Running Your Model

This section describes how you run a model built with Numerus's visual interface. It also describes several useful run-time features.

Clock Settings

Once you have built a model but before you can run it as a simulation, you must first set the clock parameters. The clock settings consist of the following 4 parameters:

• Start The time (in model time) when the simulation starts (this is usually 0).
• End The time (in model time) when the simulation ends.
• Dt The unit of increment (in model time) for updates.
• Integration Method The mathematical algorithm used to update Stocks.

Numerus currently uses an abstract unit of time ("model time") that serves as the equivalent of some model-dependent real time unit (seconds, minutes, hours, days, years). You can specify a time unit in the Unit text field.

Dt, the increment, or "delta t", value, is the amount of time that elapses between State updates. For discrete models this is fixed at 1 (see below), but can be any value otherwise (0.1, 0.01, 0.05, 2, 20).

Numerus supports 3 different integration methods

Euler

Current state values are used in computations to determine the next state. Used for all discrete models.

RK2, RK4

These Runge-Kutta algorithms treat Stocks and Variables as continuous functions and reduce the error by subdividing the update interval. Simulations run slower but with more accuracy. RK4 is the preferred option of the two.

Discrete

This is not actually an integration method; rather it sets Dt to 1 and uses the Euler method. The result is a sequence of State values rather than an attempt to approximate a continuous function.

Launching The Simulation

Once you have finished specifying the runtime parameters click the Launch button to produce the runtime canvas. This will either appear in a separate frame or as a new panel, depending on the choice made in Preferences.

Run Buttons

Once you have built a model in Nova, you can run it by clicking Capture → Load → Exec. Let’s break it down and see what each of these buttons does.

Capture

This button “captures” the structure from the components of the model you’ve built and turns it into a NovaScript program.

Load

Clicking Load loads the information captured from the model and prepares it to run.

Exec

Clicking Exec executes the run command on the model. You will be unable to select Exec until you have first selected Capture and Load.

Init

You can also run the model by clicking Capture → Load → Init. Init initializes the simulation run. After you’ve selected Init, you can either select Run to run the model at the selected speed, or click Step to run the model one time-step at a time.

When you are in timeline mode, you can use the Back button to go back exactly one time step.