Investigation Of Flow Separation Control Over A Dynamic Stall Airfoil Using Dielectric Barrier Discharge Plasma Actuators

Dynamic stall is an extremely complicated aerodynamic phenomenon.It is usually associated with helicopter rotors,turbine blades and vertical axis wind turbine airfoils.The dynamic stall can cause a series of negative effects such as aerodynamic force oscillation,sharp reduction of lift and aerodynamic damping.Therefore,in practical engineering applications,flow control techniques need to be adopted to control dynamic stall.In this work,the numerical simulation is conducted to investigate the control of dynamic stall over a periodically pitching NACA0015 airfoil using DBD(dielectric barrier discharge)plasma actuator.The two-dimensional unsteady Reynolds Averaged Navier-Strokes(URANS)method is employed to resolve the flow which is controlled by alternating current(AC)or nanosecond(NS)DBD plasma actuators.The AC and NS DBD plasma discharges are modelled by an empirical body force model and a self-similar plasma model respectively.The control of dynamic stall at two Reynolds numbers is investigated,and the comparison between AC and NS DBD plasma in terms of control mechanism and authority is carried out.For moderate Reynolds number flow at Re=2.5×10~5,it is found that the jet flow produced by AC DBD or the residual heat due to NS DBD can excite the instability of shear layer,which in turn produce large-scale spanwise vortices.These induced spanwise vortices will entrain the external high-speed airflow into the separation region and change the flow pattern over the suction surface of airfoil.The AC and NS DBD plasma actuators have comparable control performance under moderate Reynolds number flow condition.They both can achieve pronounced control authority.The periodic plasma excitation can reduce the aerodynamic force oscillation,improve the aerodynamic hysteresis and accelerate the flow reattachment process.However,for the dynamic stall at high Reynolds number of Re=7.5×10~5,a significance difference is observed between AC DBD and NS DBD in the control of dynamic stall.Under the excitation of the NS DBD plasma,the separation is overwhelmingly suppressed,the lift of the airfoil is greatly increased,and the aerodynamic hysteresis effect is nearly eliminated,while the AC DBD almost has no effect on the flow.This is due to the fact that the velocity of jet produced by AC DBD is much smaller than the freestream speed for high Re flow.However,the residual heat generated by NS DBD will move downstream along the wall.This unique thermal convection characteristic enables NS DBD plasma actuator to extend its influence region and make the control process more flexible and efficient.Finally,the influence of NS DBD plasma excitation frequency on control efficiency at moderate Reynolds number is examined.It is observed from the results that when the excitation frequency is low,a large scale spanwise vortex will form at the middle chord position.When the excitation frequency increases to a high value,the vortices induced by the repetitive discharges will merge together,forming a larger spanwise vortex on the suction surface of airfoil.At moderate excitation frequencies,the induced vortices on the suction surface of airfoil are independent of each other.They push the original separation zone to move downstream.In current study,the plasma excitation at all frequencies can increase the lift,reduce the aerodynamic hysteresis,and accelerate the flow reattachment process.However,the plasma actuations at frequencies of 5.0 and 6.25 achieve the best control effect,which can maximize the lift and reattachment angle in the reattachment stage.