Wind Turbine Aerodynamics Part B: Turbine Blade Flow Fields - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 358

Previous wind tunnel experiments employing subscale HAWT models have revealed
similarly augmented surface pressure distributions on rotating blades. These surface pres-
sure profiles took on approximately trapezoidal [Barnsley and Wellicome 1992] or triangular
[Ronsten 1992] profiles. In addition, large scale HAWT field measurements [Robinson, et
al. 1995] have detected trapezoidal surface pressure distributions.
Clearly, stall c p distributions on inboard locations of the rotating blade differ dramati-
cally from those observed farther outboard under rotating conditions. Inboard rotating blade
stall c p distributions differ in equally dramatic fashion from the stall c p distributions observed
anywhere on the stationary blade. This enhanced suction at elevated LFA has been attrib-
uted to various mechanisms mediated by centrifugal or Coriolis forces [Himmelskamp 1950,
Banks and Gadd 1963, McCroskey and Yaggy 1968, McCroskey 1977, Madsen and Chris-
tensen 1990, Barnsley and Wellicome 1992, Eggers and Digumarthi 1992, Snel et al. 1992]
and to the presence of a stationary energetic vortex structure [Eggleston 1990].
Flow Field Structure
The pressure instrumentation on the blades of the UAE Phase VI wind turbine provided
accurate portrayals of flow field states at the blade surface, but was incapable of discerning
off-surface flow field structure. To overcome this limitation, the EllipSys3D computational
fluid dynamics code was used [Michelsen 1992, Michelsen 1994, Sørensen 1995]. Code
results were first validated against UAE Phase VI surface pressure measurements. The flow
field above the blade surface was then extracted from these computations, revealing flow
structures and processes responsible for rotational augmentation [Schreck et al. 2007].
Figure 5-50 contains a two-dimensional section of the computed streamlines in the
stalled flow field at a blade radius of 0.30 R , for U ¥ = 15 m/s. This computation corresponds
to the measured data in Figure 5-46, for LFA = 40.2° and C n = 2.33. Under these conditions,
separation occurs at the leading edge. The resulting shear layer arches over the blade suction
surface and finally impinges on the aft portion of the blade chord. Between separation and
Figure 5-50. Two-dimensional section of computed UAE Phase VI blade flow field
showing a region of intense recirculation above the blade's suction surface ( U ¥ = 15 m/s,
r / R = 0.30). [ Computation courtesy of N. Sørensen, Risø DTU and Aalborg University]