Parametric Optimization Of Plasma Actuators For Drag Reduction In A Turbulent Boundary Layer

Turbulent boundary layer existing around the transportation vehicles,such as aircraft,automobile and ships,and is the main reason for the skin-friction drag.The coherent structures near the wall are responsible for the turbulence energy production,therefore,it would be useful to reduce the friction drag by controlling the coherent motions in a turbulent boundary layer.This thesis aims to investigate experimentally the drag reduction in the turbulent boundary layer based on the dielectric barrier discharge(DBD)plasma actuator.The geometric parameters of the actuator and various excitation signals were optimized in order to maximize the drag reduction.The experiments were performed in the wind tunnel with a freestream velocity U? = 2.4 m/s.A hot wire anemometer was deployed to measure the fluctuating velocity at 25 mm downstream the trailing edge of the actuator with and without control.The power consumption of the actuator was measured by using high voltage probe and current probe,aiming to estimate the control efficiency under unsteady actuator control.Smoke-wire flow visualization was performed in a plane(y+ = 21)parallel to the wall in order to display the flow structure change.In addition,the statistics distribution of the streamwise velocity U and the bursting frequency in the near-wall region was studied through analyzing the velocity signals in the turbulent boundary layer.The maximum average drag reduction(35)cf = 24% is achieved for the configuration with the dielectric layer thickness,electrode length,voltage and discharge spacing are 0.14 mm,300 mm,6 k Vp-p and 60 mm respectively.For the unsteady mode,the average drag reduction decreases with the increase of the pulse frequency fp,when the pulse frequency fp ? 100 Hz,the vortices was stable enough,so it was not sensitive to the drag reduction for turbulent control.While the duty cycle played a key role on the drag reduction.The maximum control efficiency |?(81)?????|/P is 0.018 when the voltage,pulse frequency and duty cycle are 7.4 k Vp-p,50 Hz and 50 % respectively.The bursting frequency fb+ in the near wall region was calculated with and without control.It showed that the fb+ is significantly decreased with control,and the reduced amplitude of fb+ with steady control is greater than that of the unsteady mode.It indicated that the low-speed streaks become stable and the turbulence events are suppressed,and also the steady control lead to larger drag reduction.The flow visualization revealed that the low-speed streaks were pushed away from the wall then moved toward the center of the actuator pair by the strong upwash effect of the counter-streamwise vortices induced by the steady plasma actuators.As a result,a narrow low-speed streaks ribbon is formed,and inferring the low speed flow region that contributed to a considerable drag reduction.For unsteady plasma actuator control,the duty cycle led to the low-speed streaks continuously gathered and dispersed to form a wave-like low-speed streaks ribbon,this phenomenon indicated that unsteady control was less effective than the steady optimal configuration.In this case,it also proved that only the stable streamwise vortex can effectively suppress the turbulent events in the near wall region.