Dynamics of falling films on the outside of a vertical rotating cylinder: waves, rivulets and dripping transitions
Dynamics of falling films on the outside of a vertical rotating cylinder: waves, rivulets and dripping transitions
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DOI：
10.1017/jfm.2017.657
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发表时间：
2017-10
影响因子：
3.7
通讯作者：
M. Rietz;B. Scheid;F. Gallaire;N. Kofman;R. Kneer;W. Rohlfs
中科院分区：
文献类型：
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作者：
M. Rietz;B. Scheid;F. Gallaire;N. Kofman;R. Kneer;W. Rohlfs
在板的底面或旋转的圆柱体的外部，下降的液膜受到不稳定的体力的影响。薄膜拓扑的演变是由Kapitza和Rayleigh-Taylor不稳定性之间的相互作用决定的，导致薄膜表面的复杂图案化，并最终导致流体从衬底上分离。本文通过实验研究了大半径垂直旋转圆柱体外表面薄膜的表面拓扑结构的演变。液体/空气界面处的剪切通过外部同向旋转的圆柱体来抑制。通过高速可视化技术捕捉到控制参数，即雷诺数和旋转频率对薄膜演化的影响。瑞利-泰勒不稳定性对不断增加的不稳定体力(增加圆柱体的转速)的影响越来越大，最显著的是以主导薄膜拓扑的溪流结构的起始长度缩短的形式观察到。小溪的波长和起始长度与经典Rayleigh-Taylor问题的线性稳定性分析的预测相符。在这种情况下，实验和支持的数值结果表明，对于任何非零值的不稳定体力，在给定的演化长度之后，随着体力的增加而减小，都会出现小溪。流体从基质中剥离与溪流结构的存在密切相关。根据控制参数的不同，分离液滴要么是不同相速的孤立脉冲与溪流相互作用的结果，要么是在二维波进入溪流失稳后直接观察到的，要么是立即在流体入口处观察到的。通过与该问题的积分边界层形式的线性稳定性分析所预测的对流/绝对不稳定转变的比较，表明对主要滴落机制的预测超出了线性分析的范围。
Falling liquid films on the underside of a plate or on the outside of a rotating cylinder are subject to a destabilizing body force. The evolution of the film topology is determined by interactions between the Kapitza and the Rayleigh–Taylor instability, leading to complex patterning of the film surface and eventually fluid detachment from the substrate. This study experimentally investigates the evolution of the surface topology for a film on the outside of a vertical rotating cylinder of large radius. Shear at the liquid/air interface is suppressed through an outer, co-rotating cylinder. The film evolution is captured through high speed visualization in dependence of the control parameters, namely Reynolds number and rotation frequency. An increasing influence of the Rayleigh–Taylor instability for an increasing destabilizing body force (increasing rotational speed of the cylinder) is most notably observed in the form of a decreasing inception length of rivulet structures dominating the film topology. Wavelength as well as inception length of rivulets match the predictions from linear stability analysis of the classical Rayleigh–Taylor problem. In this context, experimental and supporting numerical results suggest that the emergence of rivulets occurs for any non-zero value of the destabilizing body force after a given evolution length that decreases with increasing body force. Fluid detachment from the substrate is found to be intimately related to the existence of rivulet structures. In dependence of the control parameters, detaching droplets are either observed as a result of interactions of solitary pulses of varying phase speed on rivulets, directly after destabilization of two-dimensional waves into rivulets or immediately at the fluid inlet. By comparison to the convective/absolute instability transition predicted by linear stability analysis of an integral boundary layer formulation of the problem in question, it is shown that the prediction of a predominant dripping mechanism lies beyond the scope of linear analysis.