Page 476 - ExtendSim User Guide
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450 Advanced Topics
LP technology
The sequence of events
A change in a block’s constraints during the simulation run initiates a reevaluation of all the effective rates for the LP area at that point in time. The block’s constraint change will defi- nitely cause a recalculation of the effective rates for all directly connected rate sections, and (depending on relational constraints between the sections) might cause a recalculation of the effective rates for other rate sections. The sequence is:
1) Ablock’sconstraintchanges.
• If a block’s status changes it can affect its effective inflow and/or outflow rates. For example, the effective inflow rate for a finite Tank block that is filling up is greater than its effective outflow rate. When the Tank becomes full, it creates an event because its effective inflow rate can no longer exceed its effective outflow rate.
• When a block reacts to new parameters, the effective inflow and/or outflow rates must be reevaluated. For example, a Merge block in Select mode might choose its top inflow branch at the start of the simulation. If the block subsequently receives an order to select its bottom inflow branch, its effective rates have to be recalculated.
2) Theblockpostsazero-timeeventrequestingareevaluationofeffectiverates. 3) TheLPareaisdeterminedbasedonthepropagationofblockmessages.
• Starting with the originating block. messages are propagated through the model to all the blocks that might be affected by the change. This propagation of messages defines the boundaries of the LP area.
• TheLPareaencompassesalltheratesectionswitheffectiveratesthatmightchange during the calculation.
• ForacompletedescriptionoftheLPareaandhowitisdetermined,seepage449. 4) AstheLPareaisdetermined,theblocksupdatetheirstatus.
• EachblockthatispartoftheLPareaupdatestheamountofflowwhichhaspassed through it since the last update.
• If the block is a residence block (Convey Flow, Interchange, or Tank), the amount of flow the block is holding is also updated.
• Note: The frequency of status updates outside of an LP calculation is set in the Execu- tive. See “Update flow status” on page 437 for information.
5) BlocksreporttotheExecutivehowtheyimpacteffectiverates.
• EachblockintheLPareadeclarestheflowrules(criticalandrelationalconstraints) that it applies to the rate section it is connected to.
• Theblock’sbiasorderisstoredinalist.Eachblockdeclaresitsbiasorder(ifany)and provides coefficient information to the Executive. The coefficients allow the Executive to build an objective function that considers the effect of the bias.
• See “Types of information provided to the Executive” on page 451. 6) TheExecutivedeterminesanobjectivefunction.
• The objective is to maximize the flow rate for each rate section subject to the con- straints defined by the blocks’ flow rules.
Discrete Rate

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