Drag Reduction Of Turbulent Boundary Layers By Plasma-induced Blowing And Suction

Turbulent boundary layer is the main source of friction drag for moving aircraft,high-speed railway,ships and so on.Therefore,the drag reduction control of turbulence has always been an important subject in fluid mechanics and engineering fields.The friction drag is closely related to the coherent structure of turbulent boundary layer.Therefore,effective control of the coherent structure of turbulent boundary layer is an effective way to reduce skin-friction drag.This thesis aims to investigate experimentally drag reduction of turbulent boundary layers by DBD plasma induced blowing and suction,which applies blowing,suction and the combinations of blowing and suction to obtain the best drag reduction and control mechanism in turbulent boundary layer.This experiment is carried out in the wind tunnel at Re_?=1450.The average drag reduction was measured with floating element force balance at downstream of slots about 280 mm×100 mm area.In order to study the recovery and local drag reduction of the control scheme more carefully,a hot wire anemometer was deployed to measure the average velocity curve of vertical wall at 10 mm downstream from the slots with and without control,and the drag reduction is calculated by the velocity gradient in the linear region under each control scheme,and the recovery also can be measured.Smoke-wire flow visualization was utilized in a plane?y⁺=17?parallel to the wall to study the flow structure change.The experimental results show that the drag is decreased with blowing,blowing and suction combination;and the drag is increased by suction.At present,at the control parameters:severity parameters of blowing is 0.0379(E=12 kV_p-p),excitation frequency f=2 kHz,has the maximum local drag reduction is 58%,and the maximum average drag reduction is 31%with twin blowing by force balance,recovery zone is x⁺=933;and the maximum local drag reduction is 57%,the maximum average drag reduction is 28%with the combination of blowing and suction by force balance,maximum recovery zone is x⁺=1266 when the ratio of blowing and suction is 2.25.In addition,through increasing the area of floating element,decrease the thickness of floating element and adding a perspex box to balance the pressure difference by blowing,so the stability and accuracy of the improved force balance system have been improved,and the error between the improved force balance system and the hot-wire measurement results can be guaranteed to be about 4%.The results of flow visualization show that the suction can draw the streaky structures to the near-wall,so the interaction of the streaky structures with the wall becomes stronger,the drag increase near the slot;secondly,as for twin blowing control,the streaky structures can be lifted up the wall,and forming small-scale structures,that the interaction of the streaky structures with the wall becomes weaker,therefore the friction drag decrease.The blowing-suction control,blowing can lift the streaky structures,while the suction can draw the streaky structures to the wall,which will form“wavy”,and reduce the spacing between the low-speed streaks,so the low-speed streaks are stabilized,therefore the drag is reduced and the recovery is extended.