## 3 Results and Discussion
### Flow Control Effect by AC-DBD Plasma Actuation
#### 3.2.2 Time-Averaged Velocity Field
This further indicated that the linear AC-DBD plasma actuation could not exert any observable effects in altering the size of the recirculating bubble and the magnitude of the reverse flow in the wake region. Figure 9: Averaged shear layer angle relative to the freestream direction at different actuation frequencies. Figure 10: Normalised x-velocity contour 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. In contrast, some interesting phenomena could be observed when considering the normalised y-velocity contours (v/U) shown in Figure 12. From Figure 12, a region that shows considerably high levels of positive y-velocity (i.e., strong upward flow movement) appears between \(0.25<\) x/H \(<1.5\) and \(-1.25<\) y/H \(<-0.25\). The length and height of this region in the baseline trailer are 1.2 H and 1.1 H, respectively (Figure 12a). However, when the linear AC-DBD plasma actuation is implemented and the actuation frequency is at 7, 8, 9, and 11 kHz (Figure 12b-d,f); the size of this region is slightly smaller than that presented in the baseline trailer. This indicates that with these plasma actuation frequencies, the strong upward flow movement in the wake region is confined to a smaller area. Surprisingly, the size of this region is the largest when the actuation frequency is 10 kHz (Figure 12e) amongst all cases being studied. This could explain why the shear layer points the least downward when the actuation frequency is 10 kHz (Figure 9). Figure 11: Normalised x-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. Figure 13 shows the normalised y-velocity profiles measured at various normalised x-locations between 0.1 \(<\) x/H \(<\) 1.1 along the wake region of the trailer model. At x/H = 0.1 (Figure 13a), it can be seen that the normalised y-velocity profiles in the region between \(-\)0.8 \(<\) y/H \(<\) 0 (i.e., the beginning of the recirculating bubble) are very similar in all the cases. This suggests that using the linear AC-DBD plasma actuation could only exert limited effects in changing the flow velocity in the y-direction at this location. At x/H = 0.3 and an actuation frequency of 7 kHz (Figure 13b), the normalised y-velocity in the region between 0.2 \(<\) y/H \(<\) 0.8 (i.e., the middle of the recirculating bubble) is slightly lower than that shown in the baseline trailer. In contrast, slightly higher normalised y-velocity could be observed at the same region when the actuation frequency is 8 kHz compared to that shown in the baseline trailer. Figure 12: Normalised y-velocity contour 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. Some changes in the normalised velocity in the y-direction occurred further downstream of the trailer rear end. At x/H = 0.5 (Figure 13c), the lowest normalised y-velocity appears when the actuation frequency is 7 kHz compared to all other cases being studied. Similar levels of normalised y-velocity appeared at the same region for all other actuation frequencies and are always slightly higher than those shown in the baseline trailer. This suggests that at this location, the linear AC-DBD plasma actuation induces stronger upward flow movement in the wake region with the exception of when the actuation frequency is 7 kHz. In contrast, lower levels of normalised y-velocity could be observed between \(-0.4<\) y/H \(<-1\) in all the trailers with the linear AC-DBD plasma actuation being implemented at the normalised locations x/H = 0.7 (Figure 13d) and 0.9 (Figure 13e). This indicated that the linear AC-DBD plasma actuation, regardless of its actuation frequency, induces weaker upward flow movement in the wake region at these two locations.