Flow-measurements in the wake of an oscillating sessile droplet using laser-Doppler velocity profile sensor
Flow-measurements in the wake of an oscillating sessile droplet using laser-Doppler velocity profile sensor
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DOI：
10.1515/teme-2021-0119
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发表时间：
2022-01-29
影响因子：
1
通讯作者：
Janoske, Uwe
中科院分区：
文献类型：
--
作者：
Burgmann, Sebastian;Kraemer, Veronika;Janoske, Uwe
尽管在许多技术应用中是相关的，但通过（空气）流去除表面上的固着液滴仍然难以预测。液滴周围的流动尚未详细研究，但到目前为止，知识的流动结构是必不可少的适当的阻力相关性的评估。研究了气流中PMMA基片上的小液滴（5-40 μ l）。由于液滴的小尺寸，具有频移的激光多普勒速度分布传感器被应用于测量振荡的、仍然粘附的液滴的尾流中的流动。由于激光多普勒速度分布传感器的高空间和时间分辨率及其测量双向流动的能力，可以精确分析液滴后面的流动。额外的热膜测量用于调查的时间行为的流动。补充，数值模拟进行应用修改VOF（体积的流体）方法。实验和数值计算数据的结合给出了固着液滴尾流结构的新见解：随着雷诺数的增加，可以检测到回流的液滴尾流。来自液滴上侧的分离剪切层导致涡流脱落，并在时间平均值上形成再循环区域。与刚性半球相反，液滴的空气-液体界面的移动导致由外部流动结构驱动的内部流动，反之亦然。这是一个提示，固着液滴的阻力系数不能简单地通过分析刚性半球形结构的流动来推导。此外，液滴轮廓和尾流表现出相同的特征振荡频率。与刚性半球的斯特劳哈尔数0.28相比，相应的斯特劳哈尔数几乎恒定在0.03。因此，可以假设可能存在最终导致液滴运动的气动弹性自激效应。
Although relevant in many technical applications, the removal of sessile droplets on surfaces by an (air-) flow is still hard to predict. The flow around the droplet has not been investigated in detail so far but knowledge on the flow structure is essential for the assessment of appropriate drag force correlation. Small droplets (in the range of 5-40 mu l) on PMMA substrate in an air flow are investigated. Due to the small size of the droplet the laser-Doppler velocity profile sensor with frequency shift is applied to measure the flow in the wake of the oscillating, still adhering droplet. Thanks to the high spatial and temporal resolution of laser-Doppler velocity profile sensor and its capability to measure bidirectional flows, the flow behind the droplet can be precisely analyzed. Additional hot-film measurements are used to investigate the temporal behavior of the flow. Complementary, numerical simulation is performed applying a modified VOF (Volume-of-Fluid) method. The combination of the experimental and numerical data gives new insight in the wake flow structure of sessile droplets: With increasing Reynolds number, a backflow can be detected in the wake of the droplet. A separated shear layer stemming from the upper side of the droplet leads to a vortex shedding with formation of a recirculation region in the temporal mean. In contrast to rigid hemispheres, the movement of the air-liquid interface of the droplet leads to an internal flow which is driven by the outer flow structure and vice versa. This is a hint that drag coefficients of sessile droplets cannot be simply derived by analyzing flows of rigid hemispherical structures. Additionally, droplet contour and wake flow exhibit the same characteristic oscillation frequency. The corresponding Strouhal number is almost constant at 0.03 compared to the Strouhal number of a rigid hemisphere of 0.28. Therefore, it can be assumed that an aeroelastic self-excitation effect may be present that eventually leads to droplet movement.