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Explorations in Computational Science: Hands-on Computational Modeling using STELLA Presenter: Robert R. Gotwals (“Bob2”) Shodor Education Foundation, Inc. Neura l tp r ANS i no Humo ral tpr Sys Art Vo l P Ven Sys Pul Runoff Pul Art Vol P Ven Pu l Sys Run off ~ C sys Vo l in P Art Sys COsys Vo l ou t P Pu l Art Sys Ven Vol P Rt Hrt P L t Hrt Lt Hrt Vo l COpul Pul Ven Vol VR L t Hrt VR Rt Hrt Lt Hrt Ela stan ce National Computational Science Leadership Program (NCSLP) Rt Hrt Vol Rt Hrt Elastance 1 Session Goals First experience in computational science » Application: computational epidemiology » Algorithm: 1927 Kermack-McKendrick SIR algorithm First experience with an Architecture: STELLA on a PC » computational tool – STELLA Logistics » Short overview » Hands-on model building exercise » Extensions as time permits National Computational Science Leadership Program (NCSLP) 2 System Dynamics A method of studying dynamic (time-driven) phenomena through the use of: » Computer simulations based on ordinary differential equations » Development of causal mechanisms (feedback loops) » Analysis of the factors that affect a system (a collection of interacting elements) Examples: » Interactions of predators and prey in an ecosystem » Fate, transport, and distribution of a pharmaceutical through a patient » Photooxidation of precursor atmospheric pollutants becoming ozone National Computational Science Leadership Program (NCSLP) 3 STELLA Highlights no programming skills required Relatively short learning-curve icon-based: modelers need to understand functions of icons graphs and tables easily constructed, manipulated, exported mathematical engine underlying software fairly robust mathematics is transparent to users “authoring” capabilities, provides user-friendly graphical interface to underlying models National Computational Science Leadership Program (NCSLP) 4 Mathematical Basis of STELLA (an intro to ODE's!) we wish to be able to study events as they change over time. main question: how do different elements change the event over time? Example: how does one's height change over time? ² height ² time = some mathematical equation ( or function) Suppose the height of an animal increases by 20% every year until it is 20 years old? National Computational Science Leadership Program (NCSLP) 5 Mathematical Basis of STELLA (an intro to ODE's!) ²h ²t = 0.2h or dh = 0.2h dt Do some algebraic rearrangements: dh = 0.2 h dt then "integrate" both sides from starting time to stopping time: t20 dh 0.2hdt where h at start equals 1 inch t 0 h = e 0.2h * (20 - 0) (by definitions from integral calculus) = 54.6 inches National Computational Science Leadership Program (NCSLP) 6 STELLA Implementation National Computational Science Leadership Program (NCSLP) 7 STELLA Basic Elements Stocks » act as "accumulators", have an initial value » viewed as having some unit » are the "nouns" (things) for the system Flows » provide input/output to the stock » have value of unit/time (unit same as stock unit) » are the "verbs" for the system Converters » hold constants or change units » can be algebraic or graphical » are the "adverbs" or "adjectives" Connectors National Computational Science Leadership Program (NCSLP) 8 Case Study: Simple Epidemiology Model: Influenza Epidemic in a Boarding School Source: Mathematical Biology, J.D. Murray, Springer-Verlag, 1989. Background: » In 1978, a study was conducted and reported in the British Medical Journal (4 March 1978) of an outbreak of the influenza virus in a boys boarding school. The school had a population of 763 boys. Of these 512 were confined to bed during the epidemic, which lasted from 22 January until 4 February. It seems that one infected boy initiated the epidemic. At the outbreak of the epidemic, none of the boys had previously had influenza, so no resistance to the infection was present. National Computational Science Leadership Program (NCSLP) 9 Case Study: Influenza Epidemic Goal: create a computational model of the boarding school epidemic Algorithm: 1927 Kermack-McKendrick SIR algorithm Three “types” of students: » Susceptibles » Infecteds » Recovereds dS rSI dt dI rSIaI dt dR aI dt where: S= population of susceptible people I = population of infected people R = population of recovered people r = probability of becoming infected a = infectious period National Computational Science Leadership Program (NCSLP) 10 Case Study: Influenza Epidemic Extensions: » What is the effect of vaccinations? Add a vaccination algorithm. Use sensitivity analysis to analyze effect » What is the effect of a return to susceptibility? Add a return loop to susceptibility » Add possibility of deaths, both as a result of disease and natural deaths » Add possibility of influx of new susceptibles National Computational Science Leadership Program (NCSLP) 11