The Kutta Condition

We know that the air that flows off the back end of a sail that is trimmed properly does so with equal pressure on both sides. This fact of aerodynamics is known as the Kutta Condition after the scientist who discovered it in 1902. It's important for us to understand the Kutta Condition because it is relevant to the way the entire flow field around the sail is affected. Visualize what would happen if the Kutta Condition was not a reality and the air coming off the leeward

THE KUTTA CONDITION

Stagnati Stream!

Stagnati Stream!

speed and, by extension, the same pressure.

Stagnation Streamline

Figure 15.10

The air flow around the total region of the airfoil adjusts itself so that the air flows off both sides of the sail at the same speed and pressure.

speed and, by extension, the same pressure.

side was flowing faster than the air coming off the windward side (as many of the earliest theories claimed). There would be two different pressures on either side of the sail (per Bernoulli's theory), and once the barrier of the sail was no longer there, nothing would stop the high-pressure air from taking over and pushing into the low-pressure region thereby negating the lift created by the sail. What really happens is that the air flow around the total region of the airfoil adjusts itself so that the air flows off both sides of the sail at the same speed and pressure. In other words the Kutta Condition prevails (Figure 15.10).

So, we know that the air speeds up at the edges of the surface, and it exits the surface at the same speed. How does this relate to lift and the mechanics of a sail? Enter the boundary layer. This invisible layer around the sail has its own characteristics, and they affect the flow of the air around the surface. If no boundary layer existed, the air would quite likely be able to make the sharp turns at the edges of the surface without any problem. Instead, it is trying to make the turn within the context of the boundary layer, and scientists have discovered that while wind within the boundary layer has no problem speeding up, there is a certain amount of momentum created by the boundary layer that prevents it from slowing down and making sharp turns.

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