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Example that does not work. What is wrong?
Example that does not work. What is wrong?
Below you find a model from the Modelica 2.1 book of Peter Fritzson. It simulates but the results are not oké.
The variable CtrlValveFlow.inlet.mdot looks correct, but the variable pressureEnthalpySource.outlet.p is zero (0) but should be 1.2e5. What could be wrong?
I do get the warning:
[dck1.mo:2:3-7:20:writable] Warning: Connector dck1.FlowConnector is not balanced: The number of potential variables (2) is not equal to the number of flow variables (1).
Is this the problem?
Code:
package dck1
connector FlowConnector
import Modelica.SIunits.*;
Pressure P;
flow MassFlowRate mdot;
SpecificEnthalpy h;
end FlowConnector;
model PressureEnthalpySource
import Modelica.SIunits.*;
FlowConnector outlet;
parameter Pressure P_0=1.0e5;
parameter SpecificEnthalpy h_0=300190;
equation
outlet.P = P_0;
outlet.h = h_0;
end PressureEnthalpySource;
model ControlledValveFlow
import Modelica.SIunits;
import Modelica.Blocks.Interfaces;
FlowConnector inlet, outlet;
Interfaces.RealInput u;
parameter SIunits.Area A = 1e-4;
parameter Real beta = 5.0e-5;
equation
// Conservation of mass
0 = inlet.mdot + outlet.mdot;
// Conservation of energy
0 = inlet.mdot*inlet.h + outlet.mdot*outlet.h;
beta*(A*u)^2*(inlet.P - outlet.P) = inlet.mdot^2;
assert(inlet.P - outlet.P > 0, "Error: model not valid for backflow");
end ControlledValveFlow;
model PressureSink
FlowConnector inlet;
parameter Modelica.SIunits.Pressure P_0=1.0e5;
equation
inlet.P = P_0;
end PressureSink;
model CtrlFlowSystem
import Modelica.Blocks.Sources;
PressureEnthalpySource pressureEnthalpySource(P_0=1.2e5);
Sources.Step step(offset=1,startTime=0.5,height=-0.5);
ControlledValveFlow ctrlValveFlow;
PressureSink pressureSink;
equation
connect(pressureEnthalpySource.outlet, ctrlValveFlow.inlet);
connect(ctrlValveFlow.outlet, pressureSink.inlet);
connect(step.y, ctrlValveFlow.u);
end CtrlFlowSystem;
end dck1;
Thank you
Re: Example that does not work. What is wrong?
What version of OpenModelica are you using? The following gave the correct result in r14428
Code:
loadModel(Modelica);getErrorString();
loadFile("a.mo");getErrorString();
simulate(dck1.CtrlFlowSystem);getErrorString();
val(pressureEnthalpySource.outlet.P,0);getErrorString();
- sjoelund.se
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Re: Example that does not work. What is wrong?
I'm using r14545.
I also get a good result when I use OMShell and your script commands. (a.mo probably should be dck1.mo in your script?)
Although when I use the 'outputformat="csv" I do not get the same values in the 'csv' file!
Could you check this.
Re: Example that does not work. What is wrong?
The problem was caused by the following line:
Code:
beta*(A*u)^2*(inlet.P - outlet.P) = inlet.mdot^2;
The solution method choose for inlet.mdot the negative value as the solution. It should have been the positive value, because the inlet.P pressure is higher than the outlet.P pressure.
So i changed the code to:
Code:
sqrt(beta*(A*u)^2*(inlet.P - outlet.P)) = inlet.mdot
Now all looks oké!
Is there a way of coding that can avoid such programmer "mistakes"?
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