Expression

The Expression class in NeoFOAM holds a set of operators to express an expression and ultimately helps to formulate equations. Its core responsibility lies in the answering of the following questions:

• How to discretize the spatial terms?

  • In OpenFOAM this information is provided in fvSchemes

• How to integrate the system in time?

  • In OpenFOAM this information is provided in fvSchemes

• How to solve the system?

  • In OpenFOAM this information is provided in fvSolution

The main difference between NeoFOAM and OpenFOAM is that the DSL is evaluated lazily, i.e. evaluation is not performed on construction by default. Since, the evaluation is not tied to the construction but rather delayed, other numerical integration strategies (e.g. RK methods or even sub-stepping within an the equation) are possible.

To implement lazy evaluation, the Expression stores 3 vectors:

        classDiagram
    class Operator {
        +explicitOperation(...)
        +implicitOperation(...)
    }
    class DivOperator {
        +explicitOperation(...)
        +implicitOperation(...)
    }
    class TemporalOperator {
        +explicitOperation(...)
        +implicitOperation(...)
    }
    class Others["..."] {
        +explicitOperation(...)
        +implicitOperation(...)
    }
    class Expression {
        +temporalTerms_: vector~Operator~
        +implicitTerms_: vector~Operator~
        +explicitTerms_: vector~Operator~
    }
    Operator <|-- DivOperator
    Operator <|-- TemporalOperator
    Operator <|-- Others
    Expression <|-- Operator
    

Thus, an Expression consists of multiple Operators which are either explicit, implicit, or temporal. Consequently, addition, subtraction, and scaling with a field needs to be handled by the Operator.