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System Dynamics 3. CAP4800/5805 Systems Simulation. 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:

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system dynamics 3

System Dynamics 3

CAP4800/5805

Systems Simulation

tips for constructing models from vensim
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
vensim modeling simulation
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
vensim
VenSim
  • http://www.vensim.com/
  • Go to Download page
  • Download and install VenSimPLE
  • User’s Guide:

http://www.vensim.com/documentation.html

  • Today we will cover two examples from the User’s Guide
    • Chap 4 Causal Loop Diagramming
      • Work to do Project Model
    • Chap 6 Building a Simulation Model
      • Rabbit Population Model
equations rabbit population model
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