Identifying implicit variables

identifying implicit variables

One of the objectives of this feature is to easily identify the implicit variables of the system. There are two types of implicit variable:

• Implicit variables declared in submodels as a constraint or implicit state: these are the declared implicits.

• Constraints automatically created to resolve algebraic loops: these are the generated implicits.

Implicit variables are automatically generated by the Simcenter Amesim solver whenever there is an algebraic loop in the system. An algebraic loop occurs when there is no order in which the submodels can be called such that all the inputs of each submodel are known when the submodel is called. Simcenter Amesim overcomes this problem by introducing implicit variables to break the algebraic loop. The result is differential algebraic equations (DAEs) but they are generally easy to solve using the DASSL integrator. Technically DAEs are classified by an integer known as the index of nil-potency or simply index. Algebraic loops generate index 1 DAEs which are the easiest to solve whereas an index 2 problem is significantly more difficult.

Coming back to the State contributions information:

• Identify the blue colored lines which correspond to an implicit variable.

• Select any variable with this type and double-click the line it belongs to. The component concerned will be highlighted on your simulation sketch.

It is generally a good idea to try and remove these algebraic loops by introducing time constants or other artifices.

For more information about this subject consult:

• Algebraic loops within Simcenter Amesim

• The Submodel Editor user manual: Constraint variables

• The Submodel Editor user manual: Avoiding algebraic loops

• How can I avoid/break algebraic loops?