It is also uncertain that sufficiently detailed field

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Unformatted text preview: ield experience. It is also uncertain that sufficiently detailed field measurements exist for validating aerodynamics models against the actual three-dimensional, unsteady fluid physics. Attempting to glean aerodynamic performance data from experimental field data is an extremely difficult task. Stochastic inflow conditions vary three-dimensionally with spatial scales less than the characteristic rotor diameter. Measuring a simple performance parameter, like instantaneous angle of attack at a specific span location, becomes extremely difficult in a time varying, three-dimensional field test environment. Further, one must be cognizant of wake effects in that the data obtained in any single rotation cycle serves as an integrated result of several previous rotations. Capturing the characteristics and nuances of three-dimensional flow separation in a stochastic inflow environment on a large piece of rotating machinery poses as significant a challenge as understanding the underlying fluid physics. The following Unsteady Aerodynamics Experiment data compare the performance characteristics of two different experimental rotors tested at the National Wind Technology Center. This comparison utilizes the entire data set from both rotors, providing a global performance overview across all observed wind speeds and yaw errors. Data represent means and standard deviations computed for individual cycles. These data have been binned according to cycle averaged wind speed and yaw error. Where interesting anomalies occur in the cycle averaged data, time series data also are presented to emphasize differences between cycle averages and single cycle transient events. Highlighted are the three-dimensional effects of blade twist and resulting aerodynamic performance in attached, stalled, and yawed flow. These results provide some insight into turbine operation under normal field operation, where unsteady, separated, and strongly three-dimensional flow responses are common events. . NOMENCLATURE α angle of attack φ blade azimuth angle ϕ yaw error ρ density c chord q dynamic pressure {1/2ρV2)} p static pressure M blade moment N’ normal force per unit span r span location R blade span length S blade planform area V velocity CM flap bending moment coefficient{M/(qTSR)} CN normal force coefficient {N’/(qc)} pressure coefficient {(p-p∞)/q} Cp i local span conditions ∞ free stream conditions T blade tip conditions UNSTEADY AERODYNAMICS FIELD EXPERIMENT The Unsteady Aerodynamics Experiment horizontal axis wind turbine (Fig. 1) is well documented [2-5]. The 10.1m diameter, three-bladed downwind machine rotates at a constant 72 rpm and is capable of producing 20 kW of power. A cylindrical tower 0.4 m in diameter supports the turbine at a hub height of 17 m. This generic configuration has been used for five different phases of the experiment. The test configuration for each phase and data collected are summarized in Table 1. 2 Table I: Summary of test configurations. Phase II Configuration Total10-minute Campaigns Non-Rotating Campaigns -9° Pitch Angle Phase III S809 Air f o il Phase V 3-blade, 3-blade, 3-blade, 2-blade, Constant Chord, Constant Chord, Constant Chord, Constant Chord, Untwisted Twisted Twisted Twisted 29 19 75 74 0 2 minutes -3° Pitch Angle 3° Pitch Angle 19 13,680 cycles 29 20,880 cycles 9 90 minutes 1 720 cycles 12 8640 cycles 39 28,080 cycles 12 8,640 cycles 1 720 cycles 11 110 minutes 6 4320 cycles 11 7920 cycles 37 26,660 cycles 6 4320 cycles 1 720 cycles 10 30 20 40 50 60 70 80 90 100 % Ch o r d (0.457m Ch o r d ) 100 Fu ll Pr e ssu r e Tap Dist r ib u t io n 90 80 70 60 50 40 % Rad iu s (5.023m Rad i u s) 8° Pitch Angle 12° Pitch Angle Phase IV Op t io n al if t r ailin g e d g e t ap is b lo ck e d Pr e ssu r e Tap Pr e ssu r e Tap s at 4% an d 36% Ch o r d On ly 30 20 10 0 Figure 2: Blade pressure tap locations Figure 1: Combined experiment isometric. Data from Phase II and Phase IV were used for the comparisons documented herein. Both the Phase II and Phase IV rotors had three...
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