Mechanisms Of The Aerodynamic Improvement Of An Airfoil Controlled By Sawtooth Plasma Actuators

Flow separation phenomena results in many negative effects on structures,especially on aircraft and turbines.This phenomenon not only deteriorates the aerodynamic performance of aircraft,but also require more energy consumption and even lead to casualties.Therefore,active control of the flow separation of aircraft with great significance.Active control using plasma actuator has become an important active control technique and gained lots of concern from engineers.This control method can effectively improve lift,reduce drag,and increase the stall angle of the aircraft,which is beneficial to the improvement of aircraft performance and energy saving.Although this technique has been extensively studied,there are still many questions remained unknown.Therefore,with the aim to further improve control effect of the plasma actuator and explore the control mechanism,the present study focus on the characteristics of flow induced by sawtooth plasma actuator under pulsed excitation and investigate its performance on flow separation control.In this thesis,the sawtooth plasma actuator is adopted to control the flow separation over NACA0015 airfoil at post stall angle.The major contents including the documentation of flow induced by plasma actuator in still air,the control effect of plasma actuator on the aerodynamic force of airfoil and the exploration of control mechanism.The lift and drag force of airfoil measured by load cell at two different Reynolds numbers Re=0.77×10~5 and 1.0×10~5.It is found that the plasma actuator under unsteady actuation can further improve lift of the stalled NACA0015 airfoil.At Re=0.77×10~5,the optimum non-dimensional burst frequency F~+(=f_bc/U_?,where f_b,c and U_?are the burst frequency,airfoil chord length and free-stream velocity,respectively)0.6 and duty cycle DC=5%,respectively.The airfoil stall angle-of-attack?is delayed by3°,the lift-to-drag ratio C_L/C_D is increased by 155.4%,and the energy input is saved by 95%compared with the steady actuation.When Re=1.0×10~5,the optimal control parameters become F~+=1.0 and DC=60%,respectively,and the stall angle is delayed by 2°,the maximum lift coefficient is increased by 15%,and the lift-to-drag ratio is increased by 176.9%.Based on the PIV and hot wire measurements,it has been found that as F~+=0.6 and DC<10%,there are large-scale quasi-sequential structures formed over suction surface which biased toward suction surface.The existence of these structures fully enhances the momentum transmission of the incoming flow and the boundary layer near surface,thus greatly increasing the lift of aifoil.For F~+=0.6,DC>50%,the early transition from laminar flow to turbulence is trigued,the periodic vortex structure over suction surface generate additional lift,but these structures are more inclined to suction surface,that's the reason that the improvement effect weakened lightly.