## 3 Results and Discussion
### Flow Control Effect by AC-DBD Plasma Actuation
#### 3.2.3 Flow Steadiness and Turbulence  
Flow turbulence along the wake region of the trailer model is measured in the form of the normalised turbulence kinetic energy (tke/U\({}^{2}\)) contour, and is shown in Figure 14. In general, considerably high levels of turbulence kinetic energy appear along the upper shear layer in all cases  
Figure 13: Normalised y-velocity profiles measured at various normalised x-locations along the wake region. x/H = (**a**) 0.1, (**b**) 0.3, (**c**) 0.5, (**d**) 0.7, (**e**) 0.9, and (**f**) 1.1.  
being studied. In contrast, the turbulence kinetic energy levels remain relatively low in the region at which the recirculating bubble is present (i.e., between 0 \(<\) x/H \(<\) 0.8 and \(-\)1.5 \(<\) y/H \(<\) 0). From Figure 14, it is clear that using the linear AC-DBD plasma actuation at the trailer rear end changes the turbulence kinetic energy levels along the upper shear layer. In the baseline trailer (Figure 14a), moderate levels of turbulence kinetic energy occur along the upper shear layer in the region \(-\)0.1 \(<\) y/H \(<\) 0.1. When the linear AC-DBD plasma actuation is implemented, at 7 kHz (Figure 14b), higher levels of normalised turbulence kinetic energy could be observed along the upper shear layer compared to that shown in the baseline trailer. The levels of turbulence kinetic energy increase further by increasing the actuation frequency to 8 (Figure 14c) and 9 kHz (Figure 14d). Further increasing the actuation frequency to above 9 kHz reduces the turbulence kinetic energy levels along the upper shear as shown in Figure 14e,f for the plasma actuation frequencies of 10 and 11 kHz, respectively. Figure 14: Normalised turbulence kinetic energy downstream of the trailer rear end at various plasma actuation frequencies. (**a**) Baseline, (**b**) 7 kHz, (c) 8 kHz, (**d**) 9 kHz, (**e**) 10 kHz, and (**f**) 11 kHz. An interesting observation should be pointed out here. In Figure 14, it can be seen that a pattern of alternative high and low levels of turbulence intensity occurs in the shear layer in those cases with the linear AC-DBD plasma actuation being implemented. This phenomenon is particularly prominent at 7 (Figure 14b) and 8 kHz (Figure 14c). Since the presented contours are averaged over 2400 instantaneous PIV snapshots, the appearance of this alternative high and low turbulence intensity regions in the shear layer suggest that the acquisition rate of the PIV images corresponded to some significant frequency in the flow field. To be exact, it suggests that the linear AC-DBD plasma actuation induces a frequency in the flow. It is speculated that this frequency induced by the linear AC-DBD plasma actuation in the flow field leads to the occurrence of the increase of the flow unsteadiness in the flow field. Further investigations by employing time-resolved non-intrusive measurements are required to understand these phenomena in the presence of plasma actuation, and its effects on the overall wake flow and drag. In fact, the turbulence kinetic energy levels along the wake region could be further quantified by plotting the normalised turbulence kinetic energy profiles at various locations along the wake region as shown in Figure 15. From Figure 15, it can be seen that in the region between \(-1.5<\mathrm{y/H}<-0.2\) (i.e., below the upper shear layer), turbulence kinetic energy profiles are very similar at the locations \(\mathrm{x/H}=0.1\), \(0.3\), \(0.5\), and \(0.7\) in all cases being studied. However, stronger fluctuation of the turbulence kinetic energy levels in the recirculating bubble (i.e., the region between \(0<\mathrm{x/H}<0.8\) and \(-1.2<\mathrm{y/H}<-0.2\)) is observed in those trailers with the linear AC-DBD plasma actuation being implemented. This could be confirmed by observing Figure 14a,c,d at the locations \(\mathrm{x/H}=0.1\), \(0.5\), and \(0.7\), respectively. Figure 15: Normalised turbulence kinetic energy profiles measured at various normalised x-locations along the wake region.