Drag forces on a bed particle in open-channel flow: effects of pressure spatial fluctuations and very-large-scale motions
Drag forces on a bed particle in open-channel flow: effects of pressure spatial fluctuations and very-large-scale motions
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DOI：
10.1017/jfm.2018.1003
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发表时间：
2019-01
影响因子：
3.7
通讯作者：
S. Cameron;V. Nikora;Ivan Marusic
中科院分区：
文献类型：
--
作者：
S. Cameron;V. Nikora;Ivan Marusic
利用立体粒子图像测速技术，沿着记录了作用于明渠流床面的球形粗糙元上的脉动阻力，并同步测量了周围的速度场。配备有力传感器的目标颗粒的突起相对于六边形填充的相邻球体在零直径和二分之一直径之间系统地变化。拖曳力波动的预乘谱被发现具有双峰形状与低频（${\approx}0.5~\text{Hz}$）峰值对应的湍流中存在的超大尺度运动（VLSMs）。的高频率（$\gtrapprox 4~\text{Hz}$）区域的阻力谱不能解释的速度时间序列提取的点周围的颗粒，而是出现在上覆流场中的压力梯度的作用占主导地位。对于小颗粒突起，该高频区域贡献了大部分的拖曳力方差，而低频拖曳力波动的相对重要性随着突起的增加而增加。高频拖曳力波动的幅度调制的VLSM的颗粒突起无关。这些结果提供了一些洞察床颗粒稳定性的力学，并表明，颗粒夹带的最佳条件可能会发生时，嵌入在VLSM的高速部分覆盖颗粒的低压区域。
The fluctuating drag forces acting on spherical roughness elements comprising the bed of an open-channel flow have been recorded along with synchronous measurements of the surrounding velocity field using stereoscopic particle image velocimetry. The protrusion of the target particle, equipped with a force sensor, was systematically varied between zero and one-half diameter relative to the hexagonally packed adjacent spheres. Premultiplied spectra of drag force fluctuations were found to have bimodal shapes with a low-frequency ( ${\approx}0.5~\text{Hz}$ ) peak corresponding to the presence of very-large-scale motions (VLSMs) in the turbulent flow. The high-frequency ( $\gtrapprox 4~\text{Hz}$ ) region of the drag force spectra cannot be explained by velocity time series extracted from points around the particle, but instead appears to be dominated by the action of pressure gradients in the overlying flow field. For small particle protrusions, this high-frequency region contributes a majority of the drag force variance, while the relative importance of the low-frequency drag force fluctuations increases with increasing protrusion. The amplitude of high-frequency drag force fluctuations is modulated by the VLSMs irrespective of particle protrusion. These results provide some insight into the mechanics of bed particle stability and indicate that the optimum conditions for particle entrainment may occur when a low-pressure region embedded in the high-velocity portion of a VLSM overlays a particle.