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Energies 2020, 13, 1374 17 of 20 with the present setup; however, a simplified study can be done by considering only the lift increase ∆L. Assuming, as exposed above, a negligible variation of ∆L between the two configurations (long and short DBD), the effectiveness ε = ∆L/W would change from ε0 = ∆L/W0 to ε1 = ∆L/W1 for a shorter actuator, with an improvement ε1/ε0 = 1.57, regardless of the ∆L value. Moreover, a rough estimate of the ∆L value can be obtained by the variations of the CP coefficients presented in the previous section, accounting for the known properties of the 0015 airfoil. With steady actuation at U∞ = 30 m/s and α = 14.5◦, it is obtained an approximate value of 15 N over the whole wing. With this value, the effectiveness ratio ε0 = ∆L/W0 turns out to be about 0.09 N/W, and the correspondingdimensionlessefficiencyisη0 =0.09Vt.Clearly,inapracticalapplication,thisefficiency must be higher than one, and the relation η0 > 1 shows that the present model would need a minimum ascent velocity Vt ≃ 11.1 m/s to operate properly. More favourably, the condition η1 = ∆L Vt/W1 > 1 referred to as a shorter actuator with lower power W1 would reduce the minimal ascent velocity to Vt ≃ 7.1 m/s. A further improvement could be achieved by using a pulsed actuation with reduced frequency F+ = 1 and high duty cycle of 80%, since this combination has been proven to control the stall inception with performance very similar to the steady actuation (duty cycle 100%), with the advantage of saving 20% of power [20]. The minimal ascent velocity would be lowered in this case to about 5.7 m/s. This indicates that the technology level is appropriate for realistic applications, since the minimal ascent velocities above are lower than the rates of climb of several diffused light aircrafts under full load, as for example Lancair IV-P (10 to 15 m/s for different versions), Pipistrel Virus 912 SW100 (8.4 m/s), de Havilland DHC-6 (8.1 m/s), Cessna 400 (7.6 m/s), and Cirrus SR 22 (6.6 m/s). Of course, further study would be needed for each aircraft, including the evaluation of drag and accounting for their specific characteristics as wing profile, takeoff and climb performance, and the other important V-speeds. 4. Conclusions and Further Developments This study characterizes the effect of a multi-tip DBD actuator over a wing, consisting of an airfoil model considered as a three-dimensional body and thus intentionally placed in the wind tunnel without end-plates. Through PIV measurements, five spanwise distributed sections were studied, from the model centerline to the external section. Several tests have been run under different stall conditions: in general, the actuator-flow interaction takes place along the whole span of the model but the local effects are different, depending on the corresponding base flow. In particular, in early stall conditions, the base flow along the central section of the model is separated whereas it remains attached on the external regions. Correspondingly, the plasma discharge acts in different ways: in the central zone, it reduces the recirculating region size and shifts the separation points downstream, whereas on the external sections, it only induces slight modifications of the local streamlines. This suggests that an actuator of shorter span, with a corresponding lower power consumption, could achieve a similar separation control: assuming that the global effect of the actuator remains unaltered, the effectiveness measured in terms of N/W can increase in this way up to 57%. Finally, as regards plasma flow control experiments on airfoils, it is concluded that the validity of data collected near the centerline should be considered local, unless end/splitter plates of proper size are installed to counteract and limit the natural three-dimensional structure of the flow. Author Contributions: Conceptualization, M.B.; methodology, M.B. and A.Z.; investigation, all authors; resources, G.C.; data curation, G.Z.; writing—original draft preparation, G.Z., L.D.V., and M.B.; writing—review and editing, M.B. and G.Z.; supervision, M.B. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.PDF Image | Stall Control by Plasma Actuators: Characterization along the Airfoil Span
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