Active Drag Reduction In A Turbulent Boundary Layer Based On Plasma-actuator-generated Streamwise Vortices

Active drag reduction of turbulent boundary layers has received a great attention in the literature due to its practical and fundamental importance. This work aims to investigate experimentally the friction drag reduction in a turbulent boundary layer on flat plate based on streamwise vortices generated by using dielectric barrier discharge(DBD) plasma actuators.Four plasma actuators were explored, of which they all could generate streamwise vortices in the turbulent boundary layer. While the time-resolved PIV was used to capture the development and interaction of streamwise vortices in a plane normal to the mean flow, a surface balance technique was deployed to measure the skin friction drag.A new calibration method was proposed for this technique to resolve accurately the friction drag change. It has been found that the plasma-actuator-generated vortices and their interactions with the boundary layer led to the maximum drag reduction of 50%.The control efficiency was also estimated. Hot-wire measurement of the manipulated flow was conducted to investigate the effect of plasma induced vortices on the statistics distributions of streamwise velocity u in turbulent boundary layer and the distribution of drag reduction as well as the drag recovery behind the plasma actuators.For a selected plasma actuator(configuration B), smoke wire flow visualization performed in xz plane(y+ = 24) revealed that the low-speed streaks were pushed toward the centre of the actuator pair by the plasma force, which led to the high dissipation of turbulent energy. PIV results illustrated that the near wall streaks shrank from large- to small- scale due to the high energy dissipation of the manipulated flow, while the strength of the streaks were weakened and the average spacing between low-speed streaks were increased. On the other hand, the angle of mean quasi-streamwise vortex decreased from 9° to 4°, while the lifting of the low-speed streaks were suppressed, and the near-wall vortex regeneration cycle is weakened.