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Merging, Diverging, and Routing Flow 395
Features of the Merge and Diverge blocks
advantage; the branch that gets chosen cannot be predicted. It is used when the system does not need to control how the flow is routed.
• In the case of a Diverge block, when the upstream supply is greater than or equal to the downstream demand, the block passes as much flow through each branch as downstream demand will allow. However, when downstream demand exceeds upstream supply, the dis- tribution of flow across each branch cannot be predicted.
• In contrast, the Merge block passes as much flow as possible through each inflow branch when downstream demand exceeds upstream supply. However, when upstream supply exceeds downstream demand, the distribution of flow across each branch cannot be pre- dicted.
The Neutral mode should be used carefully but can be handy in certain cases. As a general rule of thumb, if you don't care exactly which branch has priority, but you do want maximum flow, consider using the neutral mode. The Neutral mode can also be used to resolve conflicting decision rules. For example, using the Neutral mode in a downstream Merge block would allow an upstream Diverge block in Proportional mode to control the effective rates of the inflow branches in the Merge.
Merge/Diverge blocks in Neutral mode are not always compatible with Merge/Diverge blocks in Sensing mode. Consequently, an area of the model with some blocks in Sensing mode and others in Neutral mode are prone to error. See “Cautions when using potential rates” on
page 455 for more information.
Features of the Merge and Diverge blocks
Some features available in the Merge and Diverge blocks of particular interest include: • Bias order
• Managing flow attributes when flow merges and diverges • InternalThrowandCatchconnections
• Dynamically changing parameters
These features are described in the following sections.
Bias Order – resolving competing requests for flow
As models grow in complexity, it is common for the priorities or proportions defined in one Merge/Diverge block to compete or conflict with the priorities or proportions defined in other Merge/Diverge blocks. This problem of “competing requests for flow” is resolved by assigning a bias order to the competing blocks. This is accomplished through entries in either the Model Settings tab of the individual blocks or the Discrete Rate tab of the Executive block. The fol- lowing example demonstrates one of the many ways competing requests can arise and be resolved.
☞ Because certain modes allow flexibility in the way flow is distributed, Merge or Diverge blocks set to Distributional, Priority, or Sensing modes must specify a bias order to resolve conflicts between competing preferences for flow, as discussed below. For a complete description of the bias concept and bias order, see “Biasing flow” on page 432.
Discrete Rate

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