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Creating Flow 355
Flow attributes
attribute’s values are entered. Values for flow attributes can also be set dynamically through value input connectors on the Set(R) block.
How flow attributes are propagated in the model
Flow attributes attach to sections of flow and are present throughout the model. And a discrete rate model can have many flow attributes where each flow attribute can have multiple values. However, the values of flow attributes are location specific. This means that the value of a flow attribute is unique at any time in each section of the model. (For information on sections. see “Rates, rate sections, and the LP area” on page 371.)
Flow layers in residence blocks
As mentioned earlier, flow attributes are used to organize quantities or volumes of flow into layers, where each layer represents an amount of flow that has specific and unique characteris- tics. By definition, the attribute values for every unit of flow in a particular layer must be the same.
Flow layers are stored, tracked, and distributed by the Tank, Interchange, and Convey Flow blocks in the following ways:
• Tank.TheTankhastheabilityto store incoming layers of flow by placing them on top or on bottom, or by adding the incoming flow to a pre-existing matching layer. Outgo- ing flow can be taken from the top, the bottom or from anywhere in between, as shown on the right. This flexibility allows the Tank to mimic LIFO, FIFO, and priority behavior.
• Interchange. The Interchange block can use layers to manage the filling and releasing of items. For example, there’s an option to release an item as soon as an attribute on incoming flow changes to a different value. This allows you to fill items with flow that contains a uni- form attribute value.
• Convey Flow. The Convey flow will always receive and distribute flow layers in a FIFO fashion.
For example, a layer in a Tank could be a quantity of yogurt with the Flavor attribute value “Vanilla” and the Nuts attribute value “No Nuts”. Vanilla yogurt with nuts would be in a differ- ent layer, as would Berry yogurt without nuts.
ExtendSim handles the organization of layers within the residence blocks according to options set in the blocks’ dialogs.
Propagation of attribute values outside of residence blocks
All Rate blocks get attribute values from inflow connectors and set attribute values on outflow connectors. For instance, a Merge block will receive attribute values from each of its upstream branches and, depending on these values and the rules set in the block, it will set the attribute values for the downstream flow.
If the upstream flow has the potential to satisfy downstream demand, the connectors in between will reflect the attribute values of the upstream flow even if the effective rate is zero.
Discrete Rate

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