Why does the stagnation point on airfoil move with the angle of attack?

What is the physical explanation for the reason the stagnation point is where it is? And, why does it move further down the lower side of the airfoil with an increase in angle of attack?

• I imagine it is something to do with that being the balance of point of where it is easier for the air to go up and over or instead carry on down the lower side of the airfoil? but then if that's the case why does it move, possibly to do with the decrease in pressure on the top making it easier for more air to go up and over instead of along the bottom? Jun 25, 2018 at 0:41
• You are on the right track. As the pressure difference between above and below increases, the low pressure region above sucks more air, so the stagnation point moves down. I don't think it can be quantified without running the full fluid dynamics calculation though. Jun 26, 2018 at 19:38
• @JanHudec Yes it is the only reasonable explanation i have come to i just can't find any source anywhere to justify it. Jun 28, 2018 at 10:07

At the stagnation point, the streamline is directly perpendicular to the airfoil. As angle of attack increases, the streamline is no longer perpendicular to the leading edge (which is where the stagnation point is usually located at 0 angle of attack). This is the best explanation I can come up with for why the front stagnation point moves as it does. The rear stagnation point is usually located at the trailing edge of the airfoil, thanks to the Kutta Condition which says that, "A body with a sharp trailing edge which is moving through a fluid will create about itself a circulation of sufficient strength to hold the rear stagnation point at the trailing edge." (according to wikipedia). The airflow around the airfoil, which is a body with a sharp trailing edge, keeps the rear stagnation point where it is. This moving of the front stagnation point as the AoA increases is part of the reason why lift increases with increases in AoA.

• If anyone can come up with a better explanation, please let me know. I feel this doesn't do justice to the question. Jun 25, 2018 at 8:29
• Pretty good explanation. If you consider the stagnation point where the air hits the wing (or vice versa), air above it goes over and air below it goes down. If you change the angle of the wing, the air hits at a different point. Jun 25, 2018 at 17:08
• But once the angle of attack is increased it is not just the one streamline but multiple streamlines that are perpendicular to the flow which start to go over the airfoil. So what is determining how much of the airflow that decides to go back up and over instead of carrying on below the airfoil? Jun 26, 2018 at 10:42
• The streamline may, and does, curve before hitting the wing, so while it indeed is locally perpendicular at the stagnation point, it is not enough to determine it. Jun 26, 2018 at 19:35
• @IanHodges That is determined by the pressure difference. High pressure above and low pressure below means the airflow is far more likely to travel to the lower surface. Jun 27, 2018 at 2:38

stagnation point is the point with more pressure, always it's the first point that the flow faces an object. At that point, the flow has most Kinetic energy and it loses more of it in the first collide that makes pressure.at the other points, this flow doesn't have such as that energy so less energy makes less pressure.when the AoA change, the first point that the flow face will change. if we increase the AoA, the stagnation point goes down on the airfoil because the first point that flow faces comes down And conversely.

• In fact the direction of flow near the front of the wing changes as the angle of attack changes. Jun 29, 2018 at 14:08

Although a tad late; I'll have a go at this and I will use elementary yet powerful potential flow approach for this.

Suppose we start with the non-lifting case. Then, the forward stagnation point will be x/R = -1

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Now let us rotate the cylinder clockwise. The mathematical representation is a vortex. Clearly, the stagnation point moves downward. One should view this point as the point where the induced velocity exactly opposes the velocity caused by the non rotating case. This also means that with increasing circulation, the stagnation point moves downward.

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An airfoil can be thought of as a lifting cylinder to some abstraction. So, when the angle of attack is increased and a new steady state has been achieved (i.e. effect of shed vortex is diminished), the circulation must have increased. Hence the stagnation point moves to a lower position.

EDIT: To make it physical: A change in angle of attack leads to a change in vorticity and circulation. This is basically a consequence of fluid dynamics and boundary conditions. These in turn are a consequence of a new balance between pressure and shear forces!