Experimental Investigation Of Controlling Flow Separation Over Backward-facing Step Using Plasma

The backward-facing step flow is regarded as ideal experimental object for studying separated flow control because of simple geometry structure and fixed separation point,containing main features of flow with separation and reattachment.There commonly exists this type of flow phenomenon in practical engineering applications,such as the separation flow around airfoil at high angle of attack,flow entering pipeline with sudden expansion,combustion chamber,flow passing turbine and compressor blade,flow through the bridge and high-buildings and so on.The efficiency of fluid machinery and performance of aircraft are unfavorably influenced by the occurrence of the flow with separation and reattachment.So,it is necessary to adopt the flow control manner to control the separation flow effectively,the aim of controlling flow separation is to delay the occurrence of flow separation or to reduce the separation area,to improve the forced state on object surface.It is positive meaning for improving fluid machinery efficiency,making aircraft stable work region wider,enhancement of lift and reduction of drag,restraining structure vibration and noise.In recent years,the plasma flow control technology as a new active flow control means which can effectively improve the forced state on object surface has received considerable attention.In order to investigate the control mechanism and effect of plasma excitation on separation flow,the plasma actuator is used to study the control method and mechanism over the backward-facing step separation flow.The series and parallel type of right-angle-shaped plasma actuator are developed for the flow over the backward-facing step.It is revealed that the law of the glow discharge characteristics with changing voltage and current for two types of plasma actuators,the working principle of the glow discharge of the plasma actuator is described.The induced flow velocity field of two types of plasma actuators is measured by the two-dimensional particle image velocimetry(PIV)in the stationary atmosphere.The law of induced velocity and body force by plasma actuator with changing voltage is obtained under steady and unsteady actuation.The mechanism of two types of plasma actuators on the airflow induced acceleration has been studied.The wind tunnel experimental system of the plasma actuator over the backward-facing step separation flow is constructed by using PIV and Hot-wire anemometer.The effects of the excitation parameters,such as the excitation direction,the pulse frequency and the excitation voltage,on backward-facing step separation flow are investigated by using the two-dimensional PIV measurements.The control mechanism of the plasma actuator over backward-facing step separation flow along different directions is discussed under a typical excited state.The instantaneous flow field over backward-facing step flow is reconstructed using the proper orthogonal decomposition(POD),the main influence modes over backward-facing step separation flow control by the plasma actuator are analyzed.The main works are listed below:1.For the flow control over a backward-facing step,the right-angle-shaped plasma actuators are designed.In order to make the plasma actuator in a narrow area(40mm(streamwise)/38mm(vertical direction))inducing higher wind speed,and to avoid the phenomenon of reverse discharge between electrodes due to the placement of multiple electrodes,the plasma actuator is designed in series and parallel connections of electrodes.The insulation strengthening treatment on the surface of the actuator is carried out,which can delay the stable work time under sufficient strong AC voltage source.It is confirmed that the plasma actuator in series and parallel connections of electrodes can avoid the reverse discharge between electrodes by investigating the voltage,current,glow discharge characteristics and revealed the law of the glow discharge characteristics with changing voltage and current for two types of plasma actuators.2.In the stationary air,the induced velocity field by two types of plasma actuators along 45°direction and horizontal direction is measured by PIV.The induced flow mechanism has been studied.The magnitude and spatial distribution of the body force(along 45°direction)induced by two types of plasma actuators is evaluated by solving the momentum equation,the law of variation of body force with changing voltage has been studied.3.The instantaneous velocity within the shear layer for various streamwise positions was multi-points measured.The natural instability frequencies of various streamwise positions were obtained by power spectral density analysis.It provided an important experimental basis for the flow control experiment over a backward-facing step.4.The flow over backward-facing step is subjected to periodic disturbance along45°,horizontal,vertical direction separately according to the natural instability frequencies of various streamwise positions from measurement results.Some instantaneous flow details were observed by PIV measurement results,the change of momentum thickness,time-average streamwise velocity gradient,vorticity,reattachment position is analyzed under typical excitation state.The experimental results showed that the pulse frequency is modulated as St_H?0.21,excitation voltage Vpp=24.4kV and excited along horizontal direction,the mean reattachment length decreases to minimum.Compared with the previous study results,the flow control effect has been significantly improved.5.The POD is applied on the flow over backward-facing step.The influence of various scales flow structures on instantaneous flow state is analyzed by using increasingly POD modes in reconstruction of instantaneous velocity field.The analysis showed that under typical excitation state,the low-order mode contained more energy essentially behaved as vortex motion in shear layer,which played an important role in flow evolution.