# SDBD Flexible Plasma Actuator with Ag-Ink Electrodes: Experimental Assessment
## 3 Results and Discussion
In the same work, an electrode width of 20 mm, an electrode gap of 0 mm, a frequency of 1.5 kHz, and a voltage of 40 kV\({}_{\text{pp}}\) led to a maximum speed of about 6.5 m s\({}^{-1}\). In another case [4], where a 300 mm long actuator consisting of 0.1 mm thickness Kapton\({}^{\text{\textregistered}}\) barrier, 12.7 mm wide covered grounded electrode, and 6.35 mm wide exposed high-voltage electrode were used, the time-averaged consumed power was reported to be less than 0.3 W mm\({}^{-1}\) when the actuator was driven by short nanosecond pulses at frequencies less than 3 kHz and typical peak voltage at 16 kV. The velocity field generated in still air gave ensemble-averaged speed values up to 0.5 m s\({}^{-1}\), 20 mm downstream of the discharge for repetition frequency 2 kHz. In a last instance [28], a 0.15 mm thick Kapton\({}^{\text{\textregistered}}\) barrier, a 0.025 mm thick copper foil, and a 12.7 mm wide exposed electrode were employed. For a 24 mm wide insulated lower electrode and 16 kV\({}_{\text{pp}}\) triangular (5 kHz) high voltage, a maximum fluid speed between 2.5 and 3 m s\({}^{-1}\) was recorded. Regarding the present actuator, the above speed plots provide a reasonable picture of the flow field pattern within the 2D space. This picture is validated by the schlieren images of Figure 3:1 It is noticed that schlieren images, unavoidably, correspond to side-on snapshots (xz-plane). This may point to an integration of optical events along the Y  
Figure 3:2 Mean speed along the \(z\)-axis at two different positions behind the discharge gap. (a) x = 4 mm and y = 0 mm; (b) x = 12 mm and y = 0 mm. Mean values and standard deviations are derived from two sets of measurements. Two newly constructed actuators are used (first: 16 kV\({}_{\text{pp}}\)/x = 4 mm/x = 12 mm; second: 16 kV\({}_{\text{pp}}\)/x = 4 mm/x = 12 mm). direction and their projection onto the xz-plane. However, following this angle, three different regions and regimes of flow field are readily distinguished. The first region (1) refers to the area above the high-voltage-driven electrode. This area is associated with an eddy-like flow field, which appears concentrated on the electrode edge adjacent to the discharge gap. The second region (2) is the interelectrode one, i.e., the gap itself, where a laminar flow well bounded onto the actuator surface is established. The thickness of this layer is approximately 1 mm. The third region (3) is the extended area above the grounded electrode, where the boundary laminar layer continues to propagate onto the surface. Nonetheless, it becomes progressively thicker along the \(x\)-axis, and a clear transition to intense perturbated (turbulent) flow field is observed along the \(z\)-axis. In addition, it tends to bend upwards as it approaches the end of the grounded electrode, resembling the separation effect. The perturbated field occupies more and more space as the distance along the \(x\)- or/and \(z\)-axis increases. This result is reflected in the speed and standard deviation values discussed above. The distinction of the three areas holds true for both voltage amplitudes in Figure 3:3 Nevertheless, the 16 kV\({}_{\text{pp}}\) case promotes more intense perturbations. Lastly, the flow field perturbations induced by the actuator propagate to long distances, even behind the grounded electrode. The snapshots of Figure 11 unveil perturbations far away from (i) the grounded electrode left edge (up to about 60 mm on the \(x\)-axis) and (ii) the electrode plane (up to about 20 mm on the \(z\)-axis). Such an intense directed motion of neutrals should be attributed to collisions with ions drifting into high local electric fields. The extremely thin electrodes were painted on the dielectric point to electric field enhancement as a dominant factor determining the effectiveness of momentum coupling into the surrounding air [28]. The role of temperature remains ambiguous even after the present results.