Surface velocity distribution of the NACA 2412 airfoil at 0º angle of attack.

The difference between the velocities on the upper and lower side of the NACA 2415 airfoil at 0º angle of attack is relatively small. The enclosed area corresponds to the integrated pressure difference and results in a small lifting force.

Velocity distribution on the surface of the NACA 2412 airfoil at 6º angle of attack.

Increasing the angle of attack to 6º also changes the velocity distribution: The velocity on the upper surface increases, whereas the velocity on the lower surface is reduced. The enclosed area increases and thus the lift (-coefficient). The differences are more pronounced in the leading edge region, which (in this case) contributes most to the total lift.

The image also shows, that the stagnation point (where the velocity is zero) is not exactly located at the extreme left of the leading edge, but instead has moved slightly back on the lower side. The streamlines passing along the upper surface have to turn around the leading edge, whose small radius of curvature accelerates the flow in this region rapidly (from zero to 100 in a split second). This causes the velocity peak close to the leading edge, where the flow reaches about 1.85 times the speed of the onset flow.

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