Wind Turbine Aerodynamics Part B: Turbine Blade Flow Fields - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 361

Figure 5-52. Cyclic variation of local inflow angle, LFA, with blade azimuth angle, Y,
at four radial locations, for a rotor yaw angle of 40 deg. U ¥ = 13 m/s. Azimuth angle is
zero when the blade is at the 12 o'clock position.
board on the blade. However, even at 0.84 R , the LFA oscillation amplitude remains substan-
tial. Notably, all four LFA cyclic amplitudes exceed the S809 stall LFA of approximately
21 deg, thus initiating dynamic stall.
Aerodynamic Force Amplification
An example of the aerodynamic response to the cyclic flow field is shown in Figure 5-53,
in which mean normal force coefficients along the blade, C n , are plotted as a function of blade
azimuth angle, Y. Again, the rotor yaw angle is 40 deg and the test airspeed is 13 m/s. Prior
research has shown that dynamic stall forces similar to those shown in this figure are com-
mon for a broad range of airspeeds and yaw angles [Schreck et al. 2005].
At 0.30 R , C n reached a maximum of 3.18 at Y approximately equal to zero. C n maxima
at the remaining three spanwise locations decreased progressively with distance outboard.
Nevertheless, at 0.80 R the C n maximum of 1.31 still significantly exceeded the stall C n of
1.00 for two-dimensional flow over a static NREL S809 airfoil, testifying to the presence and
influence of dynamic stall.
Figure 5-53. Variation of mean normal force coefficient, C n , with blade azimuth angle,
Y, at four radial locations, for a rotor yaw angle of 40 deg. U ¥ = 13 m/s.