Studies Of Control Techniques About Separation Flow Of Wing At High Angle Of Attack

As an important tendency of fluid mechanics, research of wing separation flow control at high angle of attack is of profound theoretical and practical significance. In this paper, a new flow control method was comprehensively researched.Firstly, starting with look through the present state of flow control, these commonly used methods were deeply studied. By absorbing the essence of these methods, a new flow control technology was introduced, called flow deflector. By researching the flow control characteristics of the multi-element airfoil, the principle of the flow control was found. The flow from the low surface of the airfoil through the gap at the leading edge and blows the boundary layer of the up surface. The energy of the boundary layer was enhanced and the separation was suppressed. The shortcoming of the method was that the pressure of the low surface of the airfoil was also reduced. Based on such consideration, some devices, fixed at the leading edge, could deflect the coming flow to control the flow separation. Then the idea of the flow deflector was born. Fixed at the leading edge, the parallel flats could make the coming flow deflect to the up surface of the airfoil. The velocity of the boundary layer was enhanced, the separation was retrained and the stall was delayed.Secondly, some experiments indicated that the flow deflector can restrain the flow separation. By increasing recognition of the flow deflector, three flow deflector models were produced to meet requirement of the research. The first model was used in the flow visualization experiment. The results validated the basic idea of controlling separation. The second model was used in the force measurement experiment. The results indicated that the flow deflector could delay the stall angle by 3 degree. As a function of the flow deflector, the peak of the pressure on the up surface was keep to angle of attack 24 degree. At last, the third model could delay the stall angle by 6 degree.The results of the PIV measurements indicated that there are many differences between the airfoil with and without flow deflector. By the control of the flow deflector, the separation vortices were retrained and the stall angle was delayed. The region where the vorticity changed extremely moved down to the up surface. The energy exchange between the coming flow and the boundary layer was sharp. The energy of the boundary layer was enhanced. And the ability of restrain the separation was improved.Thirdly, analysis of mechanism and optimization were done on the new flow control method. Deep research of flow direction changing trend and boundary layer are carried out to investigate the mechanism of flow deflector. The results indicate that the flow deflector increase the angle of flow around the leading edge of the wing and decrease the angle of the coming flow to make it deflect to the upside of the wing. The flow deflector can suppress the three dimensional effects and maintain the two-dimensional flow characteristics to some extent. Furthermore, the flow deflector can easily change the velocity profile in the boundary layer and make it more"fuller". It decreases the shape factor and increases the stability of the boundary layer to restrain the flow separation.The results of the genetic algorithm optimization demonstrated the basic idea and application prospect. In the flow control experiment, the optimal flow deflector could enhance the aerodynamic characteristics effectively. Computational fluid dynamics in con-junction with a searching algorithm like a genetic algorithm potentially offers an efficient and robust optimization method and is a promising solution for current flow control designs. With the iSIGHT a highly efficient optimization platform combining with the FLUENT software can be extended to a wider field of fluid application.In this paper, the genetic algorithm in aerodynamics experiment was put forward for the first time. The platform could provide better plan for the experiments of aircraft design and aerodynamics theory analysis. The flow deflector, restraining the separation and delaying the stall angle effectively, is a proper carrier for the genetic algorithm in aerodynamics experiment. To meet the high precision demand of the parameters'varation in genetic algorithm, the using of the precision actuate technology in flow control was put forward for the first time. This technology spread the application of the genetic algorithm in experiment and the flow deflector in actual enginerring.Lastly, an application case in engineering was give. The results of numerical simulation for the airfoil S809 show that the flow deflector can control the separation effectively and improve the aerodynamic characteristics of the airfoil. An optimum flow deflector is obtained by genetic algorithm, which enhances the ability of separation control. It is indicated that the lift coefficient of the airfoil with flow deflector is increased by 24% and the lift-drag ratio by more than 50%. The stall of the airfoil is delayed by two degrees and the stall process is slow. Most of the time, the wind turbine run in the airfoil stall state, the slow stall process could reduce the blade dithering and enhance the running quality.