This information is part of the Modelica Standard Library maintained by the Modelica Association.

Enumeration to define the discretization structure of distributed pipe models according to the staggered grid scheme:

ModelStructure.	Meaning
av_vb	port_a - volume - flow model - volume - port_b
a_v_b	port_a - flow model - volume - flow model - port_b
av_b	port_a - volume - flow model - port_b
a_vb	port_a - flow model - volume - port_b

The default is "ModelStructure.av_vb", i.e., the distributed pipe has "volumes" at its both ends. The advantage is that connections of the pipe to flow models (like fittings) lead to the desirable structure of alternating volume and flow models, which means that no non-linear algebraic equations occur.

Direct connections of distributed pipes with this option means that two volumes are directly connected together. Due to the stream concept this means that the pressures of the two connected volumes are identical, but the temperatures are not set equal (this corresponds to volumes that are connected together with a very short distance and it needs some time until different volume temperatures are equilibrated). Since the pressures of the volumes are identical, the number of states is reduced and index reduction takes place (which means that medium equations depending on pressure are differentiated and the number of required initial conditions is reduced by one).

The default option "av_vb" cannot be used, if the dynamic pipe is connected to a model with non-differentiable pressure, like a Sources.Boundary_pT with prescribed jumping pressure. The modelStructure can be configured as appropriate in such situations, in order to place a momentum balance between a pressure state of the pipe and a non-differentiable boundary condition (e.g., if the jumping pressure component is connected to port_a, use model structure ModelStructure.a_vb).

av_vb	av_vb: port_a - volume - flow model - volume - port_b
a_v_b	a_v_b: port_a - flow model - volume - flow model - port_b
av_b	av_b: port_a - volume - flow model - port_b
a_vb	a_vb: port_a - flow model - volume - port_b