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?
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.
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.