Page 192 - Wind Energy Handbook

166 AERODYNAMICS OF HORIZONTAL-AXIS WIND TURBINES

1.5
NACA0012 Re=1 000 000

1 Stalled region

C l

Attached flow region
0.5

0
0 5 10 15 20 25
α (degrees)

Figure A3.16 The Pressure Distribution Around the NACA0012 Aerofoil at Æ ¼ 58

The effect of the boundary layer is to modify the pressure distribution at the rear
of the aerofoil such that lower pressure occurs there than if the boundary is ignored.
The modified pressure distribution gives rise to pressure drag which is added to
the skin friction drag, also caused by the boundary layer.

A3.7 The Stalled Aerofoil

If the angle of attack exceeds a certain critical value (108 to 168, depending on the
Re), separation of the boundary layer on the upper surface takes place. This causes
a wake to form from above the aerofoil, reduces the circulation, reduces the lift and
increases the drag. The flow past the aerofoil has then stalled (Figure A3.17). A flat
plate will also develop circulation and lift but will stall at a very low angle of attack
because of the sharp leading edge. Arching the plate will improve the stalling

Figure A3.17 Stalled Flow Around an Aerofoil