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V. RAGHAV JOURNAL OF THE AMERICAN HELICOPTER SOCIETY results from one type of test geometry can be used to infer details on the other. The paper starts with a discussion on the phenomenon of dynamic stall. Most of the literature on explaining and predicting dynamic stall arises from interest in rotorcraft forward flight at high advance ratio, poststall maneuvering of combat aircraft, and operation of jet engine compressors at high stage pressure ratio. Research in this area has pro- gressed from predictions and experiments on oscillating “airfoil sections” in wind and water tunnels (Refs. 1–4) to rotating blades with inflow and swept wings undergoing large excursions in angle of attack and roll. Extensive studies of dynamic stall on oscillating wings have been ac- complished (Refs. 5–7) and numerical simulations have proceeded from inviscid formulations to full Navier–Stokes simulations (Refs. 8–10). Prior work Predicting the delayed onset of flow separation resulting from un- steady aerodynamics remains a tough challenge. This is especially true when the Reynolds number of the blade boundary layer is high enough so that the boundary layer upstream of separation is turbulent. In this case, studies show that stall initiation is a localized three-dimensional phenomenon, even if the blade itself has zero sweep and taper and is not rotating. Van Dommelen and Sten (Ref. 11) predicted the development of a singularity at the onset of separation, followed by focused sudden eruptions of boundary layer fluid into the outer stream, propagating the stall event. The precise timing of such events varies from cycle to cycle in the case of a helicopter in forward flight, although the flight condition is nominally steady. In the case of wind turbines, it has been observed in Ref. 12 that the dynamic stall vortex initiation strongly depends on local inflow angle. The above reasons make the timing of the dynamic stall difficult to predict and control. In addition, according to Refs. 13 and 14 the three-dimensional ef- fects that are unique to the rotating environment add critical concerns. The coupled effects of centrifugal and Coriolis forces in the rotating environment add more complexity to the flow characteristics (Ref. 15). Studies in Ref. 16 show that the dynamic stall region of wind turbine blades exhibits substantial radial pressure gradients. Radial acceleration appears to have a strong influence on poststall lift and pitching moment evolution. Moreover, deviations from the two-dimensional kinematics of dynamic stall vortex initiation on a wind turbine have been observed in Ref. 12. These three-dimensional influences include viscous interactions associated with rotational augmentation as well as inviscid ones like local sweep effects. McCroskey (Ref. 17) cites experimental evidence that the fluid in the stalled region does indeed move outward due to the centrifugal forces. Moreover, according to Corten (Ref. 16), the radial flow is pronounced on the inboard sections of a HAWT blade. Hence, understanding the nature of the near-surface flow under the influence of these coupled forces is crucial to predicting dynamic stall, to the accu- racy needed for high advance ratio rotor design as well as wind turbine design. Initial experiments at The Georgia Institute of Technology (Ref. 18) used a rotor in a hover facility, for the convenience in bringing the flow diagnostics close to the flow field. Transient stall was induced by an “inflow obstructer,” which was a plate in the shape of a sector of a circular disk, placed axially upstream of the rotor. The rotor blade was set at a pitch angle well above the static stall angle of attack of the airfoil section. When operated as a rotor, the induced inflow would ensure that the blade remained unstalled. The pitch was increased by trial and error until it was just under the angle where the rotating blade would stall as it entered the region where the inflow was obstructed. Laser sheet visualization confirmed that stall was occurring over the blocked sector, Looking upwind Top view Wind R =0 = 30 NREL wind turbine Vc Vn Vw Similar fluid flow conditions VTPP V =0Vn 022005-2 Top view = 270 GT rotor setup Fig. 1. Schematic representation of similar fluid flow conditions. but the rotor was producing thrust elsewhere over the rotor disk. PIV data showed discrete structures developing in the velocity field when viewed in the axial-radial cross-flow section. These structures suggested a mechanism whereby the accelerated flow near the blade surface could be exchanged with the nearly stagnant flow away from the surface. In this particular experiment, the inflow obstruction was much more severe than what may be encountered in any practical engineered design of a rotating-blade device. Following the “inflow obstructer” experiments, extensive PIV studies of the radial velocity field over a blade of a two-bladed teetering rotor in forward flight were conducted. The present paper is a revised, expanded, and integrated description of these studies (Refs. 19, 20). Correspondence between the forward flight and yawed wind turbine cases A small diversion is taken from the main thrust of this paper to identify the regions of flow similarity between the wind tunnel rotor dynamic stall experiment reported here and that on a full-scale HAWT. References 21 and 22 cite one limiter of the operating lifetime of horizontal axis wind turbines is the dynamic loading on blades and generators, sometimes far in excess of their design loads leading to fatigue failure. This dynamic loading is attributed to a variety of unsteady aerodynamic effects, includ- ing dynamic inflow, turbulence, wind shear, and dynamic stall. However, the problem of dynamic stall is of particular importance on wind turbines as the unsteady loads produced can be large enough to cause structural damage. Veers (Ref. 23) showed that a 30% error in prediction of airload leads to a 70% decrease in life expectancy of a wind turbine. Moreover, Huyer et al. in Ref. 24 showed that two-dimensional data on oscillat- ing airfoils from wind tunnel tests are insufficient to construct a good estimate of the structural loading on a HAWT. Such data consistently underpredicted actual loading and power output (Ref. 25). Coton et al. (Ref. 26) reported that substantial differences have been observed in the pressure distribution of the inboard section of rotating wind turbine blades compared to two-dimensional models preceding dynamic stall. In this paper the study was conducted on a rotor in forward flight. The hatched portions on the blades in Fig. 1 depict the portion of the blades that have similar flow conditions to a HAWT in yaw. The wind turbine case selected was the Grumman Wind Stream 33 Downwind HAWT used by the National Renewable Energy Laboratory in dynamic Looking from = 270PDF Image | Radial Flow Rotating Blade Retreating Blade Stall
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