Flow Control Around An Airfoil Using Plasma Actuators Over Reynolds Number Range Of 10~4 To 10~6

Flow separation often occurs on aircraft wings,producing negative effect on the aerodynamic performance,i.e.,increase in drag and reduction in lift.The experimental studies on flow separation control using DBD plasma actuators on a NACA 0015 airfoil are carried out.The mechanism behind the control of lift and stall angle-of-attack?under low Reynolds number(Re=1.0×10~4–3.0×10~5)and moderate Re(=3.0×10~5–5.0×10~6)regimes are investigated.The symmetrical discharge plasma actuator,which can simultaneously induce near-wall jets in opposite directions,is designed and applied in the present study at Re=7.7×10~4 and 3.0×10~5,respectively.The dependence of the lift enhancement and stall delay on the actuation mode,non-dimensional modulation frequency F~+(=f_bc/U_?,where f_b,c and U_?are the modulation frequency,airfoil chord and freestream velocity,respectively)and duty cycle(DC)are investigated systematically.The smoke-wire flow visualization technique is used to explore the flow control mechanisms.At Re=8.0×10~5 and 1.0×10~6,the multi-plasma actuators are designed,calibrated and applied to flow separation control.The effect of the spacing D between the actuators on the maximum velocity of the induced-jet generated by the multi-plasma actuators is studied.The influence of the actuator position and discharge direction,as well as F~+and DC,on the lift enhancement and stall delay are investigated.Besides,the physical mechanism of flow separation controlled by the multi-plasma actuators is explored using the particle image velocimetry technique.The results show that at Re=7.7×10~4,under unsteady actuation(F~+=0.6,DC=5%),the symmetric discharge actuator leads to the stall angle delay from 13°to 21°and increase the maximum lift coefficient by 23.6%,compared to no control.Pulsed jet generated by upstream discharge is overwhelmed by the oncoming flow and then forms the spanwise vortices,which interact with the jet generated by downstream discharge,thus producing the turbulent flow above the airfoil and enhancing the momentum exchange in the boundary layer.As a result,the suction pressure and hence the lift coefficient is increased.At Re=1.0×10~6,the multi-plasma actuators,which discharge in the downstream direction and locate at 27.5%and 40%of the airfoil chord,can produce the induced jet with the maximum velocity of 8.73 m/s.Under the optimum unsteady parameters(F~+=0.9,DC=50%),the actuators postpone the stall angle from 14°to 15°and increase the maximum lift coefficient by1.4%.The periodic jet generated by multi-plasma actuators increases the spanwise vorticity and enhances mixing effect near the airfoil surface,causing the reduction in the recirculation region,and as a result,the lift is increased.