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 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: • 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 • 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 • 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) • 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