Wind Turbine Aerodynamics Part B: Turbine Blade Flow Fields - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 364

Figure 5-56. Signatures of c p across a blade chord at 0. 47R tracking the moving location
of a dynamic stall vortex above the suction surface of a UAE Phase VI wind turbine blade
at three successive azimuthal positions. U ¥ = 13 m/s and rotor yaw angle = 40 deg.
Again, no flow visualization has been accomplished as yet for the UAE Phase VI turbine
blade. However, pressure signatures elicited by a dynamic stall vortex have been identified
and tracked on a UAE Phase VI blade surface. These c p data are shown in Figure 5-56 for
a station at 0.47 R , with U ¥ = 13 m/s and a 40 deg yaw angle. The dynamic stall vortex was
first detected just aft of the leading edge ( x/c = 0.04), as indicated by a high, narrow surface
pressure minimum (suction peak) centered at, when the blade was at an azimuth of -72.8
deg. Then, 35 msec later, when Y = -57.8 deg, the suction peak had moved aft, broadened,
and decreased in magnitude to -2.42. Finally, after an additional 66 msec had passed and Y
= -29.4°, the suction peak had continued aft, broadened further, and decreased in magnitude
to -1.42. Movement of the peak aft corresponds to vortex convection and peak broadening
is associated with vortex growth. Dynamic stall vortex convection was rapid, traversing 75
percent of the chord width in approximately 0.1 sec.
Using this suction peak tracking methodology, a detailed pressure history can be as-
sembled at each spanwise location of pressure taps, documenting vortex chordwise posi-
tion as a function of time. Then, for each combination of airspeed and yaw angle, vortex
convection topologies can be mapped, producing diagrams such as that illustrated in Figure
5-57 [Schreck et al. 2001 and 2005]. Detailed interpretation of these topologies shows that
the vortex structure evolves rapidly and dramatically, undergoing complex structural defor-
mations.
The dynamic stall vortex exhibited restrained convection at 0.30R, implying the exis-
tence of pinning influences at the inboard vicinity of the vortex [Freymuth 1988; Robinson
and Wissler 1988; Lorber et al. 1992; Schreck et al . 1991; Schreck and Helin 1994; Schreck
et al . 2001; Coton and Galbraith 1999]. Similarly suppressed vortex convection was observed
at the outboard extreme of the vortex, again consistent with pinning interactions. Vortex con-
vection was most active near the central region of the blade.
Dynamic Stall Modeling
The fluid dynamic structures and processes described in connection with Figure 5-55
are common to dynamically-stalled flow fields in the qualitative sense. However, quantita-
tive aerodynamic force production can vary radically in response to subtle input parameter
variations. Thus, several dynamic stall models have been developed, with most intended
originally for rotorcraft applications. Some of these dynamic stall models have been adapted