Skin Friction Drag Reduction Using Unsteady Blowing Through Two Arrays Of Streamwise Slits

Frictional drag is a significant part of flow induced drag force by submarines or surface crafts,which restrains vessels from expanding cruising range and optimizing economics.Active control actuators are reported to be the most effective in achieving drag reduction in turbulent boundary layer(TBL).Based on existed streamwise slit actuator,another array of slits was located at somewhere downstream for the purpose of expanding effective control area.The structure of jet supporting system was optimized to meet the demand of engineering and experiments.New tubes were designed to decrease their span-wise longitude and able to inject particles into jetting air.Hot-wire constant temperature anemometry,PIV and flow visualization technique were used in the research.The output velocity of a slit jet was calibrated.Drag reduction rate induced by the jets and its recovery were both detected at different working conditions with a variety of the gap between two array of actuators.Results were compared to the control effect of a single actuator and that without control.The optimistic control effect occurred when the gap from trailing edge was set as 35 mm.At x = 35 mm(x+ = 326.79)streamwise after the downstream actuator,the local drag reduction rate was up to ?(28)58.06%,and remained 12% at x = 45 mm(x+ = 420.16).Effective interference covers a streamwise range over 80 mm,ruling out the space right above the actuators.The streamwise velocity profile along the y-direction was also tested,revealing that the added slits not only could expand the down-stream-wise control area,but also interfered the flow structures at high positions above the wall up to y+ = 10 in the buffer layer of a TBL.Particle images on the x-y plane at z = 0 indicated vortices in the buffer layer were eliminated by the second actuator.Sizes of eddied were dramatically deduced.Flow structures at the x-z plane with y+ = 6.67 were recorded.Sweeping of highvelocity streaks was proved to be the main factor inducing drag force recovery.The results would be offered as guiding reference for further research with steam-wise slit jet actuators.