Aerodynamics: flow separation's effect on lift and drag

[Vasco Mena de Olivei]

In aerodynamics, how does flow separation reduces the lift and drag of a wing at the same time?

 

[Arjan82]

Because most of the drag is 'induced drag' which is drag due to lift generation. Less lift thus means less drag.

 

[harsh_raj]

When flow is separated, it creates turbulent eddies and regions of low pressure behind the separation point. These phenomena increase the drag force acting on the object, making it harder for the object to move through the air efficiently. Due to loss of pressure, the pressure difference is reduced which causes reduction in lift.

 

[Lnewqban]

In aerodynamics, how does flow separation reduces the lift and drag of a wing at the same time?

Welcome, Vasco!

That statement seems not to be totally correct (unless you are referring to induced drag only).
After flow separation occurs, the lift force gets reduced, but the total drag increases.

Think of the drag force induced in both extreme cases:
1) A thin airfoil at zero angle of attack (no lift and minimum form drag).
2) Same thin airfoil positioned at 90° angle of attack (no lift and maximum form drag).

For a wing in normal flying conditions, the main zone of low pressure is located over the top surface of it, not interfering much with the horizontal balance of forces (thrust versus induced and parasitic drags).

For a wing in abnormal AOA conditions, the main zone of low pressure is relocated from the upper zone towards the aft zone, now interfering with the horizontal balance of forces (increased pressure differential between leading edge-bottom surface of wing and trailing edge-top surface).

At the same time, more area is presented to the incoming airflow, which velocity momentarily tends to stagnation.

The rotational nature of the shear forces inside the tri-dimensional turbulent wake detached airflow consumes mechanical energy, which is stolen from the lift effect, if thrust from the engine remains constant.

Copied from:
https://eaglepubs.erau.edu/introduc...cles/chapter/introduction-to-boundary-layers/

"Flow Separation on an Airfoil
Boundary layer separation from a body, such as an airfoil section, as shown in the schematic below, can have significant consequences, including a large increase in drag and a substantial loss of lift.
This outcome is because the rear part of an airfoil creates an adverse pressure gradient, which becomes increasingly adverse with an increasing angle of attack.
At low angles of attack, the boundary layer can withstand this gradient, reaching the airfoil’s trailing edge or separating just before that point. However, as the angle of attack increases, the more severe adverse pressure gradients cause the flow to separate at a shorter downstream distance, so the separation point moves forward.
Eventually, flow separation occurs near the leading edge, and under these conditions, the airfoil is said to be stalled.
The turbulence produced in the separated flow region and wake is also a source of unsteady aerodynamic loads and buffeting on the wing.
Indeed, a characteristic of stalling the wing of an aircraft during flight is the creation of unsteady aerodynamic loads and buffeting transmitted to the airframe, warning the pilot of an impending wing stall."

 

[aeroandy7]

"Wing" in the original question is assumed to be unswept with unchanging geometry and in a steady flow field. Perfectly fine. For fun, I just wanted to add that there are cases where large-scale vortices from separated flow can reattach and result in increased lift. For example, flow over a highly-swept, small-radius (sharp) wing leading edge may separate and roll up into a so-called vortex sheet near the wing's upper surface at relatively low angle-of-attack. Kuchemann's The Aerodynamic Design of Aircraft touches on this in figure 3.5.

Another example is an oscillating wing feature such as a pop-up spoiler, other effector, or even the entire wing flapping at the appropriate Strouhal number has been found to potentially lead to reattachment and enhanced lift. I'm sure some micro-UAV developers may have developed this considerably, mimicking nature, but unfortunately have no specific example to offer.