This lesson covers the process of defining the domain of a boundary value problem, specifically focusing on the fluid region inside a nozzle. The lesson explains how to use symmetry to model a 2D axisymmetric section of the nozzle using Ansys Workbench™ simulation integration platform. It guides through the steps of setting up the project, renaming it for organization, and changing the analysis type to 2D. The lesson also demonstrates how to sketch the geometry, define it parametrically, and create a surface from the sketch. It further explains how to set up locations for applying boundary conditions using named selections. For instance, the lesson uses a nozzle flow as an example to illustrate these steps.