Drag reduction of high Reynolds number turbulent boundary layers using 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 (Re_t). To this end, the high-resolution force balance developed by Cheng et al. (2021a,b) has been substantially improved to measure accurately the space-averaged drag reduction (DR) over the control area. One array of steady blowing microjets through spanwise slits is deployed, which has been demonstrated to be able to yield a significant DR; meanwhile, its associated surface roughness incurs little additional drag. Experiments were carried out with a Re_t range from 1,000 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, reaching 25% at 66 m/s. Hotwire data analysis as well as flow visualization indicate 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 and causing local flow relaminarization. Both contribute to the DR.