Vortex Generators

If you've ever opened up the stopper in a sink full of water and watched the water swirl down the drain, you know what a vortex is. Simply put, it is an energetic swirling mass of fluid. Vortices are quite common in aerodynamics. Probably the most well-known and significant of these are the trailing vortices that are seen coming off the tips of wings in flight. These vortices are not desirable because they create a type of drag known as induced drag, or that drag induced by a surface generating lift (like a wing). Aerodynamicists often spend considerable effort trying to reduce the adverse effects of such vortices. However, vortices very similar to trailing vortices can also be used to produce beneficial effects, and one of the methods used to create beneficial vortices is the vortex generator.

In previous questions, we have discussed the concept of flow separation. When an aircraft flies at high angles of attack, the airflow over the wing can become detached, or it stops following the shape of the wing. When this happens, the lift produced by the wing will suddenly and rapidly decrease, and the wing is said to be stalled. When the flow separates from the wing, it usually means the air is moving too slowly, or there isn't enough energy in the flow to keep it moving. Since vortices are energetic, they can be used to put energy back into the flow to keep it moving in the desired direction. This is what vortex generators are designed to do.

Vortex generators are simply small rectangular plates that jut above the wing surface. They look like tiny little wings jutting up perpendicular to the wing itself. As air moves past them, vortices are created off the tips of the generators just like the trailing vortices mentioned earlier. These vortices interact with the rest of the air moving over the wing to speed it up and help reduce the possibility of separation. Vortex generators are typically used in the following applications:

Vortex generators are not the only method used to delay wing stall. Wing fences, thick trailing edges, dogtooths or sawtooths, drooped leading edges or slats, and leading-edge notches produce similar effects. Each method has its drawbacks, most notably increased drag, and they are typically only used as a last resort when re-designing the entire wing is not practical. For these reasons, they are sometimes referred to as the "vacuum cleaners of the aerodynamicist" since they are used to clean up after previous mistakes.
- answer by Jeff Scott, 14 January 2001

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