In order to produce thrust based on Newton’s third law, ramjets, scramjets and rockets use a nozzle to accelerate hot gases.
As a result, a nozzle is a simple device, in particular a duct with a specific shape.

In fact for rockets this is generally a convergent-divergent to accelerate the flow to supersonic speeds. However, using only a convergent one, the maximum achievable speed would be that of sound.

Not only our DART guys are designing a convergent-divergent nozzle for their model rocket to generate the required thrust, but also they are using Ansys Fluent (industry-leading fluid simulation software known for its advanced physical and precision modeling capabilities) so they set up the simulation to analyze flow through the nozzle.

Flow simulation with CAD softwares.
Flow simulation in the nozzle.

Furthermore this was made possible by importing the geometry (previously drawn to CAD), setting the mesh of it, the physics and the numerical model.

Mesh associated with the nozzle, using the same geometry.
Associated nozzle’s mesh.

On the other hand, It should be emphasized that the simulation nozzle is still provisional until a next engine test.