Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 134

The definition of the drag coefficient for a streamlined body, such as an aircraft wing or a wind turbine blade, because of the relevance of surface friction drag is based not on the frontal area but on the plan area. The flow past a body that has a large span normal to the flow direction is locally quasi‐2‐D, and in such cases the drag coefficient can be based upon the drag force per unit span using the streamwise chord length for the definition:

(A3.7)

The drag coefficient of an aerofoil varies with angle of attack. For a well‐designed aerofoil at moderate to high Reynolds number [O(10⁶)–O(10⁷)], the value of C_d is O(0.01) in the minimum drag range of angle of attack (called the drag bucket).

The following sections show some results for two classical NACA four‐digit aerofoils that, although not now used except exceptionally for wind turbines, do demonstrate the typical force behaviour of aerofoil sections.