Vortices Create Circulation

Scientists have discovered that any surface, be it a flat board or a curved sail, when presented to the wind starts to generate a circulation around that surface,


Figure 15.11

The momentum created by the boundary layer prohibits the air from making the turn, so instead, it continues beyond the edge, tripping up on itself until the air starts to swirl in a small vortex.


Back End of Sail

Back End of Sail

Air flow meeting Kutta Condition

Starting Gortex

Figure 15.12

The initial vortex is then swept downstream. Because there is a continual flow of air across the sail, a continual stream of vortices is created, collectively referred to as the "starting vortex."

in other words a thin layer of air starts to rotate around the surface, often in directions that are far different from the surrounding layer of air. This circulation is in fact created by the small vortices that spin off the back end of the foil (or flat surface if that's what you are working with). The circulation flow is strongest near the foil and becomes progressively weaker as it moves away from it. How the vortices create the circulation can be illustrated with a mechanical analogy (Figure 15.13). If you think of the vortex as a small cog spinning around, and know that air has some viscosity, then you will understand that the spinning air has an effect on all the air particles with which it comes into contact. Using the small cog analogy, the spinning particles start to create a circulation flow around the sail. (Note that Figure 15.13 is a schematic representation of what is going on, and as explained above, circulation takes place very close to the sail and is not a great



^ Starting Vortex I* (Small Cog)

X Circulation Around Sail (Big Cog)

Figure 15.13

The starting vortex creates a circulation around the sail.

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