In this lecture Prof H. Bijl first finishes the problem that was left unanswered in the previous lecture. The first part of the lecture is concluded with an exploration of how supersonic flows behave in a windtunnel with a changing cross sectional area. The second half of the lecture focuses on the main subject of the lecture, the concept of viscous flows. There the Reynolds number, properties of laminar and turbulent flow, and the phenomenon of flow separation are explored.