The opposite of attached flow is separation, when the air no longer travels smoothly along the sail, but is broken up by turbulence. Remember that a thin layer of air passing over a sail actually sticks to its surface and moves with the sail. This air is in fact moving in the opposite direction to the rest of the air, or the "external flow" since the boat is sailing forward while the wind is blowing aft. This negative or opposite flow exerts an influence on the adjacent air and tries to drag it along with the sail, and the slower that adjacent air is flowing, the more chance the negative air has of succeeding. The problem arises if there is too abrupt a transition between the boundary layer and external flow, for example, where the air is forced to make an abrupt turn around the leeward side of the luff. In that case the laminar flow will be broken up into speed-robbing vortices and there will be separation (Figure 15.5).
Curvature of the sail directs the force in a certain direction.
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