# SDBD Flexible Plasma Actuator with Ag-Ink Electrodes: Experimental Assessment
## 3 Results and Discussion
The identification is organized in Table 1. Interestingly, with regard to ionic species, the nitrogen molecular ion is solely observable, and despite its weak relative spectral emission, its role in the flow field formation is emphasized. Otherwise,  
Figure 4: Representative conventional photographs of the DBD developed between the Ag-ink electrodes of the actuator. The filamentary nature of the discharge is manifested: (**a**) 14 kV\({}_{\text{pp}}\). (**b**) 16 kV\({}_{\text{pp}}\). The scale is the same in both photos (the plasma visible pattern has a length of about 50 mm). Figure 5: Electrical mean power consumed by a newly constructed actuator during the first 24 h of its operation (accumulative): (**a**) 14 kV\({}_{\text{pp}}\) and (**b**) 16 kV\({}_{\text{pp}}\). Solid symbols: mean values following 10 sets of measurements (see Section 2 for details). Error bars: standard deviations derived from the 10 sets. Lines: linear fitting of the mean values. the spectrum is mainly populated by excited nitrogen molecules, whereas species such as NO\({}_{\gamma}\), OH, and N\({}_{2}\)(FPS) are not detected. Furthermore, high-resolution spectra can give the patterns of the rotational distributions of excited molecules, which thus serve as probe molecules for the gas temperature evaluation. In Figure 7b, the rotational distribution of the N\({}_{2}\) (SPS) species (337.13 nm in Table 1) is presented, being determined both experimentally and numerically. The numerical fitting of the latter to the former implies a gas temperature up to around 410 K at the maximum applied voltage (16 kV\({}_{\text{pp}}\)). Further experiments have given values around 318 K at 12 kV\({}_{\text{pp}}\) and 336 K at 14 kV\({}_{\text{pp}}\). Apart from the positive ions, the role of this rather elevated temperature in the flow field pattern should also be considered. This remark becomes more critical if one considers the local temperature, which is expected to be higher than the average one measured here. The term \"local\" means the temperature within specific elementary volumes of the plasma, contrary to the integrated emissive volume probed here (see the caption of Figure 7). In the frame of this work, the relative contribution of each factor, i.e., ions versus temperature, to the flow field modifications cannot be conclusive, but this topic remains challenging (see the further discussion below based on the schlieren imaging results). Figure 6: External top view of a newly constructed actuator (**a**) before and (**b**) after operation during 24 h (accumulative time) at 16 kV\({}_{\text{pp}}\). Figure 7: (**a**) Optical emission spectroscopy wide scan of the SDBD plasma (resolution 50 pm). For temporal evolution of the emission, see Figure 3c. (**b**) N\({}_{2}\)(SPS; \(\
u^{\prime}=0-\
u^{\prime\prime}=0\)) rotational distribution (resolution 1 pm). Solid symbols: experimental points. Line: theoretical rotational distribution, at 410 K, fitted to the experimental data. In both frames, the projection of the optical fiber acceptance cone on the actuator surface corresponds to a circle of 12.5 mm in radius, centered on the point (x = 0, y = 0, z = 0). Figure 8 presents the point-specific speed of the fluid over the actuator surface (z = 0.75 mm; y = 0 mm) at various positions far away from the discharge gap (x = 8, 18, and 24 mm) versus the time. These graphs unveil the variation in the speed that may take place as a function of the space coordinates and the time elapsed. Speed fluctuations are highly pronounced at longer distances from the discharge gap and they almost fade as the gap is approached (sequence Figure 8c \(\rightarrow\) Figure 8b \(\rightarrow\) Figure 8a). It is thus suggested that a transition from laminar to turbulent flow takes place along the x direction, even if we remain close to the surface of the actuator. Based on time-resolved speed graphs such as those of Figure 8, the mean speed is calculated from each 300 s graph, and the 2D distribution (xz-plane) of the gas mean speed is mapped as in Figure 9 (y = 0).