# SDBD Flexible Plasma Actuator with Ag-Ink Electrodes: Experimental Assessment
## 3 Results and Discussion
As regards the fluid speed close to the actuator surface (z = 0.75 mm; solid symbols in graphs), an increased speed is measured close to the discharge gap (x = 0 mm), whereas it decays monotonously along longer distances (up to x = 24 mm). This trend persists independently of the applied voltage amplitude. However, at 16 kV\({}_{\rm pp}\) (Figure 9b), a slightly higher speed is achieved compared with that at 14 kV\({}_{\rm pp}\) (Figure 9a). On the other hand, an abrupt decrease in the speed is observed at an almost double distance from the actuator surface (z = 1.4 mm; open symbols in graphs), and this is more noticeable when the actuator is driven by a lower voltage (Figure 9a). Another remarkable fact is the increased values of the standard deviations (error bars) calculated in the case of z = 1.4 mm compared with those found for the measurements closer to the surface (z = 0.75 mm). This fact should not be attributed to experimental uncertainties but, as it is mentioned above, it is rather related to the transition from a laminar flow layer to a turbulent phase. The subtleties of this assumption are considered below along with the schlieren imaging results. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Number/Label** & **Species** & **Vibrational Transition v\({}^{\prime}\) - v\({}^{\prime\prime}\) (\(\Delta\)v)** & **Theoretical \(\lambda\) (nm)** \\ \hline \multirow{3}{*}{(1)} & N\({}_{2}\) (SPS): C\({}^{3}\Pi_{\rm u}\)–B\({}^{3}\Pi_{\rm g}\) & 3\(-\)1 (+2) & 296.2 \\  & N\({}_{2}\) (SPS): C\({}^{3}\Pi_{\rm u}\)–B\({}^{3}\Pi_{\rm g}\) & 2\(-\)0 (+2) & 297.68 \\ \hline \multirow{3}{*}{(2)} & N\({}_{2}\) (SPS): C\({}^{3}\Pi_{\rm u}\)–B\({}^{3}\Pi_{\rm g}\) & 3\(-\)2 (+1) & 311.67 \\  & N\({}_{2}\) (SPS): C\({}^{3}\Pi_{\rm u}\)–B\({}^{3}\Pi_{\rm g}\) & 2\(-\)1 (+1) & 313.6 \\  & N\({}_{2}\) (SPS): C\({}^{3}\Pi_{\rm u}\)Figure 8: Real-time recordings of the point-specific, “instantaneous” speed (integration time 1 s) over 300 s time intervals. Two independent sets are obtained at x = 8 mm, \(y\) = 0 mm, z = 0.75 mm (**a1,a2**); x = 18 mm, \(y\) = 0 mm, z = 0.75 mm (**b1,b2**); and x = 24 mm, \(y\) = 0 mm, z = 0.75 mm (**c1,c2**). A newly constructed actuator is used for each set (first set: **a1-c1**; second set: **a2-c2**). Figure 9: Mean speed along the \(x\)-axis at two different positions over the actuator surface. Solid symbols: y = 0 mm and z = 0.75 mm. Open symbols: y = 0 mm and z = 1.4 mm. Driving voltage: (**a**) 14 kV\({}_{\rm PP}\) and (**b**) 16 kV\({}_{\rm PP}\). Mean values and standard deviations are derived from two sets of measurements. Four newly constructed actuators are used (first and second: 14 kV\({}_{\rm PP}\)/z = 0.75 mm/z = 1.4 mm; third and fourth: 16 kV\({}_{\rm PP}\)/z = 0.75 mm/z = 1.4 mm). The vertical (i.e., along the \(z\)-axis) variation in the mean speed for two representative positions behind the discharge gap is plotted in Figure 10. In both cases, i.e., x = 4 mm and x = 12 mm, as the actuator surface is approached, the fluid speed increases constantly, whereas it becomes negligible at a vertical distance z = 1.5-2 mm. Comparing Figure 10a,b, the turbulent nature of the flow as we move behind the discharge gap is mirrored in the increasing standard deviation values, similarly to what is implied by the data in Figure 3:0  
A direct comparison of the above measured parameters with the corresponding values appearing in the bibliography does not make sense since an identical plasma actuator operating at the same conditions has not been evaluated earlier. Nevertheless, we can suppose indicative information from experiments on SDBD plasma actuators in terms of fluid speed, electrical features, electrode dimensions, etc. For an actuator with both electrodes exposed (Al tapes of 200 mm length, 5 mm width, and 0.1 mm thickness), a 2 mm PMMA barrier, an electrode gap equal to 5 mm, and a sinusoidal (700 Hz) driving voltage at 40 kV\({}_{\text{pp}}\), a maximum speed of about 4.5 m s\({}^{-1}\) has been noted [27].