When you model processes with liquid (or gases) there are several reasons to introduce the Modelica stream concept. A major reason is to keep the model balanced at a local level. Another central reason is to allow back-flow. However, in many applications you do not need back-flow and it would be good to turn that possibility off at time of compilation. This would speed-up simulations. A simple example (not shown) of perfusion reactor made both with and without the stream concept gives a time increase of about two with the stream concept allowing for back-flow (in both cases no write on disk). I am not that sure of how representative the example is, but gives an indication.

I have heard that in MSL Media and Fluid libraries where the stream concept is used, there is a switch where back-flow can be turned-off. It would interesting to see how it is done so that I can implement a similar switch in my library. Where can i find it?

A simplified library where the stream concept together with the associated functions inStream() and actualStream() is shown below. It would be good to see how this code could be modified to include a switch to turn off the possibility for back-flow, but still keep the use of the stream concept and associated functions. (The example was used in an earlier post long time ago “How to construct a balanced connector for liquids in Modelica”)

package DEMO_v42

//  ---------------------------------------------------------------------------------------------
//     Interfaces  
//  ---------------------------------------------------------------------------------------------

    import Modelica.Blocks.Interfaces.RealInput;
    import Modelica.Blocks.Interfaces.RealOutput;

//  ---------------------------------------------------------------------------------------------
//     Equipment
//  ---------------------------------------------------------------------------------------------

    package EquipmentLib

        connector LiquidCon
            Real P                                             "Pressure";
            flow Real F                                        "Flow rate";
      stream Real c_outflow "Substance conc";
        end LiquidCon;

        model PipeType
            LiquidCon inlet, outlet;
            parameter Real area = 1;
        equation 
            inlet.F = -outlet.F;
            outlet.F = -area^2*(inlet.P - outlet.P);           // Linearized Bernoulli equation
            outlet.c_outflow = inStream(inlet.c_outflow);
            inlet.c_outflow=inStream(outlet.c_outflow);
        end PipeType;

        model FeedtankType
            LiquidCon outlet;
            parameter Real P = 0.1                             "Pressure";
            parameter Real V_0 = 100                           "Initial feed volume";
            parameter Real c_in = 1.0                          "Feedtank conc";
            Real V(start=V_0, fixed=true)                      "Feed volume";
        equation 
            outlet.c_outflow = c_in;
            outlet.P = P;
            der(V) = outlet.F;
        end FeedtankType;

        model HarvesttankType
            LiquidCon inlet;
            parameter Real P = 0.0                             "Pressure";
            parameter Real V_0 = 1.0                           "Initial harvest liquid volume";
            parameter Real m_0 = 0.0                           "Initial substance mass";
            Real V(start=V_0, fixed=true)                      "Harvest liquid volume";
            Real m(start=m_0, fixed=true)                      "Substance mass";
            Real c                                             "Substance conc";
            Real inletC=actualStream(inlet.c_outflow);
        equation 
            inlet.P = P;
            inlet.c_outflow=c;
            der(V) = inlet.F;
            der(m) = actualStream(inlet.c_outflow)*inlet.F;
            c = m/V;
        end HarvesttankType;
    end EquipmentLib;

//  ---------------------------------------------------------------------------------------------
//     Example of system 
//  ---------------------------------------------------------------------------------------------

    model Test
        EquipmentLib.FeedtankType feedtank;
        EquipmentLib.HarvesttankType harvesttank;
        EquipmentLib.PipeType pipe;
    equation 
        connect(feedtank.outlet, pipe.inlet);
        connect(pipe.outlet, harvesttank.inlet);
    end Test;

end DEMO_v42;