In this figure the two time series plots correspond

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Unformatted text preview: stall event (Fig. 10b). These effects are relatively benign when compared to the large changes introduced by yaw error (Fig. 11). In this figure, the two time series plots correspond to yaw errors of ϕ = +20° and -20° for the twisted blade at 80% span and V∞ = 20 m/s. The Cp variation can be correlated with the changes in α shown in Fig. 3. For positive yaw (Fig. 11a), α decreases with φ and reattaches prior to the tower shadow. Through the tower shadow, α and yaw error effects are additive, rapidly pitching the blade through stall. The pitch through a larger cumulative stall angle due to the yaw contribution more than likely exacerbates the peak pressure and transient load from the dynamic separation event. The flow remains separated throughout the remainder of the cycle. Behavior at ϕ = -20° (Fig. 11b) is quite different. Here, a minimum α is achieved at φ = 0°. This produces two reattachment points in the rotation cycle, φ = 0° and φ = 180° (tower shadow). 7 Figure 10(a): Surface pressure distribution versus azimuth angle (Untwisted blade, V∞ = 8 m/s, ϕ = 0°, 30% span). Figure 11(a): Surface pressure distribution versus azimuth angle (Twisted blade, V∞ = 20 m/s, ϕ = 20°, 80% Span). Figure 10(b): Surface pressure distribution versus azimuth angle (Twisted blade, V∞ = 8 m/s, ϕ = 0°, 30% span). Figure 11(b): Surface pressure distribution versus azimuth angle (Twisted blade, V∞= 20 m/s, ϕ = -20°, 80% Span). 8 Tower shadow creates another interesting performance anomaly in the near and post-stall regime. Depending on the inflow conditions and the local blade angle of attack, the interaction between the blade and tower shadow can create a bifurcation in the quasi-static pressure distribution. Passage through the tower shadow can prompt surface pressure distributions to transition from attached to separated, or from separated to attached. Alternatively, tower shadow passage can result in no significant change to the flow state, with either attached or separated conditions persisting before, during, and after tower shadow interaction. Significantly, although the perturbation elicited from the brief tower wake interaction itself may be small, the state change in quasi-static load when integrated over the rotation cycle is not. Several unsteady parameters including V∞, ϕ, unsteady wake hysteresis, and behavior of any three-dimensional stall can have an effect on the bifurcation response. Although difficult to quantify, this tower shadow effect occurs with sufficient frequency to warrant further investigation. The potential link between this bifurcated response and quasi-static stall hysteresis creating the observed state change should also be examined. Variation in normal force coefficient (CN) over the blade rotation cycle also was used to quantify each blade’s three-dimensional dynamic response. In Figs. 12 and 13, CN was obtained from direct integration of the Cp data for the 30% and 80% span locations respectively. Both cycle mean and standard deviation values for CN were binned on mean V∞ and ϕ. Responses for the twisted and untwisted blades were similar and only the twisted blade data are shown here. In Fig. 12, at higher wind speeds, average CN values exceed 2.4 at 30% span – over two times greater than static two-dimensional wind tunnel data predicts. Figure 13: Average and standard deviation for CN for twisted blade at 80% span. Figure 12: Average and standard deviation for CN for twisted blade at 30% span. The standard deviation in CN is a direct measure of the dynamic load fluctuation during the cycle for a given V∞ and ϕ. At 30% span (Fig. 12), the lowest values occur along the ϕ = 0° axis (zero yaw error). Significant dynamic effects are observed with increasing ϕ where values of 1.0 are obtained at 30° ϕ. At this level, deviations in normal force produce average fluctuations on the same order of magnitude as the maximum CN values obtained from static wind tunnel tests. The effect is asymmetric with yaw. Larger d...
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