## 3 Flow Control Devices for Gust Load Alleviation
### Fluidic Actuators
#### 3.4.1 Surface Jet Blowing  
For normal surface jet blowing, the jet flow disrupts the main flow, leading to flow separation [84]. A separation bubble emerges to encompass the surface up to the trailing edge. An alteration in the Kutta condition is then realised as the separation bubble encourages flow to be entrained from the freestream flow around the wing surface, thereby modulating the circulation created by the wing [85]. Consequently, the load on the wing is modified. Jet actuators have been investigated experimentally for load control on NACA0015 aerofoil for reshaping aeroelastic responses including limit cycle oscillation and flutter by [PERSON] et al. [86]. The results showed an improvement of more than 15% of the flutter speed by the jet actuators using a PID controlled loop [87]. Microjets being small pneumatic jets using high-speed flow blowing normal to the aerofoil or wing surface have been studied as approaches for load control. [PERSON] et al. [88] conducted numerical studies of a non-rotating NACA0018 aerofoil with microjets located near the trailing edge under freestream Mach number of 0.176. Significant changes in lift were observed for the angle of attack ranging from \(-10^{\circ}\) to \(10^{\circ}\). Moreover, the results also showed that approximately 50% of the total change in the lift could be obtained within the non-dimensional time \(s=\frac{U_{\mathrm{u}}\Delta t}{c}=1\), indicating its rapid load control response characteristic. [PERSON] et al. [66; 85] compared the load control effects of microjets and microabs deployed on the NACA0012 aerofoil trailing edge. The results showed that both concepts had a similar load control mechanism by affecting the trailing-edge flow, and therefore produced very similar aerodynamic load control effects. [PERSON] et al. [89] conducted wind tunnel tests for comparing the effects of blowing (microjets) and microtabs, and pointed out that blowing and microtabs were viable methods for load control but with very different behaviours: the blowing deflected the wake upwards thereby reducing lift, conversely the microtabs promoted separation over the upper surface resulting in lift reduction. They also noted the nearly constant lift change across all angles of attack by microjet blowing located at the trailing edge, which was constant with the result drawn by [PERSON] et al. [88]. However, for microtabs, optimal location varied according to the angle of attack. At small ones, it is preferable to place the microtabs near the trailing edge, while locations near the leading edge were better when the angle of attack is high. [PERSON] et al. [88] performed numerical studies at the steady condition on the NACA0018 aerofoil at \(M_{\infty}=0.176\) with a normal jet placed on the upper surface trailing edge and a significant lift reduction was obtained. [PERSON] et al. [43] assessed the capability of blowing for lift reduction experimentally. Two different blowing directions, normal and upstream, from the upper surface of the NACA0012 aerofoil under the steady incoming flow velocity of 20 m/s and a range of angles of attack from \(0^{\circ}\) to \(20^{\circ}\) were compared. The results indicatedthat the chordwise location of normal blowing had a dramatic influence on the load control effectiveness in terms of lift reduction. Normal blowing at \(x/c=0.95\) induced a lift coefficient decrease of 0.15 under the maximum blowing momentum coefficient (momentum coefficient is defined as \(C_{\mu}=\frac{\dot{m}U_{jet}}{q_{\infty}A}\). where, \(\dot{m}\) is the mass flow rate through the jet slot exit, \(q_{\infty}\) is the dynamic pressure of the freestream, \(A\) is the surface area of the wing and \(U_{\rm jet}\) is the jet velocity). However, moving the microjet further forward, the lift change was negligible and even no lift decrease was induced when normal blowing was placed near the leading edge.