How to set user defined Boundary Conditions in poisson equation <p>Hi, I want to define n ˙∇U = (e2/εε0kBT)σsh as my boundary condition in the Poisson boltzman equation, can some please guide me how to do this. Thank you.</p> Mon, 20 May 2019 16:26:01 +0000 4.2019-05-20 16:26:01.235591 Different results of S11 parameter for different mesh element sizes <p>Hi,</p> <p>I have achieved two very different S11 parameter plots for the same patch antenna simulation at "extremelly coars" and "normal" element sizes of mesh. I attached these two results as image files to this question. "normal" one is more similar to the measurement results. But how can I be sure which element size is enough for me. Because, the finer element sizes are very heavy for my computer.</p> <p>Thank you</p> Mon, 20 May 2019 12:04:25 +0000 4.2019-05-20 12:04:25.235581 How to apply frequency shift in acoustic module <p>Hi all, I am doing research in acoustics module on transducers.Please tell me how to apply frequency shift in acoustics module.</p> <p>Thank u.</p> Mon, 20 May 2019 09:14:48 +0000 4.2019-05-20 09:14:48.235561 Light propagation in waveguide <p><em>Can we simulate light propagation in plane tiO2/SiO2 waveguide on silicon substrate? Please help regarding the problem. Thank you.</em></p> Mon, 20 May 2019 08:34:10 +0000 4.2019-05-20 08:34:10.235551 Different results are strange with same definitions but different physics in rmm&mf <p>Dear all,</p> <p>I am simulating a pm_motor that I defined the magnets with Ampere Law. I used the same value of Remenant flux density. When I reached the result, the result in magnetic field is normal but in rmm field is zero. The magnetic insulation is in same condition. I uploaded the pictures of the results. Have you met this problem? How can I solve the problem.</p> <p>Regards</p> <p>Joey</p> Mon, 20 May 2019 08:07:03 +0000 4.2019-05-20 08:07:03.235543 Difference between contact pair and identify pair <p>I am a little confused about the difference between contact pair and identity pair when I do some simulatation of solid machanics . And are there any restrictions of the study type? When we do stationary study, what is the difference between assembly with identity pairs and union? Thanks very much!</p> Mon, 20 May 2019 03:22:27 +0000 4.2019-05-20 03:22:27.235533 resistive heating coil simulation <p>I have a resistive coil from a MEMS. The only data I have to simulate the resistive heating is the input voltage which I can use between 0.5V to 20V and the material of the coil(GOLD). I am getting error like [Failed to find a solution. System matrix is zero. Returned solution is not converged.]. How would someone perform simulation with this amount of data and the given cad model.</p> Sun, 19 May 2019 23:19:50 +0000 4.2019-05-19 23:19:50.235531 Hydrodynamic dispersion in porous media <p>Hi</p> <p>I'm currently working on a mass induced natural convection 2D problem in porous media (single phase miscible displacement).</p> <p>System description:</p> <p>At the initial condition, component 1 fills a rectangular domain. On top, a Dirichlet BC is set with mass fraction(component 2)=1. A non-monotonic density-mass fraction function is implemented, such that when component 1 is pure, it is lighter than component 2. However, when they mix due to diffusion, the mixture becomes heavier forming instabilities which lead to the classical fingering patterns.</p> <p>In order to do so i'm solving the two components mass conservation. The conservation of momentum in done via the Darcy law for porous media. Hydrodynamic dispersion is represented by the scheidegger model which states linear dependency between the mechanical dispersion and velocity (multiplied by the dispersivity coefficient). Finally, a linear viscosity-mass fraction function is implemented.</p> <p>So far so good. I currently use Comsol (finite element and implicit scheme) to do so, and manage to solve the system to get reasonable results (compared to experimental results).</p> <p>The conundrum:</p> <p>Problem starts when i increase the dispersivity coefficient. For some reason, which is counter intuitive to me, when i increase dispersivity, for the same element size, convergence becomes worse up to a point where it doesn't converge at all. I assumed that increasing dispersivity would play a stabilizing factor (decreasing the elemental Peclet number), but for some reason it is not.</p> <p>Any insights?</p> <p>Thanks!</p> Sun, 19 May 2019 08:03:37 +0000 4.2019-05-19 08:03:37.235511 Maximum value from each slice of cut planes <p>Hi,</p> <p>I'm simulating air flow through a tube and looking at difference in presure along the system. I want to graph the maximum pressure along different areas of the system. To do this, I would like to find the maximum value of cut planes along the system. I have only found a way to find a single maximum but not each maximum for each slice. Anyone have advice?</p> <p>Thanks!</p> Sun, 19 May 2019 04:25:17 +0000 4.2019-05-19 04:25:17.235502 Mumps is unable to switch to out of core mode: insufficient disk space for out of core mode <p>Hello,</p> <p>How to resolve "mumps is unable to switch to out of core mode: insufficient disk space for out of core mode" error? I am wokring on a workstation, windows 7, 48gb RAM.</p> <p>Any help would be appriciated.</p> Sun, 19 May 2019 04:11:22 +0000 4.2019-05-19 04:11:22.235493 How to simulate a mesh tube acting as a porous fliter? <p>Hi, I am currently try to simulate a chemical reaction reactor. The figure is attached. The gas flow into the reactor through a tube. This gas tube permit the gas flow inside the reactant region, the gas and the chemical powder react. The tube acts as a porous fliter and permit gas flows in and out , preventing the powder flow out. I want to know how to simulate the mesh tube? The only function of this tube is allowing the gas in and out and preventing the powder out. What physics module or what kind of material character should I add to this region?</p> Sun, 19 May 2019 02:42:52 +0000 4.2019-05-19 02:42:52.235491 How to obtain the dynamic stiffness and damping for a hydrodynamic journal bearing? <p>I am trying to model a hydrodynamic journal bearing using COMSOL and I can get the static performance of the bearing, but I can't find a way to obtain the dynamic performance of this bearing in terms of stiffness coefficient and damping coefficient. Are there any suggestions to deal with this problem?</p> Sat, 18 May 2019 15:28:26 +0000 4.2019-05-18 15:28:26.235483 Computation with Time dependent Study doesnt end? <p>Hi, I am trying to do time dependent study with Electrowetting droplet on COMSOL. My problem is after doing all the settings, the simulations goes on and on and never ends. What can be the possible problem?</p> <p>Thanks &amp; Regards,</p> Sat, 18 May 2019 04:08:21 +0000 4.2019-05-18 04:08:21.235481 Finding The Electron Transport Properties As A Function Of Reduced Electric Fields <p>Hi,</p> <p>I wanna find the transport properties of electrons as a function of reduced electric fields;As Plasma User's Guid suggestion , I have used Plasma Madule>>Boltzman Equation, Two-Terms Approximation>>Reduced Electric Fields .</p> <p>Now there are 2 questions ? 1- What range is good for "Reduced electric fields" at Study section ?</p> <p>2 - What does this error : "<em>Failed to find a solution for the initial parameter. Singular matrix. There are 1 void equations (empty rows in matrix) for the variable at coordinates: (0), ... Returned solution is not converged. Not all parameter steps returned.</em>" mean ?</p> <p>What parameter did I miss to specify at "Intial Values" section ?</p> Fri, 17 May 2019 21:02:55 +0000 4.2019-05-17 21:02:55.235472 What should I do to make the program equally understand the separator "dot" and "comma"? <p>I wrote a simple application with one input field. If I enter in the input box 1.0 (through dot) everything works. If I enter 1,0 (through comma), it gives an error (<em>Syntax error in expression</em>). Is it possible to somehow force the application to correctly read a number written with a comma?</p> Fri, 17 May 2019 16:53:17 +0000 4.2019-05-17 16:53:17.235461 High-frequency background magnetic field <p>Hello,</p> <p>I built a 2D multi-layer structure. When high-frequency magnetic field goes through this structure, I want to consider the attenuation of this field so that I can get different magnetic field dietribution for different layers. To this end, I chose H-formulation Physics and set up the prescribed background field in Dirichlet Boundary Condition. However, I am not sure if this prescribed field will force the field distribution to be the same for each layer? Should I set different background fields for distinct layers? But how?</p> <p>Thanks for your help in advance!!!</p> <p>Napo</p> Fri, 17 May 2019 15:30:58 +0000 4.2019-05-17 15:30:58.235451