A detailed explanation on this awesome tiny device-vortex generators!
Vortex generators are small fins on the surface of the wing near the leading edge which improves the plane’s aerodynamics by delaying the boundary layer separation over the wing. They are used to generate vortices just like the wing tips. when the vortices are created over the wing surface, the effectiveness of the wing increases. The vortex generators are also placed on the control surfaces such as ailerons, elevators, rudders, flaps to increase their effectiveness (effectiveness can be increased by delaying flow separation). These are also used in wind turbine blades.
How does a vortex generator work?
Before getting into the working mechanism, let’s look at what is a Boundary layer and why it is important to delay its separation?
When a solid object through the air, a small layer of air called boundary layer surrounds the object. Air is viscous, it slows down due to friction. As the fluid moves past the airfoil, the molecules right next to the surface stick to the surface. The molecules just above the surface are slowed down in their collisions with the molecules sticking to the surface.in Fact, the air right above the surface is not at all moving. This region of air where the friction slows down the air is called the boundary layer. Refer the image below,
Since the boundary layer is somewhat viscous, or sticky, and slow-moving compared to the moving aeroplane, it falls behind and separates from the object, creating a wake. This wake creates drag(wake drag) on the object and slows it down, resulting in higher energy requirements (This Boundary layer is the source of skin friction drag), as well as the potential for loss of lift, for aeroplanes.
Formation of the boundary layer with high energy keeps air flow in contact with wing surface. But at slow speed and higher angle of attack this airflow separates from the wing surface and sometimes aircraft stalls. Also, sometimes airflow separates from the wing before reaching to control unit such as ailerons. And pilot loses controls!
So to overcome such problems, Vortex Generators are used.
Consider an airfoil with no vortex generator installed and it moves in the air, at higher angles even at low speed, the flow over the airfoil gets separated and forms wake drag, which in turn reduces the speed of the aircraft. At the same time, when a vortex generator is installed, the separation is delayed by the vortex generated, now this Vortex generator when placed close to the leading edge generates a vortex and mix up the lower energy boundary layer with high energy air to delay the flow separation and as a result, stalling is delayed. At slow speed and higher angle of attack, airflow cannot remain attached with wing due to its low energy. So here come vortex generators. Vortex generators form vortices. Vortices are just small tornados. It brings high energy air above boundary layer near the surface of the wing by forming vortices and energize boundary layer. This keeps airflow attached with wing and reduce the possibilities of aircraft stalling.
How does a vortex generator look like?
Vortex Generators can be designed in various shapes, some known shapes as of now are
1.Gothic Vortex generators
2.Rectangular Vortex generators
3.Triangular vortex generators
4.Parabolic Vortex generators.
5.Ogive vortex generators.
Steps to design a Vortex generator:
From the above-mentioned shapes, choose the on that best fits your design purpose.
Determine the Reynolds number of the flow.
Determine the length of VGs and their location along the chord of your wing. (The VG’s should be placed just in front of laminar to turbulent transition, this takes place usually around 16% of chord)
Determine the height of the Vortex generators.
Calculate spanwise spacing of Vortex generators.
- Changing Stall characteristics.
- It delays the stall by delaying the flow separation.
- Lowering takeoff and landing distances
- For example, to reduce the runway length you must have the required lift coefficient, this can be achieved with the help of VGs.
- Increasing maximum takeoff weight can be achieved.
- Reducing drag on large transonic aircraft
- Noise reduction (Airbus family incorporate vortex generators under the wing of A320 aircraft to reduce the noise generated by the airflow over circular pressure equalization vents for the fuel tanks. A maximum 2 dB noise reduction is reported.