Skin-friction Drag Reduction In A Turbulence Boundary Layer Via Real-time Control Of Coherent Structures

In the engineering field,turbulent is everywhere.The fluid creates a turbulent boundary layer on the surface of the vehicles,which produces high shear resistance,resulting in waste of energy.Therefore,considering the economic value brought by the turbulent boundary layer drag reduction technology,the research on turbulent boundary layer control is particularly urgent.The traditional open-loop control system does not consider the information in the flow field.There is little research on the closed-loop control research that combines the flow field information as the system input.Aiming at the above background,this paper proposes an experimental study on the wall drag reduction of the turbulent boundary layer in real-time control of coherent structures.Based on the technology of streamwise-slit air-blowing,this paper studies the effects of different closed-loop control schemes on the drag reduction of the near-wall turbulent boundary layer.The experimental wind speed is 2.4m/s.The single point method is used to measure the local drag reduction,and the mobile hot line is used to measure different Under the control condition,the turbulent boundary layer characteristic curve is used to quantitatively analyze each speed statistic.Finally,the flow display technology is used to observe the turbulence structure before and after the control,and the physical mechanism behind the turbulent boundary layer drag reduction is explained from different parts.The experimental results show that under the control condition,the slope of the flow velocity curve was decreased,the linear region was prolonged,the rms of the near wall was decreased,and the burst strength of the near wall was decreased,indicating that the near-wall quasi-order structure was suppressed.The upward or downward trend of the physical quantity above was consistent with the amount of drag reduction obtained by each control method.The results of flow-visualization show that the large-scale vortex structure under controlled conditions is transformed into a small-scale structure,and the strip structure is more evenly distributed along the streamwise direction and the spanwise direction.The experimental results also reveal the basic principle of slit blowdown.The slit blowing has two effects on the turbulent boundary layer near the wall.Part of it is due to the injection of low-speed fluid,which causes the rise of the flow structure,which reduces the flow velocity gradient and thus reduces the drag.The other part is due to the reduction of the high resistance zone.Energy reduces drag.