Attached Flow

A condition of attached flow exists when there is a smooth transition, without excessive turbulence, from the boundary layer to the surrounding air. This condition is crucial to sail performance since without it a sail cannot obtain lift. Specifically, Bernoulli's principle will not be able to come into play.

There are three types of attached flow (Figure 15.4). Near the leading edge of an airfoil there is a smooth change of air speed within the boundary layer

Figure 15.4

There are three types of attached flow.

Figure 15.5

The wind gets to the deepest part of the sail too quickly and starts to separate.

from the airfoil surface to the edge of the boundary layer. This is called the laminar boundary layer and the flow is called laminar flow, i.e, flow in which the air molecules are both moving in parallel and interacting with one another without the interference of turbulence. Just aft of the laminar flow is an area where the air encounters a certain amount of natural turbulence. This can be caused by any number of things, from seams to a rough fabric surface. The smooth changes in speed within the laminar flow start to give way to a more erratic type of flow, and this area becomes known as the transitional region with the flow named appropriately, transitional flow. Moving further aft, the transitional flow becomes even more agitated and the boundary layer becomes known as the turbulent layer, and the flow as turbulent flow. Turbulent flow should not be confused with separation since even though the air is turbulent it is still attached to the sail, i.e., Bernoulli's principle is still in effect. Of all three, laminar flow is the most desirable, followed by transitional and then turbulent. The goal of a sail trimmer is to keep all three types of flow attached to the sail.

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