This three dimensional flow once established appears

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Unformatted text preview: hreedimensional, with flow in the separated region moving toward the blade tip. This three-dimensional flow, once established, appears very stable in the visualizations with tufts over the aft 2.5 Phase II, 63% Span Phase IV, 30% Span Phase IV, 63% Span CSU Wind Tunnel Figure 7: CN versus αi for baseline conditions. THREE-DIMENSIONAL PERFORMANCE EFFECTS Pressure coefficient (Cp) variation over a rotation cycle was used to provide a metric of the increased blade loading due to three-dimensional dynamic response. For each span location, all data cycles were binned on cycle mean averaged V∞ and ϕ. If the pressure coefficient decreased below a threshold value of Cp = -8.0, the cycle was recorded. Fig. 8 shows the percentage of cycles meeting this criterion over the range of V∞ and ϕ, for the twisted blade. In this figure, only the 30% and 80% span locations are shown. On each figure, the left boundary indicates the conditions for V∞ and ϕ where the static stall angle is reached as a maximum during the rotation cycle (α = 15.2°). The right boundary demarcates where the static stall angle is reached as a minimum during the cycle. At both span locations, the most prominent contour features lie between these two boundaries. Hence, the greatest number of dynamic events occurs between these boundaries, under near and post-stall flow conditions. 6 locations, and flow remains separated over most of the operating velocities. Tower shadow tends to cycle these locations through stall much more often than with twisted blades. The increase in activity at 63% and 80% span locations for the untwisted and twisted blades, respectively, has not been resolved. Untwisted Blade Twisted Blade 25% % of Cycles with Peak Cp < -8 % of Cycles with Peak Cp < -8 25% 20% 15% 10% 5% 0% 20% 15% 10% 5% 0% 30% Span 47% Span 63% Span E n tir e C y c le 80% Span 30% 47% 63% 80% 95% Span Span Span Span Span O u ts id e T o w e r S h a d o w Figure 9: Tower shadow effects on dynamic loading. Figure 8: Percent of cycles where Cp < -8.0 (Phase IV). Fig. 9 isolates tower shadow contribution to post-stall and unsteady aerodynamic events by showing counts of all cycles where Cp < -8.0 is reached (gray bars), and those cycles where Cp < -8.0 is reached outside tower shadow influence (speckled bars). The latter was accomplished by excluding any such events occurring at 150° < φ < 240°. Fig. 4 showed that cycle mean velocities are most numerous at lower velocities (~10 m/s) where flows tend to be attached and dynamic effects are less prevalent. Thus, the average number of total cycles exhibiting dynamic effects is skewed lower than would be expected under normal operating conditions. In Fig. 9, the first substantial difference between the blade geometries is observed. The untwisted rectangular planform blade is much more sensitive to tower shadow effects than the twisted blade. Also, dynamic effects at 80% span are more prevalent for the twisted blade and cannot be attributed to tower shadow alone. This behavior will be shown to have a direct effect on the integrated blade bending moment (CM). The untwisted blade has much higher α values at inboard span Time series data showing the chordwise Cp distribution through the rotation cycle highlight the complex aerodynamic response of the blades under different flow conditions. Both blades at zero yaw error and similar inflow velocities (V∞ = 8 m/s) exhibit similar behaviors, but for different reasons. The untwisted blade is at α = 17° and the flow should be separated (Fig. 10a). Three-dimensional cross flow at this 30% span location produces a dynamic reattachment. Reattachment followed by a slight dynamic stall event occurs as the blade passes through the tower shadow (150° < φ < 240°). In contrast, the twisted blade at the same wind speed had an angle of attack α = 4°. The flow remained attached until the blade passed through the tower shadow, again producing separation and a slight dynamic...
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