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Energies 2020, 13, 1374 10 of 20 = > # # # To test the actuator under more challenging conditions, a deep stall configuration with an angle of attack α = 22◦ was introduced. Previous studies [7,22] underlined that, to exert an effective control on a deep stall, it is necessary to directly interact with the vortex structure formation and detachment. This is typically achieved by pulsing the actuation with a repetition frequency F close to the vortex shedding one. For a DBD, the pulse frequency F represents a square wave modulation applied to the normal (carrier) frequency f . The pulses repetition is usually expressed by means of the reduced dimensionless frequency F+, defined in literature as F+ = F c/U∞ . Its most effective value for stall control is proved to be close to 1, corresponding to the time scale of a fluid element traveling along the chord c at velocity U∞. Accordingly, also in this study, F+ was set to unity while the carrier frequency f was kept at 650 Hz. Another important parameter is the duty cycle, which represents the (active time)/(pulse period) ratio, here, varied from 20% to 50%. Within this range, it does not significantly influence the effect of the actuator on the flow; however, lower values correspond to lower power consumptions. In Figure 12a, the plasma-off condition is observed. It is characterized by a separation point laying close to the leading edge and a recirculation bubble much larger with respect to early stall, expanding also outside of the captured region. The actuator is not expected to reattach the flow, but the plasma-on configuration shows a mild shift in the separation point, still represented by the red arrow and a clearly visible change in the size and shape of the recirculation bubble (Figure 12b). Thanks to the pulsed actuation mode, the bubble is shifted closer to the airfoil and a well-visible recirculation structure appears. +FBB +F ,+F O? O? = # N? > # N? $ # " ! 7I ! " # 7I N? Figure 11. (a) Cross-wake velocity profile and (b) CP values at R for α = 14.5◦ and U∞ = 20 m/s. Figure 12. (a) Plasma-off and (b) plasma-on flow field at TA for α = 22◦ and U∞ = 20 m/s. These modifications are once again confirmed by the cross-wake velocity profiles. In fact, Figure 13a indicates a significant reduction of the region interested by the momentum drop, even if the whole wake profile, remarkably large for α = 22◦, cannot be recovered by the present setup. +F ,+FPDF Image | Stall Control by Plasma Actuators: Characterization along the Airfoil Span
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