World of Science - Aerodynamics - Part 5

Drag and Lift.

They said that lift might be hard to understand, but drag is in fact harder.

There are two kinds of drag, form drag and skin drag.

Skin drag is the resistance between a moving surface and a viscous fluid. The fluid tends to stick onto the surface, thus causing skin drag. That thin layer of fluid that is directly affected by the presence of a surface is the boundary layer.

Reynold's number is a parameter that measures how much of the flow is controlled by viscousity and how much is affected by speed of the flow. At high values (Speed is more dominant), the boundary layers break away from the surface because they can't change direction along the surface fast enough, and the flow tends to become turbulent. This turbulence (or the wake) appearing behind the moving object causes form drag.

Aerofoil

This link might be useful in understanding turbulent wake flow.

At different Reynold's numbers, different flow patterns occur.

Can't find pictures of flows with increasing reynolds numbers. Instead, here is a link about drag.

I can't say anymore myself because I am not familiar with this topic.