Intro to Compressible Flows — Lesson 1

In certain fluid flow applications, the fluid density is nearly constant and the flow physics are obtained using a constant density assumption. Such flows are considered incompressible flows. However, there are other important fluid flow applications where density variations are significant. Such flows are referred to as compressible flows. Ignoring these variations often leads to erroneous conclusions. A good example of compressible flow is observed inside a four-stroke diesel engine. As the diesel-air mixture is compressed in the cylinder by the moving piston, its temperature increases leading to the ignition of the fuel inside the cylinder.


Engineers designing high-speed aircraft as well as rocket nozzles have to rely on the physics of compressibility to estimate the forces around the moving body and accurately predict the flow phenomena that are exclusive to compressible flows, such as shock waves.


In this lesson, we will introduce the concept of compressibility of a fluid. Here we will outline the difference in compressibility between a liquid and gas. Finally, we will also understand the adverse effects of ignoring compressibility in fluid flows where density variations are significant.

Alternate video link.

Here are the accompanying handout slides for this lesson.