System Dynamics 3

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System Dynamics 3

• CAP4800/5805
• Systems Simulation

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Tips for Constructing Models (from VenSim)

• When constructing a Level and Rate diagram,
  consider what variables accumulate over a period
  of time
• Another way to think about this is
• if time slowed down to zero for your system, what
  variables would still be nonzero?
• For example, in the system where you pour water
  into a glass, the water contained in the glass is
  the Level. If you froze time, the pouring (a
  Rate) would stop, but you would still see a
  quantity of water in the glass (a Level)
• Once you know what levels you need, enter them
  first and then connect the rates and auxiliaries

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VenSim Modeling Simulation

• The modeling process starts with
• Sketching a model
• Writing equations
• Specifying numerical quantities
• Then simulate the model
• Simulation output automatically saved as a
  dataset
• Analysis tools examine and discover the dynamic
  behavior of variables in the model

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VenSim

• http//www.vensim.com/
• Go to Download page
• Download and install VenSimPLE
• Users Guide
• http//www.vensim.com/documentation.html
• Today we will cover two examples from the Users
  Guide
• Chap 4 Causal Loop Diagramming
• Work to do Project Model
• Chap 6 Building a Simulation Model
• Rabbit Population Model

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Causal Loop Diagram(Work to do Project Model)
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Flow Graph(Rabbit Population Model)
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Equations(Rabbit Population Model)

• average lifetime 8
• Units Year
• birth rate 0.125
• Units fraction/Year
• births Population birth rate
• Units rabbit/Year
• deaths Population / average lifetime
• Units rabbit/Year
• Population INTEG(births - deaths,1000)
• Units rabbit