Turbulent Boundary Layer Control Based On Localized Blowing With Different Injection Orientation Angles

In the era of rapid development of transportation,the air-friction drag accounts for40%to 50%of the total drag of high-speed transport aircraft,and most of the air-friction drag comes from skin-friction drag.The skin-friction drag of high-speed transport aircraft is inseparable from coherent structures such as vortices and trips in the turbulent boundary layer(TBL).Therefore,how to suppress the coherent structure in the turbulent boundary layer has became the key to reducing skin-friction drag.In this project,a dielectric barrier(DBD)plasma actuator induced blowing jets with different frequencies and different angles in the spanwise slit are used to control the turbulent boundary layer,thus reduce skin-friction drag.This work presents an experimental investigation on the TBL control using DBD plasma-induced blowing through a spanwise slot.Under three blowing modes,including steady,unsteady and periodical with dissymmetric in time(dissymmetric ratio?_s=T_a/T_p,where T_a and T_p denote the time period with signal amplitude increase and the time period for one cycle,respectively.The subscript s denotes the actuation signal),different blowing frequencies f⁺(=0.0143?0.286,f⁺=f?/u?²,where?and u?are the kinetic viscosity of air and friction velocity,respectively)and jet angles?(=37°?143°,with respect to the longitudinal axis)have been invesigated with a view to maximize the friction drag reduction(DR).A hot-wire measurement technology and a high-resolution(10^-4)floating-element force balance have been adopted to measure the local-and spatial-averaged DR with control.The mechanism of drag reduction is explained by comparing the velocity flow profile and flow structure changes in the turbulent boundary layer with and without control through flow visualization and partical image velocity measurement technique.It has been found that for unsteady(E=12 k V,DC=80%))and dissymmetrical blowing(E=6?12 k V,?_s=0.3)modes the drag reduction(DR)increases with increasing f⁺until f⁺reaches about 0.143,and decreases with further increase in f⁺.At the optimum f⁺of 0.143,the unsteady and dissymmetrical blowing can achieved DR of 56%and 58%,repectively.In quescient air,two rows of vortex structures are generated near the blowing slit and moving along the normal direction under unsteady and dissymmetric blowing of f⁺=0.0143?0.143.The streamwise-diameter of vortex d_x⁺and the center-to-center spacing between two adjacent vortices in the same row D⁺decrease with the increasing frequency.When f⁺=0.286,there is no continuous vortices generated.Under dissymmetrical blowing of f⁺=0.143 and?_s=0.3,the spatial fraction d_x⁺/D⁺of the vortex structure is positively correlated with the f⁺,which is found to be 0.603,the highest among all cases.It is indicated that the vortices adjacent to the wall influence a large portion of the TBL over the entire forcing period.The spanwise vortices form a reverse flow near the wall,thereby reducing the velocity near the wall,thus achieving better drag reduction.In this study,local(x⁺=66.7)and spatial-averaged DR(x⁺=113.3?780)are studied at different blowing angles?(=37°?143°)under steady,unsteady and dissymmetric blowing at the optimal f⁺(=0.143).It has been found that for any given blowing modes the drag reduction(DR)increases with increasing?until?reaches about 123°,and decreases with further increase in?.The steady,unsteady and dissymmetric blowing obtain the space drag reduction of 28%,35%and 37%at?=118°,120°and 123°,respectively.Mean streamwise velocity profiles indicated that the sublayer would thickened at blowing control with various jet angles.The thickened sublayer leads to a thinner buffer layer(especially at?=123°in dissymmetrical blowing)where the energy and momentum transfer between the inner and outer layer,as well as the turbulence production,take place mostly.