Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 135

Figure A3.11 shows that on the upper surface pressure is rising as the flow moves from the suction peak towards the trailing edge. This is an adverse pressure gradient that slows the air down. It also thickens the boundary layer more rapidly, causing more velocity momentum to be lost. If the flow above (i.e. just off) the surface within the boundary layer is slowed to a standstill, the surface streamlines separate from the surface, stall occurs, and the pressure drag rises sharply. The strength of the adverse pressure gradient increases with angle of attack, and therefore the drag will also rise with angle of attack. Figure A3.14 shows the variation of C_d with α for the symmetrical NACA0012 aerofoil.

Figure A3.14 Variationof C_d with α for the NACA0012 aerofoil.

Figure A3.15 Lift/drag ratio variation for the NACA0012 aerofoil.

The efficiency of a wind turbine rotor blade is significantly affected by the lift/drag ratio of its aerofoil section(s) (as shown in Figure A3.15), and it is desirable that a turbine blade operates at the maximum ratio point.

The nature of the flow pattern around an aerofoil is determined by the Reynolds number, and this affects the values of the lift and drag coefficients. The general level of the drag coefficient increases with decreasing Reynolds number. The effect on the lift coefficient is largely concerned with the angle of attack at which stall occurs. Below a critical Reynolds number of about 200 000, the boundary layer remains laminar, usually leading to early stall or partial separation (long bubbles) at low angles of attack.. As the Reynolds number rises, so does the stall angle and, because the lift coefficient increases linearly with angle of attack below the stall, the maximum value of the lift coefficient also rises.

Figure A3.16 Variation of the drag coefficient with Reynolds number at low angles of attack.

Characteristics for the NACA0012 aerofoil are shown in Figures A3.16 and A3.17.