Direct numerical simulation of turbulent flow over wide‐rib rectangular grooves

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The turbulent flow over the wide‐rib rectangular grooves, the motions and variations of near‐wall streamwise vortices with time, and the interaction between microgroove and near‐wall streamwise vortices were investigated by direct numerical simulation method (DNS). The distributions of radius, density, and swirling strength of streamwise vortex were also studied quantitatively by using swirling‐strength criterion. It was found that the distribution of vortex radius in smooth channels can approximately be divided into three parts. The vortex radii are smaller in grooved channels than in smooth channels and almost the same when y+ > 40 for all grooved cases. Moreover, a simple prediction method was proposed to estimate the optimal height and spacing of drag‐reducing microgrooves for different fluids, and they were about 10 and 17 wall units for water, respectively. Furthermore, using the same frictional velocity uτ to normalize the shear stress is more suitable for the quantitative comparison and analysis of different longitudinal microgrooves. The drag‐reducing mechanism of longitudinal microgrooves could be considered as the competition results between the “restriction or blockage effect” of microgroove on the near‐wall vortices (causing a drag‐reducing effect) and the “tip effect” of microgrooves caused by the scouring of higher speed fluid near the groove tip (causing a drag‐increasing effect). A large number of small streamwise secondary vortices with small swirling strength within the groove valley, which are induced by microgrooves, may be the essential reason of drag reduction by microgrooves.