Lesson 14 - Unit 6 Summary

In this Unit, we discussed how to carry out a dynamic simulation in GoldSim. The key points that we covered were as follows:

• Systems can change with time due to exogenous influences and/or endogenous influences. Exogenous behavior is something that comes from “outside” the model and is not due to or explained by the model.  Endogenous behavior is something that comes from “inside” the model and is explained by the relationships within the model. Most real-world systems that you would model are controlled by both endogenous and exogenous factors.
• To solve systems that vary with time numerically, it is necessary to discretize time into discrete intervals referred to as timesteps. Care must be taken when selecting this timestep, and you may have to experiment with a model to ensure the timestep is small enough.
• The GoldSim Simulation Settings dialog provides various options for defining the duration and timestep used in a simulation.
• There are three ways to create a model that will vary with time: 1) use one or more GoldSim elements that implicitly compute their output as a function of time; 2) define one or more variables that explicitly reference time in their definition; and 3) use one or more GoldSim elements whose outputs are inherently stochastic. The first of these is by far the most important and interesting way to make a model vary with time.
• The Reservoir element is an example of an element that implicitly computes its output as a function of time. It is used to track the movement or changes in “stuff” or tangible things (e.g., materials, money, people, widgets). It is one of the most commonly used and important elements in GoldSim.
• There are three primary modes that a model can be in: Edit ModeRun Mode and Result Mode.
• There are two fundamental types of results in GoldSim: 1) Final Value results, and 2) Time History results.
• To facilitate the referencing of time in a simulation, GoldSim provides a number of special reserved names, referred to as Run Properties, that can be directly referenced in expressions.
• Selector elements provide a way to represent complex If-then logic.
• Conditional expressions in GoldSim (such as those used in a Selector element), can be complex and include And, Or and Not operators. Because using these can result in very complex conditional expressions, to make complex logic easier to understand, GoldSim provides three logical elements (And, Or, and Not).
• GoldSim provides various methods for formatting results, including adding multiple outputs to a chart, changing line styles and changing labels.
• Result elements allow you to, among other things, quickly display key results that have been formatted, and view multiple result charts or tables simultaneously.
• In some cases, rather than using the result plotting and post-processing capabilities provided by GoldSim, you may wish to plot, analyze, or store the results using a separate program, such as a spreadsheet, a database, or statistical analysis package. To facilitate this, GoldSim provides several mechanisms that allow you to export results. In this Unit, we explored using a Time History Result element to export time history results to a spreadsheet.

Now that you know how to build simple models and run a dynamic simulation, we are ready to look in more detail into one of the most common applications for dynamic simulation models: modeling material flows (such as water, people and money).  We will do that in the next Unit.