# Lesson 1 - Unit 6 Overview

Advection: The transport of a substance or quantity by bulk motion. From the Latin “advectiĆ”, the act of bringing or conveying.

In the previous Unit, we discussed modeling a single well-mixed compartment (e.g., a tank) using a Cell pathway. As part of this, we explored the use of the Species, Reference Fluid, Solid and Cell elements.  The Example (and the Exercise) we discussed in that Unit were very simple.  In particular, they were not really transport models at all, since no mass was being transported (only decaying). In this Unit, we will start to consider systems in which mass is being transported between pathways.

In particular, we will begin to discuss the process of advection between well-mixed compartments. As discussed in Unit 3, Lesson 3, advection is the transport of material via the bulk movement of the medium (typically water) in which those materials are dissolved or suspended. Advection will typically be the dominant transport process in systems that you will model.

We will begin by describing how advective transport processes can be defined between Cell pathways by specifying outflows (and inflows). We will then work through some Exercises and Examples, and describe how GoldSim solves the equations for advective mass transport between well-mixed compartments.

As we saw in the previous Unit, pathway elements allow you to define an initial condition (the initial amount of mass of each species).  In order to model many systems, we also need the ability to specify a boundary condition (i.e., the rate at which mass of each species is added to a pathway). We will discuss how this is done, and work through an Exercise to provide you some practice in doing so.

All the systems we have discussed up to this point consist of tanks (as opposed to a natural environmental compartment such as a lake or soil).  We did this because the concept of a well-mixed tank serves as a simple way to understand how Cells work.  We will start to discuss, however, how natural environmental compartments can also be represented as well-mixed compartments using Cells.

To close the Unit, we will discuss some additional computational details of how the transport equations involving advection between Cells are solved (and how you can control this). This information will be important as we discuss more advanced features in subsequent Units.

This Unit has three Exercises, and four Examples that we will walk through together. It has a total of 12 Lessons (including this overview and a summary at the end).