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 couping and use a user-defined value instead to, for example, investigate a simulation that does not converge.