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仿生V形微沟槽结构对壁湍流减阻特性影响

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

  • 摘要: 为探究仿生微结构的减阻特性,设计3种紧密排列的V形微沟槽结构(角度60°、75°和90°,深0.8 mm),采用粒子图像测速系统(particle image velocimetry,PIV)观测不同雷诺数下V形微沟槽壁面的流场变化规律,分析其壁面阻力变化,通过空间相关性分析探究壁湍流涡结构变化规律。结果表明:V形微沟槽壁面上湍流边界层平均流速出现明显分层现象,平均流速在对数律区明显上移,平均流速增大;在缓冲层,雷诺切应力随沟槽角度的增大而增大,在对数律层,雷诺切应力趋于一致;V形微沟槽结构近壁处湍流涡结构的特征长度、结构角与光滑壁面相比减小,涡结构的最小结构角为6.7°,微沟槽结构改变了壁湍流涡结构的空间相关性。60°角V形微沟槽壁面的减阻率为7.6%,减阻效果最好。V形微沟槽结构通过改变边界层内湍流涡结构的空间相关性关系实现渠道壁面湍流增速、减阻效果。研究成果对增强渠道壁面输水性能具有重要意义。

     

    Abstract: 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.

     

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