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Advanced Topics 455
Upstream supply and downstream demand
Upstream supply and downstream demand
The effective rate is not the only result a rate-based model can provide. The potential upstream supply rate and the potential downstream demand rate can also be useful in special situations – the rates determine the branch proportions for Merge and Diverge blocks in Sensing mode (as discussed in “Sensing mode” on page 393) and the Sensor block can be used to report the potential rates for making model decisions.
☞ This is an advanced topic because the concept is complex and there is an elevated potential for error, as discussed in the section “Cautions when using potential rates”, below. Careful model verification and validation, and an advanced knowledge of the ExtendSim LP technology, are required to avoid unexpected results.
Definition
The potential upstream supply rate is the theoretical rate at which an upstream source could provide flow to the beginning of a rate section if there weren’t any downstream limitations on flow movement (downstream capacity is infinite). For instance, for a not-full Tank at the beginning of a rate section, the upstream supply rate would equal the Tank’s effective inflow rate. A not-full Tank does not limit the inflow rate it can receive. In this case, the effective inflow rate is also the potential upstream supply rate.
The potential downstream demand rate is the theoretical rate at which a downstream sec- tion of the model could receive flow from the end of an upstream rate section if there were an unending upstream supply of flow (upstream source is infinite). For instance, for a not-empty Tank at the end of a rate section, the potential downstream demand rate would be equal to the Tank’s effective outflow rate. A not-empty Tank does not limit the outflow rate it can provide. In this case, the effective outflow rate is also the potential downstream demand rate.
Upstream supply and downstream demand could potentially be infinite. For example, the upstream supply rate right after a Tank (if the tank doesn't declare any constraint on its outflow rate) is infinite.
☞ When flow movement is from left to right, the information to calculate the upstream supply rate is propagated from left to right while downstream demand information is propagated from right to left.
Requirements for the supply/demand calculation
For a model’s Executive block to calculate a potential supply or demand rate, at least one of the following blocks must be part of the LP area:
• Sensorblock.TheonlypurposeoftheSensorblockistodisplaythepotentialrateswherever it is located in the model. The Sensor block only provides information; it has no direct impact on the calculation of effective rates.
• Diverge block in Demand Sensing mode. Proportions for the outflow branches are calcu- lated as a function of the potential downstream demand. For example, the downstream demand placed on an outflow branch becomes the proportion for that outflow branch.
• MergeblockinSupplySensingmode.Theblockusestheavailableupstreamsupplyrateto define the Supply Sensing proportions for each inflow branch.
Cautions when using potential rates
We strongly recommend exercising caution when using the Merge/Diverge blocks in Sensing mode, or when relying on a potential rate reported by the Sensor block, because:
Discrete Rate


































































































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