Skin Friction Reduction in a High Reynolds Number Turbulent Boundary Layer Using Spanwise Blowing Microjets

This work aims to develop a technology to reduce skin friction in the turbulent boundary layer (TBL) of a high-speed train, which is both effective and efficient even at high friction Reynolds numbers (Reτ) that occur in engineering applications say a 600 km/hr super high-speed maglev train. To this end, the high-resolution floating-element force balance developed by Cheng et al. in (J Fluid Mech 918:A24 [1]), Cheng et al. in (Meas Sci Technol 32, 2]) has been substantially modified and improved in order to measure accurately the space-averaged drag reduction (DR) over the control area. Arrays of steady blowing microjets through spanwise slits are deployed, which have been demonstrated to be able to yield a significant DR; meanwhile, its associated surface roughness incurs little additional friction drag. Experiments were carried out with a Reτ range from 1000 to 18,000. Results obtained so far indicate that the maximum spatially averaged DR over the control area may shoot beyond 70%, though the maximum net energy saving occurs when the DR is only 20–30%. This net energy saving grows with increasing free-stream velocity, reaching 15% at 66 m/s. Work is under way to perform experiments to gain the in-depth understanding of the control mechanisms.