Latest Discussions - COMSOL Forums Most recent forum discussions Thu, 02 Apr 2020 15:48:26 +0000 COMSOL Forum: Latest Discussions Terminal velocity question <p>I am trying to set up a simple test problem. A solid sphere falling through water, accelerating until it reaches terminal velocity. I am a fairly new user so I apologise in advance if I missed something obvious</p> <p>My plan was to calculate the accelaration by subtracting the drag force on the sphere at the previous time step from the gravitational acceleration. Use the accelaration and the time step length to increment inlet flow velocity. repeat until acceleration equals zero.</p> <p>I have set up the flow around the sphere no problem. I can calulate the drag force no problem. I am just a bit stuck on how to get the previous time step drag force.</p> <p>I calculate drag by integrating spf.T_stressy over the sphere surface. I have been trying to figure out how to use the prev command. I have added the domain ODE to my component. I am just stuck on what to set the ODE to and then how to use the result of the ODE to calculate last time step drag. Any guidance would be welcomed.</p> <p>Is there an easier way to calculate this? Am I taking the correct approach?</p> <p>Thanks in advance for your assistance.</p> Thu, 02 Apr 2020 15:48:26 +0000 4.2020-04-02 15:48:26.255011 How can I extract the data from a defined variable? <p>So in my study i used parametric sweep and used this expression to calculate total deformation of the system: sum(withsol('sol2',dy,setind(t1,i),setval(t,1)),i,1,20) My problem is that i cannot find a method to tabulate the values, I can plot them and they look great. But i need these values in an excel sheet or something. Help please.</p> Thu, 02 Apr 2020 09:36:47 +0000 4.2020-04-02 09:36:47.254983 Hole on a spherical shell <p>Hello, I am trying to build a metallic spherical shell with a hole on its surface. The shell must be filled with a dielectric e.g. air. Can you recommend any way of doing this?</p> Thu, 02 Apr 2020 08:21:14 +0000 4.2020-04-02 08:21:14.254981 Help Beginner! <p>Hi guys I am a beginner and I use comsol for learning purposes. So, I would like to set up a simulation but I don't know how. I have a butterfly valve that controls the inflow of exhaust gases in a water tank(aprox. 5L but does not matter at the moment).My exhaust gases will heat up the water at it will start to boil. The water bubbles will start to flow out and are also helped by the vacuum pump sucking them out at the end. I would like to have vacuum inbetween the two butterfly valves but I really don't know how to set up the simulation.What type of flow and heat transfer should I use , does anyone have an idea? Thank you in advance.</p> Thu, 02 Apr 2020 08:07:22 +0000 4.2020-04-02 08:07:22.254971 Changes in water pressure caused by underwater moving objects <p>Underwater pressure changes caused by underwater moving objects, how should this pressure change be simulated?</p> Thu, 02 Apr 2020 07:08:10 +0000 4.2020-04-02 07:08:10.254963 Electrostatic shielding by a spherical shell with a hole <p>Hello,</p> <p>I am trying to simulate what i've attached as a jpg. Essentially i want to test the electrostatic shielding of a thin conducting spherical shell <strong>filled with air</strong> with a <strong>hole</strong> on its surface. The electrostatic field must be uniform. The troubles i have are two: 1) I don't know how to put a hole on the shell and then fill the hole domain with air. 2) I don't see a way to simulate the uniform electrostatic field. Should i build a capacitor? Can anyone help? Thanks</p> Thu, 02 Apr 2020 06:48:29 +0000 4.2020-04-02 06:48:29.254961 Phase Change in Time Dependent Study <p>I have created a time dependent study to model the change in temperature of a solid material exposed to room temperature air over 24 hours. Next, I added in ice around the solid material to function as insulation. This ice is exposed to the same room temperature air and before adding the ice the study ran correctly. However, I am having difficulty incorporating the ice as a phase change component. The starting temperature of the ice should be -20 degC (253 K) but the temperature legend only goes to -3.16 degC and the ice component is still white as if it's not properly incorporated to the model (the far right polygon as seen in the picture attached). Additionally, the solid material should have a uniform initial temperature but this is not the case as there are white spots all over the area. I have tried to adjust the mesh but the problem still occurred. This may require a simple fix that I am overlooking but any help would be greatly appreciated.</p> <p>Thank you.</p> Wed, 01 Apr 2020 14:26:39 +0000 4.2020-04-01 14:26:39.254952 Modeling monoatomic layer material in comsol <p>Hello. I am working on a project to model monoatomic layered material such as graphene, standalone and with other composite materials in comsol.</p> <p>We are getting dimensional errors and warnings when trying to make the geometry and mesh, around these sizes: 1cm x 1cm x 1nm.</p> <p>Is there a workaround or can this just not be done?</p> <p>Regards.</p> Wed, 01 Apr 2020 09:37:12 +0000 4.2020-04-01 09:37:12.254941 Optimization Module for heat transfer <p>Dear all,</p> <p>lately I am working on the thermal simulation of a solid material exposed to hot gases to validate some experimental data of around 12000 seconds run time. Inside the solid we measure some local temperatures using 10 thermocouples. Now I am doing a transient simulation, where all know boundaries are defined and based on the thermocouple transient data the heat on the unknown boundary should be calculated. As mentioned I am using 10 thermocouple positions and the data are filtered/smoothed and reduced to 1 measurement/10 seconds, which means one optimization at every 10 simulation seconds. I use the lease-square objective and a local point heat for the control variable, means 10 parameters to optimize. Further I use the Levenberg-Marquardt method. The simulation works so far but it is too slow, even with a course mesh, 4 CPUs and 64 Gb ram available it won't be finished under 24h. With some more refinement even Comsol crashes due to lack of memory. So my question is, is there a way to make things faster? Did I forget to set something else? Since I need to test different meshes and runs several cases and we only have one licence, we can't afford that low speed.</p> <p>Appreciate any help. Thank you.</p> <p>Regards</p> <p>Rahand</p> Wed, 01 Apr 2020 07:57:36 +0000 4.2020-04-01 07:57:36.254921 error:singular matrix <p>When i try to compute a stationary problem of a 3d model,singular matrix occurs,like: - Feature: Stationary Solver 1 (sol2/s1) Failed to find a solution. Singular matrix.</p> <p>For mesh-case 0 there are 29578 void equations (empty rows in matrix) for the variable comp1.V. at coordinates: (-0.04,-0.04,-0.04), (-0.04,-0.032,-0.04), (-0.04,-0.036,-0.04), (-0.036,-0.04,-0.04), (-0.032,-0.04,-0.04), ... and similarly for the degrees of freedom (empty columns in matrix). Returned solution is not converged. Not all parameter steps returned. i find one of my material,air,has a 0 conductivity(default).then i change it to a tiny value,like 1,the singular matrix problem disappears.</p> Wed, 01 Apr 2020 01:41:14 +0000 4.2020-04-01 01:41:14.254913 Adjust colour range for post-processing <p>Dear all,</p> <p>I would like to adjust the colour range of the von Mises Stress such that it automatically shows the maximum and minimum stress values of the domain that is actually visible in the graphics window (i.e. exclude the higher stress values of the domains that are hidden).</p> <p>Does anyone know how to do this?</p> <p>Thank you very much!</p> Tue, 31 Mar 2020 12:41:08 +0000 4.2020-03-31 12:41:08.254891 Is it possible to change the angle of a Gaussian beam in an emfd model? <p>Hello, I was wondering if it was possible to change the angle at which a gaussian beam would come at a model's geometry. Or is it necessary to rotate the whole geometry of the model?</p> <p>I currently have a model with concentric sphere with radii in the 10^-7m range and I would like to see the effects of a gaussian beam at different upon it.</p> <p>All the best, Owen Steele</p> Tue, 31 Mar 2020 02:52:47 +0000 4.2020-03-31 02:52:47.254882 Thermal expansion mass balance. <p>Hi,</p> <p>I am working on model which involves phase change. I can model that successfully, but I noticed mass is varying as the temperature varies. (makes sense as volume is constant but density varies with T)</p> <p>I tried coupling with thermal expansion Multiphysics, but still mass is varying.</p> <p>In both the cases, I used volume average to find the mass of the domain. I also tried defining intop function and defined mass variable. Still the same.</p> <p>I also checked comsol tutorials, I noticed the same issue.</p> <p>How to build a model without mass change?</p> <p>Thank you</p> Mon, 30 Mar 2020 23:07:02 +0000 4.2020-03-30 23:07:02.254872 Mixing Time Scale in Mixer Application <p>Dear All I'm using Mixer Application, after analyzing the project it shows us the mixing time scale Does anybody know that what is the "Mixing Time Scale"? I mean what exactly tell us and how we should use it in our designs.</p> <p>Thanks Ciamak</p> Mon, 30 Mar 2020 16:16:17 +0000 4.2020-03-30 16:16:17.254861 how to apply a recoil pressure in CFD? <p>I am dealing with laser welding simulation. I use a phase field method to track the vapor/melt interface. I can link the heat source to the interface with delta function, but:</p> <ol> <li>how can I apply a recoil pressure on the interface when boiling point is reached to make the interface deform with vaporization?</li> <li>a jump conditon is needed at the interface, linking normal velocities of vapor/melt phases with their densites, how can I do that?</li> </ol> Mon, 30 Mar 2020 12:05:03 +0000 4.2020-03-30 12:05:03.254853 Reusing derived expressions <p>Hi, Is it possible to reuse the output of derived values in the expression of other derived values, to make the calculation more efficient?</p> <p>I have a model with magnetic fields, in which I excite different current patterns one-by-one in an auxiliary sweep (using 0/1 flags as multipliers). Let 'i' index the different cases. In order to calculate the self and mutual inductances between these current patterns, I have to evaluate the 3D integrals of the expressions H_i<em>B_i and (H_i+H_j)</em>(B_i+B_j), and use them in further expressions. B_i is obtained by taking the 'i'th parametric case using the 'with' operator: with(i,mf.Bx), with(i,mf.By)... etc. Since these integrals are explicitely written out in the expressions of each of the inductivities, they are (re)evaluated in each case, making the calculation extremely slow.</p> <p>I thought that if one introduces 'variables', these are evaluated after (or during?) the simulation, and they are then not re-evaluated when using them in derived expressions. Is this true at all?</p> <p>If it is true, it could be a solution. The 3D integral of H<em>B could be defined as a variable: HBintegral=intop_3D(mf.Hx</em>mf.Bx+mf.Hy<em>mf.By+mf.Hz</em>mf.Bz), and one could use it as with(i,HBintegral). However, when taking the cross-terms (i,j) I can not do this.</p> <p>So my question is, is it possible to do it in a smart way? Can I define derived values for the H*B integrals, including all cross terms as well, evaluate them, and then use these values in the further evaluation of the inductivities?</p> <p>Thanks daniel</p> Mon, 30 Mar 2020 09:35:18 +0000 4.2020-03-30 09:35:18.254851 about Multi-turn coil <p>in reality,when I wind a coil,If the coil inner diameter, wire diameter and coil height are constant,the outer diameter of the coil is constant.however,in COMSOL If I want to simulate a multi-turn coil.Should I decide the coil outer diameter based on the coil inner diameter, wire diameter and coil i know,in comsol If the coil inner diameter, wire diameter and coil height of the two coils are the same, but the coil outer diameters are different, the current distribution in the two coils is different.Does this mean that the winding of a certain coil is not tight?</p> Mon, 30 Mar 2020 00:30:32 +0000 4.2020-03-30 00:30:32.254843 Can you control fluid flow with a boundary condition? <p>Hello, First of all, thank you so much for the time you spent reading/answering my question. [Laminar flow module] I have a pipe with a diameter D,' and I reduce the diameter of the pipe to 'd' for short length, and get back to the original diameter. By doing this procedure, I get to build a control flow valve at the middle of the pipe. I can control the flow by reducing or increasing the value of 'd' ('d' has a max value of 'D'). Instead of changing the geometry every time, is there a way to control the flow at the middle of the pipe with a boundary condition?</p> Sun, 29 Mar 2020 22:13:00 +0000 4.2020-03-29 22:13:00.254842 Cannot resolve for " center of coil" error . <p>I am designing solenoid for charged particle tracing but I couldn't resolve for the error "cannot determine centre of the coil" under magnetic fields node.</p> Sun, 29 Mar 2020 16:58:27 +0000 4.2020-03-29 16:58:27.254841 Using remesh deformed configuration as geometry <p>Hello,</p> <p>I'm modeling a thermally actuated microvalve. I'm first running a study on only the actuation part, so the valve moving due to thermal actuation without fluid. I want to use the deformed mesh of that study to run a laminar flow study to calculate the flow through the (open) valve.</p> <p>I'm able to create the deformed mesh, but whenever I want to run my laminar flow study with this mesh I get the following error message: "- Feature: Compile Equations: Stationary (sol2/st1) Error in multiphysics compilation. Evaluations in finite voids are not supported when evaluating using geometries generated from a mesh."</p> <p>Can anyone help me out on this one?</p> <p>Thanks,</p> <p>Stijn</p> Sun, 29 Mar 2020 10:56:24 +0000 4.2020-03-29 10:56:24.254821