Definitions
Just as the Global Definitions branch () collects user-defined parameters, variables, and functions accessible at all levels in the Model Builder, the Definitions branch () under each Component collects the definitions of variables, functions, and other objects where the geometric scope is restricted to a single component.
Definitions are under the Component branch because several model components can separately be defined in one multiphysics file, for example, when treating certain parts of the whole model in 2D and other parts in 3D. Definitions which in some way refer to domains in a geometry, the geometry’s dimension or its coordinate names must therefore be held apart in different Component branches.
An example of the type of objects you can add under the Definitions branch is a Selection node (), which saves selections of geometric entities (boundaries, for example) that relate to a region or part of the overall geometry for reuse in operations later in the modeling process.
Add a Component node to the Model Builder, then add definitions with a local scope that apply to that specific model Component. Click or select features either using the Definitions toolbar or right-click Definitions () and choose an option from the context menu:
Select among the following definition types:
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 Variables (): Add user-defined variables to simplify the expressions of other variables and properties.
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Variable Utilities (). Add variable utilities such as matrices and matrix operations, modal participation factors, and Mass Properties, for computing quantities such as volume, mass, center of mass, and moment of inertia
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 View: Create a user-defined view to visualize the model. See User-Defined Views in the Visualization and Selection Tools chapter.
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 Functions (): Add user-defined functions based on analytic expressions or imported data, or specifying parameters for common function types such as step functions, ramps, and random functions.
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 Probes (): Add a probe to monitor the development of a scalar-valued quantity (real or complex-valued number) during a dynamic simulation.
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Component Couplings and Coupling Operators (): Add nonlocal couplings inside or between Components, in the form of integration, mapping, projection and similar user-defined operators.
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Selections (): Create a user-defined set of geometric entities for reuse throughout the model component. See Creating Named Selections in the Visualization and Selection Tools chapter.
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 Pairs: Pairs are only available and necessary when the model geometry is an assembly. See Identity and Contact Pairs. There are two types of pairs:
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Contact Pair (), which specifies two sets of boundaries that can come in contact under deformation. The contact pairs are only available if your license includes the Structural Mechanics Module or the MEMS Module.
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 Identity Boundary Pair (), which specifies two boundary selections (also available for edges and points as Identity Edge Pair () and Identity Point Pair (), respectively) that coincide while belonging to different parts of an assembly. Special boundary conditions connect the physics features in the two parts.
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Coordinate Systems (): Create coordinate systems for use in the physics interfaces.
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Extra Dimensions: Attach extra dimensions to a selection in the base geometry of a model. See Using Extra Dimensions for more information.
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Moving Mesh Features (). Add moving mesh functionality to model moving meshes in a COMSOL Multiphysics model.
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 Topology Optimization (). Add Density Model () nodes to define the density for topology optimization. This functionality requires the Optimization Module. See the Optimization Module User’s Guide for more information.
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Perfectly Matched Layer (PML) (), Infinite Element Domain(), or Absorbing Layer (): Surround your model by a perfectly matched layer, an infinite element domain, or an absorbing layer, behaving as an unbounded extension of the modeling domain. See Infinite Elements, Perfectly Matched Layers, and Absorbing Layers.
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 Ambient Thermal Properties. For specifying ambient thermal properties in heat transfer simulations as user-defined properties or from meteorological data (the meteorological data requires the Heat Transfer Module). The ambient meteorological data comes from the processing of measured data from ASHRAE Weather Data Viewer 5.0 (©2013 ASHRAE, www.ashrae.org. Used with permission.) and ASHRAE Weather Data Viewer 6.0 (©2017 ASHRAE, www.ashrae.org. Used with permission.), given as frequencies of observations, monthly and hourly averaged for several past years of observation. They provide time-dependent weather conditions for more than 8000 stations worldwide in terms of the dry bulb temperature, the dew point temperature, the relative humidity, the wind speed, and the solar direct and diffuse irradiance. See “Ambient Thermal Properties” in the Heat Transfer Module User’s Guide for more information.
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 Model Input. For specifying a value for a model input locally, overriding its global value from the Common Model Inputs node. See Model Input.