Entrainment of sediment particles by very large-scale motions
Entrainment of sediment particles by very large-scale motions
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DOI：
10.1017/jfm.2020.24
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发表时间：
2020-02
影响因子：
3.7
通讯作者：
S. Cameron;V. Nikora;M. Witz
中科院分区：
文献类型：
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作者：
S. Cameron;V. Nikora;M. Witz
利用布置在两个正交面上的立体粒子图像测速仪(PIV)捕捉明渠水流中球形床面颗粒卷吸瞬间的流动结构。实验是在直径为16 mm的共面固定球床中进行的。目标粒子的突出物设置为平均夹带率为$1/60~{S}^{-1}$。这些突起是从广泛的初步实验中建立的，这些实验利用一种自动机制将球体放置在水槽的床上，并记录它们被水流卷走所经过的时间。结果表明，在较低的流深与颗粒直径比下，床面颗粒更稳定，需要较大的凸起才能以与较大深度相同的速度卷吸。这一效应与低潜流时速度变化和阻力变化减小的观测结果是一致的。PIV测量表明，颗粒夹带与沿流向延伸至50个流动深度的超大规模运动有关。较小尺度的速度和压力空间波动的贡献被与颗粒大小相关的空间平均效应和与粒子从静止口袋中完全卷起所花费的时间相关的时间平均效应所抑制。这些观测结果与泥沙输运模型有关。然而，还需要进一步的数据来澄清颗粒升力和颗粒形状在夹带过程中的作用。
Stereoscopic particle image velocimetry (PIV) configured in two orthogonal planes was utilised to capture the flow structure at the instant of entrainment of spherical bed particles in open-channel flow. Experiments were conducted with lightweight target particles amongst a bed of coplanar fixed spheres with diameters of 16 mm. The protrusions of the target particles were set to give an average entrainment rate of $1/60~\text{s}^{-1}$. These protrusions were established from extensive initial experiments which utilised an automated mechanism to place spheres on the bed of the flume and record the time elapsed until they were entrained by the flow. The results showed that at lower flow depth to particle diameter ratios, bed particles are more stable and require larger protrusions to entrain at the same rate as at a larger depth. This effect is consistent with observations of reduced velocity variance and reduced drag force variance for lower flow submergences. The PIV measurements indicated that particle entrainment is associated with very large-scale motions which extend up to 50 flow depths in the streamwise direction. Contributions of smaller scale velocity and pressure spatial fluctuations are suppressed by a spatial averaging effect related to the particle size, and a temporal averaging effect related to the time taken to fully entrain a particle from its resting pocket. These observations are relevant to sediment transport modelling. However, further data are required to clarify the role of particle lift forces and particle shape in the entrainment process.