04 m radius all blades were of rectangular planform

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Unformatted text preview: blades of 5.04 m radius. All blades were of rectangular planform, and used NREL S809 airfoil crosssections with a 0.457 m chord. Phase II rotor blades were untwisted, while Phase IV blades had twist distributions optimized to yield a uniform 15° angle of attack at a wind speed of 8 m/s. One blade on each rotor was thoroughly instrumented with pressure transducers (Fig. 2). Phase II employed four instrumented span locations (30%, 47%, 63%, and 80% span). Phase IV added an instrumented span location at 95% span. Dynamic pressure and inflow angle were also measured at or near these four span locations. The data sample rate (521 Hz) was sufficient to capture dynamic and transient pressure events elicited from time varying inflow conditions. Wind magnitude and direction were measured 12-15 m upwind of the turbine using cup, propeller, and sonic anemometers. It is important to note that both the rectangular and twisted blade geometries used for the Unsteady Aerodynamics Experiment are atypical. Multiple Unsteady Aerodynamics Experiment program phases were designed to provide a common data set for comparing three-dimensional blade geometry effects on aerodynamic performance. Hence, the blade geometry has been altered parametrically, and all other variables have been held as constant as physical design constraints would allow. The blade that will be tested next in the Unsteady Aerodynamics Experiment program will be an optimized tapered and twisted blade. This blade was designed in accordance with current “best practice” standards and is typical of blades used by industry. This blade will be tested in both a field (NWTC) and wind tunnel (NASA Ames 80ft x 120ft) environment, providing a comprehensive data set for developing and validating new aerodynamics codes. ANGLE OF ATTACK SENSITIVITY TO INFLOW AND TURBINE GEOMETRY To fully appreciate the three-dimensional aerodynamic response of turbine rotors, it is important to consider the interrelated effects of blade geometry, pitch, and rotation rate, as well as turbine architecture, wind magnitude and direction on the local angle of attack (α). Local blade angle of attack is a function of the vector sum of the local inflow and rotor angular velocity. Small variations drive the local blade angle of attack beyond the S809 static stall angle of 15.2°. Blade rotation decreases α at span locations closer to the tip. Thus, inboard span locations experience larger variations in α for any given inflow condition. Downwind turbines, like the Unsteady 3 three-dimensional variability along the span. In fact, when inflow turbulence is added, effecting both inflow magnitude and direction, these relationships become much more complex. One notable effect that also will be seen in the experimental data, is the phase relationship between the tower shadow and α variation due to ϕ. Local AOA (deg) 80 70 60 50 40 30 20 10 0 0 0.95 0.8 90 0.63 180 Span (r/R) 0.47 270 Azimuth Angle (deg) 0.3 360 80 70 60 50 40 30 20 10 0 0 0.95 0.63 180 A 0.47 270 S (r/R 0.8 90 0.3 360 80 70 60 50 40 30 20 10 0 0 0.95 0.63 180 0.47 270 S (r/R 0.8 90 A Aerodynamics Experiment, experience an additional complexity. Tower shadow occurs when the wake from the cylindrical tower intersects the rotor plane down stream. As the blade rotates into the tower shadow (φ ≈ 180°, depending on yaw error), the wake velocity deficit first decreases α until it reaches a minimum at the tower wake center. Then, α rapidly increases as the blade moves out of the wake. The presence of the tower shadow is additive to the other variables effecting α and will be shown to have a predominant effect on the overall aerodynamic performance data reported below. The wake behind the cylindrical tower support is inherently unsteady, being dominated by vortex shedding. In addition, the local tower Reynolds number can transition between sub-critical and critical ranges in response to routine changes in wind speed. The resulting variation in blade α through the tower shadow region is correspondingly unstea...
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This note was uploaded on 11/13/2011 for the course AEE 495 taught by Professor O.uzol during the Spring '11 term at Middle East Technical University.

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