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Simulate the Model
From SystemsWiki
In this module you will actually begin interacting with the model that you've developed up to this point, though first there are just a couple additional things to define.
Contents |
[edit] Model Simulation Specs
First you have to tell the software when to start, when to end and what size steps to take along the way. Select Simulation Specs... from the Simulation menu to display Fig. 1. Set starttime = 0, stoptime = 10, and dt = 0.25. As indicated in Fig. 1. This tells the software to run for time equal 0 to 10 with a step size of 0.25, meaning the model with actually run in 40 steps. We're making the assumption that the time is in Minutes. We'll address Integration Method later in this course and Random Number Generator in another course.
[edit] Data Display
Since we're getting ready to run a simulation it is probably a good idea to set up something that helps us visualize what's happening in the model. We'll use a graphic displays of the elements to do this. Click the Graph Tool, the last icon on the Model Tools toolbar, then move your mouse to the display area below the model and click to place the graph area as depicted in Fig. 2 (no worries, the picture looks really like this).
Now Double-Click the graph area to open the Graph Editor as depicted in Fig. 3. Here are all the tools you will use to identify the variables to be displayed on the graph as well as various aspects of the display itself.
Click the Variables tab and select bathtub, draining, and filling and add them to the Selected Variables list as in Fig. 4.
Now click OK to close the Graph Editor as we'll be using the default values for all the other aspects of the graph definition.
[edit] Run the Model
To Run the simulation you can select Run from the Simulation menu, enter CRTL + R from the keyboard, or click the small green arrow to the right of Model_1 in the Toolbar. Any one of these should produce the display in Fig. 4.
The graph in Fig. 5 shows how each of the elements of the model change and the implications of the interactions between the elements of the model over a period of 10 minutes.
Congratulations. You have now just built and simulated your first model. Not difficult at all, was it? From here on everything is just a slight variation or expansion of what you've done so far. First we crawl, then we walk, then we run, and in some cases we even get to race.
At this point you can open filling and draining and assign different values and run the model to see the effect changing the values of these values has on the graph.
[edit] Summary
I hope this hasn't been too much like watching paint dry. Things will begin to move a bit faster now that you're acquainted with a foundation to build on. Here's a summary of what we've covered to this point.
- Stock & Flow are two of the elements used in developing models. Auxiliaries and Links will be covered in the next module.
- Stocks represent a collection of something. Stocks change over time based on what enters or exits through Flows. When you label a Stock use a label that helps one understand that it represents an amount of something.
- Flows represent something that is moving in time from one location to another. Flows change the amount of what's in a Stock either by adding to it, or subtracting from it, over time. When you label Flows use a label that implies something flowing or moving in time making it easier for others to understand the model.
- Equations are used to assign values to the elements of a model and define how the elements behave during a simulation. In this module equations have simply been constants. We'll use more interesting equations in the future.
The next module, Savings Account, will cover the final two model elements and begin to acquaint you with some basic structure.
[edit] References
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