A model has certain independent variables with which you can define new ones, also applied only in certain domains. After reading through the documentation, I figured that I could put a variable there, and define the variable to be a different value depending on which domain it is in. Thanks for your help. 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. Functionally, though, it is equivalent. - Function: / Failed to evaluate . Please login with a confirmed email address before reporting spam . Good luck Hi, Best regards, 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. And the I used definitions -> variables subnode for defining the variable C. I could give also other examples. 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. integrating over the spatial domain i.e. Hi You find the variable in the list of predefined expressions but under Definitions (not Electrostatics). This, however assumes that the variable you are looking for is announced as a postprocessing variable. Analytically this should be -5. Hi 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. COMSOL offers a whole range of . That is an interesting question, but likely not one which can be addressed here. COMSOL. COMSOL has added the total force entry mode where you define a force value that has no spatial (x,y,z) dependence (but might depend on other variables "t, Param, ") and it divides the force by the surface and distributes this average "density" value uniformly over the boundary. But it can not find the solution. Then you have two options: Suppose that our wafer has a thin coating on the top that strongly affects the surfaceemissivity. Posted Oct 2, 2019, 8:39 a.m. EDT -- Our laser heating model already defines a Global Parameter called emissivity, which is a constant value of 0.8 and is used within the model to define both the absorbed and radiated heat. Example: I simulate diffusion in two domains that have variables c1 and c2. Variable in Integration Expression. These variables can also be used to affect other fields, such as material properties, and can thus be used to implement hysteresis into your model. Since some releases you have added quite some functionalities under Variable Utilities but Im lacking good examples to ensure I understand them correctly and use them all at the best (this seems to be the only BLOG on one of eleven!) Regarding es.normE: It was missing as a predefined variable in 4.0 but is available in 4.0a. You probably didn't think of it, but in the expression I just mentioned, sqrt, ^, and even + are all examples of operators. To start a new discussion with a link back to this one, click here. Giulio Remo Cupilari. I think it's a bit less confusing now. It does not seem to work. listed if standards is not an option). 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? Use the Replace Expression dialog, and try to find it in a more structured manner (see screenshot). Because It took me some time to sort this out with COMSOL. These variables can also be used to affect other fields, such as material properties, and can thus be used to implement hysteresis into your model. Use the **Replace Expression** dialog, and try to find it in a more structured manner (see screenshot). In order to have access to the **Replace Expression** dialog, you must have some results. Good luck You can then find it in the Variables list in the Inner Gimbal-Spinning Disc node's Equation View subnode, where it is described as the z component of the joint axis. We encourage you to try it out and see what other applications you can come up with. looking forward to your reply and appreciate it! - Function . Really appreciated. In my understanding: For example, in this case, the heat capacity can be defined via an if statement as: if(SorL,SolidtoLiquid(T),LiquidToSolid(T)). But if I want to make a plot across the entire geometry, I define that in domain 1, C = c1 and in domain 2, C = c2. This, however assumes that the variable you are looking for is announced as a postprocessing variable. Unfortunately, unlike the pre-built physics models, there is no option "From material" for the absorption coefficient. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. Velocity expression using local variables, Initial guess leads to undefined function value, How to Use State Variables in COMSOL Multiphysics, Using the Port Boundary Condition in Acoustic Waveguide Models. 9 Replies, Please login with a confirmed email address before reporting spam. In this case, we would want to update the variable after each step. But is your "u depending on rho" linked to your structural physics ? In the current version, you can press Ctrl+Space in the edit fields for each vector component to access user-defined variables. What is puzzling me is I have seen on some forum discussions, that other people who are plotting field intensity are discussing about a predefined expression "normE". If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. to access these quantities. Posted Mar 19, 2015, 9:05 a.m. GMT+1 Allow me to elaborate: let's say that I want to replace the turbulent dissipation (ep) with the following expression ep=eptil+D. If I had attempted to type that out on my own I wouldn't have known that. At least it saves some typing. Using subdomain expression variables makes it easier to enter the subdomain properties of the model. 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 cannot select the variable for plotting and if I write it in the expression field, it is not recognized. Posted Nov 24, 2021, 5:11 p.m. GMT+1 This enables you to: See how the variables are calculated by the software See the names of variables and the quantities each one defines Access and use the variables when formulating expressions Discussion Closed This discussion was created more than 6 months ago and has been closed. now if you are in the time domain a F = F0* (t>1) to turn on the force F after 1 second is a "hard" way to go for your solver, try the heaviside functions to smoothen the turn on/off (search for heaviside in the doc good luck Ivar Version 5.2 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. Send Private Message Flag post as spam. Variables A variable 's expression can contain numbers, parameters, mathematical constants, physical constants, other variables, functions of variable expressions, spatial variables, time, unary operators, and binary operators. Check the Equation View and you will quickly understand (turn on under "File - Preferences" ) Lets start from the setting at the bottom of the window. Most variables that you as a user would be interested in can probably be found this way. The State Variables interface is available when you enable the Variable Utilities option within the Model Builder, as shown in the screenshot below. The State Variable feature simplifies the tracking of the history of a field over time. where SolidtoLiquid(T) and LiquidToSolid(T) are two different functions of temperature associated with the two different branches. The State Variable feature simplifies the tracking of the history of a field over time. Posted Nov 24, 2021, 11:11 a.m. EST Version 5.2 0 Replies . You can also make global variables here by i.e. Run a **Get Initial Value** if there is no solution yet. Suppose that we have a transient model where the temperature is fluctuating in time. Discussion Closed This discussion was created more than 6 months ago and has been closed. If you provide a description, COMSOL uses that and it appears among the other expressions in alphabetical order; otherwise the variable name appears within parentheses. you can define a global variable "Length = intop1(1)", as the operator intop1 is defined over a given Domain or Boundary it will give back a volume, surface or respective length depending on your space dimension of the model, and will be defined by default on all entities in your model. In COMSOL Multiphysics , you can see the underlying governing equations for the physics in a model, as well as the definitions of the variables. 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. Transforming variables with Jinja2 filters . But there is still a problem. Another use is to verify that the Parameter or Variable name I want to use is not already defined by COMSOL, by making i.e. Otherwise, MaxT remains equal to its current value. what about defining a variable (global variable in 3.5a) and use this ? I'm very new to this software and I'm now looking through the application libraries and tutorials. To start a new discussion with a link back to this one, click here. 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. I assume, that the parameter is recomputed every iteration step, but this is not necessary. Regarding grouping of vector components: That is something that we will look into. sqrt(realdot(es.Ex,es.Ex)+realdot(es.Ey,es.Ey)+realdot(es.Ez,es.Ez)) So, just as the temperature field varies in space and time, the state variable will also. Next, the Order controls at how many points within each element the state variables are saved. Use the Replace Expression dialog, and try to find it in a more structured manner (see screenshot). Transcendental functions such as logarithms operate on dimensionless arguments. Hi http://bit.ly/fGRVIa Hi, Your detailed explaination about the variables relieves my confusion about them a lot. Operators in COMSOL Multiphysics. -- Good luck But I don't understand the usage of the definitions -> variables subnode. 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. In one of the tutorials I come across, the modelling gyroscopic effect tutorial, there is a section where a variable is defined in the definition section of the model tree, and the expression reads mbd.hgj3.e1z, and I am expected to type this out. Because It took me some time to sort this out with COMSOL. There arent many settings here that we need to look at, but there is a lot we can do with this feature. For example, the capitalize filter capitalizes any value passed to it; the to_yaml and to_json filters change the format of your variable values. 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