Wind Turbine Aerodynamics Part B: Turbine Blade Flow Fields - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 356

Plotted in Figure 5-46 is C n vs . LFA for the inboard portion of the rotating blade at wind
speeds from 5 m/s to 25 m/s, with a fixed blade pitch angle and constant rotor speed. The C n
curve rises monotonically until LFA = 45 deg, where it attains a stalled C n of 2.41. Thereafter,
the normal coefficient undergoes a gentle decline with increasing inflow angle. Note that
this rotating blade C n represents an amplification of nearly threefold relative to the baseline
stationary blade C n for the inboard station.
Outboard Locations on Turbine Blades
Rotational augmentation undergoes progressive attenuation at radial locations farther
outboard on the blade. In Figure 5-47, C n at station 0.80 R is plotted versus LFA , for both
the stationary and rotating blades. Again, all data correspond to an axisymmetric operating
state, with the rotor plane orthogonal to the wind vector. At 0.80 R , 20 m/s yields a Reynolds
number, Re , of 0.63 x 10 6 , while at 30 m/s the Re is 0.94 x 10 6 . Normal force hysteresis was
found to play a role in shaping the post stall C n curve for the stationary blade.
Figure 5-47. Comparison of sectional normal force coefficients, C n , at an outboard sta-
tion for stationary (parked) and rotating operations of the UAE Phase VI wind turbine.
Up to a local inflow angle of approximately 25 deg, the normal force coefficients of
the rotating blade closely track those of the stationary blade. However, at higher LFA in
the post-stall regime, the rotating blade coefficients at this outboard blade station exceed the
stationary blade coefficients by as much as 45 percent.
Significant stall LFA delay and stall C n amplification were observed for the rotating
blade relative to the stationary blade at all blade span locations. Stall LFA , stall C n , and dif-
ferences between rotating and stationary C n values were all consistently larger for inboard
span locations.
Surface Pressure Distributions
To understand the physical mechanisms responsible for rotational stall delay and C n am-
plification relative to stationary conditions, surface pressure distributions were compared for
the stationary and rotating operating conditions. Figure 5-48 shows the results of this com-
parison of surface pressure coefficients, c p , which are equal to the measured local pressure
divided by the section dynamic pressure. These c p distributions correspond to C n at stall on