Latest Discussions - COMSOL Forums Most recent forum discussions Wed, 17 Oct 2018 01:37:57 +0000 COMSOL Forum: Latest Discussions Ball-screw <p>Hi everyone,</p> <p>Is anybody here know how can i model a ball-screw with nut?</p> <p>Thanks</p> Wed, 17 Oct 2018 01:37:57 +0000 4.2018-10-17 01:37:57.210372 UvsI Coil and Required Resistor in Parallel in Electric Circuit Node <p>Hello COMSOL team and users:</p> <p>I am using an AC current source to excite a UvsI coil in the Electric Circuit node and I see that I have to connect a resistor in parallel with the current source and with the UvsI coil and I would like to know what is the purpose of this resistor? In the mmf node I specify my multi-turn coil's settings and set the values for the coil wire's cross section area, coil number of turns, the coil wire conductivity, and my the length of my 2D model is specified, so I figured that the resistance of my coil is already built in my model so what is the resistor in parallel with my AC current source and UvsI coil doing and representing?</p> <p>Thank you!</p> <p>David Giglio</p> Wed, 17 Oct 2018 00:29:18 +0000 4.2018-10-17 00:29:18.210362 EHD Cone Jet <p>The below post is related to an <a href="//">archived discussion</a></p> <hr /> <p>Hi,</p> <p>Did you get any reasonable results for the EHD-cone jet simulation. If so, can you share the procedure?</p> <p>Regards, Tawsif</p> Wed, 17 Oct 2018 00:28:09 +0000 4.2018-10-17 00:28:09.210352 Can Comsol simulate unsaturated soil behaviour under cyclic traffic load? <p>For example using Barcelona Basic Model for the unsaturated behaviour simulation. Thank you.</p> Tue, 16 Oct 2018 22:14:29 +0000 4.2018-10-16 22:14:29.210332 Problem to setup an simulation case <p>Problem to setup an simulation case</p> <p>Hi there, I'm new to use this simulation software. I tried to simulate a rotating magnet which create another maganetic field among the balls in a bearing. Those bearing balls rotate themself and revolute the shaft in a parallel plan with the magnet. The round diametrical magnetized magnet has its own magnetic field, as a vector A. Those bearing balls would be generated a combined magnetic field by the rotaing magnet, as a vector B. In our experiment, the vector A shows a little diffefence direction from vector B.</p> <p>I tried to simualte this situation followed the tutorial "Application ID: 10437 Rotating Machinery 3D Tutorial". In the beginning model, I left out the bearing balls' self-rotion and only simulate the revolution of the bearing balls via a rotating magnet. But the result is not as my expectation.</p> <p>The magnet looks like having a rotation but the stream line of magnetic flux density hasn't. And the stream lines don't show up in the lower cylinder part.</p> <p>Can any one give me the suggetion of the configuration of this case? Thanks a lot!!</p> Tue, 16 Oct 2018 17:00:49 +0000 4.2018-10-16 17:00:49.210313 Heat transfer post processing <p>Hello, I have the following problem:</p> <p>after solving a comparably simple heat transfer problem I want to evaluate the temperature distribution in a domain in such a way that I want to know the temperature range (min and max temperature) of the domain after I remove 10% of the elements of the domain with the coolest temperature and 10% of the elements of the domain with the hottest temperature. Any way to do this?</p> Tue, 16 Oct 2018 15:36:12 +0000 4.2018-10-16 15:36:12.210311 Electrostatic Conditions <p>For an electrostatic problem, - what is the default boundary condition? - what is the appropriate symmetry condition and how can that condition be assigned?</p> <p>Thank you very much for any information.</p> Tue, 16 Oct 2018 14:17:14 +0000 4.2018-10-16 14:17:14.210302 Shell and tube heat exchanger simulation problem <p>Dear all,</p> <p>I'm simulating a heat exchanger with some of the tubes plugged within it.</p> <p>I have 19 tubes within the heat exchanger.</p> <p>When I 18 of 19 tubes, at different locations, I get the simulation done, But one of such cases is not getting siimulated.</p> <p>I think that the boundary conditions are correct (as it gets simulated in other cases ).</p> <p>Can you please take a look at the attached simulation?</p> <p>thank you.</p> Tue, 16 Oct 2018 13:53:25 +0000 4.2018-10-16 13:53:25.210301 Kinetics of heating a plastic wire in a furnace (beginner) <p>Good afternoon,</p> <p>I ask for your help to solve a relatively simple problem on COMSOL Multiphysics. Indeed, I would like to study the <strong>kinetics of heating a plastic wire in a furnace</strong> (without any phase change). The goal is to know how long the wire takes to reach the furnace temperature setting.</p> <p>To do this, I used the part "Heat transfer in solids" in which I simulated a solid plastic cylinder and a so-called air fluid environment (by adding a fluid domain in the study). I set the temperature of my hot air at 250 ° C and that of my wire at room temperature. I used the "initial values" section to set the initial temperatures. I also tried a calculation with the tool "temperatures", which allowed me to impose temperatures on the outer surfaces of my block of hot air and on the outer surface of the wire. However, when I start the simulation, first in stationary mode, I get an equilibrium at imposed temperatures. When I start the simulation in time mode (at different time scales) there is no temporal evolution of the temperatures and therefore no kinetics description.</p> <p>In addition, the plastic cylinder is directly embedded in my air volume. Do the booleans in the geometry section allow to perform this subtraction properly?</p> <p>I therefore ask for your help to help me solve this problem. I am a beginner in simulation and thermophysics, and I think that I do not set the right initial conditions, which leads to a wrong equilibrium.</p> <p>I thank you in advance,</p> Tue, 16 Oct 2018 13:09:45 +0000 4.2018-10-16 13:09:45.210291 Equality constraints in optimization <p>Hi,</p> <p>I have to minimize the volume of a heat transfer enhancement having geometrical parameters l1, l2 and h2</p> <p>I have to furnish the following equality constraint in my optimization:</p> <p>-26.143+4.3229<em>(l2^2)+(l1^2)+4</em>l1*l2=0 where both l1 and l2 are geometrical parameters and the value of l2 should change with that of l1 constrained by the above relation.</p> <p>However, I do not know how to write this in COMSOL 5.2. Any suggestions would be appreciated.</p> <p>Thanks, Mahvash</p> Tue, 16 Oct 2018 13:02:00 +0000 4.2018-10-16 13:02:00.210281 Gas flow into chamber <p>Hi,</p> <p>I am new in using comsol. I am working on gas sensor application. I want to simulate the gas flow into the gas chamber to check the diffusion of gas inside the chamber. I read this paper entitled " <a href="">Design and Optimization of Gas Sensor Testing Chamber</a>" and I think comsol can perform the same simulation.</p> <p>Any kind of help will be greatly appreciated.</p> <p>Thank you!</p> Tue, 16 Oct 2018 08:38:43 +0000 4.2018-10-16 08:38:43.210271 How to take into consideration shear waves <p>Hello, Please help me.</p> <p>In acoustics module, how do we specify that specific domains are solid. In order to take into consideration the shear waves. I am using pressure acoustics, frequency domain physics, like the tutorial ''ultrasound induced heating''.</p> <p>Thanks in advance,</p> <p>Nesma</p> Tue, 16 Oct 2018 08:23:20 +0000 4.2018-10-16 08:23:20.210261 What does Coordinate "System from Geometry" mean for a ellipsoid? <p>Hello everyone, I found several coordinate system variations. One of those is System from Geometry. What does that mean in case of selecting an ellipsoid randomly rotated in the room with this kind of coordinate system? Does that mean, that the vectors of the semiaxis´ of the ellipsoid I selected would be my "new" local coordinate system axis´ for the domain "ellipsoid"? (attached Picture)</p> <p>Greetings Daniel</p> Tue, 16 Oct 2018 08:22:30 +0000 4.2018-10-16 08:22:30.210251 Convergence issue when using a small constant (May have reached a singularity) <p>Hello I am solving a 2D transient Poisson and Nernst-Planck equations on two domains. The initial conditions are different in two domains. In the Poisson equations (There are 2 Poisson equations), on the right hand side I have a constant (epsilon) which is defined in Parameters. This is a small constant (Please see the attached photos) I can solve the problem when it is equal 8.85 * (10^(-8)) but the correct value of this constant is : 8.85 * (10^(-10)) . When I use this value the problem does not converge and I get this error: <strong>comsol Repeated error test failures. May have reached a singularity</strong> I am using PARDISO solver and I also changed the solver setting as recommended in this link : <a href="http://"></a> I also decreased the relative tolerance in different steps but none of these tricks was helpful. I am pretty sure there should be a way to solve this problem for epsilon = 8.85 * (10^(-10)) The thing is I can not change the value of initial or boundary conditions or any constants in my model. For example if I multiply the initial conditions by 0.001 this problem converges perfectly but I do not want to change my initial conditions.** I was hoping if using a preconditioner or changing the solver or mesh or WHATEVER could resolve my issue. ** I would really appreciate your time taking a look at my model and help me get around this problem. The file of my model and two screenshots are attached. Thanks in advance for your support.</p> Tue, 16 Oct 2018 04:28:55 +0000 4.2018-10-16 04:28:55.210232 Weak form of Naveir Stokes equations in COMSOL. <p>If a physical model is not listed in the wizard, we can use Comsol's weak form to enter PDE's (governing equations of a system) in their weak form. How can it be done ?</p> <p>for example: 2 equations of momentum (2D flow) coupled with continuity equation</p> <p><strong>ρ v.∇v=−∇p+μ∇2v</strong></p> <p><strong>∇.v=0</strong></p> <p>This model (well-known) is for calculating velocity and pressure fields. How to make weak form and then enter to comsol for solution?. Your help will be strongly appriciated.</p> Mon, 15 Oct 2018 12:22:58 +0000 4.2018-10-15 12:22:58.210213 Fluid in "swirling" motion <p>Hello,</p> <p>I am fairly new to comsol, and to start out in the fluid-simulations and fluid-structure-interaction I wanted to model the "swirling" motion of a fluid inside a circularly agitated vessel.</p> <p>I figured as the structure ist rotationally invariant, I can model this using the 2D axisymmetric component and just model a very basic container with two regions inside. I imported the structural mechanics module, the phasefield module, the laminar flow module and the two-phase-flow interface (multiphysics)</p> <p>(sry if the names are not 100 % correct, I am using the software in german)</p> <p>Using this model I should be able to model the flow while swirling by applying a time-dependent volume force. I am however quite puzzled how I should explain the software that the Force vector is rotating (phi-dependent)</p> <p>can anybody help me?</p> Mon, 15 Oct 2018 11:35:47 +0000 4.2018-10-15 11:35:47.210211 Snap to GRID? <p>Hello,</p> <p>most of the time, when I import designs from an *.stp files into COMSOL, the resulting domain does not have round coordinates, or corrdinates with 1 or 2 numbers after the comma. I found coordinates like: (7.35011993, -54.49998178, -11.8) for example. The consequence of such coordinates is that it is very hard to build the rest of the components, as other faces will either overlap or they will not be in contact with the faces of the imported domain. Is there a function like "SNAP to GRID" in order to place the imported design onto the closest grid coordinates, please? Or is there another way, like attaching the referential to the imported domain, please?</p> <p>Thank you! Regards, David</p> Mon, 15 Oct 2018 08:34:06 +0000 4.2018-10-15 08:34:06.210191 some questions about boundary conduction Flux/Source <p>Hello everyone, I have some questions about boundary conduction Flux/Source in 1D problem. Flux/Source boundary conduction is written as '-n(-c.gradient u-a.u-r)=g-q.u' or as the picture attached shown. My question is in 1D problem, (1)what does the symbol 'n' in Flux/Source boundary conduction mean? (2)Does it mean unit normal vector or just normal vector? If it means unit normal vector, n could be written as n=(-1, 0, 0) on the left boundary in 1D problem? Is that right? Thank you for your attention.</p> Sat, 13 Oct 2018 01:24:21 +0000 4.2018-10-13 01:24:21.210103 Temperature, Humidity, pressure in my AC/DC model . How ? <p>I am modeling a testing of breakdown voltage. I need to consider the Temperature, Humidity, pressure in my AC/DC model. I have already found the local properties and I insert the values but there is no effect. I believe there is mistake in the way that I use to consdier the Temperature, Humidity, pressure in the model.</p> <p>Could anyone please help me with that ?</p> Fri, 12 Oct 2018 21:13:29 +0000 4.2018-10-12 21:13:29.210092 Electric spark Simulation <p>i need to know what physics is used to simulate electric spark in between two surfaces? and how to define a air mixture?</p> Fri, 12 Oct 2018 17:30:38 +0000 4.2018-10-12 17:30:38.210081