Measurement Of The Skin-friction Drag In A Turbulent Boundary Layer And Active Control For Drag Reduction

The turbulent boundary layer exists widely in our daily life and lots of industrial applications,such as commercial airplanes,high-speed railway and oil pipes.In these applications,the skin-friction drag in the turbulent boundary layer is one of the main sources of the energy consumption.For example,the skin-friction drag in the turbulent boundary layer accounts for over 50%of the total drag for the commercial airplanes under their cruising condition.The reduction of the skin-friction drag in the turbulent boundary layer may not only lead to large economic benefits but also reduce the emission of the greenhouse gases,which is in favor of the environmental protection.Therefore,it is of great importance to develop the drag reduction techniques in the turbulent boundary layer.The accurate measurement of the skin-friction drag is an essential issue in the ex-perimental study of the turbulent boundary layer control for drag reduction.In present investigation,a high-resolution floating element(FE)-force balance is developed to mea-sure the small friction force in the turbulent boundary layer,with the resolution on the order of 10-6N.Based on the lever principle,the skin-friction drag on the FE is mechanically amplified and converted to a vertical force,which is measured by a load cell.The possible error sources are systematically investigated and countermeasures are taken to minimize or eliminate the errors.As the skin-friction force in the turbulent boundary layer is quite small,the conventional calibration method with pulley and weights can not be used for present study.A novel calibration method is proposed,in which the output voltage Eo is calibrated by the ?-particle tracking velocimetry(?-PTV)-measured skin-friction force F on the FE.Eo and F are linearly correlated.The balance-measured friction coefficient cf deviates by no more than 1.9%from the Coles-Fernholz correlation for the friction Reynolds number Re? of 570?1487.Besides,the dynamic response of the balance is analyzed in detail through step response experiment.Three plasma actuator configurations are developed in the turbulent boundary layer to generate large-scale streamwise vortices,with the view to reduce the skin-friction drag.Due to the difference in the number and arrangement of electrodes for different plasma actuator configurations,the generated large-scale streamwise vortices varies with each other in the strength,rotating direction and spanwise separation.The plasma actuator configuration B,which generates non-colliding counter-rotating streamwise vortices,can achieve the best performance in terms of both drag reduction and control efficiency among the three configurations.The maximum spatial-averaged drag reduction of 26%can be achieved with the applied voltage E-5.75kVp-p and the drag reduction lasts for more than 2000 wall units downstream of the plasma actuators.The local drag reduction downstream of the actuators is not uniform due to the plasma-generated large-scale streamwise vortices.The spanwise distributions of the local drag reduction are qualitatively the same at different streamwise locations and contain three distinct regions,i.e.the maximum drag increase region(R1),the maximum drag reduction region(R2)and the drag reduction plateau region(R3).Smoke-wire visualization results indicate that the low-speed fluids are pushed into the centerline region of the actuators,forming a large low-speed ribbon.In the meanwhile,the high-speed fluids lie between the low-speed ribbons,leading to the more stabilized streaky structures.Besides,the strength of the bursting event decreases and the turbulent kinetic energy production drops obviously.The drag reduction mechanism is proposed based on the altered flow structures under control.The plasma-generated large-scale streamwise vortices can merge the streaks and make them less meandering.Consequently,the streaky structures are stabilized.As the instability of the streaks is essential to the generation of new quasi-streamwise vortices in the turbulence regeneration cycle,the stabilization of the streaky structures may interrupt the near-wall turbulence regeneration cycle,suppressing the generation of new vortices.Therefore,the turbulent kinetic energy production is reduced and the turbulence intensity is weakened,accounting for the drag reduction.In summary,a high-resolution(10-6N)FE-force balance is developed to measure the small skin-friction drag in the turbulent boundary layer,which provides a useful tool for the experimental study of the turbulent boundary layer drag reduction.The plasma actuators are employed in the turbulent boundary layer to reduce the skin-friction drag and the maximum spatial-averaged drag reduction of 26%is achieved.Besides,the drag reduction mechanism is proposed based on the altered flow structure.This work gives a new idea for the experimental investigation of the drag reduction in the turbulent boundary layer.