Wind Turbine Aerodynamics Part B: Turbine Blade Flow Fields - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 354

Data Analyses for Physical Comprehension

In-depth analysis of the UAE Phase VI test data produced new findings regarding HAWT
aerodynamic phenomena, such as the following:

- Blade rotation can amplify maximum normal forces at inboard span stations, to

nearly three times the levels observed under nonrotating conditions.

- These amplified normal forces are associated with unconventional blade surface pres-

sure distributions that have been observed infrequently in previous experiments.

- Dynamic stall vortex structure and dynamics varied significantly along the blade

span, being highly three-dimensional under yawed operating conditions.

It is important to note that the following discussion concentrates solely on wind turbine
blade aerodynamics, and does not consider the wind turbine wake. Admittedly, the wake
plays a strong role in overall turbine aerodynamics, and wake modeling has advanced sig-
nificantly in recent years. Unfortunately, measured data containing detail comparable to that
available for the turbine blade flow field do not exist at present.

Measurement of Local Inflow Angles and Normal Force Coefficients

Determination of angles of attack and lift coefficients for rotating blades using measure-
ments on or near the blade itself remains a challenging and essential activity [Brand 1994;
Schepers 1995; Shipley et al. 1995b]. For the UAE Phase VI turbine, these difficulties were
deferred in order to simplify physical relationships and concentrate on rotational modifica-
tions to the flow field. This was accomplished by analyzing the local inflow angle , LFA , as a
surrogate for the angle of attack, and the normal force coefficient, C n , in lieu of the lift coef-
ficient. LFA is defined as the angle between the local inflow vector and the local blade sec-
tional chord. LFA was measured by five-hole probes located 0.80 chord width ahead of the
blade leading edge, at normalized radial positions of r/R = 0.34, 0.51, 0.67, 0.84, and 0.91.
The normal force coefficient, C n , is equal to the sectional aerodynamic force intensity
(force per unit of span length) at right angles to the blade chord divided by the product of the
local dynamic pressure and the local chord width. Normal force intensities were determined
by integrating surface pressures measured along the blade chordlines at r/R = 0.30, 0.47,
0.63, 0.80, and 0.95. Detailed information regarding instrumentation and procedures can be
found in Hand et al. [2001].

Rotational Augmentation of Aerodynamic Properties

Background

Prior research concerning rotational augmentation of airplane propeller and helicopter
rotor aerodynamics helped guide early work specifically directed at wind turbines. Augmen-
tation of rotating blade aerodynamic properties, including stall delay and lift enhancement,
was first observed for airplane propellers and qualitatively explained in terms of centrifugal
and Coriolis accelerations [Himmelskamp 1950]. Later, analytical modeling quantitatively
accounted for key elements of the rotating blade flow field [Banks and Gadd 1963]. Analyti-
cal modeling of helicopter rotors determined that rotationally induced cross flows played an
important role in blade lift production [McCroskey and Yaggy 1968]. Experimental research
[McCroskey 1971] suggested that centrifugal forces are important in the presence of flow
separation, but of limited influence otherwise.