Circulation:


                The theory of lift works only when there is a circulation. Now, how many birds have we seen to be violently spinning their wings while flying?? The secret lies in the shape of their wings.


 Starting Vortex:         The cross-section of the wing is shaped like an airfoil, with a sharp tip at the end. The airfoil is initially at rest. Immediately after it starts moving, within an extremely short time, a boundary layer is formed around the structure. As a result of the shear, a vortex is formed at the tip, and is left at the location where the airfoil started its motion.

This is called the starting vortex - as this vortex starts the circulation  so necessary for the lift!


Bound Vortex:        According to Kelvin’s circulation theorem, the circulation around any material must remain constant. Hence, as there was no circulation initially, the starting vortex causes a reverse circulation around the airfoil! This vortex continues to move with the airfoil - and is thus, “bound”.


Tip Vortices or Trailing Vortices:        The fluid is generally at a lower pressure on the top surface of the airfoil, as compared to the bottom surface. Right at the tip of the wing,   the fluid at higher velocity from the top meets the one with the lower velocity from the bottom (the difference in the velocities result from Bernoulli’s theorem), resulting in a trailing vortex.





 

As is apparent in the above diagram, the system of trailing vortices causes the fluid to have a downward motion (downwash) inside the vortex system, whereas, outside it, it has an upward motion (upwash). This results in a downward pull on the wing, thus increasing the angle of attack, which enhances the induced drag, discussed later.