Thanks for your response! Would you mind explaining the "circular movement" further? I am not sure what you meant by "air moves backward above, forward under, upward in front of and downward behind the wing"...
I have also heard another theory that states that the air (boundary layer???) on the upper surface is expanded while that on the lower surface is compressed, due to angle of attack. May I know why the air gets expanded/compressed when it flows along the wing?