how to define variables in comsol

Trivially we also have for dest(x)=dest(y)=0, that G = intop((dom==1)*1)=1. its the same for "solid.rho" rho is depending on the "dom" of the domains as it might change from one material block to another material block (hence one should talk of "rho_i" for the "i" domains in a complex model. Thanks for your reply. Magnus Ringh, COMSOL. Thanks . However, the expression is marked red which means that there is an error, which I could not. Discussion Closed This discussion was created more than 6 months ago and has been closed. One way I like evaluating my ranges is using normalized ranges, for example: As a few of hem are "fields" (dependent on x,y,z,t) and need to be defined accordingly w.r.t the appropriate domains (or integrated over domains to give . Posted Jan 18, 2020, 1:00 a.m. EST COMSOL Employee Sweden The problem is the use of a Global Variable in the Analytic function. The thermal expansion coefficient, alpha -- if(dom==1,epsilonr_air,if(dom==2,epsilonr_container,epsilonr_ball) ) Note: This discussion is about an older version of the COMSOLMultiphysics software. where (a,a,0) is the position of differential elements. How to Use the Find and Auto Completion Tools for Faster Model Setup, How to Use the Parameter Estimation Study Step for Inverse Modeling, Equation-Based Modeling with a Space-Time Discretization, Solving Models with Step Changes to Loads in Time. First, for a 2-D problem, how can I use the x value and y value of a point evaluated? If you want to define *pc=2sigma/r* you can do it as a local variable of your 2d axisymmetrical geometry (comp1) but not as a global variable. Is there a way to speed it up? Usually the richard's equation (for example) is solved for the capillary pressure pc and the velocities u, v, w are results of d (Pc,x), d (Pc,y) and d (Pc,z). Am I right on this one? Ivar, Dear Comsol community, In the manual, but what is the scope etc here? This would allow me to used more complicated boundaries (e.g. Then again T might well be T(x,y,z,) and so on I would be really happy if you help me, I am trying to solve the inhomogeneous wave equation on COMSOL. (dom==1)*alpha , the question is how could I know if this condition is evaluated for boundary 1 or domain 1 ? Best regards, Magnus Ringh, COMSOL Ali, Hi Good luck COMSOL is different from many other FEM tools, these physical parameters are defined on the geometric "elements" and not into the mesh, the mesh elments are mapped on the domains and boundary , and the mesh heritate the physical properties from the domains respective boundaries. Data format of CSV file is as: nx3-matrix with first column as data values, and 2nd and 3rd as x and y-coordinates. 1 Reply, Please login with a confirmed email address before reporting spam. Now you apply again "Finish - Assembly mode". C is obviously a spatio-temporal variable and at each time step the total value of C across the domain would be C_total. If the definition of myVar*t means "a constant times the time t" you can define it directly as Global Variable, where "t" is a predefined variable for time. thanks in advance. For the source term f I use: d (intX,x)-f (x,y). Best At different Pc values, subsequent variables are also calculated such as Se, krw, etc. How and where from can i define the following variable for subdomain in comsol3.5a? listed if standards is not an option). ): Here the excerpt from Comsol documentation : Usually the richard's equation (for example) is solved for the capillary pressure pc and the velocities u, v, w are results of d (Pc,x), d (Pc,y) and d (Pc,z). You need global. How can I see the equations COMSOL is defining? The information provided may be out of date. How to define a coordinate system along a customized direction. To start a new discussion with a link back to this one, click here. You can define a variable as If I use the built-in integrate()-operator for this, I get an error-message. As a simple 2D example, assume I have a square domain [0,1]*[0,1] (here called dom 1). Defining a function for a material enables the software to approximate. But now comes the tricky part, how do I write in Comsol that f(x+dest(x),y+dest(y)) = 1 only when x+dest(x),y+dest(y) \in dom 1 (else 0)? -- I am specifically looking for summation as in Fourier and Taylor series. PHYSICS SETTINGS Define the following parameters for the materials: The thermal conductivity, k The heat capacity, C The density, rho Young's modulus, E Poisson's ratio, nu The thermal expansion coefficient, alpha and Tempereature Regards, Rashedul The boolean expression did the trick. - For a variable defined under Variables, you can define an expression in a flexible way, but it must represent a scalar-valued function (that is, when called with its inputs, it returns a scalar value). COMSOL Multiphysics is a general-purpose software platform, based on advanced numerical methods, for modeling and simulating physics-based problems. You can define a variable as 2*pi*r^2 in an axisymmetric model where r is a spatial coordinate, for example. You can generate a report that displays within the software and includes this information by completing the following steps: Select and right-click the Report node Choose to generate a Complete Report Select the option in the settings to Preview All of the report How to access a variable value at a particular time in COMSOL during simulation Question 5 answers Asked 27th Apr, 2017 Saad Pasha Hi I need to solve an equation like: a = b (at time=0.2s)*exp. Use the Variables node to define expressions as user-defined variables. The boolean expression did the trick. i was looking and learning some Model in Comsol library and it was This is done by adding a "constraint" to the boundary where x=0 with reaction term R=-intX. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Version 4.1 By doing so the problem is converted from an integration to a differentiation and therefore a boundary condition needs to be set. pH = -log ( [H+] [M]). After reading this thread and the documentation, I used the''dom'' variable conveniently in my 1D simulation in 4.0a. I know I could set up a "General projection" for this but I noticed that this is computationally very expensive. Ivar Kjelberg "Retired" but still part time CSEM SA (CH) (root.mod2.at2(x+dest(x),y+dest(y),dom) == 1 Discussion Closed This discussion was created more than 6 months ago and has been closed. For integration of f(x,y) from 0 to x I use a separate "General form PDE" with a dependent variable intX on the domain of the function. Then I get small steps at the beginning but larger steps at the end but I don't need to do any math to figure out how to adjust the function. Hi, Analytic functions take one or more argument that you define directly in the Settings window for the Analytic function. I strongly suspect that the RF module can do this, but my license does not include it and I don't intend to waste any more of my advisor's funds. If you go to the topmost list (i.e Electromagnetic waves (emw) ), then go to the equation tab, there are now options to choose . . COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH), How to Use State Variables in COMSOL Multiphysics. I hope this helps. Also define a new variable, Temperature, to visualize the temperature in all domains in the postprocessing analysis. listed if standards is not an option). Thanks in advance ! Hello Magnus, thanks for the reply. Hi Mohammed, Hi Ivar , 4 Replies Last Post 4 mai 2013, 13:13 UTC+2. Best, I am trying to add this variable as a. Check out Chapter 5 of the COMSOL Multiphysics Reference Manual, version 5.2a. Good luck [QUOTE] So, you can use the calculated outcome as a variable in your boundary. Actually I want to define a variable in my variables(local) that will take the range of values but in my variables we can not use the range function so what other function we have in comsol to define a variable that takes a range of values and can be defined in my variables. listed if standards is not an option). So you probably don't want that. The relation between the temperature and the surface tension is represented in the variable called "Temperature derivative of the surface tension". That is, you can define it without specifying the actual names of the arguments that you will use when calling it. Regards, Hi Hope this helps on the way I hope this helps. Particularly when you need to think meshing, and how to ease the meshing procedure, which is easy or impossible simply by cutting up your geometry in a clever way with a few extra internal boundaries, but that is another story. Worked perfectly for me. If you want to define pc=2sigma/r you can do it as a local variable of your 2d axisymmetrical geometry (comp1) but not as a global variable. Under Component right-click Definitions and choose Variables, or click Local Variables in the Definitions toolbar (Windows users). This is done by adding a "constraint" to the boundary where x=0 with reaction term R=-intX. Then, try to change the pre-defined form to the form of your equation. Posted Dec 8, 2010, 7:32 p.m. EST I defined the 'B' in my variables as well. Once the option is enabled, you'll see a Variable Utilities button within the Definitions ribbon, and a Variable Utilities submenu when you right-click the Component > Definitions branch within the model tree. do it with integrate(expr,var,lower,upper) operator defining some variable tempInt. Your internet explorer is in compatibility mode and may not be displaying the website correctly. 5 + 7 * range(0, 0.01, 1)^2 In the GUI environment these equivalent to the Postprocessing Cross Section Plot's How do I define Pc as something that varies for a range of 'r' values. I'm trying to define a new variable in two different materials (water S=1 and glass S=1.8), and to put it in an equation as, ht.S, so I expect Comsol to pick the data while calculating in glass or water, however it's not reading the new defined variable, it keeps giving error, but when I put the properties of material which was defined automatically, it doesn't have the same problem. Share Improve this answer Follow answered Jan 7, 2021 at 1:01 Sameep Shah 11 1 Add a comment Your Answer For example, if I want to slowly increase the step size I can do: and hare you sure you define these functions and use the variables in the right environment (global definitions or local domain definitions. But I have another question related to this. The density, rho Using global parameters it is very easy to change geometry easily. Thank you so much! This can be solved very nicely. If you want to define pc=2sigma/r you can do it as a local variable of your 2d axisymmetrical geometry (comp1) but not as a global variable. The heat capacity, C "dom" is one of these variables, even an internal variable that takes the entity "ID" be it Domains, Boundaries Edges or Points. Excuse me if my explanation is not clear enough Thanks for the reply Daniel. In my model [H+] is calculated in mol/m^3 unit so, I converted the unit M (mol) for calculating pH. Discussion Closed This discussion was created more than 6 months ago and has been closed. I hope this helps. Once the option is enabled, you'll see a Variable Utilities button within the Definitions ribbon, and a Variable Utilities submenu when you right-click the Component > Definitions branch within the model tree. 9 Replies, Please login with a confirmed email address before reporting spam. Aref Shaikh. Posted Feb 10, 2011, 11:48 a.m. EST 6 Replies, Please login with a confirmed email address before reporting spam. The information provided may be out of date. Best regards, A continuous range of values takes infinitely long to evaluate. I'm not sure what you want to do, but I can see a couple of issues here: Note that while COMSOL employees may participate in the discussion forum, COMSOL software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team. Simulating the Tunneling Current Across a Graded Heterojunction. As I inject the current, the magnetic flux density that is produced is to be measured. This is a tutorial for COMSOL 4.2. Posted Jan 5, 2010, 7:56 a.m. EST Hello Dr Ivan, Your internet explorer is in compatibility mode and may not be displaying the website correctly. Under Global Definitions>Parameters, define variable "freq" and assign the same numerical value of the frequency you are using in the Study Step. I want to use these values as initial values of a variable in comsol. You can define an Analytic, Interpolation, or Piecewise Function to describe any material property in COMSOL Multiphysics. Thank you so much! For the function I tipped different solution in the variable expression: spf.sr e (n-1) or spf.sr*e (n-1) or exp (log (spf.sr) (n-1) Actually I want to define a variable in my variables(local) that will take the range of values but in my variables we can not use the range function so what other function we have in comsol to define a variable that takes a range of values and can be defined in my variables. Posted May 19, 2015, 3:04 a.m. EDT PHYSICS SETTINGS Is the nojac()-operator somehow useful? I don't understand what you want to simulate! You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Thanks! The homogenous, time harmonic case of the wave equation is simply the Helmholtz equation, which is already built in COMSOL. : listed if standards is not an option). is a range from 5 to 12 with 101 points. Ivar. This way if the value of your variable goes above the limit, comsol will just use the value of the limit. Hi Mohammed, You have to study the chapter of "Integration Coupling Variables" in the doc, there are different types, not only on a border or a subdomain (projection or extrusion items), in the worst case you add a few specific interiour subdomains, sometimes this is simpler. By the way in V4 the GUI are rather different, you should consider if its worth to spend too much time on the older 3.5, rather from jumping straight into 4.1 or newer To start a new discussion with a link back to this one, click here. What I want to do is that to define a variable that varies for continuous range of values like the range function does the same but with a step size(discontinuity). I have a problem to define a variable. For f(x) from 0 to x, is it int(f(x),0,x)? Discussion Closed This discussion was created more than 6 months ago and has been closed. In the settings I set all coefficients of the PDE to zero exept for the source term f. For the source term f I use: d(intX,x)-f(x,y). Subdomain Expressions : -- Cheers, Hans. [/QUOTE] In this way COMSOL looks after the continuity on the "interiour boundaries" for you and you have only to deal specifically with the two overlapping points do define some variables to link them together the way you want (or to apply specific Boundary physics on them) Because the microrobot legs consists of several materials with different material properties, you have to define subdomain settings for each materials. While I am defining the variable as pH = -log ( [H+] [M]), it is showing. The range function returns a list (vector) of values, and it is therefore not applicable in the Variables definitions, so it should not work in any of the cases you describe. Thanks for the reply Daniel. Model Library path: Structural_Mechanics_Module/Thermal-Structure_Interaction/microrobot_3d If yo check your geometrical "Object" list you should end up with two Entities. Poissons ratio, nu Hi Roger, So physical parameters are entered on the domains principally. Now, the problem is that I want to vary the absorption coefficient "a" according to the material of the regions I define. In matlab, we can use int(f(x),x0,x1) which represents the integration of f(x) from x0 to x1? listed if standards is not an option). Materials, Parameters, Variables, & Functions, COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH), How to Use State Variables in COMSOL Multiphysics, Mode Analysis for Electromagnetic Waveguides in COMSOL. How to find the in built functions such as integrate. It is useful where you specify lists of values: In the Times field for the output times from a time-dependent study, for example. Is there a better way to integrate over fixed boundaries? r=sqrt((xo-a)^2+(y0-a)^2+(z0-0)^). Usually the richard's equation (for example) is solved for the capillary pressure pc and the velocities u, v, w are results of d(Pc,x), d(Pc,y) and d(Pc,z). In comsol, how can we get this kind of integration? I don't understand what you want to simulate! I believe it is rather important to catch the concept of "Geometrical Objects" and "FEM Entities" in COMSOL. y is my shear rate and can also be written as "spf.sr" and n is a variable I defined already. Hi Hi I am using Coefficient Form PDE physics. I think I got the essentials from your explanations, such a simple geometry is good for the learning process (easier to go throuh all cases): Thanks Dr. Ivar for your interesting reply that sheds light on a matter that we can't find by reading the docs, but only through experience. The source term (f) is -d(d(Pc,x),x). 2*pi*r^2 You should then remain with two boundaries, overlapping) where you apply your "fan effect" or equations to link velocity or whatever variable discontinuity you need. Ivar. ), or other global variables. where a ranges from 0 to 0.707. Note that while COMSOL employees may participate in the discussion forum, COMSOL software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team. I find that these could be better explained, as it took me very long before I started to feel confortable, so I do understand your issue there :) It demonstrates how to define global parameters and variables. Using subdomain expression variables makes it easier to enter the subdomain properties of the model. in an axisymmetric model where r is a spatial coordinate, for example. In such a case, only damping coefficient and source term are non-zero. In my understanding: 1) under "Global - Definition - Variables" you define variables that are INDEPENDENT of the spatial dimension (x,y,z) but these variables might be dependent on time or any other (sweeping) parameter "t, Param, freq . Do you know if it is possible to write such an expression for existing boundaries? Note: This discussion is about an older version of the COMSOLMultiphysics software. How to define region heat source with 3D variables using the If else condition? The case I am working on is a conductor of length 1m lying at 45 degrees with the x-axis. Jeff. With more than 30 add-on products to choose from, you can further expand the simulation For example with dest(x)=2*dest(y)=1/4 the above example should give the answer G = 0.6563 (=21/32). For the source term f I use: d (intX,x)-f (x,y). I am trying to add this variable as a function of T in COMSOL>Model builder>definitions>parameters. You can define probes, coupling variables, and can execute post-processing integrations as well, among other possibilities. So you probably don't want that. A continuous range of values takes infinitely long to evaluate. By doing so the problem is converted from an integration to a differentiation and therefore a boundary condition needs to be set. I found something like: In this way COMSOL looks after the continuity on the "interiour boundaries" for you and you have only to deal specifically with the two overlapping points do define some variables to link them together the way you want (or to apply specific Boundary physics on them) A nice thing about doing a 0 to 1 range is it's easy to modify the progression. 7 Replies, Please login with a confirmed email address before reporting spam. Peter. I would appreciate your help in setting up this problem in COMSOL.

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