Page 352 - ExtendSim User Guide

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326 Introduction
Simulating discrete rate systems
move instantaneously from one point in the model to another; this is never correct. For this reason, flow movement must be constrained by rates and conditions that are built into the model, and a lack of appropriate constraints is a modeling error that will stop the simulation run.
Discrete rate models
Discrete rate models are concerned with flows, constraints, rates, events, storage capacity, and routing.
• Flow is what is stored in and moves through a discrete rate system. Flow can be almost any- thing: liquids, electronic transactions, and cereal can all flow in a model. Flow is expressed and measured in flow units – either generic units or defined units such as packets, gallons, transactions, boxes, etc.
• Flowmovesthroughflowconnectionsinonedirection,fromoneblock’soutflowconnector to another block’s inflow connector. It moves at a rate that is expressed as a quantity of flow per time unit – the number of packets per second, gallons per minute, boxes per hour, and so forth.
• The discrete rate architecture maximizes the movement of flow. Unless limited in some manner, flow would approach infinity and overwhelm the system. Because the discrete rate architecture maximizes the movement of flow, every model must contain one or more con- straints (typically a Valve) to limit the rate of flow to something less than infinity. Some examples of constraints include the presence or absence of flow in a Tank, the maximum flow rate defined by a Valve block, and the rule chosen to distribute flow in a Diverge block.
• While constraints determine the maximum rate that flow can move, the effective rate is the actual rate of movement. The effective rate for each section of the model is determined using linear programming (LP), given the set of constraints that has been defined by the model's structure. The model’s set of effective rates define how fast flow actually moves from one section of the model to another. As the simulation clock advances from one event to the next, the quantity of flow which has moved is updated.
• Thestateofadiscreteratemodelchangesonlywhenaneventoccurs.Aneventmightbea Tank that becomes empty or full, a maximum rate that changes during the run, a block that changes its output proportions, and so forth. Each time an event occurs, ExtendSim makes a calculation to determine, at that moment, what the effective rates are in each part of the model. Any portion of the model that can potentially be impacted by the new event has its effective rates recalculated. This takes into consideration the constraints put on the rates, the location of the flow, and storage capacity in the system.
• Each discrete rate model is conceptually divided into unit groups, rate sections, and LP areas. These divisions are handled automatically and internally, and are determined by the type of blocks used in a model, how the blocks are connected, the settings in the blocks, and so forth. Unit groups are introduced on page 329, rate sections are described on page 371, and LP areas are discussed on page 374.
A connection between two rate-based blocks can thus be viewed as an infinitely small pipe that is always full of something at a constant pressure – as soon as the effective rate is more than 0, the pipe’s contents move at the highest rate possible based on all the constraints given by the system. When the effective rate is 0, the pipe is still full but the flow instantly stops.
Discrete Rate
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