Latest Discussions - COMSOL Forums Most recent forum discussions Mon, 19 Aug 2019 22:41:01 +0000 COMSOL Forum: Latest Discussions Frequeny <p>Hi I have been trying to get the vortex shedding frequency and the St number from the standard COMSOL model of flow past cylinder.</p> Mon, 19 Aug 2019 22:41:01 +0000 4.2019-08-19 22:41:01.241091 microring resonator simulation <p>Hello, recently, I try to simulate the ring part of the microring resonator with 2D axisymmetric referring the "models.woptics.step_index_fiber_bend." case in the app library. The aim is to obtain a stable and effective mode, but the light field always appears on the boundary. If the clad (envelope) size and core size are expanded close to 1 mm, a relatively good effective mode can be found. But my material is silicon, and I want the minimum bending radius is 3 microns. In the simulation process, only the structure and material are changed, other settings are consistent with the example, I had tried to change the boundary conditions, but the results have not changed much. I want to ask whether such a smal size is not supported in 2D axisymmetric? How can I get a better effective model? can someone give me some help? !!</p> Mon, 19 Aug 2019 07:17:54 +0000 4.2019-08-19 07:17:54.241073 Meshing & structure problem <p>I built a 3d model to study the flow characteristics of microfluidics, but I couldn't get accurate results due to differences in size. The size of the structure in the z direction is much larger than the dimensions in the x and y directions. I think it's a problem with meshing because it's hard to split the appropriate mesh. As shown, the flow direction of the fluid is along the x direction. The graphics obtained with the physics-controlled grid are strange, and sweeping the grid along the z-direction yields erroneous results. I want to know how this structure can simplify or divide the correct grid?</p> Mon, 19 Aug 2019 01:54:48 +0000 4.2019-08-19 01:54:48.241063 Structure & Meshing Problem <p>I built a 3d model to study the flow characteristics of microfluidics, but I couldn't get accurate results due to differences in size. The size of the structure in the z direction is much larger than the dimensions in the x and y directions. I think it's a problem with meshing because it's hard to split the appropriate mesh. As shown, the flow direction of the fluid is along the x direction. The graphics obtained with the physics-controlled grid are strange, and sweeping the grid along the z-direction yields erroneous results. I want to know how this structure can simplify or divide the correct grid?</p> Mon, 19 Aug 2019 01:42:30 +0000 4.2019-08-19 01:42:30.241053 fsi ,breaking the incompressibility condition <p>Hi What is the reason for breaking the incompressibility condition in a numerical study of the effect of fluid-structure interaction on transient natural convection in an air-filled square cavity with constant fluid properties with 2 boundary condition Warm left and right walls and The upper flexible wall?</p> Sat, 17 Aug 2019 19:06:06 +0000 4.2019-08-17 19:06:06.241032 Offset function beam <p>Hi everyone. I would like to know if there is a function OFFSET BEAM in comsol.</p> <p>Enrico</p> Sat, 17 Aug 2019 14:39:41 +0000 4.2019-08-17 14:39:41.241031 Different Result ! Boundary surface and cut-plane <p>I have a simple T-junction micro-mixer</p> <p>Geometery: 200*200 micrometer - lenght 2.7 mm <img src="https://" alt="" /></p> <p>Concentration Contour at outlet boundary using "surface" : <img src="" alt="" /></p> <p>Concentration Contour at outlet boundary using "cut-plan" : <img src="" alt="" /></p> <p>The problem is that when I use boundary surface for exctracting results I attain some results which are compeletly different from when I use cut-plan at same location. As you see in pictures, concentration countors are not identical in quantities and distribution! What is the problem? and why? would you please recommand a solution?</p> <p>Sincerely,</p> Sat, 17 Aug 2019 10:16:01 +0000 4.2019-08-17 10:16:01.241022 Model converges below a certain physical dimension size <p>Hi,</p> <p>I'm trying to make a 2-D model of an electrolysis cellwith fluid flow into a 3-D model. All the model parameters remain the same only a depth to the model is added to make it 3-D. If the model is kept physically small like 2x2 mm the model converges. If the model is increased to a size of for example 30x30 mm the model does not converge, and error's are given stating that a matrix is not defined. I'm wondering what causes this effect. I assume this has something to do with the computational load of the model (the kinetics are quite complex).</p> <p>What is the best method of making the model converge with larger physical dimensions? Should I use some other type of solver (now using the segregated solver)? Should I use some sort of special mesh (now using physics controlled mesh)?</p> <p>Best,</p> <p>Alex</p> Fri, 16 Aug 2019 20:13:33 +0000 4.2019-08-16 20:13:33.241012 Particles are not falling down under gravity rather they are following a parabolic profile through out the domain <p>hi, im using particle tracing with fluid flow coupled with single phase laminar flow, here my base fluid is air and particles are given density of 2200kg/m3. I ran the simulations by switching on the gravity, drag force while i maintain the inlet velocity is as low as 0.02m/s . What i expected is that the partcles should follow a falling trajectory and settle down on the bottom wall, but to my surprise particles are continued to flow along the channel with same straight path as if they are neutrally buoyant particles. Is any one faced this?? someone please explain why it is behaving like this.</p> <p>I used a 2d domain with l b = 2m 0.1m particle size is =100 microns.</p> Fri, 16 Aug 2019 15:42:47 +0000 4.2019-08-16 15:42:47.240992 Pintwise periodic boundary condition <p>Hi I want to model a cube with periodic boundary conditions and uniaxial tension. I need to apply the following equation as boundary conditions to each two surfaces. For example for the right and left surfaces it is like this: U_left=U_right+U_masterNode where master node is a node in the corner of the left surface. I would really appreciate if someone could tell me how to couple the difference in the displacement of two surfaces with the displacement of a node?</p> Fri, 16 Aug 2019 13:16:05 +0000 4.2019-08-16 13:16:05.240982 Boundary conditions <p>I want to apply gaussian heat flux to A 2-D heat transfer in solid problem. Actually I am looking to do simulation of crater formed during micro Electron Discharge Machining. How can I proceed?</p> Fri, 16 Aug 2019 06:53:03 +0000 4.2019-08-16 06:53:03.240981 contours circular domain <p>I´m modeling a crushing pipe test and i model it as 2D plain stress. The test consists of loading the pipe according to its upper generatrix and the support is materialized in two lower generatrices located 15 degrees from the vertical . I model the inferior support with a roller restriant and simetric restraint in the vertical axes. The lower support covers a small 12 degree circle arc .After process, the result of the reaction in inferior support ( small arc) don´t coincide with the radial direction . When I check shear tesions in the small arc of the support a graph show non cero values. What a mistake I am making?</p> <p>Thanks a lot.</p> Fri, 16 Aug 2019 01:28:46 +0000 4.2019-08-16 01:28:46.240962 microfluidic <p>hi I want to model a Y-micromixer with uniform magnetic field? in down inlet is ferrofluid, Can someone tell me how can I simulate it and couple magnetic field from down side ?</p> <p>Regards</p> Thu, 15 Aug 2019 19:33:42 +0000 4.2019-08-15 19:33:42.240953 "Error: Cannot evaluate expression." in AC Capacitor introductory example <p>Hello,</p> <p>I am just starting out using COMSOL, so I am using <a href="">this</a> tutorial, and others, to help me get acquainted with the software. Everything goes OK until I get to the "Global Evaluation 1" and "Global Evaluation 2" on page 12. When I try to evaluate the stated expressions (i.e. "ec.Y11" and "imag(ec.Y11)/"), I get an error dialoge box:</p> <p>"Error Cannot evaluate expression. Undefined variable. - Variable: ec.Y11 - Global scope Failed to evaluate expression. - Expression: ec.Y11"</p> <p>When clicking the "more information" link, nothing happens, but as I understand, the variable "ec.Y11" has not been created yet. However, I don't know how to create that variable. Has anyone run into this error before? Can someone help me fix this? I have very little experience with COMSOL so any information would be helpful!</p> <p>I've uploaded the file I'm using for reference, but I had to use 7zip to make the file small enough to upload.</p> <p>Thanks!!</p> Thu, 15 Aug 2019 14:43:24 +0000 4.2019-08-15 14:43:24.240952 What is the physical meaning of scaling factor in PML setting? <p>Hi all,</p> <p>I am currently using pml to absorb light to prevent back refletion. In my simple model, I incident the light by using a port to a mirror (using perfect electric conductor BC), and let the reflected light to enter the pml behind the port. Periodic boundary conditions are used on the sides of the rectangles, and scattering BC is used at the end of the pml. Before entering the pml, I also have a boudary to integrate the poynting vector perpendicular it.</p> <p>I expect, if I do it correctly, all the light gets reflected by the mirror and I will get all of the incident light. But I also realized that there may be significant reflection if the light enters the pml with a very large angle. I started to tune the parameters in the pml settings and found that when I am using rational stretching and a very large scaling factor (~200), the results are pretty good. The settings worked well in another slightly more complicated model which I used to collect the light by using port's diffraction orders.</p> <p>However, I have no idea what I have been done phyiscally by tuning the scaling factor in pml. Can anyone help me with this?</p> <p>Many thanks!</p> Thu, 15 Aug 2019 11:00:12 +0000 4.2019-08-15 11:00:12.240943 Mositure absorption in desiccant <p>hi, i am trying to simulate absorption of moisture in aqueous solution of Licl (mass fraction of LiCl in range of 0.3-0.4). it would be great if i can get some help in adding aqueous LICL as material and also the physics for the mass transfer</p> Thu, 15 Aug 2019 06:37:32 +0000 4.2019-08-15 06:37:32.240912 How to get maxop1(T,x) to return one value in time-dependent problem? <p>Hello,</p> <p>I have a 3D time-dependent heat transfer model and I am trying to find the location of the maximum temperature.</p> <p>I have defined the coupling operator maxop1 in the Definitions tab under Component. However, when I try to get the x-coordinate of the maximum temperature using maxop1(T,x), I believe I am getting a list of values, one for each solution time.</p> <p>Is there an expression to retrieve only ONE x-coordinate corresponding to the maximum temperature over all times instead?</p> <p>Thank you,</p> <p>Jeremy</p> Thu, 15 Aug 2019 03:04:59 +0000 4.2019-08-15 03:04:59.240902 Problem with meshing <p>I am using COMSOL for modelling the movement of the surface water. Figures are attached. My input data is in the form of topographic points, with X, Y and Z, and I am using 2D modelling (figure 1). The geometry is a square with the size of 6000 in 6000 meters. I also introduce a river to the geometry by defining the points of the river (figure 2). The only type of meshing that models the topography correctly is the mapped meshing with fixed distribution of elements. As I use other meshing techniques, the simulation shows water on the surface of the ground even if I consider the source term equal to zero (figure 3, the picture shows running the model with source term of zero). The problem of using mapped mesh in my case is that I cannot define an inner boundary (i.e. a stream) in the geometry. What do you recommend me to do so that I can use a meshing in my model, without having the problem of showing water on the surface even if the source term is zero?</p> Thu, 15 Aug 2019 01:43:52 +0000 4.2019-08-15 01:43:52.240892 Mode Analysis, Eigenfrequency, and Frequency-Domain Modal <p>Hello,</p> <p>I am working with comsol 4.4, and I know I can use the mode analysis to get the modes of a waveguide or a resonant cavity. However, I wanted to perform a mode analysis of a patch antenna to get the characteristic modes over frequency. Could anyone tell me how to run the mode analysis or the Freuqency-Domain Modal solver with conductive structures? I am currently running a model like that, but none of the mode solutions make any sense.</p> <p>Thank you.</p> Thu, 15 Aug 2019 00:48:05 +0000 4.2019-08-15 00:48:05.240882 Interpolation function with cylindrical coordinates <p>Hi everyone! I am trying to implement a Custom surface charge on a cylinder in my comsol Model. I'm having a file with cylindrical coordinates %r ,%phi,%a and a Value which is giving the charge and I am trying to Import it but i can't find a working method. (So it would be 3 arguments and Function with position 1)</p> <p>I deselected "use spatial coordinates as arguments" and under Units i put the Arguments m,deg,m and Function C/m^2 I made a cylindrical coordinate system (sys2) but I cant find out how to use the sys2.r,sys2.phi,sys2.a instead of the x,y,z... so if i plot the interpolated data it is always interpreted as cartesian coordinates instead of cylindrical..</p> <p>Please help me!! Thanks in advance!</p> Wed, 14 Aug 2019 12:59:04 +0000 4.2019-08-14 12:59:04.240862