Flow Control With The Vortex Induced By DBD Plasma In Boundary Layer Flow

The dielectric-barrier-discharge(DBD)plasma actuator is currently of considerable interest as one of the active control techniques.When sufficient voltages are applied to the actuator,plasma would be generated on the surface of the actuator.This plasma-based technique has been proved to be effective in enhancing lift,exciting the boundary layer instabilities,controlling dynamic stall on the airfoils and so on.Compared with other active control methods,the DBD plasma actuator has its own distinct advantages such as no moving parts which promise s a potential long lift,withstand high g-loading and high dynamic response.It can be also easily placed at most receptive locations.Since then,it has been well demonstrated in the area of flow control,such as flow separation,transitional flow,turbulence flow and the film cooling.In this work,a PIV experiment was conducted at first to study the flow over the flat plate induced by the DBD plasma actuator.From the experimental results,an increase in velocity up to 5.5% of free stream velocity is found around the leading edge of the embedded electrode,and after that ?U remain constant while ?? has a slight rise.The relationship between the free stream and the actuation strength of the DBD actuator has a deep influence on the flow.When the free stream stays the same,the influence area will be enlarged with the increase of the actuation strength.But when the actuation strength is remain unchanged,the influence area will narrow down with the increase of the free stream.The experimental results were also used to calibrate the parameters of the simplified model which was applied to establish a simulation method for plasma control to the flat plate flow.By simulating the flow under various conditions respectively and analyzing the velocity and vorticity field downstream the actuator,the mechanism of the actuator effect on the flat plate flow was investigated.It can be noticed that the actuator not only causes a local rise in near-wall velocity significantly,but also introduces a decrease in the displacement thickness of the boundary layer efficiently.The actuator-induced vorticity is generated to intensify the energy exchange between main flow and the boundary layer,thus a direct dynamic energy is added to the low-energy fluid by the actuator.The simulation model was then applied to study the flow structures over a flat plate and a wall-mounted hump induced by the unsteady dielectric-barrier-discharge(DBD)plasma.A series of vortex pairs which indicated dipole formation and periodicity distribution was generated in the boundary layer when the plasma was applied to the flow over a flat plate.They would increase mean turbulent kinetic energy of the flow to enhance the energy exchanged between the near wall region and the free stream.When the actuator was engaged in the flow over a hump,the vortex pairs were produced as well,which was in a position to delay flow separation as well as to promote flow reattachment,achieving the goal of reducing dissipation and decreasing flow resistance.At last,the DBD plasma actuator was regards as a vortex generator(DBD VG)for flow control.In this part,the formation of streamwise vortices by different arrangement of the DBD plasma actuator were investigated firstly.Results indicated that streamwise vortex could be generated when the DBD plasma actuator was aligned with the primary flow.The steamwise vortex was formed by twisting and folding the spanwise vorticity in the oncoming boundary layer into the outer shear layer of the spanwise wall jet.When a pair DBD VG were opposed setup in the flow,two spanwise vortex would be generated which were opposite to each other.These two spanwise vortices have the same strength and mutual promotion to lift off the wall.In this study,the Q-criterion method was used to identify the vortex structure around the cooling hole.The iso-surface Q shows that large vortex structure could be figured out such as the counter rotation vortex pairs(CVP),horse vortex and the wall vortex when blowing ratio M=1.0 without DBD plasma.The CVP seems to be the dominant flow feature and persists downstream.However,the cooling gas would be lifted off from the surface,which diminishes the film cooling and resulting in a bad film cooling performance.Then a pair of DBD actuators placed on the contrary were used to improve the film-cooling performance.The usual configuration which orients DBD actuator along the spanwise direcion was also calculated to make a comparison with the DBD VG configuration.Results showed that all the DBD plamsma actuation configurations have the ability to improve the performance of the film cooling.But the DBD VG configuration could reduce the CVP which is induced by the cooling jet more significantly.This leads to the weaken of the mixing stength of main flow and jet flow.The jet flow could be more closer to the surface and improve the effectiveness of the film cooling.