ABSTRACT
Before deriving the Blade Element Momentum method it is useful to examine a simple one-dimensional (1-D) model for an ideal rotor. A wind turbine extracts mechanical energy from the kinetic energy of the wind. The rotor in this simple 1-D model is a permeable disc. The disc is considered ideal; in other words it is frictionless and there is no rotational velocity component in the wake. The latter can be obtained by applying two contra-rotating rotors or a stator. The rotor disc acts as a drag device slowing the wind speed from Vo far upstream of the rotor to u at the rotor plane and to u1 in the wake. Therefore the streamlines must diverge as shown in Figure 4.1. The drag is obtained by a pressure drop over the rotor. Close upstream of the rotor there is a small pressure rise from the atmospheric level po to p before a discontinuous pressure drop ∆p over the rotor. Downstream of the rotor the pressure recovers continuously to the atmospheric level. The Mach number is small and the air density is thus constant and the axial velocity must decrease continuously from Vo to u1. The behaviour of the pressure and axial velocity is shown graphically in Figure 4.1.
