粒子通过密度过渡层沉降时的弹跳行为
Bouncing behaviour of a particle settling through a density transition layer
AI总结：
本研究通过实验和数值模拟探讨了粒子在三层分层流体中沉降时的弹跳行为，发现尾流脱离后产生的向上射流是引发弹跳的必要条件，且该行为主要由下雷诺数决定。
摘要：
本工作聚焦于粒子在无中性浮力位置情况下通过三层分层流体沉降时的一种特定弹跳行为，该行为由 Abaid, N., Adalsteinsson D., Agyapong A. & McLaughlin, R.M. (2004) 在盐度诱导的分层中首次发现。我们进行了实验和数值模拟。在实验中，通过在粒子中心平面用激光片照射，我们很好地捕捉到了其弹跳行为。我们发现弹跳过程在尾流从粒子脱离后开始。PIV 结果表明，尾流破裂后，在粒子后方的中心轴线上会产生一股向上射流。通过进行力分解程序，我们量化了由尾流浮力（$F_{sb}$）和流动结构（$F_{sj}$）引起的增强阻力。值得注意的是，$F_{sb}$ 在早期阶段对增强阻力起主要贡献，但在尾流脱离后其主导作用减弱。相比之下，$F_{sj}$ 在反转粒子运动方面起关键作用。我们推测射流是发生弹跳运动的必要条件。然后，我们在 $1 \leq Re_l\leq 125$、$115 \leq Re_u\leq 356$ 和 $2 \leq Fr\leq 7$ 的范围内，通过改变下雷诺数 $Re_l$、弗劳德数 $Fr$ 和上雷诺数 $Re_u$ 来检验粒子的最小速度（发生弹跳时为负值）。我们发现弹跳行为主要由 $Re_l$ 决定。在我们的实验中，发现弹跳运动发生在临界下雷诺数约 $Re^ \ast _{l}=30$ 以下。在数值模拟中，该临界数的最高值为 $Re^ \ast _{l}=46.2$，受限于当前研究的参数范围。
英文摘要：
The present work focuses on a specific bouncing behaviour as a particle settling through a three-layer stratified fluid in the absence of neutral buoyant position, which was firstly discovered by Abaid, N., Adalsteinsson D., Agyapong A. & McLaughlin, R.M. (2004) in salinity-induced stratification. Both experiments and numerical simulations are carried out. In our experiments, illuminated by a laser sheet on the central plane of the particle, its bouncing behaviour is well captured. We find that the bouncing process starts after the wake detaches from the particle. The PIV results show that an upward jet is generated at the central axis behind the particle after the wake breaks. By conducting a force decomposition procedure, we quantify the enhanced drag caused by the buoyancy of the wake ($F_{sb}$) and the flow structure ($F_{sj}$). It is noted that $F_{sb}$ contributes primarily to the enhanced drag at the early stage, which becomes less dominant after the detachment of the wake. In contrast, $F_{sj}$ plays a pivotal role in reversing the particle's motion. We conjecture that the jet flow is a necessary condition for the occurrence of bouncing motion. Then, we examine the minimal velocities (negative values when bounce occurs) of the particle by varying the lower Reynolds number $Re_l$, the Froude number $Fr$ and the upper Reynolds number $Re_u$ within the ranges $1 \leq Re_l\leq 125$, $115 \leq Re_u\leq 356$ and $2 \leq Fr\leq 7$. We find that the bouncing behaviour is primarily determined by $Re_l$. In our experiments, the bouncing motion is found to occur below a critical lower Reynolds number around $Re^ \ast _{l}=30$. In the numerical simulations, the highest value for this critical number is $Re^ \ast _{l}=46.2$, limited in the currently studied parametric ranges.
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