Page 191 - Wind Energy Handbook

DEFINITION OF LIFT AND ITS RELATIONSHIP TO CIRCULATION 165

α

Figure A3.14 Flow Past an Aerofoil at a Small Angle of Attack

velocities and pressures above and below the aerofoil at the trailing edge must be
the same, the particles which meet there are not the same ones that parted company
at the leading edge; the particle which travelled above the aerofoil reaches the
trailing edge first because it is speeded up by the circulatory flow.
The pressure variation (minus the ambient static pressure of the undisturbed
flow) around an aerofoil is shown in Figure A3.15. The upper surface is subject to
suction (with the ambient pressure subtracted) and is responsible for most of the lift
force. The pressure distribution is calculated without the presence of the boundary
layer.
Figure A3.16 shows the same distribution with the pressure coefficient (C p ¼
2
p p 1 =[1=2]rU ) plotted against the chord-wise co-ordinate of the aerofoil profile:
the full line shows the pressure distribution if the effects of the boundary layer are
ignored, and the dashed line shows the actual distribution.

Figure A3.15 The Pressure Distribution Around the NACA0012 Aerofoil at Æ ¼ 58