Page 474 - ExtendSim User Guide

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Advanced Topics
What this chapter covers
The earlier chapters in the Discrete Rate module discussed important concepts you need to know to build discrete rate models. This chapter provides an overview of the way discrete rate calculations are made and describes the underlying functioning of the Rate library. This infor- mation is not necessary to build discrete rate models but will be of interest to advanced users.
It is highly recommended that you read the previous discrete rate chapters before this one, par- ticularly the chapter “Rates, Constraints, and Movement”.
What this chapter covers
• TheLPtechnologythathasglobaloversightoveradiscreteratemodel • HowtheLPareaisdetermined
• ThesequenceofeventsfortheLPcalculation
• How bias affects the calculation
• ThetypesofinformationprovidedtotheExecutive
• How a relational constraint is calculated
• The potential rates “upstream supply” and “downstream demand” • Messaging in discrete rate models
LP technology
Linear programming problems involve the optimization of an objective function subject to a set of constraints. The purpose of solving a linear programming problem is to maximize or minimize selected variables in the objective function.
LP technology is the method ExtendSim uses to provide global oversight to maximize the movement of flow throughout a discrete rate system. The discrete rate architecture employs an integrated LP Solver DLL to solve a series of equations to optimize effective rates at each point of the simulation run.
The purpose of the LP calculation is to determine the maximum effective flow rates in the sys- tem given the constraints defined by block settings and the structure of the model. After all the rules for storage capacity and movement have been declared in the model, ExtendSim uses the Executive block’s LP calculations to cause as much flow as possible to move through the sys- tem. This calculation is handled automatically and internally.
Overview
In a discrete rate model, the Rate library blocks communicate with each other and with the Executive block. In turn, the Executive communicates with an integrated linear program (LP Solver). The Rate library blocks are dependent on each other, have an effect on one another, and are part of a global LP system that evaluates the entire model to calculate its effective flow rates.
• Aratesectionisanetworkofconnectedblocks,allpossessingthesameeffectiverate. Established at the beginning of the simulation run, rate sections do not change. (See “Rates, rate sections, and the LP area” on page 371 for more information.)
• An LP area is made up of one or more rate sections; the actual configuration can change during the simulation. A change in a block’s constraints during the simulation run initiates a propagation of messages through all the rate sections whose effective rates might change. This propagation defines the LP area at that point in time.
Discrete Rate
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