Variable Classification and Geometric Scope
COMSOL Multiphysics provides a set of variables that you can use in expressions when specifying a model and for visualizing and analyzing the solution. A number of variables are always defined everywhere in a Component, for example, the coordinate variables x, y, and z (for 3D and planar 1D and 2D geometries).
Every physics interface also has its own set of variables to represent quantities relevant to the physics or equations that it covers. Characteristics of variables include:
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Parameters and geometric variables are always available.
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The choice of physics interfaces and the dimension of the geometries in the model affect the set of available field variables and special variables.
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Equations can be active in different domains, which also affects the set of available variables. Variables corresponding to certain equation terms are available only in the particular part of the geometry (such as domains, boundaries, or points) where the equation is active.
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Variables defined on boundaries, edges, and points are active if the adjacent domain is active.
Variables are divided into the following general categories:
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User-defined (local) variables
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Built-In Global Variables
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Physical Constants (predefined universal constants)
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Geometric Variables, Mesh Variables, and Variables Created by Frames (variables that characterize geometric properties).
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Field variables (dependent variables and variables derived from them). For example, Shape Function Variables and Predefined Physics Variables.
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Nonlocal Couplings and Coupling Operators
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Solver Variables (available only during the solution process)