# A New Flow Control Method of Slat-Grid Channel-Coupled Configuration on High-Lift Device
## 1 Introduction  
For the commonly used three-stage high-lift device, the flow characteristic is that the leading slat and trailing-edge flaps work together to improve the lift-drag characteristics of the entire airfoil. [PERSON] [1] reviewed the recent developments in aerodynamic design and analysis methods for multi-element high-lift systems in transport airplanes, including necessity, the major design objectives, and constraints. For the commonly used three-stage high-lift device, the flow characteristic is that the leading slat and trailing-edge flaps work together to improve the lift-drag characteristics of the entire airfoil. In 1970, [PERSON] [2] analyzed the aerodynamic principle of the multi-element configuration, and believed that it was affected by the slat effect, circulation effect, off-the-surface pressure recovery, and boundary layer effect of each wing segment. It is well recognized that the usual function of the slot is that of a boundary layer control device, permitting highly adverse upper-surface pressure gradients to be sustained without incurring severe separation. There appear to be five primary effects of gaps, and here we speak of properly designed aerodynamic slots. According to the current classical active and passive flow control theories, the mechanism of the three-segment high-lift device can be classified into the passive control method. The passive flow control technology does not add energy to the flow, but changes the flow boundary or pressure gradient through some methods to control the flow, such as optimizing the geometric parameters of the airfoil, adding or combining flow control devices, and through fixed-point displacement or deformation under specific working conditions. Compared with active flow control technology [3], it is widely valued for its advantages of simple structure, lack of additional energy consumption or lower energyconsumption, and low cost. For the three-stage high-lift device, the leading-edge slat can accelerate the airflow through the slots formed with the main wing, forming a similar effect to the jet flow, which can favorably disturb the airflow of the main wing and improve its wind resistance. However, the practical application of this three-element high-lift device has many limitations. For example, the aircraft is significantly affected by the ground effect during takeoff and landing. For the high-lift device, the flaps are deflected closer to the ground [4], meaning that the impact is greater. For amphibious aircraft [5; 6], flap failure may even occur due to body splash. In order to further improve the aerodynamic characteristics and practical value of the high-lift device, slats are the preferred component for optimization and improvement. The basic principle of slat lift enhancement is that there is a gap between the slat and the leading-edge surface of the main wing. Specifically, the velocity of the airflow on the lower surface of the airfoil increases in the gap and flows to two places, respectively. Part of the airflow accelerates from the gap between the tail of the slat and the leading edge of the main wing, flows backwards with the airflow on the upper surface of the airfoil, and accelerates the flow to the convergence of the upper surface of the main wing. Moreover, the velocity of airflow in the boundary layer on the upper surface of the wing is increased to eliminate a large number of vortices [7]; the other part flows to the cavity of the concave pressure surface of the slat, forming a recirculation zone. The velocities of these two streams differ greatly; a strong free shear layer is, therefore, formed at the interface. This shear layer originates from the sharp lower trailing edge and develops downstream with the separation vortex. When the separation vortex hits the trailing edge wall of the slat, a part of the vortex will flow back into the concave cavity of the slat, interfering with the new shear layer near the lower trailing edge [8] and forming return feedback [9].