Many physics interfaces cannot use the same equations for all study types. For example, time derivative terms must be included in a time dependent study, but not in a stationary study; in a frequency domain study, the time derivatives must be converted into multiplication by i*omega. By default, the physics interface’s equation form is chosen depending on the type of the study step being computed.

When you add a physics interface to a Component, its supported equation forms are listed in the Equation form list in the Equation section of the interface’s Settings window. Study controlled is the default. The physics interface will then select a compatible equation form based on the study step that is currently being computed. Among the supported equation forms, you will typically find ones corresponding to a subset of the most common study step types: Stationary, Time dependent, and Frequency domain. These equation forms will be selected not only when the study step is of the exact corresponding type, but also when the study step is a special-purpose version of one of these basic types. For example, the Adaptive Frequency Sweep study step is a special case of Frequency Domain and will make physics interfaces select Frequency domain as study-controlled equation form.

Many physics interfaces also allow one or more special-purpose equation forms. These typically correspond directly to a particular special-purpose study step type. Examples include Wall Distance Initialization for turbulent fluid flow and Linear Buckling for structural mechanics. These equation forms will be selected only for the exact matching study step type. And conversely, no other equation form will be considered compatible with the corresponding study step types.

If no compatible study-controlled equation form can be found, then the physics interface will not be solved for in that study step. Instead, its dependent variables will be defined according to the Values of variables not solved for settings in the study step. If these settings point to a previously computed solution, then the equation form used for defining postprocessing variables will be the one used when generating that solution. When there is no prior solution to look at, postprocessing variables will instead be generated for the physics interface’s default equation form which is the first one in the list, after Study controlled.

If you want the dependent variables in a physics interface to be solved for in a study step for which the physics interface can find no compatible equation form, then you can choose an equation form manually. Either select one of the available equation forms from the Equation form list in the physics interface Settings, or enable Modify model configuration for study step in the study step Settings, select the physics in the tree and choose the desired Equation form.

Some equation forms require additional parameters which are normally set by the study step or solver. When the Equation form setting is Study controlled, such equation forms are selected only for study steps which define the required parameters. But when an equation form with parameters is selected manually, there is no guarantee that the study step or solver can provide sensible values. Therefore, additional settings appear in the Equation section for the following equation forms:

When the manually selected Equation form is Frequency domain, you must also choose a Frequency. By default, the frequency is taken From solver, which will result in the same behavior as if the frequency domain equation form has been selected as Study controlled. Select User defined to enter another value or expression (SI unit: Hz). This can, for example, be necessary when including a frequency domain physics in a transient or stationary simulation, or when you want to solve for multiple frequencies simultaneously in the same model.

Mode Analysis and Boundary Mode Analysis solve an eigenvalue problem for a propagation constant in space for waves of a fixed frequency. You normally specify this frequency in the corresponding study Mode Analysis or Boundary Mode Analysis study step. If you have selected the Mode Analysis or Boundary Mode Analysis equation form explicitly, you also need to provide the Mode analysis frequency. Enter a value or expression in the field (SI unit: Hz).

The Boundary Mode Analysis equation form in addition to Mode analysis frequency also requires a Port name. If there are multiple ports in an RF Module Electromagnetic Waves interface, the port name setting decides on which port to set up the boundary mode equations. The Port name value must be a positive integer matching the Port name setting in a Port feature.