Research On Effect Of Streamwise Slit Width On Turbulent Boundary Layer Drag Reduction Using Blowing

The moving vehicle and aircraft which directly contact with air will produce skin friction,of which the amount occupies a large part of total drag.For example,when aircraft is cruising at subsonic speeds,about half of total drag comes from air friction.By decreasing their skin friction,it not only reduces energy consumption and CO₂emissions,but also improves its hydrodynamic performance.Therefore,it has important research and application value for turbulent boundary layer（TBL）drag reduction control.Streamwise slit blowing is a simple and efficient drag reduction method for active control of TBL.In this project,based on the existed streamwise slit width b⁺=3.3 blowing array（b⁺=b/（v/u_τ）,where b means slit width,v means kinematic viscosity（m²/s）,u_τmeans wall friction velocity（m/s））,the influence of streamwise slit width of actuator on drag reduction of TBL is studied.It mainly experimentally studies and analyzes on the local,streamwise,spanwise reduction rate distribution of the actuators with streamwise slit width b⁺=5,6.7 and8.3 under steady and unsteady blowing.The TBL under different control conditions were compared and analyzed from the perspectives of TBL velocity profile,sudden event detection and fluid flow state.Under steady blowing,the local drag reduction of b⁺=5,6.7,8.3 respectively reach about 69%,72%,68%,and it is relativeliy larger when b⁺=6.7 at the blowing rateσ=0.032.It is found that increasing the streamwise slit width only reduces the overshoot strength of downstream,and has no effect on the drag-reduction and overshoot range.For unsteady blowing,with slit width increase,the increase trend of local drag reduction rate tends to be flat with blowing rate increasing.Under low blowing rate,increasing the slit width will make the local drag reduction rate under unsteady blowing better than that obtained under steady blowing,there is the most obvious phenomenon when b⁺=6.7 and the downstream control range is 15.8%larger than steady blowing,and the drag increase range is 27.2%smaller.Particle Image Velocimetry measurement results show that a counter-rotating vortex pair generates between two adjacent jets of streamwise slit,when b⁺=6.7,the vortex pair scale is comparable to viscous sublayer and more suitable for drag reduction and turbulence intensity inhibition.Based on results of flow visualization,when b⁺=6.7,it is found that slit blowing will destroy streaks of TBL and inhibit the turbulence generation cycle,that it is replaced by a regular streamwise small-scale stable structure with the largest control range along the downstream than other slit width.The experimental results show that when b⁺=6.7,the jet generated by the slit has more effective control for turbulence generation.