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. You should have received an upgrade to 4.0a or should get one anyday now. Discussion Closed This discussion was created more than 6 months ago and has been closed. But when I define a Variable under "global definitions" it doesn't. If I had attempted to type that out on my own I wouldn't have known that. In 4.0, you can define it yourself as a variable using the same expression as in 4.0a: Regarding the error, my guess is that you have defined the variable as a global variable under Global Definitions instead of a local (to the active model) variable under, for example, Model 1>Definitions. 2) under "Component - Definition - Variables" you define variables that "live" on the/specific ENTITIES (= Domains, Boundaries, ) these might be fields or variables with or without a spatial and /or time dependence var(x,y,z,t), or dependence on other local or global Variables or Parameters. In my understanding: Then when you use "Length" in your model it will take the value that is defined on the corresponding Entities, or give you an error because its not locally define somewhere in your analysed geometry. -- Lets start from the setting at the bottom of the window. Good luck Comsol Trial For Student Code To DownIoad Can you tell me how can I have got a demo passcode to downIoad the comsol 5.1 for home windows (test version). Your detailed explaination about the variables relieves my confusion about them a lot. To implement such a model, we need to introduce a switching state variable to decide if we should be following the lower or the upper branch. hence if you wanf to use myFunc (myVar) as a force input in Newtons, you should write: 1 [N]*myfunc (myVar [1/myVar_Units]) or just. br myfunc (myVar [1/myVar_Units]) [N] if myVar are in microns, and you write myVar [1/m] then the microns are scaled to meters and entered into the function as meter value, but without units. However, some variables might only be used in the post-processing and be of no specific use in the solving process, unfortunately these are also defined "high" up in the tree, and not under a (logical for me) "Definition node" at the "Results" or "Data Set" level, hence clearly defined as post-processing variables to work on the resulting Data Sets Functionally, though, it is equivalent. The information provided may be out of date. Perhaps you will find your reply if you write this out. Download two of the examples featured in this blog post by clicking the button below. Would be nice Generally I know that a parameter is a fixed value and a variable can change. We encourage you to try it out and see what other applications you can come up with. In my understanding: Variables - COMSOL Multiphysics It does not seem to work. Regards, So in specific time step of solving u, is the time derivative of variable rho solved by chain rule using time derivative of dependent variable w, or equal to zero since rho is a specific value given w is solved? Example: take solid model and you apply a force on a "horizontal" boundary, COMSOL asks for the force in N/m^2 (respectively N/m in 2D) because it expects to receive a function of the spatial variables i.e "5[N]+3[N/m]*x" a variable force along the boundary. Magnus Ringh If you do not hold an on-subscription license, you may find an answer in another Discussion or in the Knowledge Base. If so, I was talking about the variables I can define under the definitions node. From the Home (Windows users) or Main (macOS and Linux users) toolbar click Variables and choose Global Variables or Local Variables. Analytically this should be -5. Is that not strange? The State Variable feature simplifies the tracking of the history of a field over time. Use the Replace Expression dialog, and try to find it in a more structured manner (see screenshot). Variable in Integration Expression. Suppose that we have a transient model where the temperature is fluctuating in time. Too bad that comsol does not allow the grouping of user-defined vector component variables. Replace variable with scalar expression - COMSOL So, just as the temperature field varies in space and time, the state variable will also. Use the Replace Expression dialog, and try to find it in a more structured manner (see screenshot). Sincerely 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 ), or other global variables. I assume, that the parameter is recomputed every iteration step, but this is not necessary. Run a Get Initial Value if there is no solution yet. I only have version 4.0 but I tried it nevertheless. to access these quantities. Once youve added this feature, you can define variables on any geometric entity level, including: The settings for this feature are shown in the screenshot below. I was wondering what that is and how do I know that exactly. Using Variables Ansible Documentation 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. Run a **Get Initial Value** if there is no solution yet. I was wondering if it is possible to replace a variable in COMSOL like spf.rho using an scalar expression define in the "Definitions->Variables" so it applies to the whole model. I only have version 4.0 but I tried it nevertheless. listed if standards is not an option). The state variable can be updated either before each time step (or parameter step) or after. In this scenario you know that you are looking for the orientation of the axis of Inner Gimbal-Spinning Disc, but not what its variable is called. Version 4.4 When you then add `mbd.hgj3.e1z`, you will find that it is a variable for the Inner Gimbal-Spinning Disc feature. Looks like the variable can not be present in the expression of the integral. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Note: This discussion is about an older version of the COMSOLMultiphysics software. Right-click Global Definitions and choose Variables. tst = eps => if I get a value then "eps" is a COMSOL defined name (this might not be sufficient, as certain variables are only defined by COMSOL ad hoc during the solving process or during the model tree analysis, once you have arrived further "down" in the tree structure ExpressionBase. Discussion Closed This discussion was created more than 6 months ago and has been closed. This, however assumes that the variable you are looking for is announced as a postprocessing variable. For example, I define a physical property like density rho which is only a function of dependent variable w. the time derivative of rho is employed to solve another dependent variable u. [QUOTE] The State Variable feature simplifies the tracking of the history of a field over time. integrating over the spatial domain i.e. Lets next look at some examples of how to use this functionality. The State Variables interface is available when you enable the Variable Utilities option within the Model Builder, as shown in the screenshot below. How to Use State Variables in COMSOL Multiphysics Variables You can add Variables () from either the Global Definitions node or the Definitions node under the Component (local) nodes. Please login with a confirmed email address before reporting spam . Magnus Ringh, COMSOL, Magnus suggestion is the way to go for 'reverse engineering', but what if you were to set up that variable 'theta' without knowing that you were looking for mbd.hgj3.e1z? The variable becomes available for plotting and evaluation after re-computing or updating the solution. Difference between Variable and Parameter - COMSOL Multiphysics Suppose that our wafer has a thin coating on the top that strongly affects the surfaceemissivity. Hi Good luck It's the same for node fields where you fill in values or expressions, these are mostly expressed as "densities" to be understood as what applies directly to a dx*dy*dz volume of the Domain entity (respective surface for boundaries) and we humans mostly work in "total field/flux" mode, that is the integration over the selected entities of any density value. In order to have access to the Replace Expression dialog, you must have some results. However, it is found that the solutions (such as stress and strain) at previous step cannot be accurately recorded for a model with heterogeneous material properties, no matter what Order you select. How to Use State Variables in COMSOL Multiphysics Thank you. With the release of version 5.5 of the COMSOL Multiphysics software, there is a feature for tracking the state, or history, of your model. The State Variables functionality makes some of our modeling work a lot more convenient and easier to use. Ivar. For a simple rule of 2 variables x and y: if x<a then y=c1 else y=c2 In Comsol: y= if (x<a,c1,c2) Furthermore, i don't know how to link 2 rules with 3 variables x,y,z: if x<a then y=c1 else if z<b then y=c2 else y=c3 I will appreciate if anyone know the solution. So how to solve this problem? I am using global equations in Global ODE and DAE module in comsol. Variables & Functions in Comsol 4.1 It works well for homogeneous materials. Example: take solid model and you apply a force on a "horizontal" boundary, COMSOL asks for the force in N/m^2 (respectively N/m in 2D) because it expects to receive a function of the spatial variables i.e "5[N]+3[N/m]*x" a variable force along the boundary. For plots that take vector inputs you must use the defined variables directly in the edit fields for the vector components. 0 Replies, Please login with a confirmed email address before reporting spam.

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