Vensim chapter 6 v1

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  • 1. Vensim e s Chapter 6 Building a Simulation ModelVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 2. Naming Conventions g Levels – initial capital letters; e.g., Population Rates, auxiliaries, constants, lookups, data variables – all lower  Rates auxiliaries constants lookups data variables – all lower case; e.g., average lifetimeVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 3. Sketch Conventions Sketch Conventions Levels or stocks are entered with the Box Variable tool Levels or stocks are entered with the Box Variable tool Rates are entered with the Rate tool Box Variable - Level RateVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 4. Sketching the Rabbit Model g File > New Model 1 5 2 Type the name Population and press 3 enter t 4Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 5. Sketching the Rabbit Model g 9 7 8 6 6 5 6 8 7 7 A positive feedback loop from Population to births increases  Population, and a negative feedback loop from deaths  decreases Population decreases PopulationVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 6. Writing Equations g q births = population * birth rate Birth rate has not causes; it is constant in the model Bi th t h t it i t t i th d l Equations All the variables in the model will turn black. The highlights indicate which variables still require equations or have incomplete equationsVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 7. Checking y g your model Model > Units CheckVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 8. Variable Type: Auxiliary yp y Click on variable births 2 1 3 4Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 9. Variable Type: Level yp Click on variable population equation is already present in the equation editing box Rates constructed by clicking first outside, and then on the Level are considered positive (inwards) flows; rates constructed by clicking on the Level, and then outside the Level are considered 2 negative (draining) flows1 3 4 Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 10. Variable Type: Constant yp birth rate 2 1 3 this means that the fractional birth rate is measured in fraction (of rabbits) per year. 4Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 11. Variable Type: Constant yp average lifetime 1 2Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 12. Variable Type: Auxiliary yp y Click on variable deaths 1 2 3Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 13. Checking y g your model Model > Check ModelVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 14. Checking y g your model Model > Units CheckVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 15. Unit Equivalents ( y q (Synonyms) y ) Fail in Units check because Vensim does not see the plural  form as the same word form as the same word Model > Settings... 1 2 3Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 16. Simulating the Model g 2 1Double click on the Runname editingbox on the Toolbar and type equilib for yp qthe first run name. Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 17. Model Analysis y Click on the Level Population in the sketch. Click on the Graph t l A Cli k th G h tool. A graph of Population i h fP l ti is generated: t d2 1 GraphVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 18. Model Analysis y Population appears as a flat line at the top of the graph at  P l i fl li h f h h 1000 rabbits. To check that no change is occurring, Click on the Table tool l k h bl l Output window shows that Population is unchanging.Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 19. Simulation Experiments p Click on the SyntheSim button . Answer yes to  whether you want to overwrite the existing run. whether you want to overwrite the existing run The toolbar will change to  and you will see variable behavior for all elements on the  diagram. diVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 20. Simulation Experiments p Click on the Runname editing box and replace equilib with the  name growth. name growth Drag the slider beneath birth rate up till it shows 0.2 as its  value. The blue lines show the current run and the red line the  equilib run results run results. Click on the Stop button to stop SyntheSim. View>Show Behavior to see behavior graphs again. Vi >Sh B h i t b h i h iVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 21. Alternative Simulation Setup p Click on the Set up a Simulation button The Toolbar changes to the simulation toolbar. The Toolbar changes to the simulation toolbar notice that the constants turn into yellow words with blue  ti th t th t t t i t ll d ith bl background Also, the sketch tools are grayed out, preventing work in the  Also the sketch tools are grayed out preventing work in the sketch window. Click on the variable birth rate and in the editing box type the value 0.2 This is a temporary change for this run only and does not permanently alter the value in your model. d lVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 22. Causes Strip Graph p p Click on Population to select it into the workbench. Click on the Control Panel button Cli k th C t l P l b tt 1 2 The last run you made (growth) is loaded first (at the top of the dialog).Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 23. Causes Strip Graph p p Click on the Graph tool. A graph will show both runsVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 24. Causes Strip Graph p p Now click on the Causes Strip tool.Causes Strip Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 25. Runs Compare p Click on the Runs Compare tool. shows the differences in the Constant birth rate for runs  equilib and growth.Runs Compare Population grew in the growth run because the birth rate was  set to a higher value than the equilibrium value. This made the  positive feedback loop through births stronger than the  positive feedback loop through births stronger than the negative feedback loop through deaths, resulting in Population  growth over time. Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 26. Exponential Decay p y make changes to a model Constant to generate exponential  decay or decline in the population decay or decline in the population 1 1 4 2 Drag the Slider under average lifetime till it displays di l a value of 4 l f 4. Click on the Stop Button 3 Click the Simulate button and the model will simulateVensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 27. Exponential Decay p y Click on the Graph tool and compare the three runs. 1 double click on the run equilib in the right box; this will h i h b hi ill unload the run so the 2 Analysis tools will not examine it it.Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 28. Exponential Decay p y Click on the Causes Strip tool  to show Population and its  to show Population and its causes— births and deaths, for  the two last runs. the two last runsPopulation d liP l i declines i the d in h decay runbecause the average lifetime was set to alower value than the equilibrium value.This made the negative feedback loopthrough deaths stronger than the positivefeedback loop through births, resulting in p g gPopulation decline over time.Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 29. Input and Output Objects p p j Click on the Control Panel button and then select the  Graphs tab tab. 1 2 3Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 30. Input and Output Objects p p j 1 Do the same for births (select births from the list) and f h li ) d deaths (select deaths 3 from the list) 4 2 5 6 5Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 31. Input and Output Objects p p j Select the Input Output Object sketch tool 4 2 1 3Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010
  • 32. Input and Output Objects p p j Click the SyntheSim button Experiment with moving the sliders around and seeing  E i t ith i th lid d d i the results on the graph.Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010