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Energies 2020, 13, 1374 7 of 20 3. Results In this section, the results of the PIV analysis are presented, beginning with the study of the central sections at the design velocity of U∞ = 20 m/s. Through these measures, the setup was validated and the DBD effect was tested. Successively, the freestream speed was increased to U∞ = 30 m/s and the actuator effect was studied over the whole wing, considering several spanwise sections. Figure 7 reports a sketch of the actuator and of the PIV measurement planes. In what follows, the term root indicates the middle point of the inter-tip spacing, namely the one at d/2 = 18 mm from each tip, as shown by the yellow line in Figure 7. E@@A IF= 4 6) 218 F=AI 6* 6+ 6, 6- N A=@ECA@CA !$ #" $ $ !" % Figure 7. Sketch of the exposed electrode geometry with PIV measurement planes, seen from below in Figure 2. 3.1. Central Sections Analysis at U∞ = 20 m/s The first measures were performed on the central sections as usual for this type of investigations, starting from tip TA. This PIV plane was initially studied in early stall conditions, with an angle of attack α = 14.5◦ at the design velocity of U∞ = 20 m/s. The actuator was supplied with a steady sinusoid of frequency f = 650 Hz. Here and in all the PIV measurements, the mean flows are obtained by averaging on a suitable time scale at least 100 velocity fields obtained by corresponding pairs of images. All the PIV planes are shown on a x y domain with the origin on the leading edge, made dimensionless with respect to the chord c and its projection c′ = c cos α. Figure 8a shows the plasma-off mean flow field. A recirculation bubble is present and extends up to the trailing edge. The separation starts at about 30% of the chord, and the relevant point is represented here and in all of the reported mean flow fields by a red arrow. The separation is detected by an approximation method, as follows. First, a zoomed-in field of the relevant flow region is built in order to obtain a detailed representation of the local streamlines, consistent with the PIV resolution. Among these streamlines, the algorithm identifies the one which is approached by the wall flow on both sides with opposite directions. This streamline is assumed to start from the separation point on the wall; however, this procedure cannot describe the boundary layer details, so this streamline is identified only outside the boundary layer, up to a small distance from the airfoil and then extended through extrapolation until it intersects the solid surface. The point identified in this way is considered as a suitable approximation of the separation point: the introduced error is considered acceptable since the final aim is that of defining the relative modification of the flow comparing the plasma-on and plasma-off configurations and not the absolute locations of the separation points. In the plasma-on configuration, Figure 8b, a strong shift in the separation point and a significant reduction of the recirculation bubble can be seen. Under the plasma effect, they are both confined =JAH=HE>PDF Image | Stall Control by Plasma Actuators: Characterization along the Airfoil Span
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