## 3 Flow Control Devices for Gust Load Alleviation
### Fluidic Actuators
#### 3.4.1 Surface Jet Blowing
For the influence of jet-slot location and jet-slot width on the lift reduction effects by normal blowing, the authors conducted numerical studies based on the NACA0012 airfoil [30]. Figure 3 gives the results of lift coefficient reduction (\(\Delta C_{L}=C_{L_{u}\;with\;jet}-C_{L_{u}\;without\;jet}\)) against the jet-slot locations under \(M_{\rm jet}=M_{\infty}=0.3\) at \(\alpha=0^{\circ}\) and \(3^{\circ}\). It is clear that the magnitudes of the reduction in lift coefficient increase with microjets moving towards the trailing edge, and this trend is captured both at \(\alpha=0^{\circ}\) and \(3^{\circ}\). The trend of positioning the jet towards the trailing edge enhancing lift is consistent to the findings by [PERSON] and [PERSON] et al. [90], [PERSON] and [PERSON] [91]. At \(\alpha=0^{\circ}\), the reduction in lift coefficient of \(\Delta C_{L}=-0.09\) is obtained due to the microjet blowing at \(x/c=0.4\), and this value reaches to \(\Delta C_{L}=-0.33\) when the microjet moves to \(x/c=0.95\). Noticeably, the magnitudes of lift coefficient reduction increase almost linearly with the microjet location moving from \(x/c=0.7\) to \(0.95\) for both \(\alpha=0^{\circ}\) and \(3^{\circ}\). For the influence of jet-slot width, it is shown that the magnitude of lift reduction increases with the increase in jet slot width. It is more obvious when the jet slot width is below 0.5%\(c\), as the value of lift reduction tends to be stable when the slot width increases from 0.5%\(c\) to 1.0%\(c\). Meanwhile, it is undeniable that a smaller width of the jet exit will be preferable since it is undeniably that the slots will bring discontinuity to the wing surfaces [92]. Figure 4 presents the comparisons of the surface pressure coefficients between the baseline model and the models with microjet blowing. Figure 5 displays the velocity flow fields of the models with microjets located at \(x/c=0.4\) and \(x/c=0.9\) at \(\alpha=0^{\circ}\). Figure 3: Influence of Microjet location on lift coefficient reduction with \(M_{\rm jet}=M_{\infty}=0.3\) (from [30]). Also shown is the region of interest around the microjets and the trailing edges. From these results, it can be concluded that blowing generates a separation region near the jet location, and the separation region is more apparent after the jet location than that before it. This separation region deflects the streamlines upwards near the jet location and blocks the flow over the upper surface. This increases the upper surface pressure coefficients ahead the blowing slot. However, behind the jet slot, the pressure recovers rapidly. From Figure 5, it can be seen that this separation not only deflects the streamline above the upper surface, but also entrains the flow from the lower surface upwards. This entrainment accelerates the flow under the lower surface and results in a reduction in the pressure coefficients on the  
Figure 4: Comparisons of pressure coefficients between the baseline model and microjet blowing models at \(M_{\infty}=0.3\), \(\alpha=0^{\circ}\) (from [30]). (**a**) Lower surface; (**b**) Upper surface. Figure 5: Velocity flow fields and streamlines of the baseline model and models with microjet blowing for \(M_{\rm jet}=M_{\infty}=0.3\), at \(\alpha=0^{\circ}\) (from [30]). (**a**) Microjet blowing, \(x/\zeta=0.4\); (**b**) microjet blowing, \(x/\zeta=0.9\). lower surface. Also shown is that the entrainment capability is stronger when the blowing is placed towards the trailing edge, as slight decreases in pressure coefficients are noticed with blowing slot moving towards the trailing edge as demonstrated in Figure 4b. The combined effects explain the reduction in lift coefficient with the normal microjet blowing relative to the baseline model. In [92], the authors also demonstrated that normal microjet blowing has a stronger capability for load control at transonic incoming flow as normal microjet blowing show strong influence on the shock strength on the airfoil upper surface.