Influence of bionic V-shaped micro-riblet structures on the drag reduction characteristics of wall turbulence

Bionic microstructures can be expected to reduce the wall drag effect of irrigation channels in modern agriculture. However, there was a different drag reduction of bionic microstructures with different structures and sizes. This study aims to explore the influence of bionic V-shaped micro-riblet structures on the drag reduction of wall turbulence. Three types were closely arranged with angles of 60°, 75°and 90°, and a depth of 0.8mm. Particle Image Velocimetry (PIV) was used to measure the flow field at different Reynolds numbers. A systematic analysis was implemented to determine the average flow velocity, Reynolds shear stress and drag reduction effect on the V-shaped micro-riblet wall. Spatial correlation analysis was also conducted to explore the evolution of the turbulent vortex structures in the wall region. The results showed that: 1) There was stratification behavior in the average velocity of the turbulent water flow on the V-shaped micro-riblet wall, indicating the significant upward trend in the log law region. There was an increase in the constant term B of the fitted average velocity formula, corresponding to the increasing average flow velocity on the V-shaped micro-riblet wall. The average velocity distribution was ranked in descending order of 60°, 75°, and 90°, respectively, along the V-shaped micro-riblet wall. 2) In the buffer layer, the Reynolds shear stress increased as the riblet angle increased. In the log-law layer, the Reynolds shear stress tended to be consistent. There was a decrease in the Reynolds shear stress of the buffer layer on the V-shaped micro-riblet wall, compared with a smooth wall. 3) There were smaller characteristic length and structure angle of turbulent vortex structures on the V-shaped micro-riblet wall, compared with the smooth wall. The minimum angle of the turbulent vortex structure was 6.7°. The reduction rate of the characteristic length for the vortex structures was within 10 % in the flow direction, whereas the reduction in the normal direction was more significant (ranging from 7.69 % to 23.8 %), indicating that the V-shaped micro-riblet wall strongly inhibited turbulent vortex structures in the normal direction. V-shaped micro-riblet wall altered the spatial correlation of the turbulent water vortices, thereby inhibiting the development of vortex structures. 4) Given the same dimensionless micro-riblet spacing s+, a smaller bionic riblet angle caused a higher drag reduction rate. Among them, the maximum drag reduction rates were 7.60%, 6.11% and 4.94%, respectively, for the 60°, 75° and 90° V-shaped micro-riblet walls, indicating that the drag reduction rate was significantly influenced by the micro-riblet angle. The drag reduction rate was 7.6% for the V-shaped micro-riblet wall of 60° angle, within the range of the experiment Reynolds number, indicating the best among all the tested riblet structures. The speedup and drag reduction were within the boundary layer. The band-like stripes on riblets inhibited the generation of transient vortex structures and near-wall turbulence. The bionic V-shaped micro-riblet wall was applied to the water conveyance channel for the wall drag reduction. There was an interaction between the turbulent water flow and the bionic wall. Meanwhile, the V-shaped micro-riblet wall has the drag reduction properties. The average flow velocity at the wall increased, and the water conveyance capacity of the irrigation channel increased.The findings can lay a theoretical foundation for the V-shaped micro-riblet wall in irrigation channels.