Vensim-PLE is a software packet for System Dynamics modeling that offers an easy-to-understand and easy-to-use point-and-click graphical user interface (GUI) which allows depicting stocks and their dynamics (flows) with intuitively understandable symbols. The following gives a very concise overview of the basics of Vensim-modeling on the example of a simple model for the simulation of exponential growth as discussed in respect to bank-account dynamics.A slightly more elaborate example can be found here.

Vensim allows to generate symbol-based representations of system models which can be loaded with equations, parameters and some computer commands, like shown in the illustration below.   

The rectangle with the inscription Stock represents the actual bank-account, the black-and-white arrow with subscript Flow represents the inflow into this account which is fed by the variable γ (here g) times the current state of the account. That’s what the two blue arrows indicate.

Vensim Exponential growth

When using Vensim, clicking on the rectangle Stock or on the arrow with subscript Flow opens up dialogs like the one shown on the right side for defining parameters.

Vensim input

Putting in the values as given below (left side) and pushing the sim-setup-button setup followed by the simulate-button simulate (after having told Vensim where to save its outputs), makes the model run and integrate the flow to calculate the stock. Clicking on the rectangle Stock followed by the graph-button Graph button makes the software generate the graph as shown in the plot on the right side.

Input exponential growth
Exponential growth

In mathematical terms the above Vensim-model stands for

\(Stock=\int\limits_{0}^{t}\!Flow\,dt\) respectively \( N=\int \limits_{0}^{t}\gamma N\, dt\)