Turbulent Boundary Layer Drag Reduction Using Periodic Blowing Through Streamwise Slits

Whether it is aircraft or marine vehicles,surface friction drag accounts for a large part,and excessive surface friction drag has a huge impact on their fuel economy.Therefore,it is very necessary for turbulent boundary layer control to realize the reduction of surface friction drag.Periodic blowing disturbance has many advantages such as simplicity,high efficiency and easy control of excitation intensity.In this paper,the drag reduction effects of different number of streamwise slit arrays are studied.According to the existed 31 streamwise slits actuator,an actuator with 170 streamwise slits was designed.The local drag reduction rates of different control parameters were measured by hot-wire anemometer at x+=33,and it was found that 73% of the maximum local drag reduction rates were obtained when the control parameters were optimal(blow intensity A+ = 1.85,blow frequency f + = 0.14).Moreover,under the optimal control parameters,the drag reduction range of 31 slits actuator has 450 wall units and the drag increase range has 700 wall units.Compared with 31 slit actuators,the drag reduction range of 170 slit actuators was greatly increased to 650 wall units,and the drag increase range was greatly reduced to 300 wall units.It can be seen that as the number of slits increases along the spanwise,it has a great influence on the downstream control range,and the drag reduction range will increase and corresponding drag increase range decrease.The local drag reduction is distributed in a parabolic shape along the spanwise.The results show that the large-scale structure near the wall is pushed away from the wall by periodic blowing,and a low-speed gas layer with regular height is introduced near the wall to prevent the high-speed streak from sweeping down to the wall,thus achieving drag reduction.The increasing number of slits prevents the streak from drifting towards the center of the control area,which makes the drag reduction effect more obvious.However,as the streamwise distance increases,the strength of the blowing disturbance decreases,and the streak gradually moves towards the central region.In addition,during the drift,the streak will be torsional and curved,resulting in the influence of approximate streamwise vortex.The drag increases slightly in the downstream.