Simplifying h_outflow calculation when allowFlowReversal = false

[Mathadon]

Currently Dymola is able to simplify some calculations when allowFlowReversal = false. Consider for instance the following equation.
H_in = semiLinear(m_flow, inStream(port_a.h_outflow), inStream(port_b.h_outflow))
allowFlowReversal typically sets the min attribute of m_flow equal to zero. Dymola can then detect that mass flow will never become negative and that therefore inStream(port_a.h_outflow) should always be used in the equation:
H_in = m_flow*inStream(port_a.h_outflow)
Since the equation now no longer depends on inStream(port_b.h_outflow) this can lead to smaller algebraic loops.

However there are some cases where the min and max attributes are not propagated properly by Dymola and therefore these simplifications are not made. Moreover OpenModelica does not make these simplifications at all. This can lead to large algebraic loops, which deteriorate simulation speed and stability.

I propose to make some changes in the code such that this simplification is always made. To this end I would like to change Annex60.Fluid.Interfaces.StaticTwoPortConservationEquation, Buildings.Fluid.BaseClasses.PartialResistance, Annex60.Fluid.Sensors.BaseClasses.PartialFlowSensor and similar models.

In the case of Annex60.Fluid.Sensors.BaseClasses.PartialFlowSensor the change would be to replace

  port_a.h_outflow = inStream(port_b.h_outflow);
  port_b.h_outflow = inStream(port_a.h_outflow);
  port_a.Xi_outflow = inStream(port_b.Xi_outflow);
  port_b.Xi_outflow = inStream(port_a.Xi_outflow);
  port_a.C_outflow = inStream(port_b.C_outflow);
  port_b.C_outflow = inStream(port_a.C_outflow);

by

  port_a.h_outflow = if not allowFlowReversal then Medium.h_default else inStream(port_b.h_outflow);
  port_b.h_outflow =  inStream(port_a.h_outflow);
  port_a.Xi_outflow = if not allowFlowReversal then Medium.Xi_default else  inStream(port_b.Xi_outflow);
  port_b.Xi_outflow = inStream(port_a.Xi_outflow);
  port_a.C_outflow = if not allowFlowReversal then Medium.C_default else  inStream(port_b.C_outflow);
  port_b.C_outflow = inStream(port_a.C_outflow);

It may actually be interesting the create a new partial that contains these equations since they are now repeated in a few partials.

I will add an example that demonstrates that this works.

[Mathadon]

See example Annex60.Fluid.Interfaces.Examples.ReverseFlowAdvanced on https://github.com/Mathadon/modelica-annex60/tree/issue281_floRevCalc
Heat is injected in the mixing volume based on the upstream temperature of the fluid. Since mass flow is always in the same direction this temperature is known a priori. However since dymola thinks the flow can reverse (even with allowFlowReversal=false) a non-linear system is formed:

Sizes of nonlinear systems of equations: {3}
Sizes after manipulation of the nonlinear systems: {1}

This can be removed by setting

port_a.h_outflow = if not allowFlowReversal then Medium.h_default else inStream(port_b.h_outflow);

in Annex60.Fluid.Sensors.BaseClasses.PartialFlowSensor.

[Mathadon]

If you agree with the proposed changes I can propose an implementation.

[Mathadon]

Illustration:

[Mathadon]

I need to make a new pull request for this.

[rfranke]

Note that OpenModelica now considers the simplification. In fact any Modelica tool must do so in order to produce correct results for fluid models with absolute sensors, like Modelica.Fluid.Examples.{DrumBoiler, TraceSubstances.RoomCO2}.
See: https://trac.openmodelica.org/OpenModelica/ticket/3430

[Mathadon]

Merged in #312