下降液幕中的反射波模式
Propagative waves pattern in a falling liquid curtain
摘要：
我们已经做了液幕从水平的、湿的、被两个垂直的金属丝横向约束的管中落下的实验。流体运动几乎减少到自由落体，在管下方具有非常低的分离速度。因此，帘幕包含大的亚音速区域，即其中弯曲波比流体行进得更快的域。由于上边界在横向上不受约束，我们观察到当流速逐渐减小时，出现了迄今未报道过的不稳定性：幕的顶部进入一种类似漩涡的运动，与幕的波动的传播模式相耦合，结构像棋盘。这种模式的相速度和频率的测量报告。数据与简单的维度参数一致，表明波速与表面张力除以每单位长度液体的质量通量成正比。跨音速点处的流体速度也遵循这种比例关系，即流体速度等于正弦波速度的点。最后，我们讨论了这些结果对全球稳定性的下降窗帘的影响。
英文摘要：
We have preformed experiments on a liquid curtain falling from a horizontal, wetted, tube and lateraly constrained by two vertical wires. The fluid motion nearly reduces to a free-fall, with a very low detachment velocity below the tube. Thus, the curtain contains a large subsonic area, i.e. a domain where the sinuous waves travel faster than the fluid. The upper boundary not being constrained in the transverse direction, we have observed the appearance of an up to now unreported instability when the flow rate is progressively reduced: the top of the curtain enters a pendulum-like motion, coupled to a propagative pattern of curtain undulations, structured as a chessboard. Measurements of the phase velocity and frequency of this pattern are reported. Data are in agreement with a simple dimensional argument suggesting that the wave velocity is proportional to the surface tension divided by the mass flux of liquid per unit length. This scaling is also that followed by the fluid velocity at the transonic point, i.e. the point where the fluid velocity equals that of sinuous waves. We finally discuss implications of these results on the global stability of falling curtains.
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