Effect of streamwise vortex induced by streamwise grooves on drag reduction and heat transfer performance

Based on the speed streaks spacing of 100 times the dimensionless viscous length. In a rectangular channel, the streamwise grooves were arranged at the corresponding positions where the low-speed streaks are likely to occur. The influence of streamwise grooves on flow structure and the comprehensive thermal performance are numerically investigated. The results show that the streamwise grooves inhibited the spanwise movement of the speed streaks and destroyed the self-sustainability of the turbulence, which results in the achievement of the drag reduction rate of 11.7%–16.78%. The velocity gradient inside the streamwise grooves reduced, which contributed to the obtaining of drag reduction rate of 75.53%, in spite of occupying only 20.7% of channel bottom area. In addition, the effect of the evolution of the hairpin vortex on the shear stress is paid attention as well. It's found that the occurrence of head and leg of hairpin vortex would lead to the reduction of shear stress, while the dramatic development of hairpin vortex would lead to sharp increase in shear stress. Furthermore, by quantifying the frequency of turbulent motion, it's found that the redistribution of Reynolds stress increases the ejection by 58% and decreases sweep by 45% at Reynolds number of 3745. Consequently, the heat transfer increases by 7.69%, the drag reduction rate reaches 11.7%, and the comprehensive thermal performance coefficient reaches 1.122. With the increase of Reynolds number, the influence of large-scale vortex induced by grooves on heat transfer is gradually weakened, but the drag reduction could still be obtained, and the comprehensive thermal performance evaluation indexes are greater than 1.