Flow separation

Boundary layer separation is an aerodynamic effect on wings. He has a significant influence on the flight performance of an aircraft, as they strongly impede the flow around the wing. While separation bubbles at high speeds mainly occur on the lower wing surface, while slow flight, the flow on the upper surface of the wing is particularly vulnerable. In both cases, they are in the region of the pressure increase observed ie behind the thickest part of the profile.

Among other effects, the formation of a separation bubble is particularly dependent on the Reynolds number, that is the flow velocity, the chord length, the density and the dynamic viscosity. Looking at the profile of the air flow around an airfoil, it can be seen with increasing angle of attack, the flow partially detaches from the profile contour and applying again after a certain running track. The greater the angle of attack here, the sooner, the flow separates from the top until they can no longer create. The aircraft undergoes a so-called stall, which is not covered in this article.

Laminar separation bubble is called the bubble because the flow until they are replaced is laminar and the flow after the bubble reattaches to the profile contour. This reapplication is possible because in the area of the separation bubble, the flow turns to the turbulent state. Since the molecules of the turbulent flow are higher in energy, they can follow the contour lighter, but this energy accumulation acts in an increase in the resistance of.

Simple terms, the energy loss in the boundary layer is the reason for this behavior of the flow when passing over the profile contour. The flow is to follow after a certain run-length no longer capable of the profile contour so that it leaves the contour and separates from her. Such separations are preferably formed at the locations at which the geometry of the profile is tapered again. In other words, at the points at which an increase in pressure, according to Bernoulli will be forced. This applies to both the top and the bottom of the profile, so that peeling bubbles can form on both sides.

With the help of turbulators, it is possible to prevent the flow intentionally turn into a turbulent boundary layer. Due to the turbulence the momentum exchange increased. This is easier to follow close to the wall energy and the flow, the profile contour. The separation bubble is thus prevented. Since, however, depends on the location of the separation bubble, the angle of attack, a general optimal location for the turbulators on the wing surface over the entire angle of attack range is not clearly determined. A compromise between good flight performance at low speeds and high-speed flight is practically possible only on the bottom face. Here arises only at high speeds, ie at low angles of attack, a separation bubble. On the top of the wing there is no sense of flow turbulence to force, as due to the geometry of the profile of the area of ​​detachment of the vane over the entire angle of attack is too large.

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