Wind Turbine Aerodynamics Part B: Turbine Blade Flow Fields - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 362

Aerodynamic Force Unsteadiness and Repeatability
Mean aerodynamic force records exhibit prominent time variations, which prompt ques-
tions regarding repeatability and statistical significance. Figure 5-54 shows a superposition
of UAE Phase VI C n records for 36 blade rotation cycles, at 0.30 R and 0.80 R , for U ¥ = 13
m/s and a rotor yaw angle of 40 deg. The mean C n data for these two radial locations also
are shown, as thin white traces in the approximate center of the multiple overlapping black
traces.
These plots reveal the presence of C n oscillations having frequencies of approximately
30 to 50 Hz. Peak-to-peak amplitudes of these higher frequency components correspond to
approximately 50 to 70 percent of mean levels. Both the once-per-revolution and the higher-
frequency time variations of C n associated with dynamic stall constitute significant compo-
nents of the aerodynamic load spectrum.
Figure 5-54. Instantaneous C n records for 36 blade rotation cycles at two radial loca-
tions. U ¥ = 13 m/s and yaw angle = 40 deg.
Flow Field Structure
Visualization of dynamically stalled flow fields reveals flow field structures responsible
for the dramatic C n amplification. Unfortunately, dynamic stall visualization is prohibitively
difficult on large-scale rotating turbine blades, and successful execution continues to elude
turbine aerodynamicists. However, analogous dynamically stalled flow fields occur on non-
rotating aerodynamic surfaces subjected to dynamic pitching, and are readily visualized.
Figure 5-55 shows smoke flow visualization of a two-dimensional airfoil with a 0.15
m chord, pitching from 0 to 60 deg at a constant rate of 115 deg/s. The airfoil pitch axis is
located at the quarter-chord point. Air flows from left to right at 6.1 m/s. The three panels
show instantaneous angles of attack of 20°, 25°, and 30°, from top down. The upper surface
of the airfoil is visible in each panel as a brightly lit arc inclined with respect to the panel
borders.
In the upper panel of Figure 5-55, the disordered smoke filaments immediately above the
airfoil indicate that separation has enveloped the entire upper surface. However, the smoke
line emanating from the leading edge is well defined, curving back toward the airfoil, and
impinging on the surface at approximately 20 percent of the chord width from the leading
edge. Subsequent panels show this structure to be an energetic dynamic stall vortex . The
dynamic stall vortex produces a surface pressure minimum of significant magnitude on the