雨滴形成的甜甜圈状气泡
Donut-shaped Bubbles Formed by Raindrops
摘要：
单自由落体淡水滴产生无初速度的皮下注射针头，在海拔3.61米以上的一个仍然淡水表面。获得了撞击动态的高分辨率高速视频（0.13 mm/像素，500帧/秒）。在形成通常观察到的空腔、冠状物和冠状物（Prosperetti等人，1993年）后的几毫秒内，直径通常大于3.8 mm的水滴在撞击水面时持续产生环形（甜甜圈形状）气泡。从不同角度连续拍摄撞击动态的视频，并使用105 mm镜头聚焦于事件的不同区域。这些允许对观察到的现象进行定性描述和假设性解释，这些现象与实际视频片段一起呈现在由此显示的流体动力学视频中。视频显示了实验装置的卡通，随后是从2个不同的视角（正面和顶部）同时运行的水滴撞击水面的两个视频剪辑。由于事件发生时间不到1秒，视频被减慢到12帧/秒的帧速率。画面显示，空气被封闭的树冠的收敛边缘（或冠状）所捕获。空腔的最底部迅速上升，并与完全收敛的树冠边缘提供的下落的水体相撞。这顺便防止了二次射流的形成。不稳定的环形气泡的中心（或圆环孔）最终向外移动，从而产生半球形气泡。
英文摘要：
Single free-falling freshwater drops were generated with no initial velocity by hypodermic needles, at an altitude of 3.61 m above a still freshwater surface. High resolution high speed videos (0.13 mm/pixel, 500 frames/second) of the dynamics of the impact were acquired. A few milliseconds after forming the usually observed cavity, canopy and coronet (Prosperetti et al. 1993), drops of diameters typically greater than 3.8 mm consistently generated toroidal (donut-shaped) air bubbles upon impact at the water surface. Videos of the dynamics of the impingement were successively taken from different angles, and with a 105 mm lens focused on different regions of the event. These allowed for a qualitative description and hypothetical explanation of the observed phenomena, which are presented alongside the actual video footage, in the hereby displayed fluid dynamics video. The video displays a cartoon of the experimental setup, followed by two simultaneously running videoclips of the drop impact upon the water surface, from 2 different viewpoints (front and top). As the event occurs over less than 1 second, the videos were slowed down to a frame rate of 12 frames/sec. The footage suggests that the air is trapped by the converging rim (or coronet) of the closing canopy. The very bottom of the cavity then rises rapidly, and collides with the falling water mass provided by the fully converged canopy rim. This incidentally prevents the formation of a secondary jet. The center (or donut hole) of the unstable toroidal air bubble eventually moves outward, thus yielding a hemispherical air bubble.
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