# Active Flow Control of a Supercritical Airfoil and Flap with Sweeping Jets
## 1 Introduction  
The lift-up system is one of the key components of the wings of modern aircraft. One of the critical design indicators of the system is to allow an aircraft with transonic cruising speed to land safely at low speeds. However, owing to the large number of components, heavy weight, complexity, intensive maintenance, and long design-test time, the lifting system has a significant effect on the cost and safety of the aircraft [1]. It is now common to use lift-boosting devices such as Fowler flaps, which are slits at the trailing edge of the wing. These devices are complex in structure and require additional fairing as they protrude from the wing, resulting in an overall increase in weight and additional cruise resistance [2, 3]. A lift-boosting system with active flow control (AFC) combined with simple flaps was proposed as an alternative that could provide sufficient lift while reducing associated external drag. In the 1990s, [PERSON] et al. [4] explored a range of AFC applications designed to improve the efficiency of the next generation of commercial transonic transport aircraft. However, manufacturers view simplified lift systems with enhanced AFC as a high-risk though high-yield technology that reduces aircraft target weight and fuel consumption. Therationale behind AFC is that it will reduce wing separation when typical lift-up systems are replaced with mechanically simpler and lighter variants. To this end, researchers have carried out a series of flap-lift experiments or vertical tail enhancement using AFC methods such as synthetic jets [5] and oscillatory jets [6; 7; 8; 9]. Among them, owing to the particularity of its own structure, an oscillating jet can generate a sweeping jet at its exit [8], thus facilitating a larger control area on the wetted surface, making this kind of actuator application unique to AFC [10]. As one kind of effective AFC technology, the sweeping jet has been considered a more practical flow controller than other existing technologies [11], and it is widely applied in other different sceneries. For example, the dynamics of oscillating jet emitted in confined and non-confined backward-facing step geometries were investigated with TR-PIV to resolve the mechanism of flow separation and reattachment that occurs at a sharp corner, which will help the application of fluidic oscillators in heat transfer enhancement or cooling process [12]. In addition, an experimental study on the flow control capability of sweeping jet actuators in an aggressive offset S-shaped duct with a compressed incoming flow was conducted to suppress flow separation in the duct and flow distortion at the aerodynamic interface plane. With the sweeping jet actuators controls, the absolute pressure recovery increased by nearly 5%, and the mean value and RMSE of the cross-flow velocity declined by 54.9% and 48.1%, respectively [13]. In aerospace engineering applications, in 2016, Boeing successfully used a sweeping jet to enhance the control characteristics of the vertical tail on the 757 environmental protection verification machines [14]. Based on the success of this application, a lifting system using AFC combined with simple flaps was proposed. This alternative can provide sufficient lift while reducing the associated external drag, and the scheme can effectively reduce the weight of the lifting system [15]. The scheme uses single-hinged flaps, which produce a higher deflection angle (\(\delta\)). Experimental studies show that if the lift-increasing system of AFC combined with simple flaps is used to obtain the same lift-increasing effect as Fowler flaps, the fuel consumption can be reduced by up to 2.25% [16]. Subsequently, the scheme of oscillating jets combined with simple flaps has been systematically studied as an alternative to the high-lift system. In 2017, [PERSON] et al. [17] used numerical simulation to prove that AFC on flaps using sweeping jets could greatly reduce the required mass flow rate.