Model Inputs and Multiphysics Couplings

Model inputs can appear in an equation or material model node’s 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 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). Common model inputs are always available. You can define scalar values for common model inputs under for use throughout the model (see Common Model Inputs).

For frequently used multiphysics couplings, predefined multiphysics coupling nodes are available under the 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 window’s section. For example, with a Heat Transfer in Fluids (ht) interface and a Laminar Flow (spf) interface, you can select , which the node in the branch defines, from the list in the section of the window for the node under . The velocity field from the Laminar Flow interface then becomes the velocity field for the convective heat transfer. You can also choose to use its value to define the velocity field, or choose to enter a user-defined velocity field.

When you have selected a model input from one of the lists, click the 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 dialog box. Then click to move to the selected node.

You can also, if you have selected , click the button (

) to create a local 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 common model input.

By default, the section in the node under the node is empty. But if you, for example, add a temperature-dependent material property such as the dynamic viscosity, a list appears in the section where you can explicitly choose the field or use a user-defined temperature or a like any other model input.

A list in the 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 list is unavailable in the node under . This automatic connection selects the 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 button (

). Using this option can be useful in order to, perhaps temporarily, break a multiphysics couping and use a user-defined value instead to, for example, investigate a simulation that does not converge.