Moreover, they always require a complex mechanical adjustment mechanism and control systems. However, active trailing-edge flaps and dynamic-droop leading edges tend to cause obvious changes to the center of gravity and load. Active control methods primarily include active trailing-edge flaps, dynamic-droop leading edges, synthetic jets, and plasma jets. Active control methods have many advantages over passive control methods they can be used at the required time and position and can be actively adjusted. However, these methods only have a good control effect on the suppression of the dynamic stall of the rotor under the design conditions when the actual conditions deviate from the design conditions, the control effect is poor. Passive control methods include vortex generators, Gurney flaps, fixed leading-edge slats, and fixed-droop leading edges. The current flow control methods of the suppression of the dynamic stall of the rotor include both passive and active control methods. Therefore, it is of great significance to research the methods and mechanisms of rotor dynamic stall suppression. , which will introduce many adverse effects to helicopters. Moreover, the obvious aerodynamic hysteresis effect of the rotor blade caused by dynamic stall will lead to stall flutter, a surge in vibration load, noise enhancement, etc. The dynamic stall leads to sharp increases in the drag coefficient and moment coefficient of the rotor blades, which seriously restricts the improvement of the aerodynamic performance and forward flight speed of helicopters. The dynamic stall of three-dimensional rotors and two-dimensional airfoils is investigated which indicates that the dynamic stall has a great influence on their aerodynamic performance. The retreating blades of the rotor generally work in an aerodynamic environment with a large angle of attack (AoA), and the complex unsteady dynamic stall phenomenon caused by the AoA of most sections of the blade exceeds its stall AoA. The limitation of the forward flight speed of helicopters is closely related to the flow characteristics of the airflow on the rotor blade surface. However, the low flight speed of conventional helicopters greatly limits their application range. IntroductionĪs compared with a fixed-wing aircraft, helicopters have unique advantages, such as vertical take-off and landing, high maneuverability, and hovering in the air, and they are widely used in both military and civilian fields. Moreover, the position of the injection slot is found to have a greater effect on the dynamic stall suppression, while the size of the injection slot and the position and size of the suction slot have little effect. It is found that there is a jet momentum coefficient that optimizes the suppression effect of the dynamic stall of the rotor airfoil. On this basis, the control parameters of the CFJ are further studied, including the influences of the jet momentum coefficient and the positions and sizes of the injection and suction slots on suppressing the dynamic stall of the rotor airfoil. The diffusion and blending of the turbulent shear layer between the CFJ injection jet and the main flow excite the main flow and enhance its ability to resist the reverse pressure gradient this suppresses the generation and development of the separation vortex, thereby enhancing the aerodynamic characteristics, improving the hysteresis effect, and increasing the system stability. Via the study of the typical conditions of CFJ control to suppress the dynamic stall of the OA212 rotor airfoil, it is verified that this method has a good effect on dynamic stall suppression. The numerical methods are validated by a NACA0012 pitching airfoil case and a NACA6415 airfoil case based on the CFJ, and good agreement with experiments is found. The effect of the CFJ on the unsteady dynamic stall characteristics of the rotor airfoil is numerically investigated via numerical simulations of the unsteady Reynolds-averaged Navier-Stokes (URANS) equations coupled with the Spalart-Allmaras (S-A) turbulence model. In this study, a dynamic stall control strategy, called the co-flow jet (CFJ), is applied to the rotor airfoil.
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