I’m very certain that the curved lines are streamlines.
The black arrow is labeled “Free stream velocity” meaning that, in absence of any perturbation, that is the direction that the air would freely follow. I.e. it would go in a straight line.
There is a mention of flow turning as the source of lift here [0] but the page you mention is taking the more mathematical approach of defining lift as the integration of pressure over the wing area.
I agree that the diagram could be better, most aerodynamics textbook diagrams could be :)
EDIT: I just reread the page with a bit more time and I have to disagree, it’s a very good explanation of lift, although it is a bit more technical than a layman’s. I can’t see anything obviously wrong with it, except that it doesn’t mention more advanced topics (e.g. turbulence).
The black arrow is labeled “Free stream velocity” meaning that, in absence of any perturbation, that is the direction that the air would freely follow. I.e. it would go in a straight line.
There is a mention of flow turning as the source of lift here [0] but the page you mention is taking the more mathematical approach of defining lift as the integration of pressure over the wing area.
I agree that the diagram could be better, most aerodynamics textbook diagrams could be :)
[0] https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/wha...
EDIT: I just reread the page with a bit more time and I have to disagree, it’s a very good explanation of lift, although it is a bit more technical than a layman’s. I can’t see anything obviously wrong with it, except that it doesn’t mention more advanced topics (e.g. turbulence).