Drag Reduction Of Turbulent Boundary Layer By Using Spanwise Plasma-induced Unsteady Jet

Vehicles such as airplanes and high-speed trains that are often seen in daily lives experience large skin-friction drag.Overcoming this skin-friction drag requires a lot of energy.The reason for the large skin-friction drag on the surface of moving objects is that there exist coherent structures in the turbulent boundary layer on the surface.The control the coherent structures in the turbulent boundary layer is the key to reduce the frictional resistance.In this thesis,the drag reduction is achieved through manipulating the coherent structures by a spanwise unsteady jet generated by a plasma actuator.In this thesis,by using DBD plasma induced jet to study the effects of different actuation conditions?including steady-state actuation,unsteady actuation and asymmetric actuation?on the drag reduction of the turbulent boundary layer.The Reynolds number based on the momentum thickness of the standard boundary layer?SBL?is 1450 at the test section.The hotwire anemometer is used to measure the drag reduction effect downstream of the actuator,and the velocity signal before and after control also is statistically analyzed,and then the flow visualization technique is used to observe the change of turbulent structure near the wall before and after control.It is found that for unsteady state actuation condition,the drag reduction rate increases first and then decreases with the increase of the duty cycle and pulse frequency,and the local drag reduction rate is the largest when the duty ratio is80%,pulse frequency is 100Hz and actuation voltage is 12kV_p-p.The maximum local drag reduction rate under those conditions is 55%,which is 12%larger than the steady condition.The drag reduction recovery area is 970 wall units and is nearly the same with the steady condition.There will be a drag increase in the downstream.Under asymmetric condition,the drag reduction of the asymmetry ratios equal to 0.2 and 0.3 is obviously better than that of 0.1,0.4 and 0.5.The trend of the drag reduction rate with the pulse frequency is the same as that of the unsteady state.It increases first and then decreases with the increase of pulse frequency.When the asymmetry ratios are 0.2 and 0.3,the drag reduction rate reaches the maximum at pulse frequency is 100Hz,and the maximum drag reduction is obtained when actuation voltage is 12kV_p-p,pulse frequency is100Hz and asymmetry ratio is 0.3.The maximum local drag reduction rate is58%,and the drag reduction recovery area is 1130 wall units,which effectively weakened the drag increase after the drag reduction recovery area.It can be concluded that the unsteady state condition can increase the drag reduction rate,while the asymmetric condition can both increase the drag reduction rate extend the drag reduction recovery area and can suppress the downstream drag increase phenomenon.