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:
|housing stock||housing investment||housing depreciation|
|CO2 in atmosphere||tons emitted||tons sequestered|
|fuel tank||refueling||fuel consumption|
The arrow show the direction of the flow.
create a flow from one stock to another
In case there are some other elements mentioned in the formula, you should draw links from those elements to this flow first. There is however, link autocreation mechanism supported by AnyLogic - you may just specify the formula - when finished typing, the error indicator will be shown to the left of the formula. Click this indicator to show the popup menu with items offering you to create missing links from the mentioned elements to this flow. Choosing some item from the list you create the corresponding missing link. You can easily subsequently create all missing links in such a way:
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 level – If selected, the variable is also visible on the upper level 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.
External – 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 reevaluated formula but is equal to the specified value (that can be changed by the user during the simulation).
– 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.
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.
properties are used to define dimensions that make up the array
Please refer to the Defining an array variable section for details.
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.
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
To add the salient
To remove the
To move the
move the flow
System Dynamics tutorial
System Dynamics modeling
Viewing variables at runtime