Flow

System Dynamics element. Flow (also known as rate) as well as stock is a fundamental element of stock and flow diagram.

While stocks define static part of the system, flows define rate of change for the stocks - how values of stocks change in time and thus define the dynamics of the system.

You can find more information on stocks and flows in Wikipedia: http://en.wikipedia.org/wiki/Stock_and_flow

Here are some examples of stocks and flows:

• Flow may flow out of one stock and flow in another. Such a flow is outflow for the first stock and inflow for the second one at the same time, see the figure below:

• Flow may flow into a stock from nowhere. In this case the cloud (denoting "source") is drawn at the starting point of the flow.

• And symmetrically, flow may flow out from a stock to "nowhere". In this case the cloud (denoting "sink") is drawn at the end point of the flow.

The arrow shows the direction of the flow.

To create a flow from one stock to another

1. Double-click the Flow element in the System Dynamics palette so that its icon turns into .
2. Right after that, go to the graphical editor and click the stock where the flow flows out (or just somewhere in the diagram, in case the flow originates outside the model bounds).
3. If you want to draw a flow not with a straight line, but with a shape of some custom form, you can add any number of intermediate salient points by clicking on the diagram.
4. Finally click the stock where the flow flows in (or double-click on the diagram, in case the flow flows to nowhere).
5. Go to the Properties view to set up properties of the flow.
6. Specify the name of the flow in the Name edit box.*
7. Type the formula calculating the value of the flow in the flow_name = edit box. (you may use code completion here to insert names of other elements).

8. If you do not want this flow to get its value according constantly re-evaluated formula but otherwise want it to be equal to some specified value (that can be changed during the simulation), check the Constant check box. Constant flow is drawn with an equal sign over its icon: .
9. If you explicitly specify units of measurements for elements of your stock and flow diagram, do it for this flow. Go to the Advanced section of its properties and select the System dynamics units checkbox and type the unit name in the edit box (you can find more information on units and unit checking here.)

Properties

General properties

Name – The name of the flow. The name is used to identify and access the flow in formulas.

Show name – If selected, the name of the flow is displayed on the presentation diagram.

Ignore – If selected, the flow is excluded from the model.

Visible on upper agent – If selected, the variable is also visible on the upper agent where this agent lives. This may be needed if you are creating hierarchical system dynamics models by exposing system dynamics variables onto interface of the agent type and connecting interface variables of agents.

Show at runtime – If selected, the flow is visible on the presentation at runtime.

Color – The fill color of the flow icon and arrow.

Array – If selected, the flow is of array type. Properties for array variables look slightly different. Instead of one <flow_name>= field defining the formula for the flow, there can be several fields defining formulas for some particular elements or sub-arrays of the array variable. To add new section allowing to define one more formula, click the Add formula button. Defining formulas for array elements and sub-arrays is described here.

Dependent – If selected, the flow will get value of the connected variable constantly.

Constant – If selected, the flow is a constant, i.e. its value does not change according constantly re-evaluated formula but is equal to the specified value (that can be changed by user during the simulation).

flow_name = – The formula defining the value of the flow.
In the case this flow is of array type,there can be several fields defining formulas for some particular elements or sub-arrays of the array variable. To add new section allowing to define one more formula, click the Add formula button. Defining formulas for array elements and sub-arrays is described here.

Array properties

Array properties are available only if this flow is of array type. To make the flow an array, check the Array check box on the General property page.

Available dimensions – The list of dimensions defined in this model.

Selected dimensions – The list of dimensions that will act as dimensions of this array variable.

These properties are used to define dimensions that make up the array variable.
Please refer to the Defining an array variable section for details.

Advanced

System dynamics units – If selected, you will be able to specify units of measurements for this variable in the edit box to the right. Having specified units for all elements of your stock-and-flow diagram, you may perform unit checking to find out dimension inconsistencies in your model.

At model runtime you can view flow value and chart as well as copy the collected history of values to Clipboard, you can find more information here.

Customizing the flow appearance

You can customize the flow appearance, namely you can change its color and change the form of the flow by adding some intermediate salient points.

To change the flow color

1. Select the flow by clicking it in the graphical editor or in the Projects view.
2. Go to the Properties view and choose the required color from the Color control.

To add the salient point

1. Double-click the flow arrow in the graphical editor, right in the place where you want to put the point.
2. Now you can move this point aside by dragging it with the mouse.

To remove the salient point

1. Double-click the salient point.

To move the end/salient point

1. Drag this end/salient point aside.

To move the flow

1. Drag the flow (but not its end/salient point, because it will move this end/salient point only, not the whole flow, see above).

Related topics

System Dynamics tutorial

System Dynamics modeling

Stock

Viewing variables at runtime