The SD Model in Depth

Flows

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Flows transport goods in the model. Goods are delivered between stocks in a flow. Goods are delivered to a transformer in a flow.

Unrestricted Flows

An unrestricted flow delivers a tangible or intangible good in unlimited quantities at unlimited rates of flow.

The Unrestricted Flow’s symbol is a stylized “hollow arrow”. The Unrestricted flow symbol may optionally be drawn with enforced 90° transitions or as a semicircle. The following is an unrestricted flow symbol:

Unrestricted Flow

Attributes configured by the Model Builder user for any unrestricted flow:

•none

Unrestricted flows typically connect from a transformer or from a production factory to another model component. The following component Interfaces on an unrestricted flow are allowed by the Model Builder process:

•from or to any type of stock
•from or to any type of transformer
•from or a factory according to the unique rules of the factory

Error Conditions detected and prohibited by the Model Builder process:

•ILLEGAL UNRESTRICTED FLOW CONNECTION
    – between an unbounded stock and any type of stock
    – between any two stocks unless one (or both) are queued stocks
    – from an unbounded stock to any type of transformer
    – from a factory or to a factory that violates its unique rules
•INCONSISTENT GOOD OR UNITS
    – an attempt to connect from and to stocks, transformers and factories
    when GOOD or UNIT has already been defined on both sides of a flow
    and they are inconsistent

Other Behaviors and Characteristics:

•none

Modeled Flows

A Modeled Flow is a homogeneous flow of a tangible or intangible good in the model with a control to manage the flow rate. A modeled flow incorporates the computational elements of a converter.

Three modeled flow types are defined for the Model Builder process:

•Adjusted Flow is a simple flow type that flows a calculated quantity of a good from the inflow to the outflow at every time increment
•Delayed Flow is a simple flow type that flows a good from the inflow to the outflow after a configurable time delay
•Custom Flow is a complex flow type that incorporates the combined characteristics of both adjusted and delayed flows, and adds other refinements to their models such as flow leakage and flow direction reversal

The modeled flow’s symbol is a stylized pair of concentric circles overlaid upon the unrestricted flow symbol. The arrow part of the modeled flow symbol may optionally be drawn with enforced 90° transitions or as a semicircle. The following are modeled flow symbols:

Adjusted Flow

Delayed Flow

Custom Flow

Attributes configured by the Model Builder user for any modeled flow:

•ANIMATE - a value of “NO” (default) or “YES”
•GRAPH - display a high quality formatted graph of OUTFLOW; a value of “NO” (default) or “YES”
•TABLE - display a high quality formatted table of OUTFLOW; a value of “NO” (default) or “YES”
•ABSTRACT - publish OUTFLOW to abstraction layers; a value of “NO” (default) or “YES”
•COST - publish COST attributes to abstraction layers; a value of “NO” (default) or “YES”

Attributes optionally configured by the Model Builder user for any modeled flow to describe the corresponding Cost Abstraction Layer basis:

•COST ACCUM PERIODIC` - accumulate cost for the period described in the simulation specification; a value of “NO” (default) or “YES”
•COST FIRST - a one-time “first cost”
•COST TIME - a cost per time interval
•COST GOOD - a cost per unit of good outflowed

Attributes configured by the Model Builder user for an adjusted modeled flow:

•OUTFLOW FORMULA - the formula to calculate the flow’s outflow rate
•OUTFLOW THRESHOLDS - “NONE” (default) or values for MN range, MJ range, CR range

Attributes configured by the Model Builder user for an delayed modeled flow:

•DELAY INITIAL - the outflow’s initial delay
•DELAY FORMULA - the formula to calculate the flow’s delay value
•DELAY THRESHOLDS - “NONE” (default) or values for MN range, MJ range, CR range

The attributes of the custom modeled flow are a “superset” of attributes including those of the adjusted modeled flow and the delayed modeled flow defined above. In addition, the following attributes are configured by the Model Builder user for a custom modeled flow:

•WASTE - a value of “NO” (default) or “YES-IN” to leak a portion of the inflow or “YES-OUT” to leak a portion of the outflow
•WASTE FORMULA - the formula to calculate the leakage of the flow’s outflow
•WASTE THRESHOLDS - “NONE” (default) or values for MN range, MJ range, CR range
•CONSTRAIN MAX - flow OUTFLOW > CONSTRAINT MAX not allowed; a value of “NO” (default) or “YES”
•CONSTRAINT MAX - the maximum allowed flow OUTFLOW value; mandatory if CONSTRAIN MAX = “YES”
•CONSTRAIN ZERO - flow OUTFLOW < 0 not allowed; a value of “NO” or “YES” (default)
•CONSTRAIN MIN - flow OUTFLOW < CONSTRAINT MIN not allowed; a value of “NO” (default) or “YES”
•CONSTRAINT MIN - the minimum allowed flow OUTFLOW value; mandatory if CONSTRAIN MIN = “YES”
•OUTFLOW INCREMENT - the incremental number of units outflowed at a time; a value of “NONE” or a specific numeric value

Component Interfaces on a modeled flow allowed by the Model Builder process:

•from or to any type of stock
•from or to any type of transformer
•from or to a factory according to its unique rules
•from or to a model component as an internal variable value
•from an external variable or an adjusted variable

Error Conditions detected and prohibited by the Model Builder process:

•INCONSISTENT GOOD OR UNITS
– an attempt to connect from and to stocks, transformers and factories when GOOD or UNIT has already been defined on both sides of a flow and they are inconsistent

Other Behaviors and Characteristics:

•a delayed flow is strictly FIFO by definition
•a delayed flow is interchangeable with a queued stock in the model
•the GOOD and UNIT of a flow are implicit if the flow is connected between stocks where GOOD and UNIT have been previously defined
•the flow arrow shows the direction of the flow; the arrow for a custom flow must change directions when the value of the flow rate is negative and CONSTRAIN LOWER is “NO”