The fourth video deals with the effect of viscosity. Dye, smoke, suspended particles, and hydrogen-bubbles are used to reveal the velocity field.
Various combinations of Couette and plane Poiseuille flow introduce the principles of lubrication. Axisymmetric Poiseuille flow and development of the flow around an elliptic cylinder are related to variation in the Reynolds number, and the growth of the boundary layer along a flat plate is shown.
Instability in boundary layers and pipe flow is shown to lead to turbulence. The eddy viscosity and apparent stress are introduced by hotwire-anemometer indications. The processes of turbulence production, turbulent mixing, and turbulence decay are considered.