Chapter 14 Simulation What Is Simulation Simulation A

Chapter 14 Simulation

What Is Simulation? Simulation: A model of a complex system and the experimental manipulation of the model to observe the results. Systems that are best suited to being simulated are dynamic, interactive, and complicated. Model: An abstraction of a real system. It is a representation of the objects within the system and the rules that govern the interactions of the objects. 2

Constructing Models Constructing a model requires the identification of a small subset of features of a system that sufficiently describe the system’s behaviour. There’s a fine line between not having enough characteristics in the model and having too many. 3

A Discrete Event Simulation… … consists of entities, attributes, and events. n Entity: the representation of some object in the real system that must be explicitly defined n Attribute: some characteristic of a particular entity n Event: an interaction between entities 4

Discrete Event Simulations Constructing a good model involves choosing the entities that define the system and correctly determining their rules of interaction. The Pareto principle states that, for many events, roughly 80% of the effects come from 20% of the causes. 5

Discrete Event Simulations But which 20% are needed? A model must allow “experimental manipulation of that model to observe the results. ” Which results? The determination of the desired result leads to the selection of the entities and rules required to produce the result. * 6

An Example Queuing system: a discrete-event model that uses random numbers to represent the arrival and duration of events. n n The system is made up of servers and queues of objects to be served. The objective is to utilize the servers as fully as possible while keeping the wait time within a reasonable limit. 7

Queuing Systems To construct a queuing model, we must know the following four things: n the number of events and how they affect the system o in order to determine the rules of entity interaction n the number of servers n the distribution of arrival times o n in order to determine if an entity enters the system the expected service time o in order to determine the duration of an event 8

Queuing Systems There are different types of queues. o The most common is a FIFO queue – First In, First Out o In a priority queue, each item is assigned a priority and the order in which they are processed depends on their priority, not their arrival time. o Short/Long service time queues… 9

A Continuous Simulation… …treats time as continuous and expresses changes in terms of a set of differential equations that reflect the relationships among the set of characteristics. Meteorological models fall into this category. 10

Meteorological Models… …are based on the time-dependent, partial differential equations of fluid mechanics and thermodynamics. Initial values for the variables are entered from observation, and the equations are solved to define the values of the variables at some later time. 11

Meteorological Models Computer models are designed to aid the weathercaster, not replace him or her. n The outputs from the computer models are predictions of the values of variables in the future. n It is up to the weathercaster to determine what the values mean. 12

Hurricane Tracking The modules for hurricane tracking are called relocatable models, because they are applied to a moving target. The Geophysical and Fluid Dynamics Laboratory (GFDL) developed the most widely used hurricane model in order to improve the prediction of where a hurricane would make landfall. 13

Hurricane Tracking 14

Computational Biology… …is an interdisciplinary field that applies techniques of computer science, applied mathematics, and statistics to problems of biology. “CB” encompasses several fields: n Bioinformatics n Computational biomodelling n Computational genomics n Molecular modelling n Protein structure prediction 15