Numerical investigation on the flow past circular cylinders with modification of partially covered V-shaped riblets in the subcritical regime using large eddy simulation

A numerical investigation is conducted on the flow past circular cylinders partially covered with circumferential V-shaped riblets using large eddy simulation at a subcritical Reynolds number of 3900. Simulations covered the spanwise converge ratio φ from 15% to 35% in 5% increments, with the bare cylinder serving as the validation reference. The effects of flow control and the drag-reduction mechanism are discussed from several aspects. It is found that the riblets modify the flow field by elongating the wake length and inducing semi-circular regions of spanwise flow. The three-dimensional flow pattern is characterized by spanwise zigzag-shaped separation lines and the delayed flow separation at riblet valleys. Compared with the bare cylinder, a maximum drag reduction rate of 6.81% is achieved at φ = 25%, proved to be owing to the modified separated free shear layers and stabilized wake. Then, the applicability of the secondary vortex group theory on the cylinder is validated with an efficient evaluation method of drag performance based on the streamwise vorticity. Combined with spanwise correlation analysis and the significant reduction in the lift fluctuation, it is concluded that the riblets-covered cylinders could obtain a better resistance to the vortex-induced vibrations.