The airflow around the upper surface of the wing (never mind the bottom side, because it contributes only a small part of the lift at useful angles of attack) first accelerates, moving from higher to lower pressure and thus converting its pressure surplus into speed (pressure energy into kinetic energy to be exact). In the process, an area of low pressure is created at the top of the curve, where the wing and the airflow are tugging at each other. The wing is sucking it down, and it exerts a reaction force on the wing-this is lift. Jan's comment and AEhere's explanation are essentially the correct answer, but let me rephrase it in plainer terms of energy, without explicit math.Īs flow is deflected downward by the wing, its inertia resists being redirected. This process can also force a laminar-turbulent transition, and a turbulent boundary layer can reattach. Note that the flow is not guaranteed to fully separate at the point described above. The air out of the layer can still advance, because it has higher momentum, but the bottom of the layer is forced to invert its direction, detaching from the surface.Īs to where does this adverse pressure come from, it is because the accelerated air over the wing is at a lower pressure than the rest of the free airflow, so the air at the trailing edge pushes against the air over the wing (but cannot overcome its momentum). In simpler terms, this means that the boundary layer on the upper surface of the wing is progressively slowed as it travels down the chord, until it cannot push against the higher pressure downstream. Where $s,y$ are streamwise and normal coordinates.Īn adverse pressure gradient is when $dp/ds > 0$, which then can be seen to cause the velocity u to decrease along s and possibly go to zero if the adverse pressure gradient is strong enough. The streamwise momentum equation inside the boundary layer is approximately stated as You can find a basic mathematical explanation in the linked article: The flow separation commences at the boundary layer, due to adverse pressure gradients ( from Wiki):
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