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   Go to Download page  Download and install VenSimPLE  User’s Guide:  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

Causal Loop Diagram (Work to do Project Model)

Flow Graph (Rabbit Population Model)

Equations (Rabbit Population Model)  average lifetime = 8  Units: Year  birth rate =  Units: fraction/Year  births = Population * birth rate  Units: rabbit/Year  deaths = Population / average lifetime  Units: rabbit/Year  Population = INTEG(births - deaths,1000)  Units: rabbit