**3. DBD aerodynamic actuators: flow manipulation**

Another possibility to increase actuator performance is the adoption of a third exposed electrode supplied with high-voltage DC fields. In this way, a sliding DBD is generated. Depending on the sign of the DC field, it is possible to modify the morphology of the induced

DBD actuators can be arranged to produce normal or vectorized jets, too. These devices can be used to mimic classical synthetic jets and are usually called plasma synthetic jet actuators (PSJA). Typical geometries are annular and linear one [48, 59–62]. In both cases, tangential wall jets collide merging in a unique-induced jet able to generate perturbations far away from the actuator surface (**Figures 10** and **11**). When the linear configuration is adopted, by supplying exposed electrodes with different voltages, it is possible to produce an induced jet with an arbitrary inclination. Three-dimensional flows can be also produced by using a classical DBD actuator, but with an exposed electrode characterized with serpentine or serrated geometries, instead of the usual linear one [63, 64]. This particular electrode geometry generates small tangential jets that collide and propagate downstream pushed by the EHD interaction body

**Figure 10.** Induced normal jet of linear (left-hand side) and annular (right-hand side) PSJA.

**Figure 11.** Schlieren images of PSJAs of linear (a) and annular (b) geometries. Electrode distance (a) and diameter (b) is

ionic wind [57, 58].

64 Recent Progress in Some Aircraft Technologies

force.

30 mm.

In the last decade, DBD aerodynamic actuators have been extensively studied in the active flow control domain. Position and actuation strategies of these devices are key points in their effectiveness in flow control over aerodynamic surfaces. Many studies have been accomplish‐ ed over airfoils [65–72], diffusers [73, 74], and wind [10, 11 and 75] and turbine [8, 9] blades, in order to enhance fluid-dynamic performance. Moreover, the adoption of these devices over landing gears and trailing edge surfaces have demonstrated the possibility to obtain a noise reduction effect [76–78].

DBD actuators can be usually positioned over aerodynamic surfaces in spanwise and stream‐ wise directions. In the former, the induced body force is in the same direction as the incoming flow. In the latter, induced thrust is perpendicular to the free stream direction. In this case, the composition of these two flows produces vorticities propagating in the downstream direction.

In **Figure 12**, a NACA 0015 airfoil equipped with four spanwise DBD plasma actuators is shown. The four plasma regions are clearly visible in the figure as bluish strips. In this configuration, actuators produce a tangential wall jet directed downstream. The effect of this plasma device in the recovery of stall condition is depicted in **Figure 13**, where smoke visualization is reported. Experiments show how the most effective actuator is the one positioned on the airfoil leading edge, just before the region where separation occurs [66, 67, 79, 80].

**Figure 12.** Airfoil equipped with four spanwise plasma actuators [65].

**Figure 13.** Flow structure around NACA0015 airfoil at Re = 15000 without (left-hand side) and with (right-hand side) plasma actuation [65].

When DBD actuators are streamwise mounted, induced and incoming flows combine together producing vorticities propagating in the downstream direction (**Figure 14**). Such a device can be used as a plasma vortex generator (PVG)

**Figure 14.** Vortex formation mechanism induced by a streamwise plasma actuator [8].

Changes in drag coefficient (CD) and lift coefficient (CL) by using a PVG are reported in **Figure 15**. At low Reynolds numbers, actuator effectiveness is very pronounced, leading to noticeable CD reduction with a parallel increment in the stall angle.

**Figure 15.** Drag (a) and lift coefficient enhancement (b) with counter-rotating PVGs on a NACA 4418 airfoil [8].

**Figure 13.** Flow structure around NACA0015 airfoil at Re = 15000 without (left-hand side) and with (right-hand side)

When DBD actuators are streamwise mounted, induced and incoming flows combine together producing vorticities propagating in the downstream direction (**Figure 14**). Such a device can

Changes in drag coefficient (CD) and lift coefficient (CL) by using a PVG are reported in **Figure 15**. At low Reynolds numbers, actuator effectiveness is very pronounced, leading to

**Figure 15.** Drag (a) and lift coefficient enhancement (b) with counter-rotating PVGs on a NACA 4418 airfoil [8].

plasma actuation [65].

66 Recent Progress in Some Aircraft Technologies

be used as a plasma vortex generator (PVG)

**Figure 14.** Vortex formation mechanism induced by a streamwise plasma actuator [8].

noticeable CD reduction with a parallel increment in the stall angle.

**Figure 16.** Pressure coefficient contours with streamlines over an airfoil with 15° angle of attack: plasma actuator off (a) and on (b) [47].

Effectiveness in flow control by means plasma actuators has been demonstrated by numerical works, too [47, 81–87]. In **Figure 16**, pressure coefficient contours with streamlines obtained over an airfoil with a 15° angle of attack are displayed. In **Figure 16a**, plasma actuator is switched off, and separation occurs. When the plasma device is activated, reattachment of the flow is achieved (**Figure 16b**).

Plasma actuators have demonstrated to strongly improve their ability in flow manipulation when operated with a duty cycle strategy [47, 66, 71, 72, 88]. With this approach, the DBD device is turned on and off with intermittence by following a particular duty cycle frequency. This frequency is usually chosen in the range 5–100 Hz and it is strictly related to natural vorticities developing over the aerodynamic surface. The percentage ratio between the period in which discharge is fed and the whole duty cycle period is called duty cycle percentage. If it

**Figure 17.** Lift recovery in percentage for different actuators location with a free stream velocity of 11 m/s: steady oper‐ ation (a) and duty cycle operation (b) [66].

is fixed to 50%, it means that discharge is ignited half of the time. On a parallel plane, this actuation strategy leads to lower power consumption.

**Figure 17** shows lift recovery in percentage of a stalled NACA0015 airfoil equipped with spanwise vectorized actuators located in different airfoil positions [66]. Jet 5 is generated by a DBD actuator located in the leading edge. When actuators are continuously operated, lift increments are limited to about 15%. When operated with a duty cycle strategy, lift increments are close to 50%.

Plasma actuators can also be used to reduce noise induced by aerodynamic surfaces, especially by landing gears and trailing edges [76–78]. Studies on bluff bodies have demonstrated the ability of DBD actuators to reduce downstream turbulence, leading to the suppression of particular tones or to an overall noise mitigation up to 4 dB.
