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Energies 2020, 13, 1374 9 of 20 An estimation of the CP was obtained through suitable elaboration of the collected PIV data. This calculation was performed applying the Bernoulli theorem to the attached flow areas, namely beginning from the proximity of the leading edge and stopping the evaluation in the separation zone. Through this simplified procedure, only an approximation of the coefficient was obtained: for the PIV field of Figure 8, it is reported in Figure 9b, where a significant off-to-on growth of the CP peaksintheorderof35%isobserved. Unfortunately,thisestimationcanhardlybecomparedwith experimental or numerical reference data since it is known that narrow CP peaks are difficult to obtain from PIV data because of resolution limits that easily lead to underestimate the desired value [32]. In fact, the peak is related to the maximum over-speed of the flow, of which the values are usually found in a small region very close to the airfoil that would require a locally very fine grid of PIV data to be properly resolved. However, the same order of approximation affects both the on and off configurations, so that the off-to-on variations can be considered here to give further information about the actuator capabilities for given conditions of airspeed, angle of attack, and spanwise position. Both the effects of vortex generation and momentum injection strongly influence the flow around the airfoil. As described in Section 2, the direct momentum injection is principally due to the tips thanks to the high local electric field they develop. The inter-tip spacing, instead, is characterized by the presence of the vortical structures. This induces different local modifications on the flow that are evidenced in the following analysis through the description of the measures performed at the same α = 14.5◦ and U∞ = 20 m/s conditions on the root plane R. In Figure 10a, that represents the root plane with plasma off; the flow is quite similar to the one at the tip, indicating that the passive effect of the unpowered actuator is quite weak at this Reynolds number and can be neglected. Instead, in the plasma-on case of Figure 10b, the shift in the separation point appears to be milder as is the streamlines acceleration effect. This may be explained considering that, at the root plane. the ionic wind has also a component orthogonal to the wall and directed toward the external flow, whereas the tips induce a wind almost parallel to the wall. Moreover, the induced velocity at TA is generated in a downstream chordwise position with respect to the roots. Considering that the reduction of the recirculation bubble is significant also at the root R, it can be concluded that the local flow pattern generated by the actuator keeps trace of the shape of the exposed electrode. O? = # N? > # N? $ # " ! 7I Figure 10. (a) Plasma-off and (b) plasma-on flow field at R for α = 14.5◦ and U∞ = 20 m/s. The cross-wake velocity profiles, computed as previously described and reported in Figure 11a, confirm the effectiveness of the actuator, showing a reduction of the momentum drop similar to the one downstream of the tip TA in the actuated case; thus, the wake region appears to be less affected by the electrode shape. The pressure coefficient trends, visible in Figure 11b, are instead more influenced by the actuator shape, since the attached flow region is mainly located over the active electrode. In particular, the peak value of CP exhibits only a slight increase in the plasma-on configuration, lower with respect to the one measured at the tip.PDF Image | Stall Control by Plasma Actuators: Characterization along the Airfoil Span
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