Skin-friction reduction in a high-Reynolds-number turbulent boundary layer using spanwise-microjet arrays

Abstract: This work aims to develop a technology to reduce skin friction in the turbulent boundary layer of a high-speed train, which is both effective and efficient even at high friction Reynolds numbers (Re). To this end, a high-resolution force balance has been innovatively developed to measure accurately the drag reduction (DR) over the control area. The arrays of wall-normal microjets through spanwise slits are deployed, which have been demonstrated to be able to yield a significant DR in spite of additional drag due to slit-associated surface roughness. Experiments were carried out with a Re? range from 1000 to 18,000. Results obtained 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 30-40%. This net energy saving grows with increasing free-stream velocity; its maximum may currently reach 25% at 40 m/s. Flow visualization data analysis indicates that the microjet blowing lifts up the streamwise vortices in the TBL. Furthermore, the microjets are of zero streamwise momentum, acting to decrease the near-wall streamwise velocity gradient. Both contribute to the appearance of local flow relaminarization and the significant DR. Keywords: Turbulent boundary layer, skin-friction drag reduction, high Reynolds number