Model inputs can appear in an equation or material model node’s Model Inputs section. Model inputs are typically fields such as temperature and velocities that act as inputs for material models and model equations, but they can be any available physical property. They appear in the Model Inputs section if a material is defined so that a material property becomes a function of the temperature, for example. The COMSOL Multiphysics software connects the model input to an existing field (dependent variable) within the physics interface (but not to available fields in other physics interfaces). Default model inputs are always available. You can define scalar values for default model inputs under Global Definitions for use throughout the model (see Default Model Inputs).

For frequently used multiphysics couplings, predefined multiphysics coupling nodes are available under the Multiphysics node (see The Multiphysics Branch). The following part of this section is mostly useful for cases when such predefined couplings are not available.

With more than one physics interface in the model, coupling of the fields is easy: all applicable fields that can serve as inputs in another physics interface automatically appear in the other physics interface’s Settings window’s Model Inputs section. For example, with a Heat Transfer in Fluids (ht) interface and a Laminar Flow (spf) interface, you can select Velocity field (spf), which the Fluid Properties 1 {fp1} node in the Laminar Flow branch defines, from the Velocity field list in the Model Inputs section of the Settings window for the Fluid node under Heat Transfer in Fluids. The velocity field from the Laminar Flow interface then becomes the velocity field for the convective heat transfer. You can also choose Common model input to use its value to define the velocity field, or choose User defined to enter a user-defined velocity field.

When you have selected a model input from one of the lists, click the Go to Source button () next to the list to move directly to the node in the other physics interface that provides the model input. If more than one node contribute to the model input, choose which one to go to in the Contributing Entities dialog box. Then click OK to move to the selected node.

You can also, if you have selected Common model input, click the Create Model Input button () to create a local Model Input node in the current component (see Model Input) for defining a model input for some or all of the geometry in the current component, overriding the default model input.

By default, the Model Input section in the Fluid Properties node under the Laminar Flow node is empty. But if you, for example, add a temperature-dependent material property such as the dynamic viscosity, a Temperature list appears in the Model Input section where you can explicitly choose the Temperature (ht) field or use a user-defined temperature or a Common model input like any other model input.

A list in the Model Inputs section becomes unavailable if the physics itself defines the field because it is then automatically connected to that field. For example, with a Heat Transfer in Fluids (ht) interface the Temperature list is unavailable in the Fluid node under Heat Transfer in Fluids. This automatic connection selects the Temperature (ht) field. As long as the list is unavailable, you cannot change it. If you want to use another temperature field or an expression, you first make the list editable by clicking the Make All Model Inputs Editable button (). Using this option can be useful in order to, perhaps temporarily, break a multiphysics coupling and use a user-defined value instead to, for example, investigate a simulation that does not converge.