Wind Turbine Aerodynamics Part B: Turbine Blade Flow Fields - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 357

Figure 5-48. Surface pressure coefficient distributions measured on a stationary blade
at stall. The horizontal axis is the normalized distance aft of the leading edge. Note that
negative pressure is plotted above the horizontal axis.
the stationary blade. Note that negative or suction pressures are plotted above the horizontal
axis. All four of these distributions contain suction peaks on the suction surface (or upper or
low-pressure surface ) of the blade, near the leading edge.
Extending aft of the leading edge, also on the blade's suction surface, are adverse pres-
sure gradients ( i.e. pressures that retard the blade's forward motion) of progressively de-
creasing magnitude. On the blade pressure surface ( lower or high-pressure surface ) the c p
distributions are remarkably similar. Overall, these surface pressure distributions strongly re-
semble those commonly observed on two-dimensional airfoils in the neighborhood of stall.
The rotating blade stall c p distributions shown in Figure 5-49 were characterized by
mild pressure gradients extending over significant portions of the suction surface chord. The
rotating blade distributions at 0.30 R , 0.47 R , and 0.63 R all differ considerably from those on
the stationary blade at the same spanwise locations. However, the rotating blade c p distribu-
tion at 0.80 R closely resembles that shown in Figure 5-48 at the same spanwise location on
the stationary blade. This is consistent with the strong similarity between the stationary and
rotating blade C n vs. LFA curves at 0.80 R near stall. The magnitude of the rotating blade
suction c p levels, coupled with the mild gradients, is not consistent with conventional two-
dimensional lifting surface flows, thus pointing to three-dimensional mechanisms.
Figure 5-49. Stall surface pressure coefficient distributions measured on a rotating blade.